Chapter 10: Haloalkanes and Haloarenes ; Haloalkanes and Haloarenes 10
10.0 Introduction **10.7 Stereochemistry
10.1 Classification 10.8 Nucleophilic substitution mechanism
10.2 Monohalogen derivatives of alkanes 10.9 Haloarenes
10.10 Nature of C X bond in haloarenes 10.3 Nomenclature of haloalkanes *10.11 Preparation of haloarenes 10.4 Nature of C X bond in haloalkanes 10.12 Physical and chemical properties of
10.5 Preparation of haloalkanes haloarenes
10.6 Physical and chemical properties of 10.13 Uses and environmental effects of some haloalkanes haloalkanes and haloarenes * marked section is only for JEE (Main) ** marked section is for NEET UG
10.0 Introduction
Halogen derivatives of alkanes or of arenes: When one or more hydrogen atoms of alkanes or arenes are replaced by corresponding number of halogen atoms, the resulting compounds are called halogen derivatives of alkanes (haloalkanes) or halogen derivatives of arenes (haloarenes) respectively.
Haloalkanes: The halogen derivatives of saturated aliphatic hydrocarbons are called as haloalkanes or alkyl halides. OR Haloalkanes are obtained by replacing one or more hydrogen atom(s) of an alkane with the corresponding number of halogen atom(s).
eg. H3C Cl (Chloromethane) In haloalkanes, halogen atom(s) is/are bonded to sp3 hybridised carbon atom(s) of an alkyl group.
Haloarenes: The halogen derivatives of aromatic hydrocarbons are called as haloarenes or aryl halides. OR Haloarenes are obtained by replacing one or more hydrogen atom(s) of an arene with corresponding number of halogen atom(s). In haloarenes, halogen atom(s) is/are bonded to sp2 hybridised carbon atom(s) of an aryl group.
Note: Several organic compounds containing halogen exist in nature and some of them are clinically useful.
Substance Contains halogen atom Description i. Chloramphenicol (Antibiotic) Chlorine a. Produced by soil micro-organisms. b. Used in treatment of typhoid fever. ii. Thyroxine (Hormone) Iodine a. Produced inside our body. b. Deficiency causes goiter. iii. Chloroquine (Synthetic halogen Chlorine Used in treatment of malaria. compound) iv. Halothane Used as an anaesthetic during surgery. v. Certain fully fluorinated Fluorine Being considered as potential blood compounds substitutes in surgery. 1
Chemistry Vol ‐ 2.2 (Med. and Engg.)
10.1 Classification
Haloalkanes and haloarenes are classified as monohalogen derivatives or polyhalogen (di-, tri-, etc.) derivatives of alkanes and arenes respectively, based on the number of halogen atoms in their structure.
Classification of haloalkanes on the basis of the number of halogen atoms:
Haloalkanes
Monohaloalkanes Polyhaloalkanes
(Monohalogen derivatives of alkanes) (Polyhalogen derivatives of alkanes)
One hydrogen atom of an alkane is More than one hydrogen atom of alkanes are
substituted by one halogen atom. substituted by corresponding number of
General formula: halogen atoms.
C H X [n is an integer] n 2n+1 R X [X = F, Cl, Br, I and R = alkyl group]
eg. CH3 CH2 Br Ethyl bromide (Bromoethane)
Dihalogen derivatives Trihalogen derivatives Tetrahalogen derivatives Two hydrogen atoms of an Three hydrogen atoms of Four hydrogen atoms of an
alkane are substituted by an alkane are substituted alkane are substituted by
two halogen atoms. by three halogen atoms. four halogen atoms. General formula: General formula: General formula: CnH2nX2 CnH2n1X3 CnH2n2X4 [X = F, Cl, Br, I and ‘n’ is [X = F, Cl, Br, I and n is an [X = F, Cl, Br, I and n is an an integer] integer] integer]
eg. CHI3 eg. CCl 4 Iodoform Carbon tetrachloride
Geminal dihalides Vicinal dihalides
Both the halogen atoms are Both the halogen atoms are attached to same C-atom. attached to adjacent
eg. H (vicinal) C-atom.
eg. H2C CH2
H3C C Br Br Br Br Ethylene dibromide Ethylidene bromide (1,2-Dibromoethane) (1,1-Dibromoethane)
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Chapter 10: Haloalkanes and Haloarenes
Classification of monohalocompounds on the basis of nature of CX bond:
Monohalocompounds
Compounds containing Compounds containing 2 sp3CX bond sp CX bond
Alkyl halides Allylic halides Benzylic halides Vinylic halides Aryl halides (Haloalkanes) Halogen atom is Halogen atom is Halogen atom is (Haloarenes) 3 3 Halogen atom is bonded to sp - bonded to sp - bonded to sp2- Halogen atom is bonded bonded to an hybridised carbon hybridised carbon hybridised carbon to sp2-hybridised carbon alkyl group. atom next to C=C atom next to an atom of C=C. atom of an aro-matic General formula: i.e., to an allylic aromatic ring. eg. ring. X CnH2n+1X. carbon. eg. CH2X eg. X eg.
CH2X
10.2 Monohalogen derivatives of alkanes Monohalogen derivatives of alkanes (alkyl halides) are obtained by substituting one hydrogen atom of an alkane by one halogen atom and are further classified as follows:
Alkyl halides
Primary alkyl halide (1) Secondary alkyl halide (2) Tertiary alkyl halide (3)
Halide group is attached to Halide group is attached to secondary Halide group is attached to tertiary
primary carbon atom of an carbon atom of an alkyl group. carbon atom of an alkyl group.
alkyl group. eg. eg. CH3 CH3
eg. CH3CH2CH2Br n- Propyl bromide H3C C Br H3C C Br (1-Bromopropane)
H CH3 tert-Butyl bromide Isopropyl bromide (2-Bromopropane) (2-Bromo-2-methylpropane)
10.3 Nomenclature of haloalkanes
Common and IUPAC names of some monohalogen derivatives: No. Formula Common name IUPAC name
i. CH3Br Methyl bromide Bromomethane
ii. CH3CH2Cl Ethyl chloride Chloroethane
iii. CH3CH2CH2Br n-Propyl bromide 1-Bromopropane iv. Br Isopropyl bromide 2-Bromopropane | (sec-Propyl bromide)
CH3 CH CH3 v. CH3CH2CH2CH2Cl n-Butyl chloride 1-Chlorobutane
vi. CH3CH CH2CH3 sec-Butyl bromide 2-Bromobutane | Br
vii. CH3 Isobutyl chloride 1-Chloro-2-methylpropane CHCH2Cl CH3 3
Chemistry Vol ‐ 2.2 (Med. and Engg.) viii. Br tert-Butyl bromide 2-Bromo-2-methylpropane |
CH3 C CH3 | CH3 ix. Br tert-Pentyl bromide 2-Bromo-2-methylbutane |
CH3 C CH2 CH3 | CH3 x. CH3 Isobutyl bromide 1-Bromo-2-methylpropane
CHCH2Br CH3 xi. CH3 Neopentyl iodide 1-Iodo-2,2-dimethylpropane |
H3C C CH2I | CH3 xii. CH2 = CHCl Vinyl chloride Chloroethene
xiii. CH2 = CH CH2 Br Allyl bromide 3-Bromopropene
xiv. CH2Cl2 Methylene chloride Dichloromethane
xv. CHCl3 Chloroform Trichloromethane
xvi. CCl4 Carbon tetrachloride Tetrachloromethane
xvii. CH2Cl Benzyl chloride Chlorophenylmethane
10.4 Nature of C X bond in haloalkanes i. In an alkyl halide, highly electronegative halogen atom is bonded to less electronegative carbon atom. Therefore, C X bond in alkyl halide is polar in nature. ii. The carbon atom carries partial positive charge (+) as it is less electronegative than halogen and halogen atom carries a partial negative charge (). +
C X iii. In the formation of CX bond, sp3 hybrid orbital of carbon atom overlaps with half filled p-orbital of halogen atom. iv. C X bond strength decreases down the group 17 of the periodic table because orbital overlap is most efficient between orbital of same principle quantum number (i.e., in the same row of periodic table) and efficiency decreases as difference in principle quantum number increases.
Halogen atom Its overlapping orbital in C–X bond
F 2pz Cl 3pz Br 4pz I 5pz v. The size of the halogen atom increases from F to I, as a result of which, the bond length also increases and
the bond formed is weaker. Hence, C X bond strength in CH3 X decreases in the order: CH3F > CH3Cl > 3 CH3Br > CH3I as the 2sp orbital of carbon cannot penetrate into the larger p-orbitals sufficiently to form strong bonds.
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Chapter 10: Haloalkanes and Haloarenes
Bond enthalpy, bond length and dipole moment of CX bond in CH3X: Bond Bond Enthalpy (kJ/mol) Bond Length (Å) Dipole moment (Debye)
CH3 F 452 1.42 1.847
CH3 Cl 351 1.77 1.860
CH3 Br 293 1.91 1.830
CH3 I 234 2.12 1.636
10.5 Preparation of haloalkanes
Monohalogen derivatives of alkanes (haloalkanes) can be prepared by the following methods: i. From halogenation of alkanes: a. Direct halogenation of alkanes in the presence of UV light, heat or suitable catalyst gives the corresponding alkyl halides. b. The displacement of H-atom from hydrocarbon during halogenation follows the order: Benzylic allylic > 3 H-atom > 2 H-atom > 1 H-atom > H-atom of methane > vinylic arylic c. The reactivity of halogens decreases in the order: F2 > Cl2 > Br2 > I2 d. Fluorination of alkanes is highly exothermic and violent, resulting in the cleavage of CC bonds. Chlorination is fast and exothermic while bromination is slow, as bromination of alkanes is less exothermic than chlorination. Direct iodination is not possible as reaction is reversible and highly endothermic. 1. Chlorination: Alkanes react with chlorine in the presence of UV light or diffused sunlight or at high temperature to yield the corresponding alkyl chlorides.
h ,UV light R H + Cl2 orΔ R Cl + HCl Alkane Alkyl chloride
h,UVlight eg. CH3 H + Cl2 or CH3 Cl + HCl Methane Methyl chloride 2. Bromination: Alkanes are heated with bromine in the presence of anhydrous aluminium tribromide to give the corresponding alkyl bromides.
Anhydrous AlBr3 R H + Br2 R Br + HBr Alkane Alkyl bromide
Anhydrous AlBr3 eg. CH3 CH2 H + Br2 CH3 CH2 Br + HBr Ethane Ethyl bromide Note: i. Direct halogenation of an alkane is a chain reaction and follows free radical mechanism. ii. This method of preparation gives the mixture of mono, di, tri and tetra halogen derivatives of an alkane and it is difficult to separate each component in pure form. eg. Preparation of methyl chloride by direct halogenation method results in the formation of mono, di, tri and tetra chloromethane derivatives.
h Cl2 Cl2 Cl2 CH4 + Cl2 HCl CH3 Cl HCl CH2Cl2 HCl CHCl3 HCl CCl4 Methane Methyl Dichloromethane Trichloromethane Tetrachloromethane
` chloride Therefore, halogenation (chlorination and bromination) of an alkane is not useful for laboratory preparation of alkyl halide, because it gives mixture of different alkyl halides which are difficult to separate. Consequently, the yield of any one component is less due to the formation of other component. 5
Chemistry Vol ‐ 2.2 (Med. and Engg.)
3. Iodination: i. Alkanes react with iodine to form the corresponding alkyl iodides. ii. This reaction is reversible and endothermic because hydroiodic acid (HI) during the course of reaction acts as strong reducing agent and reduces alkyl iodide back to alkane.
R H + I2 R I + HI Alkane Alkyl iodide Hydroiodic acid
eg. C2H5 H + I2 C2H5 I + HI Ethane Ethyl iodide Hydroiodic acid iii. So, this reaction is carried out in the presence of oxidising agent like mercuric oxide (HgO), iodic acid (HIO3), dilute nitric acid (HNO3), etc., which reacts with HI and prevents backward reaction. iv. In the presence of mercuric oxide (HgO): eg. 2CH4 + 2I2 + HgO 2CH3 I + HgI2 + H2O Methane Mercuric Methyl oxide iodide v. In the presence of iodic acid (HIO ): 3 eg. 5C2H5 H + 2I2 + HIO3 5C2H5 I + 3H2O Ethane Iodic Ethyl acid iodide vi. In the presence of dilute nitric acid (HNO ): 3 eg. 8C2H5 H + 4I2 + dil.HNO3 8C2H5I + 3H2O + NH3 Ethane Nitric Ethyl acid iodide Note: Iodination stops at monoiodo stage. 4. Fluorination: Alkanes react with fluorine in an explosive manner. Halogenation of alkanes is not a suitable method for preparing alkyl fluorides as the byproduct formed (hydrofluoric acid) is poisonous and corrosive. ii. From halogenation of alkenes: a. When alkenes are treated with Br2 or Cl2 in the presence of solvent like CCl4, the addition reaction takes place across the double bond to give vic-dihalides.
CCl4 C = C + X2 X C C X
Alkene Vic-dihalide
CCl4 eg. H2C = CH2 + Br2 Br CH2 CH2 Br Ethene 1,2-Dibromoethane b. This reaction is used for detection of unsaturation (multiple bond) in an organic compound. The disappearance of reddish brown colour of bromine due to formation of colourless vic-dibromide indicates the presence of a multiple bond. Note: i. The reaction of alkenes (except ethylene) with Cl2 or Br2 at higher temperature (about 773 K) gives substitution reaction product instead of addition reaction product. This is because at higher temperature, the addition reaction is reversible and the substitution reaction is irreversible. The hydrogen atom of allylic carbon is replaced with the halogen atom to form allylic halides and the reaction is called as allylic halogenation. 773K eg. H3C CH = CH2 + Cl2 Cl CH2 CH = CH2 + HCl Propene 3-Chloropropene (Allyl chloride)
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Chapter 10: Haloalkanes and Haloarenes ii. Allylic halogenation is also carried out by using specific reagents like N-bromosuccinimide (NBS) and sulphuryl chloride at 473 K in the presence of light and peroxide as initiator. eg. a. O O
h + N Br + N H peroxide Cyclohexene Br O O NBS 3-Bromocyclohexene Succinimide
b. HC CH = CH + SO Cl 473K Cl CH CH = CH + SO + HCl 3 2 2 2 h ,Peroxide 2 2 2 Propene Sulphuryl 3-Chloropropene chloride iii. By addition of hydrogen halides to alkenes: a. Alkyl halides can be obtained by the electrophilic addition of hydrogen halides like HCl, HBr, HI across the double bond of alkene.
H X
C = C + HX C C
Alkene Hydrogen Alkyl halide halide
Order of reactivity of hydrogen halides is: HI > HBr > HCl > HF b. In case of symmetrical alkenes, alkyl group or number of hydrogen atoms present on either side of the doubly bonded carbon atoms is same, therefore during addition of HX, only one type of product is formed.
eg. H 3C CH = CH CH3 + HCl H3C CH2 CH CH3 But-2-ene Cl 2-Chlorobutane c. In the case of unsymmetrical alkenes, carbon atoms involved in double bond are non-equivalent, so the addition of HX in unsymmetrical alkene takes place according to Markownikoff’s rule. d. According to Markownikoff’s rule, “during addition of an unsymmetrical reagent across the double bond of an unsymmetrical alkene, the negative part of reagent attacks on the carbon atom with less number of hydrogen atom(s) (more substituted carbon) and positive part of the reagent attacks on carbon atom with more number of hydrogen atom(s) (less substituted carbon)”. eg. Cl Cl HCl 1. H3C CH = CH2 Markownikoff ’s rule H3C CH CH3 + CH3 CH2 CH2 2-Chloropropane 1-Chloropropane Propene (Major product) (Minor product)
H I H H I
1 2 3 4 1 2 3 4 4 3 2 1 2. HI H3C C = C CH3 Markownikoff ’s rule H3C C C CH3 + H3C C C CH3
CH3 CH3 H CH3 H 2-Iodo-2-methylbutane 2-Iodo-3-methylbutane 2-Methylbut-2-ene (Major product) (Minor product) e. But the addition of HBr to an unsymmetrical alkene in the presence of peroxide like Na2O2, H2O2, benzoyl peroxide (C6H5CO)2O2 follows Anti-Markownikoff’s rule. 7
Chemistry Vol ‐ 2.2 (Med. and Engg.) f. According to Anti-Markownikoff’s rule, “during addition of HBr in the presence of peroxide, the negative part of reagent attacks on C-atom with more number of hydrogen atom(s) while positive part of reagent attacks on C-atom with less number of hydrogen atom(s)”. This rule is also known as Peroxide effect or Kharasch effect or Kharasch-Mayo effect. eg. Br
3 2 1 3 2 1 Peroxide 1. H3C CH = CH2 + HBr Anti-Markownikoff ’s rule H3C CH2 CH2 Br + H3C CH CH3
Propene 1-Bromopropane 2-Bromopropane (Major product) (Minor product)
2. CH3 CH3 CH3 3 2 123 1 3 2 1 (C65 H CO) 2 O 2 CH3 C = CH2 + HBr Anti-Markownikoff ’s rule H3C C CH2 Br + H3C C CH3 2-Methylpropene H Br 1-Bromo-2-methylpropane 2-Bromo-2-methylpropane (Major product) (Minor product)
Note: Peroxide effect is observed only in case of HBr. HI and HCl follow Markownikoff’s rule even in the presence of peroxide. iv. From Alcohols: Alkyl halides can be prepared from alcohols by substituting the hydroxy group of alcohols with halogen atom. Following three types of reagents can be used to carry out this reaction. a. halogen acids b. phosphorus halides or c. thionyl chloride a. Reaction with halogen acids: 1. Chloroalkanes: i. Alcohols react with Lucas reagent (solution of concentrated HCl and anhydrous zinc chloride) to form the corresponding alkyl chlorides.
Anhydrous ZnCl2 R OH + HCl R Cl + H2O Alcohol (conc.) Alkyl
chloride
ii. Primary and secondary alcohols react with concentrated HCl and anhydrous ZnCl2 to give the corresponding alkyl chlorides. This process is called “Groove’s process”. Anhydrous ZnCl2 eg. a. CH3 OH + HCl CH3 Cl + H2O Methanol (conc.) Methyl chloride CH3 CH3 Anhydrous ZnCl b. CH3 C OH + HCl 2 CH3 C Cl + H2O Room temperature (conc.) H H Propan-2-ol 2-Chloropropane
iii. Tertiary alcohols readily react (simply by shaking) with concentrated HCl even in the absence of anhydrous ZnCl2.
CH3 CH3 Room temperature eg. H3C C OH + HCl H3C C Cl + H2O (conc.) CH3 CH3 tert-Butyl alcohol tert-Butyl chloride
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Chapter 10: Haloalkanes and Haloarenes
Note: Anhydrous ZnCl2 acts as a catalyst by helping in cleavage of C O bond. It is Lewis acid so it easily abstracts hydroxyl group of an alcohol by coordinating with oxygen of OH group. Due to this, weakening of C O bond takes place and it finally breaks to form carbocation. Chloride ion then readily reacts with carbocation to form chloroalkanes. 2. Bromoalkanes: i. Alkyl bromides are prepared by heating alcohol with hydrobromic acid (generated in situ by treating sodium bromide or potassium bromide with conc. H SO ). 2 4 NaBr Conc.H24 SO R CH2 OH Reflux R CH2 Br + H2O + NaHSO4 Alcohol Alkyl bromide
NaBr Conc.H24 SO eg. C2H5OH Reflux C2H5Br + H2O + NaHSO4 Ethyl alcohol Ethyl bromide ii. In the preparation of secondary and tertiary bromides from respective alcohols, conc. H2SO4 is not used (as it may result in the dehydration of secondary and tertiary alcohols to form alkenes); instead dil. H SO is used. 2 4
OH Br KBr dil.H24 SO R CH R R CH R + H2O + KHSO4 Secondary Secondary alcohol alkyl bromide
R R KBr dil.H24 SO R C OH R C Br + H2O + KHSO4
R R Tertiary Tertiary alkyl alcohol bromide
where R, R and R can be same or different alkyl groups. eg. a. OH Br H C CH CH KBr dil.H24 SO H C CH CH + H O + KHSO 3 3 3 3 2 4 Isopropyl alcohol Isopropyl bromide
CH3 CH3 KBr dil. H24 SO b. H3C C OH H3C C Br + H2O + KHSO4
CH3 CH3 tert-Butyl alcohol tert-Butyl bromide 3. Iodoalkanes: i. Alkyl iodides are prepared by heating respective alcohols with conc. hydroiodic acid (57 %).
R OH + HI R I + H2O
Alcohol (conc.) Iodoalkane
(57%)
ii. Hydroiodic acid can be prepared in situ by reacting potassium iodide with 95% phosphoric acid. 9
Chemistry Vol ‐ 2.2 (Med. and Engg.) eg. a. CH3CH2CH2OH + KI + H3PO4 CH3CH2CH2I + KH2PO4 + H2O Propan-1-ol Phosphoric 1-Iodopropane acid (95%)
b. CH3 CH CH2 CH3 + KI + H3PO4 CH3 CH CH2 CH3 + KH2PO4 + H2O Phosphoric OH acid (95%) I Butan-2-ol 2-Iodobutane 4. Fluoroalkanes: Fluoroalkanes are not practically prepared by this method as hydrogen fluoride is least reactive. Note: i. The order of reactivity of alcohols in this reaction: Allyl alcohol > tertiary > secondary > primary. This is because of +I effect of alkyl group(s) attached to the - carbon atom of an alcohol, which facilitates the cleavage of C O bond of an alcohol and increases the reactivity of alcohol. ii. The order of reactivity of halogen acids with alcohols is: HI > HBr > HCl > HF. This order is in accordance with bond dissociation energies. (Bond dissociation energy of HI is less than that of HBr which is in turn less than that of HCl).
b. Reactions with phosphorus halides: Haloalkanes are prepared by heating alcohols with phosphorus trihalides or phosphorus pentahalides. 1. Chloroalkanes: Alkyl chlorides are prepared by treatment of phosphorus pentachloride (PCl5) or phosphorus trichloride (PCl3) on respective alcohols.
R OH + PCl5 R Cl + POCl3 + HCl Alcohol Alkyl Phosphorus chloride oxychloride
eg. CH3 OH + PCl5 CH3 Cl + POCl3 + HCl Methanol Methyl Phosphorus chloride oxychloride
3R OH + PCl3 3R Cl + H3PO3 Alcohol Alkyl Phosphorus chloride acid
eg. 3C2H5OH + PCl3 3C2H5 Cl + H3PO3 Ethanol Ethyl chloride Phosphorus acid
2. Bromoalkanes and iodoalkanes: i. Alkyl bromides and iodides are prepared by action of phosphorus tribromide (PBr3) or phosphorus triiodide (PI3) on alcohols. ii. PBr3 is unstable and can be generated in situ by action of red phosphorus on Br2.
2P + 3Br2 2PBr3 Red Bromine Phosphorus phosphorus tribromide
red P Br2 3R OH + PBr3 3R Br + H3PO3 Alcohol Alkyl bromide Phosphorus acid
red P Br2 eg. 3CH3 CH2 CH2 OH + PBr3 3CH3 CH2 CH2 Br + H3PO3 Propan-1-ol Phosphorus 1-Bromopropane Phosphorus (n-Propyl alcohol) tribromide (n-Propyl bromide) acid
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Chapter 10: Haloalkanes and Haloarenes
iii. PI3 is also unstable and it can be generated in situ as follows:
2P + 3I2 2PI3 Red Phosphorus
phosphorus triiodide
red P I2 3R OH + PI3 3R I + H3PO3 AlcoholPhosphorus Alkyl iodide Phosphorus triiodide acid
eg. 3CH3 (CH2)3 CH2 OH + PI3 3CH3(CH2)3 CH2I + H3PO3 Pentan-1-ol Phosphorus 1-Iodopentane Phosphorus (n-Pentyl alcohol) triiodide (n-Pentyl iodide) acid Note: i. Good yield of primary alkyl halides is obtained by this method. ii. Secondary and tertiary alcohols undergo dehydration to form alkenes and hence good yield of secondary and tertiary alkyl halides is not obtained by this method. iii. In laboratory, lower alkyl bromides and iodides are prepared by this method. iv. PBr5 and PI5 does not exist. c. Reactions with thionyl chloride (sulphonyl chloride): 1. This reaction is used for the preparation of alkyl chlorides. When alcohols are refluxed with thionyl chloride in the presence of pyridine base, corresponding alkyl chlorides are formed.
Pyridine R OH + SOCl2 Reflux R Cl + SO2 + HCl Alcohol Thionyl Alkyl chloride chloride
Pyridine eg. CH3 OH + SOCl2 Reflux CH3 Cl + SO2 + HCl Methanol Thionyl Methyl chloride chloride 2. Chloro compounds obtained by this method can be easily isolated as both the byproducts of reaction (SO2 and HCl) are gases and escape easily leaving behind pure alkyl chloride. Note: i. This process is also known as “Darzen’s procedure”. ii. Thionyl bromide is unstable and thionyl iodide does not exist, thus alkyl bromides and alkyl iodides cannot be prepared by this method. v. By Halogen Exchange: a. This method is used for the preparation of alkyl iodides. Alkyl chlorides or bromides are heated with solution of sodium iodide in dry acetone to give corresponding alkyl iodide. This reaction is known as “Finkelstein reaction”. b. Sodium bromide and sodium chloride are less soluble in dry acetone and thus they get precipitated. c. These precipitates are removed by filtration and thus backward reaction is also prevented. d. Primary alkyl bromides and chlorides give best results by this reaction.
R X + NaI Dryacetone R I + NaX Alkyl Sodium Alkyl Sodium halide iodide iodide halide
(X = Cl, Br)
Dryacetone eg. CH3 CH2 Br + NaI CH3 CH2 I + NaBr Ethyl Sodium Ethyl iodide bromide iodide 11
Chemistry Vol ‐ 2.2 (Med. and Engg.)
e. Alkyl fluorides can also be prepared by this method; by the action of mercurous fluoride (Hg2F2), silver fluoride (AgF), cobalt fluoride (CoF2) or antimony trifluoride (SbF3) on alkyl chloride or bromide.
2 R X + Hg2F2 2R F + Hg2X2 Alkyl Mercurous Alkyl fluoride halide fluoride (Fluoroalkane) (X = Cl, Br)
This reaction is known as “Swarts Reaction”. eg. 2CH3 Cl + Hg2F2 CH3 F + Hg2Cl2 Methyl chloride Mercurous Methyl fluoride fluoride Note: When the organic halides contain two or three halogen atoms on the same carbon atom, SbF3 or CoF2 are used.
eg. Cl F 3CH3 C CH3 + 2SbF3 3CH3 C CH3 + 2SbCl3 Cl F 2,2-Dichloropropane 2,2-Difluoropropane
Physical and chemical properties of haloalkanes 10.6 Physical properties: i. Physical state: a. Lower members of haloalkanes (when pure) are colourless gases at room temperature while higher members are liquids or solids. b. Bromides and iodides develop colour when exposed to light. ii. Smell: Lower members of haloalkane series are sweet smelling liquids. iii. Solubility: Alkyl halides are very slightly soluble in water but readily soluble in organic solvents like methanol, acetone, etc. iv. Density of haloalkanes: a. Bromoalkanes, iodoalkanes and polychloro derivatives of alkanes are heavier than water whereas chloroalkanes and fluoroalkanes are lighter than water. 1 b. Density of haloalkane size of halogen atom and density of haloalkane . size of the alkylgroup c. Densities increase in the order: Fluoride < Chloride < Bromide < Iodide. v. Melting and boiling points of haloalkanes: a. Melting and boiling points of alkyl halides are greater than corresponding hydrocarbons. b. Due to the polarity of C X bond and high molecular mass, intermolecular forces of attraction (dipole-dipole-London force and van der Waal’s force) between molecules of haloalkanes are stronger and results in increase in melting and boiling point. c. Hence, boiling points of haloalkanes having same alkyl group increase in the order:
H3C F < H3C Cl < H3C Br < H3C I d. In case of isomeric haloalkanes, branching results in decrease in boiling point. vi. Inflammable nature: Haloalkanes are less inflammable than hydrocarbons. They give green edged flame with copper wire on heating (Beilstein test).
Chemical properties: i. Reactivity of an alkyl halide (for the same alkyl group) decreases in the order given below: R I > R Br > R Cl > R F
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Chapter 10: Haloalkanes and Haloarenes ii. Reactivity of alkyl halide depend on the polarity of C X bond as electronegativity of halogen atoms decreases in the order of F > Cl > Br > I; so strength of C F bond is more due to large difference in electronegativities (therefore it is more stable) whereas C I bond is less stable and shows high reactivity compared to other halogens. iii. The order of reactivity among 1, 2 and 3 alkyl halide is: 3 alkyl halide > 2 alkyl halide > 1 alkyl halide. This is due to +I effect of an alkyl group which increases bond polarity of C X bond.
Substitution reactions: Reactions in which an atom or a group of atoms is substituted by another atom or a group of atoms respectively are known as substitution reactions. An alkyl halide shows nucleophilic substitution reaction due to polarity of C X bond. R X + Y R Y + X Alkyl Nucleophile Substituted halide alkane i. Hydrolysis: a. Alkyl halides on boiling with aqueous alkali hydroxide (KOH/NaOH) undergo hydrolysis to form the corresponding alcohols.
R X + KOH Boil R OH + KX Alkyl halide (aq.) Alcohol Potassium halide
b. During the course of reaction, X group of an alkyl halide gets substituted by OH to form an alcohol. Boil eg. CH3 Cl + KOH CH3 OH + KCl Methyl chloride (aq.) Methyl alcohol
c. Alkyl halides on boiling with moist silver oxide undergo hydrolysis to form the corresponding alcohols.
moist Ag2 O R X + AgOH Boil R OH + AgX Alkyl halide Alcohol
moist Ag2 O eg. CH3 (CH2)2 I + AgOH Boil CH3 (CH2)2 OH + AgI nPropyl iodide nPropyl alcohol Note: i. Silver hydroxide does not exist. ii. Silver oxide suspended in water behaves as silver hydroxide. ii. Formation of alkyl cyanides (alkane nitriles): a. Alkyl halides on boiling with alcoholic potassium cyanide form corresponding alkyl cyanides or alkane nitriles.
R X + KC N boil R C N + KX Alkyl halide (alc.) Alkyl cyanide
b. Halogen atom is substituted by nucleophile cyanide (CN) to form product, because of strong basic nature of KCN, cyanide attacks through C-atom. eg. Cl C N boil CH3 CH CH3 + K C N H3C C CH3 + KCl | 2-Chloropropane (alc.) H 2-Methylpropanenitrile 13
Chemistry Vol ‐ 2.2 (Med. and Engg.) c. The product formed in the above reaction has one more carbon atom than the haloalkanes. Thus, the reaction is a good method for increasing the length of carbon chain. iii. Formation of alkyl isocyanides (R N C): a. Alkyl halide reacts with alcoholic silver cyanide (AgCN) to form corresponding alkyl isocyanide.
R X + Ag C N R N C + AgX
Alkyl halide (alc.) Alkyl isocyanide
eg. C2H5 Br + Ag C N C2H5 NC + AgBr Ethyl bromide (alc.) Ethyl isocyanide (Carbylaminoethane) b. In this reaction halide group is substituted by nucleophilic cyanide group to form product. c. In the presence of silver salt, nucleophilic attack takes place through N-atom of cyanide. iv. Formation of amines (ammonolysis): a. Alkyl halide on heating with alcoholic ammonia under pressure undergoes substitution reaction to give corresponding primary amine. b. In this reaction, halide group is substituted by an amino (–NH2) group.
R X + H NH2 under pressure R NH2 + HX Alkyl halide (alc.) Primary amine
eg. CH3 Cl + H NH2 under pressure CH3 NH2 + HCl Methyl (alc.) Methylamine chloride (Primary amine) Note: ‘R’ group in alkyl halide can be primary, secondary or tertiary.
c. Order of reactivity of haloalkanes (for the same alkyl group) with NH3 is RI > RBr > RCl. d. When an alkyl halide is in excess, mixture of primary amine, secondary amine, tertiary amine and quaternary ammonium salt is obtained.
,under pressure RX RX RX + R X + NH3 HX R NH2 ,under pressure, R2NH ,under pressure, R3N ,under pressure [R4N] X HX HX Quaternary Alkyl 1 amine 2 amine 3 amine ammonium halide salt eg. ,under pressure CH25 Cl CH25 Cl C2H5 Cl + NH3 HCl C2H5 NH2 ,under pressure (C2H5)2NH ,under pressure (C2H5)3N Ethyl Ethylamine HCl Diethylamine HCl Triethylamine chloride (1 amine) (2 amine) (3 amine)
(1 Alkyl halide) , under
pressure C2H5 Cl
+ [(C2H5)4N] Cl Tetraethyl ammonium chloride
(Quaternary salt) e. This reaction is known as “Hoffmann’s ammonolysis reaction” or alkylation of ammonia. f. When excess of ammonia is used, primary amine is obtained as a major product. v. Formation of ethers (Williamson’s synthesis): a. Alkyl halide is heated with alkali alkoxide (KOR/NaOR) to give corresponding ether. This reaction is known as “Williamson’s synthesis”.
14
Chapter 10: Haloalkanes and Haloarenes
R X + Na OR R O R + NaX Alkyl halide Sodium Ether alkoxide
b. In this reaction, halide group undergoes substitution with alkoxy OR group. c. Sodium alkoxide can be prepared by action of sodium metal on alcohol. 2R OH + 2Na 2R ONa + H 2 Alcohol Sodium alkoxide eg. CH3 I + NaOCH3 CH3 O CH3 + NaI Methyl Sodium Dimethyl ether iodide methoxide d. When haloalkanes are heated with dry silver oxide, symmetrical ethers are obtained.
2R X + Ag2O R O R + 2AgX Alkyl halide Dry Ether silver oxide
eg. 2C2H5 CH2Cl + Ag2O C2H5 CH2 O CH2 C2H5 + 2AgCl n-Propyl chloride Dry Dipropyl ether (1-Chloropropane) silver oxide (1-Propoxypropane) Note: Silver oxide used should be completely dry, as traces of moisture may result in alcohol formation. vi. Formation of esters: a. Ethanolic solution of silver salt of a fatty/carboxylic acids on heating with haloalkanes give corresponding esters.
O
CHOH25 R X + Ag O C R R O C R + AgX Alkyl halide Ester O Silver salt of carboxylic acid
b. In this reaction, halogen group is substituted by carboxylate (R COO ) group. c. During this reaction, carboxylate ion (R COO) acts as a nucleophile. O
CHOH25 eg. CH3 I + Ag O C CH3 CH3 O C CH3 + AgI Methyl acetate Methyl iodide O Silver acetate vii. Formation of alkyl nitrite and nitroalkanes:
a. Alkyl halide (R X) on treatment with KNO2 forms alkyl nitrite (R O N = O) whereas on treatment with AgNO2 forms nitroalkane (R NO2). b. The nitrite ion possesses two nucleophilic centres (i.e., it is an ambident nucleophile). c. The linkage through oxygen results in the formation of alkyl nitrites whereas the linkage through nitrogen results in the formation of nitroalkanes.
R X + K+O – N = O Heat R O N = O + KX Alkyl Potassium Alkyl nitrite halide nitrite
R X + AgNO2 R NO2 + AgX Alkyl Silver Nitroalkane halide nitrite
15
Chemistry Vol ‐ 2.2 (Med. and Engg.)
Note: Nucleophilic substitution of alkyl halides (R – X): Reagent Product Reagent Product i. KOH / NaOH / R – OH viii. RCOOAg RCOOR moist Ag2O ii. Alcoholic KCN R – CN ix. NaI R – I iii. Alcoholic AgCN R – NC x. KNO2 R – O – N = O iv. Alcoholic NH3 R – NH2 xi. AgNO2 R – NO2 v. NaOR R – O – R xii. LiAlH4 R – H + vi. H2O R – OH xiii. R – M R – R vii. Dry Ag2O R – O – R Elimination Reactions: Elimination reactions are those reactions in which a molecule loses two atoms or groups attached to neighbouring carbon atoms with formation of double bond between carbon atoms. OR The reaction in which two atoms or groups are removed from adjacent carbon atoms in a molecule to form an unsaturated compound is called an elimination reaction. Dehydrohalogenation (formation of alkenes): i. When alkyl halides are heated with alcoholic solution of alkali hydroxide (KOH/NaOH), halogen atom from -carbon atom and a hydrogen atom from adjacent -carbon atom gets eliminated to form corresponding alkenes. ii. This reaction is also called as “dehydrohalogenation of an alkyl halide”.
H H H H
R C C X + K+OH R C = C + H O + KX 2
H H (alc.) H Alkyl halide Alkene iii. As hydrogen atom is eliminated from -carbon atom, it is also known as “-elimination reaction”. eg. CH CH CH I + K+OH CH CH = CH + H O + KI 3 2 2 3 2 2 n-Propyl iodide (alc.) Propylene iv. In dehydrohalogenation of secondary and tertiary alkyl halides there is possibility of formation of two isomers of alkene, in such a case elimination takes place according to Saytzeff’s rule. v. According to Saytzeff’s rule, “when there is a possibility of formation of two types of alkenes by dehydrohalogenation of alkyl halide, then H-atom is eliminated preferentially from C-atom having least number of H-atom(s)”. In other words, in dehydrohalogenation reaction, more substituted double bond formation is always preferred. vi. In dehydrohalogenation, reactivity of alkyl halide is in the following order: RI > RBr > RCl > RF (when same alkyl group is present). Ease of dehydrohalogenation in case of haloalkanes follows the order: Tertiary > secondary > primary (when same halogen group is present). alc.KOH eg. H3C CH2 CH CH3 HBr H3C CH = CH CH3 + CH3 CH2 CH = CH2 But-1-ene Br But-2-ene (80%) (20%) 2-Bromobutane Reaction with metals: Alkyl halides react with metals such as sodium to form corresponding higher saturated hydrocarbon and with magnesium to form organometallic compounds. i. Reaction with sodium or Wurtz synthesis: a. Haloalkanes when treated with metallic sodium in the presence of dry ether form corresponding symmetrical higher alkanes.
16
Chapter 10: Haloalkanes and Haloarenes
2R X + 2Na dryether R R + 2NaX Alkyl halide Higher alkane
b. This reaction is called as “Wurtz synthesis”. c. The product formed contains more number of carbon atoms than reactants; thus, this method is preferably used for the preparation of higher alkanes. dry ether eg. 2CH3 CH Br + 2Na H3C CH CH CH3 + 2NaBr CH3 CH3 CH3 2-Bromopropane 2,3-Dimethylbutane d. Tertiary halides do not undergo this reaction. ii. Reaction with magnesium or formation of Grignard’s reagent: a. Grignard’s reagent can be prepared by reaction of alkyl halide with pure and dry magnesium in the presence of dry ether.
R X + Mg dry ether R Mg X Alkyl halide (Dry) Alkyl magnesium halide (Grignard’s reagent)
b. Grignard’s reagent is chemically known as alkyl magnesium halide and represented by general formula R Mg X. dryether eg. CH3 I + Mg CH3 Mg I Methyl (Dry) Methyl magnesium iodide iodide c. The Grignard reagents are very reactive compounds and react with any source of proton to form corresponding hydrocarbons. R Mg X + ZH R H + ZMgX Grignard reagent Alkane where, Z = OH, OR, NH ,etc. 2 Note: i. Compound in which less electropositive carbon atom is directly attached to highly electropositive metal atom is called “Organometallic compound”. ii. In this compound, C-atom has partial negative charge and metal atom has partial positive charge. iii. In Grignard’s reagent, C Mg bond is highly polar and Mg X bond is ionic in nature. Hence, Grignard’s reagent are highly reactive towards organic as well as inorganic reagents. + R Mg X iv. a. Traces of moisture (if remained during preparation of Grignard’s reagent) readily react with Grignard reagent to form corresponding alkane; also Grignard’s reagent in free state is explosive in nature. b. Hence, Grignard’s reagent is never stored and always prepared at the time of requirement. c. It is used in the absence of air, under inert atmosphere like dry ether (as a solvent).
Stereochemistry 10.7 Stereochemistry plays an important role in deciding the product of any reaction specially nucleophilic substitution reaction. Some basic stereochemical notations and concepts are given below: Ordinary Light: Ordinary light consists of electromagnetic radiations of different wavelengths, vibrating in all possible directions in space and perpendicular to direction of propagation of light.
Monochromatic Light: i. Ordinary light after passing through monochromator (prism or grating monochromator) emerges out as a ray of single wavelength and is called as “Monochromatic ray of light”. ii. Monochromatic ray of light vibrates in different planes, perpendicular to the direction of propogation of light. 17
Chemistry Vol ‐ 2.2 (Med. and Engg.) Plane Polarized Light: i. A beam of light vibrating in only one plane in space is called “Plane Polarized Light”. ii. Ordinary beam of light after passing through Nicol’s prism (crystalline calcium carbonate) emerges out as plane polarized light. iii. Nicol’s prism is called as polarizer in which vibrations in all other planes are cut off except one plane. iv. Nicol’s prism is combination of two prisms made of calcite crystals and fused base to base by Canada balsam.
Ordinary light Nicol’s prism Plane Polarized light (Polarizer)
Plane Polarized Light
Optical Activity: i. When solution of certain organic compounds come in contact with plane polarized light, they rotate the plane of plane polarized light by some angle either in clockwise or anticlockwise direction. This property of organic substance is called as “Optical activity”.
or
Sample containing Clockwise Anticlockwise Plane polarized light optically rotation rotation Analyser active substance by by Rotation of Plane Polarized Light due to an Optically Active Substance
ii. Polarimeter is the instrument used to measure optical activity (i.e., to measure the magnitude and the direction of the rotation of plane of plane polarized light) of an optically active compound. iii. The polarimeter consists of a light source, two nicol prisms and the sample tube to hold the substance. The prism placed near the source of light is called polariser while the other placed near the eye is called analyser.
Optically Active Molecule: If a molecule is capable of rotating plane of plane polarized light in either clockwise or anticlockwise direction, it is called as “Optically Active Molecule”. eg. Lactic acid, 2-Iodobutane, glucose, fructose, etc. d - l configuration: Depending upon the behaviour of molecules of the compound towards plane polarized light; they can be differentiated as follows: i. Dextro Rotatory Molecules: a. If a molecule is capable of rotating plane of plane polarized light to the right i.e., in the clockwise direction then it is called as “Dextro Rotatory Molecule”. (Latin, dexter = right) b. These are designated as (+) or (d).
eg. 1. I 2. COOH H C* OH H3C C* C2H5 CH3 H (+)/(d)–2-Iodobutane (+)/(d)-Lactic acid
ii. Laevo Rotatory Molecules: a. If a molecule is capable of rotating plane of plane polarized light to the left i.e., in the anticlockwise direction then it is called as “Laevo Rotatory Molecule”. (Latin, Laevus = left)
18
Chapter 10: Haloalkanes and Haloarenes
b. These are designated as () or (l). eg. 1. I 2. COOH H5C2 C* CH3 HO C* H
H CH3
()/(l)-2-Iodobutane ()/(l)-Lactic acid
iii. Racemic Mixture: a. Equimolar mixture of dextro and laevo form of the same compound is known as “Racemic Mixture” or “Racemic modification” or “Racemate”. b. The process of conversion of enantiomer into a racemate is called as racemisation. c. Racemic mixture is optically inactive and does not rotate the plane of plane polarized light. d. When dextro and laevo forms of molecule cancel each other’s rotation (which is equal but in opposite direction), it is known as “External Compensation”. e. It is designated as (dl) or (). eg. dl -Lactic acid, 2-Iodobutane, etc.
Optically Inactive Molecules: Optically inactive molecules are those which do not rotate the plane of plane polarized light. eg. Ethyl chloride, water, etc. Chirality: i. Four valencies of carbon atom are arranged along the four corners of regular tetrahedron. If all 4 atoms or groups attached to such carbon atom are different, then it is called as “Asymmetric” or “Chiral carbon atom” or “Stereocentre” and it is denoted by star or asterisk (*) on it . ii. When molecule contains asymmetric carbon atom, the symmetry of molecule is lost, i.e., its mirror image is non-superimposable with itself, such molecule is known as “Asymmetric molecule”. eg. 2-Iodobutane I I
C* C*
H5C2 CH3 H3C C2H5
H H
A B a. 2-Iodobutane contains asymmetric carbon atom, its mirror image is non-superimposable on each other.
b. B is the mirror image of A. Position of CH3 group in A does not coincide with the position in mirror image B. c. Same is the case for ethyl group also. Under such condition, mirror image is non-superimposable on each other. iii. Therefore, it can be said that molecule on whole must be non-superimposable on its mirror image, such a molecule is called as “Chiral Molecule” and the property of non-superimposability is called “Chirality”. iv. Chiral molecule exists as d and l form and is an optically active molecule whereas molecule which is superimposable on its mirror image is called “Achiral molecule” and it does not exist as d and l form, therefore are optically inactive.
Vant Hoff Le Bel Theory: i. Phenomenon of optical activity was discovered by French physicist Biot in 1815. ii. Though optical activity was discovered, for many years direct correlation between optical activity and the structure of molecule was not known. iii. First convincing explanation for this corelation was given by Dutch scientist J. Vant Hoff and French scientist Le Bel in 1874. iv. They independently put forward the theory of optical activity. 19
Chemistry Vol ‐ 2.2 (Med. and Engg.) v. Almost all the scientists until 1874 believed that all the molecules are always two dimensional i.e., they are flat entities. vi. Van’t Hoff and Le Bel for the first time proposed the three dimensional structure of molecules. vii. According to this theory: a . I n s p 3 hybridized carbon atom, all four valencies are pointed towards four corners of regular tetrahedron. b. If all the four valencies of carbon atom are satisfied by different atoms or groups of atoms, then C- atom is known as asymmetric carbon atom. c. 1. In an asymmetric carbon atom, two bonds are on plane of the paper shown by ordinary line. 2. One bond is below the plane of paper (i.e., away from observer) and is shown by dotted line. 3. The other bond is above the plane of paper (i.e., towards the observer) and is shown by thick wedge. eg. 2-Iodobutane I
C*
H5C2 CH3
H d. Asymmetric centre is denoted by asterisk (*) on it. e. Molecules containing asymmetric carbon atom always exists as a pair of isomers which are non- superimposable mirror images of each other. Optical Activity of Lactic Acid: i. Number of possible stereoisomers shown by a molecule is dependent upon the number of asymmetric carbon atoms present. ii. Number of possible stereoisomer is given by a formula = 2n where, n = number of asymmetric carbon atom(s) present in that molecule. iii. Lactic acid contains one asymmetric carbon atom which is attached to COOH, OH, CH3 groups and H atom. Therefore, Number of stereoisomers = 2n = 21 iv. Lactic acid shows 2 stereoisomers i.e., d and l form which are non-superimposable mirror images of each other. COOH COOH
C* C*
H OH HO H
CH3 CH3 d-lactic acid l-lactic acid Note: i. Stereoisomers which are non-superimposable mirror images of each other and rotate the plane of the plane polarized light through the same angle but in opposite directions are known as enantiomers or enantiomorphs. eg. d and l forms of lactic acid are called enantiomorphs of each other. ii. Enantiomers have identical physical properties (except the direction of rotation of plane polarized light, though the amount of rotation is same) and chemical properties (except towards optically active reagents.) R, S Configuration: i. R, S nomenclature system was devised by Cahn, Ingold and Prelog which indicates configuration i.e., arrangements of atoms or groups around chirality centre. ii. Rules for R, S nomenclature to determine the priority of groups attached to chiral centre are given as follows: a. Groups arranged around chiral centres are given a priority order. Higher the atomic number of an atom directly attached to chiral centre, higher its priority. b. If two groups have identical atom directly attached to the chiral centre then the next atom in group is considered to determine the priority.
20
Chapter 10: Haloalkanes and Haloarenes eg. Cl
H C CH3 Order of priority: Cl > C2H5 > CH3 > H C2H5 2-Chlorobutane This is because carbon atom in methyl group is attached to 3 hydrogen atoms, whereas in ethyl group it is attached to 2 hydrogen and 1 carbon. Hence ethyl group is given higher priority. c. When group(s) having multiple bonds (CHO, C N, >C = C<) is/are attached to chiral centre, then atoms attached to double or triple bond are considered as duplicate or triplicate. Priority is given by considering triplicate and duplicate structure as shown below: H H H H H H C = C C C ; C = O C O
(C) (C) (O) (C)
(N) C C
C N C N (C) ; C = O C O
(N) (C) eg. (O) (C) C C
C = O is given higher priority over C = C because in C = O , carbon is attached to H H
2 oxygen atoms (one oxygen + one phantom oxygen) and in C = C carbon atom is attached to other 2 C atoms (1 C-atom and other phantom C-atom).
d. In the case of benzene ring, it is considered as one of the resonating structure. (C) C (C)
HC CH (C)
e. The order of priority is as follows:
I > Br > Cl > SO3H > F > OCOCH3 > OH > NO2 > NH2 > COOCH3 > COOH > CONH2 > COCH3 > CHO > CH2OH > CN > C6H5 > C2H5 > CH3 > D > H iii. Tetrahedral structure of molecules can be drawn as follows: Two bonds in the plane of paper (indicated by line), one bond above the plane of paper (indicated by thick wedge) and one bond below the plane of paper (indicated by dotted line). iv. The group attached to the dotted line should have the least priority.
eg. 2-Chlorobutane (order of priority Cl > C2H5 > CH3 > H) 1 1 Cl Cl
* * 2 C C 4 4 2 H5C2 C2H5 3 H H 3 CH3 CH3 (a) (b) 21
Chemistry Vol ‐ 2.2 (Med. and Engg.) v. It is like holding dotted line in a hand and looking at the structure from opposite side (as if viewing a bouquet of flowers) then with this view, order of priority is 1 2 3 4. vi. In structure (b), order of priority is in clockwise direction i.e., from right therefore it is R-configuration (Latin word rectus meaning right). vii. In structure (a), order of priority is in anticlockwise direction i.e., from left therefore it is S-configuration (Latin word sinister meaning left). viii. Two more examples: a. 1 1 OH OH
* * 2 C C 2 HOOC 4 4 COOH 3 H H 3 CH3 CH3
S –Lactic acid R –Lactic acid b. 1 1 I I
C* C* 2 2 H C 4 4 5 6 H H C6H5 3 3 C(CH3)2 C(CH3)2
S –1-Iodo-2-methyl-1-phenylpropane R-1-Iodo-2-methyl-1-phenylpropane
10.8 Nucleophilic substitution mechanism
Nucleophilic substitution reaction: i. A reaction in which one nucleophile is substituted by other nucleophile is known as “Nucleophilic Substitution Reaction”. ii. Nucleophilic substitution reaction can proceed by two different paths depending on the nature of substrate, the nucleophile, the leaving group and solvent. The two paths are: 1 2 a. SN b. SN mechanisms. Note: i. Mechanism of reaction: Mechanism of reaction is a step by step description of exactly how the reactants are transformed into product in as much details as possible. ii. Transition state: During the course of reaction, reactants change from one form to other through certain state. This state is known as Transition state. The minimum energy necessary to fulfill all the conditions for the formation of transition state is called as the energy of activation of the reaction (Eact). iii. Energy Profile Diagram: The energy changes of chemical reaction are depicted by energy profile diagram which shows the progress of the reaction along a path from reactants to the product. iv. Rate Determining Step (R. D. S): a. Slowest step in the reaction mechanism which determines the rate of reaction is known as Rate Determining Step. b. The rate determining step involves breaking of bond which requires input of energy and hence it is the slowest step in the course of the reaction.
22
Chapter 10: Haloalkanes and Haloarenes SN2 Mechanism: i. When primary alkyl halide reacts with aqueous alkali, corresponding primary alcohol is formed. The reaction is called as hydrolysis of an alkyl halide. R CH2 X + OH R CH2 OH + X 1 Alkyl halide (aq) 1 Alcohol eg. H3C Br + NaOH(aq) CH3 OH + NaBr Methyl bromide Methyl alcohol In this reaction, OH nucleophile substitutes halide ion of 1 alkyl halide to form 1 alcohol. H3C Br + OH H3C OH + Br Methyl bromide Methyl alcohol ii. In this reaction, the rate of formation of 1 alcohol is found to be proportional to the concentration of an alkyl halide and also to that of base used. Rate [H3C Br] [OH ] Rate = k [H3C Br] [OH ] (where k = proportionality constant) The rate of reaction is dependent on the concentration of both the reactants. Therefore, it is second order i.e., bimolecular reaction. Hence this reaction is known as “ Nucleophilic Substitution Bimolecular Reaction” and denoted as “SN2”. iii. Mechanism: a. It is one step concerted mechanism in which the formation of C OH bond and breaking of C Br bond takes place simultaneously. b. The formation of transition state is slow step i.e., rate determining step. c. Transition state (T. S.) is the highest energy state in the course of reaction. d. It may change into product or may go back to reactants. e. In the transition state, both incoming nucleophile (OH) and outgoing halide group (X) share the negative charge and C-atom carry partial positive charge. f. When the C OH bond forms completely, at the same instant C Br bond breaks completely and the reaction is completed. iv. Energy Profile Diagram:
R = reactants: CH Br + OH T.S. 3 T. S. = transition state Eact = Activation energy ΔH = Heat of reaction Eact P = Products: H3C OH + Br
R H
Potential energy P Reaction co-ordinate Energy profile diagram for SN2 mechanism
In SN2 mechanism, heat of reaction (ΔH) is negative, hence it is exothermic reaction. The product formed is of much lower energy than reactant, therefore product is more stable. v. Stereochemistry: a. In SN2 mechanism, nucleophile (OH) attacks the C-atom of 1 alkyl halide from backside, this is due to the following reasons: 1. It is least hindered (crowded) site for attack of OH. 2. Electrostatic attraction between carbon atom (with + charge) and OH (with charge). 3. Electrostatic repulsion between OH and Br is minimum. 23
Chemistry Vol ‐ 2.2 (Med. and Engg.) H 1 H H1 1
+ Fast * * slow step C Br C + Br HO + C R.D.S. HO H2 H2 Br HO H H3 2 H H3 3 1alkyl halide Transition state 1 alcohol Backside attack of nucleophile (Inversion of configuration) b. SN2 mechanism results in the inversion of configuration i.e., in the product, OH occupies position exactly opposite to that of Br and positions of H2 and H3 atoms are exactly opposite in product to that in reactant. Note: i. Order of reactivity for halide atom is I > Br > Cl > F; because as size of atom increases, the bond dissociation energy decreases. ii. Reactivity of an alkyl halide in SN2 mechanism is in the following order: CH3X > 1 alkyl halide > 2 alkyl halide > 3 alkyl halide.
H H H H H H
H C C C H H H
C Nu X Nu C Nu C X Nu C X X H H H H H C H H C H C H H Ethyl Methyl halide halide (1) H H H H
Isopropyl tert-Butyl halide (3) halide (2) (Maximum steric hindrance)
Steric hindrance increases iii. In 1896, Paul Walden theoretically anticipated inversion of configuration. iv. In 1935, Ingold and Hughes gave experimental evidences for inversion of configuration. v. The inversion is known as ‘Walden’ inversion.
SN1 Mechanism: i. When tertiary alkyl halide reacts with aqueous alkali, tertiary alcohol is formed. R1 R1 R C X + NaOH R2 C OH + NaX (aq.) R 3 R3 3 Alkyl halide 3 Alcohol eg. CH3 CH3 H3C C Br + NaOH H3C C OH + NaBr
(aq.)
CH3 CH3 tert-Butyl bromide tert-Butyl alcohol (3 Alkyl halide) (3 Alcohol)
24
Chapter 10: Haloalkanes and Haloarenes ii. Study of reaction kinetics shows that, the rate of formation of 3 alcohol is proportional to the concentration of only 3 alkyl halide.
Rate [(H3C)3C X]
Rate = k [(H3C)3C X] It is the first order reaction i.e., unimolecular reaction. Hence this reaction is known as “Nucleophilic Substitution Unimolecular Reaction” and denoted as “SN1”. iii. Mechanism: A two step mechanism has been proposed for this type of substitution. a. The first step is a slow (rate determining) step, which involves heterolytic cleavage of C X bond to form a carbocation as an intermediate.
CH3 CH3
* Slow step + C R.D.S. C + X
CH X 3 CH3 CH3
CH3 3 Alkyl halide Carbocation b. Second step is fast, in which nucleophile OH attacks highly reactive carbocation, to form a product. CH3 CH3 + Fast * C + OH C
CH3 OH CH3 CH3 CH3 Carbocation 3 Alcohol iv. Energy Profile Diagram: R
R = reactant i.e., R C X
T.S.1
T.S.2 R T.S.1 = Transition state of first step
Eact2 R = Carbonium ion/carbocation Eact1 + R + C (where R, R and R may C be same or different.) R R R R R T.S.2 = Transition state of second step ΔH Eact1 = Activation energy of first step Potential energy P E = Activation energy of second step act2 Reaction co-ordinates ΔH = heat of reaction R 1 Energy profile diagram for SN mechanism P = product i.e., R C OH
R From energy profile diagram it is clear that, in SN1 mechanism ΔH is negative. Hence, it is an exothermic reaction. v. Stereochemistry: a. In this reaction, carbocation formed has planar structure (C-atom is sp2 hybridized) therefore nucleophile OH can attack from both front and back side of carbocation. b. Back side attack of OH results in the inversion of configuration i.e., OH occupies position opposite to halide ion and position of remaining group is opposite to that of reactant. 25
Chemistry Vol ‐ 2.2 (Med. and Engg.) c. Front side attack of OH results in the retention of configuration i.e., position of X is taken up by OH and remaining groups are exactly at the same position as that of reactant. d. In SN1 mechanism of optically active reactants, the two configurations formed are non- superimposable mirror images of each other i.e., enantiomers and they are formed in nearly equal proportions. Therefore product formed is a racemic mixture () which is optically inactive.
R 1 R1 * Slow step + C R.D.S. C + X
R2 X R R 2 3 R 3 Carbocation 3 Alkyl halide
R R 1 1 R1 * * C Back side + Front side C C attack attack HO R2 R2 OH R2 R3 R3 HO R3 Inversion of Retention of configuration (50%) configuration (50%) Note: i. In SN1 mechanism, the reactivity of the halide, R-X, follows order: R–I > R–Br > R – Cl > R–F; because as size of atom increases, the bond dissociation energy decreases. ii. Stability order for carbocation is 3 > 2 > 1. CH3 eg. a. HC, H C CH b. H C CH c. 3 3 2 3 + C 1 Carbocation (Least stable) CH3 H3C CH3 2 Carbocation 3 Carbocation
(Most stable) iii. 3 alkyl halides prefer SN1 mechanism; 2 alkyl halides show mixed mechanism whereas 1 alkyl halides prefer SN2 mechanism. iv. Halides in which halogen atom is bonded to a sp3 hybridized carbon atom next to an aromatic ring are called benzylic halides.
CH2X
(where X = F, Cl, Br, I)
Benzylic halide
eg. a. CH 2Br b. CH3 H3C C Br Bromophenylmethane (1 benzylic halide)
2-Bromo-2-phenylpropane (3 benzylic halide)
26
Chapter 10: Haloalkanes and Haloarenes v. Benzylic halides form carbocation which undergoes stabilization through resonance as follows: + + CH2 CH CH CH2 2 2 + + (I) (IV) (II) (III) 3 vi. Halides in which halogen atom is bonded to a sp hybridized carbon atom next to a carbon carbon double bond are called as allylic halides.
CH2X 3-Haloprop-1-ene (Allyl halide)
eg. Cl CH3 a. H2C = CH CH2 b. H3C CH = CH CH I 3-Chloroprop-1-ene 4-Iodopent-2-ene (1 allylic halide) (2 allylic halide)
3-Haloprop-1-ene forms carbocation which undergoes stabilization through resonance as follows: + + CH2 = CH CH2 H2C CH = CH2 vii. Benzylic and allylic halides may be primary, secondary or tertiary in nature; but they undergo SN1 mechanism.
2 1 Comparison between SN and SN : No. Factor SN2 SN1 i. Kinetics 2nd order 1st order ii. Molecularity Bimolecular Unimolecular iii. Number of steps One step Two steps iv. Bond making and Simultaneous First the bond in the reactant breaks bond breaking and then a new bond in product is formed v. Transition state One step, one transition state Two steps, two transition state vi. Direction of attack Only back side attack Back side attack and front side attack of nucleophile vii. Stereochemistry Inversion of configuration Racemisation (If substrate is optically (If substrate is optically active) active) viii. Type of substrate Mainly 1 substrates Mainly 3 substrates ix. Polarity of solvent Non-polar solvent favourable Polar solvent favourable x. Nucleophile Strong Nucleophile favourable Weak Nucleophile favourable xi. Intermediate No intermediate Intermediate involved 10.9 Haloarenes
Haloarenes: i. The halogen derivatives of aromatic hydrocarbons are called as haloarenes or aryl halides. OR Haloarenes are obtained by replacing one or more hydrogen atom(s) of an arene with corresponding number of halogen atom(s). ii. They are obtained by substituting H-atom of aromatic ring with halogen atom.
X2 Ar H Lewis acid Ar X + HX (where X = F, Cl, Br, I) 310K 320K Benzene Haloarene 27
Chemistry Vol ‐ 2.2 (Med. and Engg.) Classification: Depending upon the number of halogen atom(s) attached to an aromatic ring, they are classified as follows:
Haloarenes
Monohaloarenes Dihaloarenes Trihaloarenes Polyhaloarenes One halogen atom is Two halogen atoms Three halogen atoms More than three halogen attached to benzene. are attached to are attached to atoms are attached to benzene. benzene. benzene. eg. eg. Cl eg. eg. Cl Cl Cl Cl Cl Cl Cl
Cl Cl Chlorobenzene 1,2-Dichlorobenzene 1,2,3-Trichlorobenzene 1,2,3,5-Tetrachlorobenzene
Nomenclature: Common and IUPAC names of some of the haloarenes:
Sr. No. Structure Common Name IUPAC Name Cl Chlorobenzene Chlorobenzene i.
CH3 o-Chlorotoluene 1-Chloro-2-methylbenzene or ii. 2 Cl 2-Chlorotoluene 1
CH3 m-Chlorotoluene 1-Chloro-3-methylbenzene or 3 3- Chlorotoluene iii. 1 Cl
CH3 p-Chlorotoluene 1-Chloro-4-methylbenzene or 4 4-Chlorotoluene iv. 1
Cl v. Br Bromobenzene Bromobenzene
Br o-Dibromobenzene 1,2-Dibromobenzene vi. 1 2 Br
Br m-Dibromobenzene 1,3-Dibromobenzene 1
vii. 3 Br Br p-Dibromobenzene 1,4-Dibromobenzene 1 viii.
4 Br
28
Chapter 10: Haloalkanes and Haloarenes
Br m-Bromochlorobenzene 1-Bromo-3-chlorobenzene 1 ix.
3 Cl Br sym-Tribromobenzene 1,3,5-Tribromobenzene 1 x. 5 Br Br 3 Br — 1,2,3,5-Tetrabromobenzene Br Br xi.
Br
Nature of C X bond in haloarenes 10.10 i. In haloarenes, halogen atom having p-orbital with unpaired electron overlaps with sp2 hybrid orbital of C- atom of benzene ring to form C X bond. ii. Lone pair of electrons from halogen atom is involved in electron system of aromatic ring, showing extended conjugation. As result of resonance, C X bond shows following structures: + + + X X X X (where X = F, Cl, Br, I)
iii. Thus, in aryl halides, C X bond acquires partial double bond character making itself stronger and shorter in length than in alkyl halides.
Preparation of haloarenes 10.11 Haloarenes can be prepared by the following methods: i. By Electrophilic Substitution: a. Chloroarenes and bromoarenes can be prepared from benzene or aromatic hydrocarbons by treatment with Cl2 or Br2 in the presence of Lewis acid like iron, FeCl3, FeBr3, BCl3, AlCl3, etc. at ordinary temperatures (310 K 320 K).
H X Lewis acid + X2 310K 320K + HX dark Benzene Halobenzene
b. This method is called as direct halogenation of aromatic compounds. c. Lewis acid acts as catalyst and halogen carrier for electrophilic substitution. H Br FeBr3 + HBr + Br2 310K 320K dark Benzene Bromobenzene d. If excess of reagent is used, then second halogen atom is introduced at ortho or para position with respect to the first halogen. This is because halogens are o-, p-directing groups. Br Br Br Br FeBr3 + + HBr + Br2 310K 320K excess dark Bromobenzene o-Dibromobenzene Br p-Dibromobenzene The ortho and para isomers can be easily separated due to large difference in their melting points. 29
Chemistry Vol ‐ 2.2 (Med. and Engg.) e. Direct iodination of benzene ring is a reversible reaction due to the HI (strong reducing agent) which is formed as a byproduct, hence reaction is carried out in the presence of strong oxidising agent (like nitric acid or iodic acid or mercuric oxide). I + I2 + HI
Benzene Iodobenzene The above reversible reaction can proceed in forward direction in presence of an oxidising agent.
eg. 1. 5HI + HIO3 3H2O + 3I2 Iodic acid
2. 2HI + HgO HgI2 + H2O Mercuric oxide
f. Fluorine reacts violently and uncontrollably with benzene or other aromatic hydrocarbons. Thus, fluoroarene compounds cannot be prepared by direct fluorination method. ii. Sandmeyer’s reaction: a. When primary aromatic amine (like aniline) is treated with sodium nitrite and dilute HCl at 273 K – 278 K, it results in the formation of benzene diazonium salt. b. Reaction of freshly prepared diazonium salt solution with cuprous (I) salt (cuprous chloride or cuprous bromide dissolved in corresponding halogen acids) results in the formation of chloro or bromobenzene respectively. This reaction is known as “Sandmeyer’s reaction”. eg. NH2 N NCl Cl NaNO2 /dil.HCl Cu22 Cl /HCl 1. 273K 278K, HCl + N2 HO2 Aniline Benzene Chlorobenzene diazonium chloride
NH2 N NCl Br 2. NaNO2 /dil.HCl Cu22 Br /HBr 273K 278K, HCl + N2 HO2
Aniline Benzene Bromobenzene diazonium chloride
c. Diazonium salt on treatment with KI gives iodobenzene. eg. N NCl I + KI + N2 + KCl
Benzene Iodobenzene diazonium chloride
10.12 Physical and chemical properties of haloarenes
Physical properties: i. Density of haloarenes: a. Bromo, iodo and polychloro derivatives of arenes are heavier than water. b. The density of haloarenes increases with increase in number of carbon atoms, halogen atoms and atomic mass of halogen atoms. ii. Melting and boiling points of haloarenes: Boiling points of isomeric dihalobenzenes are nearly the same. However, the melting point of para-isomer is higher as compared to ortho- and meta-isomers. It is because of the symmetry of para-isomers which fit in the crystal lattice better as compared to ortho- and meta isomers.
30
Chapter 10: Haloalkanes and Haloarenes
eg. Cl Cl Cl Cl
Cl o-Dichlorobenzene m-Dichlorobenzene Cl p-Dichlorobenzene Boiling point (K) 453 446 448 Melting point (K) 256 249 323
Chemical properties: The reactions of haloarenes include: i. Substitution reactions ii. Reactions with metals Substitution reactions: i. Haloarenes undergo substitution reactions which can be either nucleophilic substitution or electrophilic substitution. ii. Aryl halides are less reactive than alkyl halides and do not undergo nucleophilic substitution reactions easily due to the following reasons: a. Resonance effect: 1. In aryl halides, the C X bond acquires partial double bond character due to resonating structures. 2. This makes the C X bond cleavage in aryl halide more difficult than the C X bond cleavage in akyl halide. b. Different hybridization states of C-atom in C X bond:
Type of State of hybridization of % s-character Bond length of Strength of CX compound C-atom in a CX bond C X bond bond Alkyl halide sp3 Less (25%) Longer Weaker Aryl halide sp2 More (33.33%) Shorter Stronger eg. 1. X-ray analysis confirms that CCl bond length in chlorobenzene is 169 pm while that in methyl chloride is 177 pm. 177 pm
Cl
H3C Cl 169 pm 2. Reduction in bond length imparts stability, making bond cleavage difficult in aryl halides. c. Polarity of CX bond: 1. sp2 hybridised C-atom; Less tendency to release e s towards X-atom. 2. sp3 hybridised C-atom; More tendency to release e s towards X-atom. 3. Thus sp2 hybridized C-atom is more electronegative than sp3 hybridized C-atom. eg. Dipole moment Compound 1.73 D Chlorobenzene (Aryl halide) 2.05 D Chloroethane (Alkyl halide)
4. Polarity reactivity Lesser polarity of aryl halides, lesser is their reactivity compared to alkyl halides. d. Repulsion: Electron rich arenes repel the attacking nucleophile (which is also electron rich); resulting in the lesser reactivity towards substitution reactions. e. Instability of phenyl cation: 1. Phenyl cation formed due to self ionization of aryl halide will not be stabilized by resonance. 31
Chemistry Vol ‐ 2.2 (Med. and Engg.) 2. Unstable phenyl cation (carbocation) cannot undergo SN1 reaction, thus ruling out the possibility of SN1 mechanism. 3. Pi () electrons of aromatic ring blocks the backside attack of nucleophile, thus ruling out the possibility of SN2 mechanism. iii. However, under drastic conditions, aryl halides undergo nucleophilic substitution reactions. Nucleophilic Substitution Reactions: Under drastic conditions like high temperature and under pressure, halide group attached to arenes can be
replaced by OH, CN or NH2 group. i. Dow’s Process: When aryl halide reacts with NaOH at 623 K under pressure of 200 atm300 atm, forms sodium phenoxide which on acidification gives phenol. This process is known as “Dow’s process”. Cl ONa OH
623K, HCl dil.HCl + NaOH 200atm 300atm NaCl
Chlorobenzene Sodium phenoxide Phenol ii. Chlorobenzene on heating with anhydrous copper cyanide and sodium cyanide at 473 K under pressure gives cyanobenzene. Cl CN
anhydrousCuCN NaCN + CN 473 K,pressure + Cl
Chlorobenzene Cyanobenzene iii. Chlorobenzene on heating with aqueous NH3 in the presence of catalyst cuprous oxide, at 473 K under pressure gives aniline.
Cl NH2
473K 2 + 2CuCl + H2O + 2NH3 + Cu2O under pressure 2
Chlorobenzene Aniline iv. Effect of substituents on the reactivity of haloarenes:
a. It has been found that presence of electron withdrawing groups like NO2, COOH, CN at o – and/or p – position with respect to halogen atom greatly activates haloarenes to undergo nucleophilic displacement reactions. eg. o and p – Nitrochlorobenzene easily undergo nucleophilic attack of OH to give o and p – Nitrophenol.
1. NO2 group is at ortho position with respect to halogen atom: Cl Cl O + O + O + O HO Cl + HO Cl HO N N N HO N O O O slow step O
o-Nitrochlorobenzene Resonating structures
OH HO Cl + O + O N N O O fast step + Cl
Resonance hybrid o-Nitrophenol
32
Chapter 10: Haloalkanes and Haloarenes
2. NO2 group is at para position with respect to halogen atom: Cl HO Cl Cl HO Cl HO HO slow step + + + + N N N N O O O O O O O O p-Nitrochlorobenzene Resonating structures OH HO Cl
fast step + Cl
+ + N N O O O O Resonance hybrid p-Nitrophenol b. From the above mechanism it is clear that, carbanion formed by the attack of OH gets stabilized because of electrons of benzene ring as well as negative charge on C-atom attached to electron withdrawing NO2 group. c. Hence, o- and p-substituted aryl halides show greater reactivity towards nucleophilic attack. d. But in the case of m-substituted aryl halide, there is no negative charge at m-position in the resonating structures; due to this the presence of electron withdrawing group at m-position has no effect on reactivity. e. It is observed that as number of electron withdrawing groups at ortho - and para-position (with respect to halogen atom) increases, the reactivity of haloarenes also increases. eg. 1. Cl OH
NO2 NO2
+ OH (i)NaOH (ii)H ,368K + Cl
NO2 NO2 1-Chloro-2,4-dinitrobenzene 2,4-Dinitrophenol (55% yield)
2. Cl OH
O N NO2 O2N NO2 2 warm + OH + Cl HO2
NO NO2 2 1-Chloro-2,4,6-trinitrobenzene 2,4,6-Trinitrophenol (Picric acid) (93% yield) Electrophilic Substitution Reactions: i. In the case of chlorobenzene, following resonating structures are obtained. + Cl Cl+ Cl Cl+
ii. In chlorobenzene, electron density is more at o and p – position (since chlorine is o and p directing). 33
Chemistry Vol ‐ 2.2 (Med. and Engg.) iii. It is observed that, halogen atoms are highly electronegative, they pull electrons of benzene ring towards themselves due to – I effect and hence aryl halides show reactivity towards electrophilic attack. iv. Hence, weaker resonating structures control o , p orientations and stronger inductive effect controls reactivity of aryl halides. v. When an electrophile (E) attacks on ortho and/or para positions of aryl halide; more stable chloronium ion is formed as follows: + Cl+ Cl H E
H E Carbocation at Carbocation at o-position/ p-position/ chloronium ion chloronium ion vi. Attack of an electrophile at meta position forms comparitively less stable chloronium ion. vii. Thus, electrophilic substitution reaction in aryl halide (i.e., chlorobenzene) occurs slowly and under drastic conditions compared to benzene.
viii. Halogenation: Chlorobenzene reacts with Cl2 in the presence of anhydrous FeCl3 or sunlight to give o-dichlorobenzene or p-dichlorobenzene. Cl Cl eg. Cl Cl anhydrousFeCl3 + Cl2 orsunlight + + HCl
Chlorobenzene o-Dichlorobenzene Cl (Minor product) p-Dichlorobenzene (Major product) Note: a. Benzene when treated with chlorine in the presence of bright sunlight or ultraviolet light, adds up three molecules of chlorine to give benzene hexachloride/BHC (C6H6Cl6). H H Cl | C C H Cl H C C H C C H Cl + 3Cl bright sunlight 2 or UVlight H H C C H Cl C C C Cl | H C H
Benzene Cl H Benzene hexachloride (BHC) b. This is an addition type of reaction. c. Benzene hexachloride is commercially known as BHC. d. It exists in eight isomeric forms. e. The gamma () isomer is called “Gammexane” or “Lindane” which is used as an insecticide.
ix. Nitration: Chlorobenzene reacts with nitrating mixture i.e., conc.HNO3 and conc. H2SO4 to give 1-Chloro- 4-nitrobenzene (major product) and 1-Chloro-2-nitrobenzene (minor product). This reaction is known as “Nitration”. Cl Cl Cl
NO2 conc.H24 SO + HNO3 + + H O 2 (conc.) Chlorobenzene NO2 1-Chloro-4-nitrobenzene 1-Chloro-2-nitrobenzene (Major product) (Minor product)
34
Chapter 10: Haloalkanes and Haloarenes x. Sulphonation: Chlorobenzene on heating with conc. H2SO4 yields 4-chlorobenzene sulphonic acid (major product) and 2-chlorobenzene sulphonic acid (minor product). Cl Cl Cl
SO3H + H SO + + H O 2 4 2 (conc.) Chlorobenzene 2-Chlorobenzene SO3H 4-Chlorobenzene sulphonic acid
sulphonic acid (Minor product)
(Major product) xi. Friedel-Craft’s Reaction: a. Introduction of an alkyl or acyl group in the haloarene ring or in the substituted benzene ring in the presence of anhydrous aluminium trichloride is known as “Friedel-Craft’s Reaction”. b. The reaction can be carried out by reacting aryl halide (i) with alkyl chloride (Friedel-Craft’s alkylation reaction) or (ii) with acyl chloride (Friedel-Craft’s acylation reaction). eg. Cl Cl
CH Cl CH3 3 Chloromethane + + HCl
Cl 1-Chloro-2-methylbenzene anhydrous CH3 1-Chloro-4-methylbenzene (Minor product) AlCl 3 (Major product) O
Chlorobenzene Cl Cl O H3C C Cl Ethanoyl chloride C CH3 (Acetyl chloride) + + HCl
2-Chloroacetophenone C CH3 (Minor product)
O 4-Chloroacetophenone (Major product) Reaction with Sodium Metal (Wurtz Fittig Reaction): i. When an aryl halide is heated with alkyl halide, it undergoes coupling reaction in the presence of sodium metal and dry ether to give alkyl benzene. This reaction is known as “Wurtz Fittig Reaction”. eg. dryether Cl + 2Na + Cl CH3 CH3 + 2NaCl
Methyl Methyl benzene Chlorobenzene chloride (Toluene) ii. In the above reaction, along with toluene, ethane (obtained by coupling of two methyl groups) and diphenyl (obtained by coupling of two phenyl groups) are also produced as byproducts. dryether a. 2H3C Cl + 2Na CH3 CH3 + 2NaCl Methyl Ethane chloride Cl
dryether b. 2 + 2Na + 2NaCl
Chlorobenzene Diphenyl Note: Reaction of haloarenes with sodium metal is called as “Fittig reaction”. 35
Chemistry Vol ‐ 2.2 (Med. and Engg.)
10.13 Uses and environmental effects of some haloalkanes and haloarenes
Physical and chemical Name and Structure Uses Environmental Effects Properties Dichloromethane It is volatile, colorless, i. It is widely used as i. Exposure to high level (methylene sweet smelling liquid organic solvent for causes nausea, dizziness dichloride) having low boiling chemical reaction and for and numbness in fingers point. extraction, in the and toes. H Cl manufacturing of drugs ii. Exposure to low level
C and food industries. causes impaired hearing H Cl ii. It is used as paint and vision. remover and metal iii. Dichloromethane have cleaning and finishing harmful effects on solvent. central nervous system. iii. It is used as propellent in iv. On direct contact with aerosols and as fumigant skin, it causes burning
pesticides. and mild redness of skin. iv. Used as refrigerent and v. Direct contact with eyes dewaxing agent. can even burn the cornea. Trichloromethane i. It is volatile, oily i. It is used as solvent for i. It is stored in dark, (Chloroform) colorless and non- organic reactions and amber coloured bottle to inflammable liquid for separation of prevent oxidation. H Cl with sickly smell. compounds. ii. C Because of toxicity, ii. Chloroform on ii. Used in the preparation chloroform is not used Cl Cl exposure to air or of chloropicrin, as an anaesthetic.
light get oxidised to [CCl3(NO2)] (tear gas) iii. If it is inhaled for long poisonous iii. It is used in preparation time, it affects central compound of chlorofluoro nervous system (CNS). phosgene (carbonyl methane, Freon iv. If mixed with air causes chloride). [0.6 % to refrigerant R-22. headache and fatigue 1 % ethanol is iv. Used as a source of due to the formation of mixed in CHCl3 to dichlorocarbene (CCl2) poisonous phosgene. convert phosgene group. into non-toxic v. It is also used as solvent diethyl carbonate.] in production of pesticides and dyes.
Tetrachloromethane i. It is colourless oily i. It is used as solvent in i. It causes depletion in (Carbon liquid. production of ozone layer which tetrachloride) ii. It is non refrigerants. affects human skin
Cl Cl inflammable. ii. It is used as a solvent leading to cancer. for oils, fats, waxes and ii. When carbon C iii. It is insoluble in water but soluble in for the manufacture of tetrachloride comes in Cl Cl organic solvents. pharmaceuticals. contact for longer time, iii. It is used as pesticide it causes eye irritation,
and insecticide. nausea, dizziness, damages nerve cells, iv. It is used as dry cleaning agent. unconsciousness or v. It was used as a fire death. extinguisher under the name pyrene.
36
Chapter 10: Haloalkanes and Haloarenes
Iodoform i. It is yellow colored i. It is used as disinfectant It has very strong smell. (Triiodomethane) crystalline solid on small scale. having unpleasant It is used in H I ii. odour. manufacture of C It is soluble in pharmaceuticals. I ii. I organic solvent like iii. It is used as antiseptic ethanol, ether but due to the liberation of
insoluble in water. free iodine.
Freons i. Chlorofluoro i. They are used as i. Freons are chemically Dichlorodifluoro compounds of refrigerants in stable hence non- refrigerator and air methane methane and ethane degradable and remain in conditioners. (CCl2F2) are known as atmosphere. ii. They are used as (Freon 12) “Freons”. ii. In stratosphere, it propellants in aerosol ii. These are highly undergoes photo chemical Cl F stable, low boiling, products of food, decomposition and causes
C non-inflammable, cosmetics and depletion in ozone layer non-toxic, non- pharmaceuticals. which affects human skin Cl F iii. Bromo freons are used as corrosive easily leading to cancer. liquefiable and fire extinguishers. Trichlorofluoro highly unreactive iv. They are used as cleaning compounds. agents for clothes and methane metallic surface. (CCl3F) v. They are used as foaming
Cl F agent in preparation of foamed plastics. C
Cl Cl
Chlorodifluoro methane (CHClF2) H F C
Cl F
DDT DDT is insoluble in i. DDT is mainly used as i. DDT is non- p,p- water but moderately an insecticide against biodegradable malaria. dichlorodiphenyltrich soluble in polar solvents. compound. It remains in ii. Because of its high and loroethane OR environment for long specific toxicity, it is time (owing to it’s 2,2-bis(p- used to kill mosquitoes chemical stability). chlorophenyl)-1,1,1- and houseflies. trichloroethane ii. DDT is fat soluble and
gets deposited and Cl CH CCl 3 stored in fatty tissues therefore causing health problems in animals and Cl fishes.
37
Chemistry Vol ‐ 2.2 (Med. and Engg.)
Quick Review
Preparation of alkyl halides: Reactions of alkyl halides: (Haloalkanes) (Haloalkanes) aq.KOH
Hydrolysis boil Cl2, sunlight (h) C H OH 2 5 UV light or Δ AgOH Ethanol
Br moist H C CH 2 3 3 Anhydrous AlBr Ag2O, Ethane 3 I2
HIO3/HNO3/HgO alc.KCN C2H5CN (dil.) boil Ethanenitrile
alc. AgCN HCl C2H5 NC Carbylaminoethane HBr H2C = CH2 Ethene HI alc.NH 3 CH CH NH , pressure 3 2 2 Ethanamine HCl
Anhydrous ZnCl2, Δ Na O CH3 H5C2 O CH3 Methoxyethane HBr
(NaBr + conc.
H2SO4), Reflux Dry Ag2O C H C H Conc.HI (57%) OR 2 5 2 5 n-Butane (KI+95%H3PO4) HI, Δ O C 2H5OH PCl3 C2H5X O Ethanol Haloethane Δ Ag O C C2H5 [X=Cl, Br, I] C2H5 O C C2H5 Red P/X Ethylpropanoate 2 (Ethyl propionate) X2 = Br2, I2 alc. KOH PCl5 H2C = CH2 Δ Ethene
SOCl2 2Na Pyridine, C H C H 2 5 2 5 reflux Dry ether n-Butane NaI dry acetone Mg C2H5 Mg X AgF Dry ether Ethyl magnesium halide CoF2 C2H5X KNO2/ C2H5X Haloethane C2H5 O N = O Haloethane SbF3 Ethyl nitrite (X= Cl,Br) [X = I, F] AgNO2 C2H5 NO2 Hg2F2 Nitroethane
38
Chapter 10: Haloalkanes and Haloarenes Markownikoff’s rule and Anti-Markownikoff’s rule:
H HBr + H3C CH2 CH2Br H3C CH = CH2 Markownikoff ’s rule H3C C CH3 1-Bromopropane Propene Br (Minor product) 2-Bromopropane (Major product) H
HBr, (C H CO) O H C CH = CH 65 2 2 H C CH CH Br + H C C CH 3 2 Anti Markownikoff ’s rule 3 2 2 3 3 1-Bromopropane Propene (Major product) Br
2-Bromopropane (Minor product)
Preparation of Haloarenes: Reactions of Haloarenes:
Cl2 FeCl 3 OH (310K320K) + Br2 NaOH, H FeBr 3 623K, Benzene (310K320K) 200-300 atm Phenol I2 X CN HIO3 Nucleophilic If substitution NaCN+CuCN Haloarene reactions 473K,pressure NH2 N NX Cu X X = Cl 2 2 (X=Cl, Br, I) Cyanobenzene NaNO HX X=Cl,Br 2 NH2 273 278K KI NH3, Cu2O Aniline Benzene diazonium 473K,pressure halide Aniline
39
Chemistry Vol ‐ 2.2 (Med. and Engg.) Reactions of Haloarenes: Cl Cl Cl Cl2, anhydrous FeCl3 +
Cl 1,4-Dichlorobenzene 1,2-Dichlorobenzene (Major) (Minor) Cl Cl NO HNO3 2 + conc. H2SO4
NO2 1-Chloro-2-nitrobenzene 1-Chloro-4-nitrobenzene (Minor) (Major) Cl Cl
conc. H2SO4 SO3H +
SO H 2-Chlorobenzene 3 4-Chlorobenzene sulphonic acid sulphonic acid (Minor) (Major)
X Cl Cl If CH3Cl CH3 Haloarene X = Cl anhydrous AlCl3 + (X=Cl, Br, I) 1-Chloro-2- CH3 methylbenzene 1-Chloro-4- (Minor) methylbenzene (Major) O Cl Cl C CH3 CH3COCl + anhydrous AlCl3
2-Chloroacetophenone C O CH (Minor) 3 4-Chloroacetophenone R (Major) RX, Na
dry ether Alkyl benzene
CH Na, CH3Cl 3 dry ether Methyl benzene (Toluene) OH NaOH, 623 K
Pressure H+ Phenol
Formulae For a given optically active compound; Number of possible stereoisomers = 2n (where n = number of asymmetric C-atom(s) present in the molecule of that particular compound).
40
Chapter 10: Haloalkanes and Haloarenes 7. If two halogen atoms are attached to same Multiple Choice Questions carbon atom of an alkane, it is called as
______. 10.0 Introduction (A) alkyl halide (B) alkylene halide
(C) alkylidene halide (D) aryl halide 1. ______are obtained by replacing one or more hydrogen atom(s) of an alkane with the 8. Geminal dibromide is ______. corresponding number of halogen atom(s). [RPMT 2000] (A) Haloarenes (B) Haloalkanes (A) CH3C(Br)(OH)CH2Br (C) Alkenes (D) Alkynes (B) CH3CBr2CH3 (C) CH2(Br)CH2CH3 2. In haloalkanes, halogen atom is bonded to (D) CH2(Br)CH2Br ______hybridized carbon atom of an alkyl group. 9. In vicinal dihalides, two halogen atoms are (A) sp (B) sp2 present on ______. (C) sp3 (D) sp3d (A) adjacent carbon atoms (B) same carbon atom 3. Match the compounds given in column I with (C) terminal carbon atom the uses given in column II. (D) middle carbon atom
Column I Column II 10. Which of the following is an example of i. Halothane a. Treatment of vic-dihalide? [NCERT Exemplar] malaria (A) Dichloromethane ii. Chloroquine b. Anaesthetic (B) 1,2-dichloroethane iii. Chloramphenicol c. Treatment of (C) Ethylidene chloride (D) Allyl chloride typhoid fever d. Treatment of 11. Ethylene dichloride and ethylidene chloride cancer are ______. e. Blood (A) chain isomers substituent (B) functional isomers
(C) position isomers (A) (i) – (b), (ii) – (a), (iii) – (c) (D) stereo isomers (B) (i) – (c), (ii) – (d), (iii) – (e) (C) (i) – (e), (ii) (c), (iii) – (a) 12. Haloforms are trihalogen derivatives of (D) (i) – (a), (ii) – (b), (iii) – (c) ______. [CPMT 1985] (A) C2H6 (B) CH4 10.1 Classification (C) C3H8 (D) C2H4
4. The general molecular formula of 13. Tetrahalogen derivatives have the general monohalogen derivatives of alkanes is formula ______.
______. (A) CnH2n+1X (B) CnH2nX2 (A) R-X (B) CnH2n + 1X (C) CnH2n1X3 (D) CnH2n2X4 (C) CnH2nX (D) all of these 14. Which of the following is allylic halide? 5. The general formula of dichloroalkane is [GUJ CET 2015] ______. (A) Benzyl chloride (A) CnH2n+1Cl2 (B) CnH2n+2Cl2 (B) (1-Bromoethyl)benzene (C) C H Cl (D) C H Cl n 2n 2 n 2n-2 2 (C) 1-Bromobenzene 6. In geminal dihalides, the halogen atoms are (D) 3-Chlorocyclohex-1-ene
attached to ______. 10.2 Monohalogen derivatives of alkanes (A) same carbon atom (B) adjacent carbon atoms 15. Isopropyl halide is ______alkyl halide. (C) each other (A) primary (B) secondary (D) all of these (C) tertiary (D) all of these 41
Chemistry Vol ‐ 2.2 (Med. and Engg.) 16. Which of the following is a primary alkyl Cl halide? [DCE 2004] |
(A) Isopropyl iodide (C) CH3 C CH3 (B) sec-Butyl iodide (C) tert-Butyl bromide CH3 (D) Neohexyl chloride CH3 17. Number of secondary alkyl halides formed by | compound C5H11Cl are ______. (D) CH3 CH2 C Cl (A) 2 (B) 3 (C) 4 (D) 5 |
18. Among the following, an example of tertiary CH3 alkyl halide is ______. 23. CH3 CH2 CH CH3 is ______. (A) CH3 CH2 C(CH3)2 | Cl Cl (A) isobutyl chloride (B) sec-Butyl chloride Cl (C) n-butyl chloride (B) CH3 C Cl (D) both (A) and (B) 24. What should be the CORRECT IUPAC name Cl for diethylbromomethane? (C) CH3 CH2 CH C(CH3)3 [NCERT Exemplar] | (A) 1-Bromo-1,1-diethylmethane Cl (B) 3-Bromopentane (C) 1-Bromo-1-ethylpropane (D) CH CH CH Cl 3 2 (D) 1-Bromopentane | 25. The IUPAC name of isobutyl chloride is ______. CH3 (A) 1-Chloro-2-methylbutane 19. Number of carbon atoms present in the (B) 1-Chloro-2-methylpropane simplest secondary and tertiary alkyl halides (C) 2-Chlorobutane are ______. (D) 2-Chloro-2-methylpropane (A) 3, 4 (B) 2, 3 (C) 4, 4 (D) 4, 5 26. Which is the CORRECT IUPAC name for CH – CH – CH – Br ? 20. C H Br can represent ______. 3 2 4 9 (A) a tertiary bromoalkane C2H5 [NCERT Exemplar] (B) a secondary bromoalkane (A) 1-Bromo-2-ethylpropane (C) a primary bromoalkane (B) 1-Bromo-2-ethyl-2-methylethane (D) all of these (C) 1-Bromo-2-methylbutane 21. An alkyl chloride containing quaternary (D) 2-Methyl-1-bromobutane
carbon atom(s) would be ______. 27. CH – CH CHCl CH is ______. (A) tert-butyl chloride 3 3 (B) neohexyl chloride C2H5 (C) sec-butyl chloride (D) n-propyl chloride (A) 3–Chloro2–ethylbutane (B) 2–Chloro3–ethylpentane 10.3 Nomenclature of haloalkanes (C) 2–Chloro3–ethylbutane (D) 2–Chloro3–methylpentane 22. Structural formula of isobutyl chloride is ______. (A) CH3 – CH CH2Cl 28. IUPAC nomenclature of the compound (CH3)2C(CH2CH3)CH2CH(Cl)CH3 is ______. CH3 (A) 5-chloro-3,3-dimethylhexane (B) 4-chloro-2-ethyl-2-methylpentane (B) CH3 – CH CH2CH3 (C) 2-chloro-4-ethyl-4-methylpentane (D) 2-chloro-4,4-dimethylhexane CH3
42
Chapter 10: Haloalkanes and Haloarenes
29. IUPAC name of 10.4 Nature of C X bond in haloalkanes
C2H5 Cl CH3 35. In CH3–CH2–CH2Br, CBr bond is formed by H3CCH2CHCHCHCH2CH2CH3 is the overlapping of ______orbitals of C-atom ______. and ______orbitals of Br-atom respectively. 3 3 (A) 4-Chloro-3-ethyl-2-methylheptane (A) 2sp , 2pz (B) 2sp , 3pz 3 3 (B) 4-Chloro-3-ethyl-5-methyloctane (C) 3sp , 3pz (D) 2sp , 4pz (C) 4-Chloro-5-methyl-3-ethyloctane (D) both (A) and (B) 36. Which of the following is the CORRECT order for strength of C-X bond? [GUJ CET 2014] 30. The IUPAC name of ethylene dichloride and ethylidene chloride respectively are ______. (A) CH3F > CH3Cl > CH3Br > CH3I (A) 1,1-Dichloroethane and (B) CH3F < CH3Cl < CH3Br < CH3I 2,2-Dichloroethane (C) CH3I > CH3F > CH3Cl > CH3Br (D) CHCl > CH Br > CH F > CH I (B) 1,1-Dichloroethane and 3 3 3 3 1,2-Dichloroethane 37. Among the halomethanes, the CX (C) 1,2-Dichloroethane and (X = halogen) bond energy increases in the 1,1-Dichloroethane order ______. [Assam CEE 2015] (D) 2,2-Dichloroethane and (A) CHF < CH Cl < CH Br < CH I 1,1-Dichloroethane 3 3 3 3 (B) CH3I < CH3Br < CH3Cl < CH3F 31. IUPAC name of the compound (C) CH3F < CH3Br < CH3Cl < CH3I (CH3)2C(Cl)C(CH3)2Br is ______. (D) CH3Cl < CH3F < CH3Br < CH3I (A) 3-Bromo-2-chloro-2,3-dimethylbutane (B) 2-Bromo-3-chloro-2,3-dimethylbutane 38. The CORRECT order of C – X bond polarity (C) 2-Bromo-4-chloro-2,4-dimethylbutane is ______. [RPMT 2000] (D) 3-Bromo-4-chloro-2,4-dimethylbutane (A) CH3Br > CH3Cl > CH3I 32. The IUPAC name of allyl chloride is ______. (B) CH3I > CH3Br > CH3Cl (A) 1-Chloroethane (C) CH3Cl > CH3Br > CH3I (B) 3-Chloroprop-1-yne (D) CH3Cl > CH3I > CH3Br
(C) 3-Chloroprop-1-ene 39. Which of the following has the least dipole (D) 1-Chloropropene moment? 33. IUPAC name of the compound (A) CH3 F (B) CH3 Cl
H (C) CH Br (D) CH I H3C 3 3 H 10.5 Preparation of haloalkanes H Br H 40. Chlorination of ethane is carried out in is ______. [KCET 2016] presence of ______. [MH CET 2015] (A) 1-Bromobut-2-ene (A) anhydrous AlBr3 (B) mercuric chloride (C) ultraviolet light (D) zinc chloride (B) 2-Bromo-2-butane (C) Bromobutane 41. The main product of the reaction of propane (D) 1-Bromobut-3-ene with chlorine at 25 ºC, in the presence of 34. The IUPAC name of the following compound sunlight is ______. is ______. [Assam CEE 2015] (A) 1-chloropropane (B) 2-chloropropane Br (C) chloroethane (D) chloromethane
42. If n-Butane is monochlorinated, the number of Br products formed are ______. (A) 2 (B) 3 (C) 4 (D) 6 (A) 3,4-dibromo-5-phenylpentane (B) 2,3-dibromo-1-phenylpentane 43. 2,6-Dimethylheptane on monochlorination (C) 3,2-dibromo-1-phenylpentane produces ______derivatives. [DPMT 2001] (D) 4,3-dibromo-5-phenylpentane (A) 5 (B) 6 (C) 3 (D) 4 43
Chemistry Vol ‐ 2.2 (Med. and Engg.) 44. 2-Methylpropane on monochlorination under 51. Halogenation of alkanes is ______. photochemical condition gives ______. [KCET 2002] [WBJEE 2013] (A) a reductive process (A) 2-Chloro-2-methylpropane as major (B) an oxidative process product (C) an isothermal process (B) (1:1) Mixture of 1-chloro-2- (D) redox process methylpropane and 2-chloro-2- methylpropane 52. The addition of HCl to pent-1-ene will give (C) 1-Chloro-2-methylpropane as a major major product as ______. product (A) 3-chloropentane (D) (1:9) Mixture of 1-chloro-2- (B) 2-chloropentane methylpropane and 2-chloro-2- (C) 1,2-dichloropentane methylpropane (D) 1-chloropentane
45. In the chlorination of isobutane, which 53. Order of reactivity of HX in the reaction product will be formed in excess? C = C + HX RX is ______. (A) (CH3)2CHCH2Cl (B) (CH3)3CCl (A) HI HBr HCl HF (C) CH3 – (CH2)2 – CH2 – Cl (B) HBr HCl HI HF (D) CH – CHCl – CH 3 3 (C) HCl HBr HI HF 46. Which of the following statement is (D) HF HBr HCl HI
CORRECT? 54. According to Markownikoff’s rule, negative (A) Neopentane forms only one monochloro part of reagent adds to that carbon atom of an derivative. unsymmetrical alkene ______. (B) Neopentane forms four monochloro (A) which carries lesser number of carbon derivatives. atoms (C) Neopentane has higher boiling point (B) which carries more number of hydrogen than n-pentane. atoms (D) Neopentane shows optical isomerism. (C) which carries lesser number of hydrogen 47. Which of the following yields only one type of atoms monosubstituted chloroalkane upon chlorination? (D) which carries no hydrogen atoms (A) Isobutane (B) Cyclopentane (C) n-Butane (D) Propane 55. Propene on treatment with HBr gives ______. [CPMT 1986] 48. Alkanes can be brominated to give the (A) isopropyl bromide corresponding alkyl bromides by heating with (B) n-butyl bromide ______. (C) propylene dibromide (A) Br and HBr 2 (D) all of these (B) Br2 and anhydrous AlBr3 (C) Br2 and red phosphorus 56. What is the major product of the reaction (D) both (B) and (C) between 2-methylpropene with HBr? 49. Direct iodination of alkanes is NOT possible [RPMT 2002] because ______. (A) 1-Bromobutane (A) the reaction requires a catalyst (B) 1-Bromo-2-methylpropane (B) the reaction is reversible (C) 2-Bromobutane (D) 2-Bromo-2-methylpropane (C) I2 is a weak reagent (D) alkanes do not react with I 2 57. Which of the following does NOT observe the 50. In direct iodination of an alkane, the reverse Markownikoff’s addition of HBr? reaction due to HI is prevented by using (A) But-2-ene ______. (B) Propene (A) mercuric oxide (B) iodic acid (C) Pent-2-ene (C) dil. nitric acid (D) all of these (D) But-1-ene
44
Chapter 10: Haloalkanes and Haloarenes
peroxide 58. The reaction of C6H5CH = CHCH3 with HBr (B) CHCH = CH + HBr produces ______. [AIPMT 2015] 3 2 CH3CH2CH2Br (A) C6H5CHCH2CH3 peroxide (C) CH3CH = CH2 + HCl Br CH3CH2CH2Cl (B) C6H5CH2CHCH3 peroxide (D) CH3CH = CH2 + HI CH CHI–CH Br 3 3 64. Lucas reagent is an equimolar mixture of ______. (C) CH CH CH CH Br 6 5 2 2 2 [MP PMT 1996; MP PET 1992, 95; (D) CH=CHCH3 CPMT 1986, 89; AIIMS 1980;
Kurukshetra CEE 2002; BCECE 2014]
(A) concentrated HCl + anhydrous ZnCl 2 (B) dilute HCl + hydrated ZnCl 2 (C) concentrated HNO + anhydrous ZnCl Br 3 2 (D) concentrated HCl + anhydrous MgCl 59. This will give 2,2-dibromopropane: 2 [BCECE 2014] 65. Which of the following is CORRECT? (A) Primary alcohol + HCl Room temperature (A) CH CH + 2HBr Chloroalkane + H2O (B) CH3C CH + 2HBr (B) Secondary alcohol + HCl Room temperature (C) CH3CH = CH2 + HBr (D) CH3 – CH2 – CH = CH2 + HBr Chloroalkane + H2O 60. Peroxide effect takes place in the presence of (C) Tertiary alcohol + HCl Room temperature
______. Chloroalkane + H2O (A) phosphorus trichloride (D) all of these. (B) benzoyl peroxide 66. Which of the following alcohols will yield the (C) phosphorus pentoxide corresponding alkyl chloride on reaction with (D) thionyl chloride concentrated HCl at room temperature? 61. Assertion: Reaction of but-1-ene with HBr [NCERT Exemplar] gives 1-bromobutane as major product. (A) CH3CH2 – CH2 – OH Reason: Addition of hydrogen halides to (B) CH3CH2 – CH – OH alkenes proceeds according to Markownikoff’s rule. CH3 The CORRECT answer is ______. (C) CH 3 CH 2 – CH – CH2OH [TS EAMCET (Engg.) 2015]
(A) Assertion and Reason are correct. Reason CH3
is the correct explanation of Assertion. CH3
(B) Assertion and Reason are correct but (D) Reason is not the correct explanation of CH3CH2 – C – OH
Assertion.
(C) Assertion is correct but Reason is not CH3 correct. 67. Which of the following reactions can be used (D) Assertion is not correct but Reason is for the preparation of alkyl halides? correct. anhydrous ZnCl2 (I) CH3CH2OH + HCl 62. Which of the following acids adds to propene (II) CH3CH2OH + HCl in the presence of peroxide to give anti- (III) (CH3)3COH + HCl Markownikoff’s product? [MP PET 2003] (IV) (CH ) CHOH + HCl anhydrous ZnCl2 (A) HF (B) HCl (C) HBr (D) HI 3 2 [AIPMT RE-TEST 2015] 63. Which of the following reactions follows (A) (IV) only AntiMarkownikoff’s addition? (B) (III) and (IV) only (A) CH3CH = CH2 + HBr (C) (I), (III) and (IV) only peroxide CH3CHBr CH3 (D) (I) and (II) only 45
Chemistry Vol ‐ 2.2 (Med. and Engg.) 68. When ethanol is distilled with potassium 75. The best reagent which is used in the bromide and concentrated H2SO4, we get conversion of butan-1-ol to 1-bromobutane is ______. ______. (A) ethyl bromide (A) anhydrous ZnBr2 (B) Br2/H2O (B) propyl bromide (C) PBr3 (D) conc. H2SO4 (C) acetylene dibromide 76. C H OH P/I2 C H I (D) both (B) and (C) 2 5 2 5 In the above reaction, the other product would 69. ______is prepared in situ by reacting be ______. potassium iodide with 95% phosphoric acid (A) PI3 (B) H3PO3 (H PO ). 3 4 (C) POI3 (D) H3PO4 (A) HI (B) HIO3 77. Ethyl alcohol on treating with thionyl chloride (C) KHPO (D) HPO 2 4 3 3 gives off the ______gas. 70. Butan-2-ol on reacting with concentrated (A) Cl2 (B) C2H6 hydroiodic acid gives ______. (C) SO3 (D) SO2 (A) 1-iodobutane (B) 2-iodobutane 78. C H OH + SOCl Pyridine C H Cl + SO + (C) 1,2-diiodobutane (D) all of these 2 5 2 Reflux 2 5 2 HCl 71. The order of reactivity of following alcohols The above reaction is known as ______. with halogen acids is ______. [AIIMS 2002] (i) CH3CH2 – CH2 – OH (A) Groove’s process (ii) CH3CH2 – CH – OH (B) Darzen’s procedure (C) Williamson’s synthesis CH3 (D) Wurtz synthesis CH3 (iii) 79. In the preparation of alkyl chloride from alcohol by using SOCl2, the solvent used is CH3CH2 – C – OH ______. (A) CH (B) CH N CH3 6 6 5 5 (C) C H Cl (D) C H NO [NCERT Exemplar] 6 5 6 5 2 (A) (i) > (ii) > (iii) (B) (iii) > (ii) > (i) 80. Ethanol is converted into ethyl chloride by (C) (ii) > (i) > (iii) (D) (i) > (iii) > (ii) reacting with ______.
72. In which one of the following conversions, [MP PET 1991; MP PMT 1990; BHU 1997] phosphorus pentachloride is used as a reagent? (A) Cl2 (B) SOCl2 (C) HCl (D) both (B) and (C) [EAMCET 1997] (A) H2C = CH2 CH3CH2Cl 81. Which of the following is best reagent to (B) C2H6 C2H5Cl convert alcohol into the corresponding alkyl (C) CH3CH2OH CH3CH2Cl chloride? (D) All of these (A) PCl3 (B) HCl
(C) PCl5 (D) SOCl2 73. In the reaction, R–OH + PCl3 the products formed are alkyl chloride and 82. Which reagent CANNOT be used to prepare ______. an alkyl halide from an alcohol? (A) phosphorus oxychloride [CPMT 1989, 94] (B) phosphorus acid (A) HCl + ZnCl2 (B) NaCl (C) PBr (D) SOCl (C) phosphoric acid 3 2 (D) hydrochloric acid 83. Name the following reaction: DryAcetone 74. The reaction of phosphorus tribromide with CH3CH2Cl + NaI CH3CH2 I + NaCl ethanol gives ______. [GUJ CET 2015] (A) monohalogen derivative (A) Swarts reaction (B) dihalogen derivative (B) Finkelstein reaction (C) bromine gas (C) Wurtz reaction (D) alkene (D) Hell-Volhard-Zelinsky reaction
46
Chapter 10: Haloalkanes and Haloarenes 84. The synthesis of alkyl fluorides is best (B) 1-Iodobutane < 1-Bromobutane accomplished by ______. < 1- Chlorobutane < Butane [JEE (MAIN) 2015] (C) Butane < 1-Iodobutane (A) free radical fluorination < 1-Bromobutane < 1-Chlorobutane (B) Sandmeyer's reaction (D) Butane < 1-Chlorobutane (C) Finkelstein reaction < 1-Iodobutane < 1-Bromobutane (D) Swarts reaction 92. Which of the following alkyl halides has the 85. Which one is the Swarts reaction from the lowest boiling point?
following? [GUJ CET 2014] (A) CH3 CH2 Br Acetone (A) CH3Cl + NaI CH3I + NaCl (B) CH3 (CH2)2 CH2Br Acetone (B) CH3Br + NaI CH3I + NaBr (C) H3C CH Br (C) CH3Br + AgF CH3F + AgBr | Dry ether CH3 (D) 2CH3Cl+2Na CH3CH3+2NaCl (D) CH3Br 86. The most suitable reagent(s) required to prepare 1-iodobutane from but-1-ene is/are ______. 93. Arrange the following compounds in (A) Raney Ni, increasing order of their boiling points.
(B) HI in the presence of H O CH3 2 2 (a) (C) KI CH – CH2Br
(D) HBr in the presence of H2O2 and NaI in CH3 dry acetone (b) CH3CH2CH2CH2Br
10.6 Physical and chemical CH3
properties of haloalkanes
(c) H3 C – C – CH3 87. Which of the following is liquid at room temperature? [AFMC 2005] Br [NCERT Exemplar]
(A) CH3I (B) CH3Br (A) (b) < (a) < (c) (B) (a) < (b) < (c) (C) (c) < (a) < (b) (D) (c) < (b) < (a) (C) CH3Cl (D) C2H5Cl 88. Alkyl halides are insoluble in water because 94. The arrangement of following compounds: they ______. i. bromomethane (A) are covalent compounds ii. bromoform (B) have low polarity iii. chloromethane (C) do not form hydrogen bonds with water iv. dibromomethane (D) have tetrahedral structure in the increasing order of their boiling points
is ______. [KCET 2015] 89. The alkyl halide with the lowest density is (A) iii < i < iv < ii (B) iv < iii < i < ii ______. (C) ii < iii < i < iv (D) i < ii < iii < iv (A) methyl iodide (B) ethyl iodide (C) n-butyl iodide (D) n-propyl iodide 95. Most reactive alkyl halide (for the same alkyl group) is ______. 90. For a given alkyl group, the densities of the (A) R Cl (B) R – I halides follow the order: (C) R Br (D) R – F [MP PMT 1997] 96. General order of reactivity of R-X is ______. (A) RI < RBr < RCl (B) RI < RCl < RBr o o o o o o (C) RBr < RI < RCl (D) RCl < RBr < RI (A) 3 > 2 > 1 (B) 1 > 2 > 3 (C) 1o > 3 > 2o (D) 2o > 3o > 1o 91. Which is the CORRECT increasing order of boiling points of the following compounds? 97. Conversion of an alkyl halide to alcohol can 1-Iodobutane, 1-Bromobutane, be done by ______. 1-Chlorobutane, Butane (A) oxidation [NCERT Exemplar] (B) hydration (A) Butane < 1-Chlorobutane (C) alkaline hydrolysis < 1-Bromobutane < 1-Iodobutane (D) acidic hydrolysis 47
Chemistry Vol ‐ 2.2 (Med. and Engg.)
98. Which one of the following is boiled with 107. Cl ethyl chloride to form ethyl alcohol? NH3 [MNR 1982] EtOH (A) Alcoholic KOH (B) Aqueous KOH Br (C) Alcoholic KCN (D) Alcoholic NH 3 The product of the above reaction is ______. 99. Alkyl halides can be hydrolysed to alcohols by [WB JEEM 2015] NH2 reacting with ______. (A) (A) moist silver oxide (B) alcoholic alkali NH2 (C) alcoholic ammonia Cl (D) dilute sulphuric acid
100. Ethyl bromide can be converted into ethyl (B) alcohol by ______. [KCET 1989]
(A) heating with dilute hydrochloric acid NH2 NH (C) 2 and zinc (B) boiling with an alcoholic solution of KOH
(C) the action of moist silver oxide Br
(D) refluxing with methanol NH2 (D) 101. When alkyl halides form alkane nitriles, halogen is substituted by ______. OEt (A) N C group (B) NO2 group (C) C N group (D) NH2 group 108. Chloromethane on treatment with excess of ammonia yields mainly ______. 102. In which case, formation of butane nitrile is [NCERT Exemplar] possible? [Orissa JEE 2004] (A) N,N-Dimethylmethanamine, (A) CHBr KCN (B) CHBr KCN CH3 37 49 CH3 – N (C) C37 H OH KCN (D) CHOH49 KCN CH 3 103. Which of the following reagents is used to (B) N-methylmethanamine, CH –NH–CH step up the C-chain in alkyl halides? 3 3 (C) Methanamine, CH NH (A) HCN (B) KCN 3 2 (D) Mixture containing all these in equal (C) NHCN (D) AgCN 4 proportion
104. An alkyl isocyanide can be prepared from a Br respective alkyl halide by heating with ______.
(A) aqueous KOH (B) moist silver oxide Alc.NH3 109. CH3 C CH3 Y (C) alcoholic AgCN (D) alcoholic KCN HBr 105. Ethyl chloride on heating with alcoholic silver CH3 cyanide forms a compound X. The functional Y in this reaction is ______. isomer of X is ______. [MH CET 2013] [EAMCET 1997; KCET 2005] (A) 2-methylpropan-2-amine (A) C2H5NC (B) C2H5CN (B) 2-methylpropene (C) HC – NH – CH (D) CH NH 3 3 2 5 2 (C) 2-amino-2-methylpropane (D) but-2-ene 106. Ammonolysis of ethyl chloride using alcoholic ammonia will yield ______. 110. When sodium salt of ethanol is treated with [AIIMS 1992] ethyl bromide, the product formed is ______. (A) ethyl amine (A) methoxyethane (B) triethyl amine (B) ethoxypropane (C) tetraethyl ammonium chloride (C) propoxypropane (D) all of these (D) ethoxyethane
48
Chapter 10: Haloalkanes and Haloarenes 111. In a Williamson’s synthesis reaction, the 118. Saytzeff’s rule states that ______alkene is products were found to be 1-methoxypropane major product in dehydrogenation of an alkyl and sodium bromide. The reactants are ______. halide.
(A) C2H5 Br and C2H5ONa (A) more substituted (B) less substituted (B) CH – Br and C H ONa (C) less branched (D) dehydrated 3 7 2 5 (C) CH – Br and CH ONa 3 7 3 119. Reactivity order of halides for (D) CH – Br and CH ONa 2 5 3 dehydrohalogenation is ______. 112. An alkyl halide was reacted with dry silver [KCET 2016] oxide but it was found that the silver oxide (A) R F > R Cl > R Br > R I was containing traces of moisture. Product (B) R I > R Br > R Cl > R F expected in such a reaction is ______. (C) R I > R Cl > R Br > R F (A) symmetrical ether (D) R F > R I > R Br > R Cl (B) unsymmetrical ether (C) monohydric alcohol 120. Among the following, the most reactive (D) symmetrical ketone towards alcoholic KOH is ______. [AIIMS 2004] 113. Methyl chloride reacts with silver acetate to (A) CH3Br yield ______. [BVP 2003] (B) CH3CH2Br (A) acetaldehyde (B) acetyl chloride
(C) ethyl acetate (D) methyl acetate (C) CH 3 – CH – CH3 114. For the preparation of ethyl propionate from | ethyl bromide, the other reactant used can be Br ______. CH3 (A) silver acetate | (B) propionic anhydride (D) CH3 – C – Br (C) propanoyl chloride | (D) silver propionate CH3 115. Alkyl halide can be converted into 121. 1-Chlorobutane reacts with alcoholic KOH to corresponding alkenes by ______. form ______. [BCECE 2005] [IIT-JEE 1991; AFMC 1998] (A) nucleophilic substitution reaction (A) but-1-ene (B) but-2-ene (B) elimination reaction (C) butan-1-ol (D) butan-2-ol (C) both nucleophilic substitution and 122. 12.3 g 1-bromopropane is treated with elimination reaction (D) rearrangement alcoholic KOH. What mass of propene is obtained if yield is 50%? [Assam CEE 2015] 116. Dehydrohalogenation is a process ______. (A) 6.05 g (B) 12.3 g (A) in which hydrogen is removed and (C) 4.2 g (D) 2.1 g halogen is added (B) in which both hydrogen atom and 123. 2-Bromopentane is heated with potassium halogen atom are removed from the hydroxide in ethanol. The major product same molecule (C) in which dehalogenation occurs in the obtained is ______. [CBSE PMT 1998] presence of hydrogen (A) pent-1-ene (B) cis pent-2-ene (D) in which dehydration occurs in the (C) trans pent-2-ene (D) 2-ethoxypentane presence of halogen 124. 1-Phenyl-2-chloropropane, on heating with 117. Alcoholic solution of KOH is a specific alcoholic KOH, gives mainly ______. reagent for ______. [Assam CEE 2015] (A) dehydration (A) 1-phenylpropene (B) dehalogenation (B) 3-phenylpropene (C) dehydrohalogenation (C) 1-phenylpropan-2-ol (D) dehydrogenation (D) 3-phenylpropan-1-ol 49
Chemistry Vol ‐ 2.2 (Med. and Engg.) 125. Which one of the following CANNOT 133. Alkyl halides can be converted into Grignard undergo dehydrohalogenation reaction? reagents by ______. [KCET 1989] [J & K 2005] (A) boiling them with Mg ribbon in (A) Isopropyl bromide alcoholic solution (B) Ethanol (B) warming them with magnesium in dry (C) Ethyl bromide ether (D) n-Butyl bromide (C) refluxing them with MgCl2 solution (D) warming them with BaCl2 126. The isomer of C4H9I which is capable of producing but-2-ene with alc. KOH solution is 134. Identify organometallic compound(s). ______. [MH CET 2013] [BCECE 2015] (A) 1-Iodobutane (A) CH3ONa (B) C2H5SNa (B) 2-Iodobutane (C) CH3MgI (D) all of these (C) 1-Iodo-2-methylpropane (D) 2-Iodo-2-methylpropane 135. Grignard reagents are obtained by using dry reactants because they react with water to 127. In Wurtz reaction, alkyl halide reacts with form ______. ______. [MH CET 2004] (A) ketone (B) an alkane (A) magnesium in dry ether (C) an aldehyde (D) alcohol (B) sodium in dry ether (C) zinc metal 136. Dry ether HO2 (D) anhydrous ZnCl Br + Mg A B 2 H 128. An alkyl bromide (X) reacts with Na in dry The product ‘B’ is ______. [KCET 2015]
ether to form 4,5-diethyloctane. Compound X
is ______. [Roorkee 1999] (A) OH (B) MgBr (A) CH3(CH2)3Br
(B) CH3(CH2)5Br (C) (D) OH (C) CH3(CH2)3CH(Br)CH3 (D) CH3(CH2)2CH(Br)CH2CH3 10.7 Stereochemistry 129. Which of the following does NOT undergo Wurtz reaction? 137. A ray of light consisting of a single (A) C2H5F (B) C2H5Br wavelength vibrating in all planes (C) C2H5Cl (D) C2H5I perpendicular to the direction of propagation is called ______. 130. The compound which is NOT formed when a (A) plane polarized light mixture of n-butyl bromide and ethyl bromide (B) polarized light treated with sodium metal in presence of dry (C) monochromatic light ether is ______. [MHT CET 2016] (D) ultraviolet light (A) Butane (B) Octane (C) Hexane (D) Ethane 138. Ordinary light is converted into plane polarized light by passing through a ______. 131. The alkane that will NOT be formed in Wurtz (A) Nickel prism (B) glass prism synthesis from isopropyl bromide and ethyl (C) Nicol’s prism (D) glass slab bromide together or separately with sodium in ether is ______. 139. What is the chemical composition of Nicol’s (A) isopentane prism? [MH CET 2015] (B) n-hexane (A) Al2O3 (B) CaSO4 (C) 2,3-dimethylbutane (C) CaCO3 (D) Na3AlF6 (D) n-butane 140. The property by virtue of which a compound can rotate the plane of plane polarized light in 132. Which is NOT present in Grignard reagent? either clockwise or anticlockwise direction is [CBSE PMT 1991] known as ______. (A) Alkyl group (B) Magnesium (A) photolysis (B) phosphorescence (C) Halogen (D) COOH group (C) optical activity (D) polarization
50
Chapter 10: Haloalkanes and Haloarenes 141. The instrument used to measure the optical 149. Asymmetric carbon is usually ______activity is ______. hybridised. (A) potentiometer (B) polarimeter (A) sp (B) sp2 (C) galvanometer (D) both (A) and (B) (C) sp3 (D) sp3d2
142. d and l form of an optically active 150. If ‘n’ represents total number of asymmetric compound is also denoted by ______signs carbon atoms in a compound, the possible respectively. number of optical isomers of the compound is (A) (+), () (B) (), (+) ______. [MHT CET 2016] 2 (C) (+), (+) (D) (), () (A) 2n (B) n (C) 2n (D) 2n + 2 143. Racemate is optically ______. (A) active (B) laevo rotatory 151. If a molecule has 64 optical isomers, the (C) dextro rotatory (D) inactive number of asymmetric carbon atoms in that molecule would be ______. 144. Which of the following is an optically active (A) 4 (B) 5 (C) 6 (D) 16 compound? [BCECE 2015] (A) Butan-1-ol (B) Propan-1-ol 152. Lactic acid is a classical example of ______. (C) 2-Chlorobutane (D) 4-Hydroxyheptane (A) position isomerism (B) geometrical isomerism 145. In which of the following molecules carbon (C) optical isomerism atom marked with asterisk (*) is asymmetric? (D) chain isomerism [NCERT Exemplar] 153. The IUPAC name of lactic acid is ______. H D [BCECE 2014] C* C* I Cl I Cl (A) 2-hydroxyethanoic acid (B) 2-hydroxypropanoic acid Br Br (a) (b) (C) 3-hydroxybutanoic acid (D) 2-hydroxybutanoic acid H H 154. The reason for the loss of optical activity of * * C C lactic acid when OH group is replaced by H- HO CH3 H CH3 atom is that ______. C H C H 2 5 2 5 (A) asymmetry of molecule is destroyed (c) (d) (B) symmetry of the molecule is destroyed (A) (a), (b), (c), (d) (B) (a), (b), (c) (C) spatial arrangement is changed (C) (b), (c), (d) (D) (a), (c), (d) (D) structural change occurs
146. The number of asymmetric carbon atoms in 155. Two possible stereo-structures of 2,3,4-trichloropentane is ______. CH CH(OH)COOH, which are optically (A) 0 (B) 1 (C) 2 (D) 3 3 active, are called ______. 147. A molecule is said to be chiral if it ______. [AIPMT RE-TEST 2015] (A) contains a plane of symmetry (A) enantiomers (B) mesomers (B) contains a centre of symmetry (C) diastereomers (D) atropisomers (C) cannot be superimposed on its mirror image 156. Enantiomers have same ______. (D) can be superimposed on its mirror image (A) molecular formula (B) structural formula 148. The necessary and sufficient condition for a (C) configuration molecule to exhibit optical activity is (D) both (A) and (B) ______. (A) molecular symmetry 157. d and l forms of an optically active (B) molecular disymmetry or chirality compound differ in ______. (C) superimposability on its mirror image (A) boiling points (B) relative density (D) tetrahedral nature of carbon atoms (C) specific rotation (D) refractive index 51
Chemistry Vol ‐ 2.2 (Med. and Engg.)
158. If one of the enantiomers has optical rotation 162. Order of priority of groups OCOCH3, COCH3 of 30, then its mirror image shows optical and COOCH3 in R,S configuration is rotation of _____. ______. o o (A) 30 (B) – 30 (A) COOCH3 > COCH3 > OCOCH3 (C) – 15o (D) 45 (B) OCOCH3 > COOCH3 > COCH3 (C) COCH3 > COOCH3 > OCOCH3 159. Among saturated hydrocarbons (C H , (D) OCOCH3 > COCH3 > COOCH3 n 2n+2 where n is an integer), what is the formula of 163. CORRECT order of priority among functional the compound of lowest molecular weight, groups – OH, – CN, – COOH and – CH3 in R,S which on monohalogenation could configuration is ______. demonstrate optical activity in at least one of (A) – CH > – CN > – COOH > – OH its structural isomers? 3 (B) – OH > – COOH > – CN > – CH3 (A) C4H10 (B) C5H12 (C) – COOH > – OH > – CH3 > – CN (C) C9H20 (D) C7H16 (D) – OH > – CN > – COOH > – CH 3 160. Which of the following structures is 164. While assigning R, S configuration, the enantiomeric with the molecule given below? CORRECT order of priority of groups H attached to chiral carbon atom is ______. CH3 [MHT CET 2016] C (A) CONH2 > COCH3 > CH2OH > CHO
H5C2 (B) CONH2 > COCH3 > CHO > CH2OH Br (C) COCH3 > CONH2 > CHO > CH2OH [NCERT Exemplar] (D) CHO > CH2OH > COCH3 > CONH2 H (A) C2 H5 165. Which among the following functional groups C has been given the highest priority while assigning R,S configuration? H3C Br [MH CET 2015] (A) C6H5 (B) CN CH3 (B) H (C) C2H5 (D) CH3 C 166. If the order of priority is in clockwise Br direction in an optically active molecule in C2H5 which the lowest priority group is below the (C) H plane of paper, then it is named as ______Br configuration. C (A) R (B) S (C) d (D) l H C 3 167. If molecule is in R configuration, then its non- C2H5 superimposable mirror image is in ______(D) Br configuration. H (A) R (B) S (C) d (D) l C
168. What is the configuration (in terms of R, S H5C2 CH3 configuration) of the following molecule?
161. R, S nomenclature system indicates Cl arrangements of atoms or groups around * ______. C (A) double bond H CH3 (B) chiral centre H5C6 (C) triple bond (A) R (B) S (D) symmetric C-atom (C) R and S both (D) None of these
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Chapter 10: Haloalkanes and Haloarenes 175. In the energy profile diagram of SN2 10.8 Nucleophilic substitution mechanism mechanism difference in energies of products 169. Transition state is defined as the state of a and reactants is denoted as ______. reaction, which possesses ______. (A) ΔH (B) ΔE
(A) minimum energy, minimum stability (C) Eact (D) U (B) maximum energy, minimum stability 2 (C) minimum energy, maximum stability 176. SN reaction of a compound containing an (D) maximum energy, maximum stability asymmetric carbon atom always gives ______. [IIT 2001] 170. R X NaOH ROH NaX (A) an enantiomer of the substrate The above reaction is classified as ______. (B) inversion of configuration [BHU 1982; CBSE PMT 1991; RPET 2000] (C) a mixture of cis-trans isomers (A) nucleophilic substitution (D) none of these (B) electrophilic substitution (C) reduction 177. The order of reactivities of the following alkyl 2 (D) oxidation halides for a SN reaction is ______. [IIT JEE (screening) 2000] 171. A primary alkyl halide would prefer to (A) RF > RCl > RBr > RI undergo ______. (B) RF > RBr > RCl > RI [NCERT Exemplar] (C) RCl > RBr > RF > RI (A) SN1 reaction (B) SN2 reaction (D) RI > RBr > RCl > RF (C) -Elimination (D) Racemisation
172. In SN2 mechanism, the rate of reaction is 178. Which of the following alkyl halides is more proportional to concentration of ______. reactive towards nucleophilic substitution (A) only substrate reaction? [MH CET 2013] (B) only reagent (A) H3C CH2 F (B) H3C CH2 Cl (C) HC CH Br (D) H C CH I (C) both substrate and reagent 3 2 3 2 (D) electrophile 179. The CORRECT order of reactivity of the 173. Assertion: Methyl bromide reacts with following iodides in SN2 reaction is ______. aqueous alkali to form methyl alcohol. The [TS EAMCET (Med.) 2015] 2 reaction is called as SN reaction. (i) CH3CH2CH2CH2I Reason: The product formation takes place (ii) CH3 CI in two steps. 3 (A) Assertion and Reason both are true. (iii) CH3CH2CHCH3 Reason is a correct explanation of |
Assertion. I (B) Assertion and Reason both are true. (A) (i) > (ii) > (iii) (B) (i) > (iii) > (ii) Reason is not a correct explanation of (C) (ii) > (i) > (iii) (D) (ii) > (iii) > (i) Assertion. 180. In S 2 reactions, the CORRECT order of (C) Assertion is true. Reason is false. N (D) Assertion is false. Reason is true. reactivity for the following compounds CH3Cl, CH3CH2Cl, (CH3)2CHCl and 174. In alkaline hydrolysis of 1-Bromopropane (CH3)3CCl is ______. with NaOH, which of the following statement [JEE (MAIN) 2014] is TRUE? (A) CHCl > (CH ) CHCl > CH CH Cl > (A) Reaction is endothermic. 3 3 2 3 2 (CH ) CCl (B) Rate of reaction is independent of 3 3 (B) CHCl > CH CH Cl > (CH ) CHCl > concentration of NaOH. 3 3 2 3 2 (CH ) CCl (C) Rate of reaction is doubled, if 3 3 concentration of any reactant is doubled. (C) CH3CH2Cl > CH3Cl > (CH3)2CHCl > (D) Rate of reaction is doubled as (CH3)3CCl concentrations of both the reactants are (D) (CH3)2CHCl > CH3CH2Cl > CH3Cl > doubled. (CH3)3CCl 53
Chemistry Vol ‐ 2.2 (Med. and Engg.) 181. Which of the following alkyl halides will (C) rate is doubled, as concentration of undergo SN1 reaction most readily? nucleophile is doubled [NCERT Exemplar] (D) rate is independent of concentration of (A) (CH3)3C – F (B) (CH3)3C – Cl the reactants
(C) (CH3)3C – Br (D) (CH3)3C – I 188. The SN1 mechanism for the hydrolysis of an 182. Ethyl bromide undergoes the following reaction: alkyl halide to an alcohol involves the C2H5Br + KOH C2H5OH + KBr formation of ______. Ethyl (aq.) Ethyl (A) carbonium ion bromide alcohol (B) pentavalent carbon in the transition state Which of the following is a WRONG statement? 2 (C) carbanion (A) The reaction follows SN mechanism. (D) free radical dc (B) = k [C2H5Br] [OH ]. 189. Alkaline hydrolysis of a tertiary alkyl halide dt involves ______. (C) In the energy profile diagram, there are (A) only one step (B) two steps two peaks. (C) three steps (D) four steps (D) There is inversion of configuration. 183. Which of the following statements is 190. The geometry of a carbonium ion is ______. INCORRECT for bimolecular nucleophilic (A) planar (B) tetrahedral substitution reaction (SN2)? (C) pyramidal (D) linear
[GUJ CET 2014] 191. In case of carbon-bromine bond fission, (A) It is a second order reaction. 2 (CH ) CBr undergoes ______. (B) In SN reaction the substrate does not 3 3 undergo heterolytic fission. (A) homolysis (C) The rate of SN2reaction does not depend (B) heterolysis to produce carbocation on concentrations of both substrate and (C) heterolysis to produce carbanion nucleophilic reagent. (D) no fission 2 (D) SN reaction occurs in single step without 192. Carbocation acts as a/an ______. forming intermediate. (A) electrophile 2 184. Inversion of configuration observed in SN (B) Lewis acid mechanism is also known as ______inversion. (C) nucleophile (A) Ingold (B) Walden (D) both (A) and (B) (C) kinetic (D) Van’t Hoff 193. During SN1 reaction mechanism of alkyl 185. In alkaline hydrolysis of tert-butyl bromide, halides, the change observed is ______. the order of reaction with respect to nucleophile is ______. [MH CET 2013] (A) only retention of configuration (A) zero (B) first (B) only inversion of configuration (C) pseudo (D) second (C) both retention and inversion of configuration 186. In the alkaline hydrolysis of tert-Butyl bromide, (D) retention of geometry which of the following is CORRECT? (A) Rate = k [(CH3)3CBr][OH ] 194. (+)-2-Chloro-2-phenylethane in toluene racemises slowly in the presence of small (B) Rate = k [(CH3)3CBr] + [OH ] amount of SbCl5, due to the formation of (C) Rate = k [(CH3)3CBr] (D) Rate = k [OH] ______. [WB JEE 2013] (A) carbanion (B) carbene 187. In alkaline hydrolysis of tertiary butyl (C) free-radical (D) carbocation bromide, ______. (A) rate of reaction is doubled as the 195. The hydrolysis of optically active concentration of the substrate is doubled 2-bromobutane with aqueous NaOH results in (B) rate of reaction is halved as the the formation of ______. [KCET 2015] concentration of any one of reactant is (A) (+) butan-2-ol (B) () butan-2-ol doubled (C) (±) butan-1-ol (D) (±) butan-2-ol
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Chapter 10: Haloalkanes and Haloarenes 196. Alkaline hydrolysis of which among the (C) (iv) > (ii) > (iii) > (i) following compounds leads to the formation (D) (iii) > (ii) > (i) > (iv)
of a racemate? [MH CET 2014] 203. Which of the following statements is FALSE (A) 1-Bromo-1-phenylethane about SN1 mechanism? (B) 1-Chloro-3-methylbutane (A) It is a first order reaction. (C) Bromoethane (B) It is favoured by higher concentration of (D) 1-Chloropropane the nucleophilic reagent. (C) It is favoured by polar solvents. 197. Amongst the following, the most stable (D) It results in the racemisation. carbocation is ______. 204. Choose the FALSE statement. + + (A) Alkaline hydrolysis of benzylic halides (A) CH3 (B) H3C CH2 and allylic halides follow SN1 mechanism. + + (B) The attack of nucleophiles on carbonium (C) H3C CH (D) H3C C CH3 | | ion is considered as the faster step in SN1 mechanism. CH3 CH3 1 (C) SN reaction follows second-order 198. The reactivity of various types of alkyl halides kinetics. 1 towards SN reaction ______. (D) In alkaline hydrolysis of alkyl halides, (A) primary > secondary > tertiary the final products are an alcohol and an (B) secondary > primary > tertiary alkali halide irrespective of whether the (C) tertiary > secondary > primary reaction follows SN1 or SN2 mechanism. (D) secondary > tertiary > primary 205. An INCORRECT statement with respect to 199. An optically active halide when allowed to SN1 and SN2 mechanisms for alkyl halide is: react with CN gives a racemic mixture. The [KCET 2014] halide is most likely to be ______. (A) A strong nucleophile in an aprotic (A) primary (B) secondary solvent increases the rate or favours SN2 (C) tertiary (D) quaternary reaction. (B) Competing reaction for SN2 reaction is 200. Reaction of C6H5CH2Br with aqueous sodium hydroxide follows ______. rearrangement. 1 [NCERT Exemplar] (C) SN reactions can be catalysed by some (A) SN1 mechanism Lewis acids. (B) SN2 mechanism (D) A weak nucleophile and a protic solvent 1 (C) Any of the above two depending upon increases the rate or favours SN the temperature of reaction reaction.
(D) Saytzeff rule 10.9 Haloarenes
201. Which of the following undergoes 206. In aryl halide, halogen atom is attached 1 nucleophilic substitution exclusively by SN ______. mechanism? [CPMT 2005] (A) to a sp2 hybridized carbon atom of (A) Benzyl chloride (B) Isopropylchloride benzene ring (C) Methyl chloride (D) Ethyl chloride 2 (B) to a sp hybridized carbon atom of side 202. The decreasing order of reactivity of the following chain 3 compounds in SN2 reactions is ______. (C) to a sp hybridized carbon atom of benzene ring C6H5CH2Br C6H5CH(C6H5)Br (D) to both benzene ring and side chain (i) (ii) C6H5CH(CH3)Br C6H5C(CH3)(C6H5)Br 207. When four or more halogen atoms are (iii) (iv) attached to an aromatic ring; the compound is [Assam CEE 2015] classified as ______. (A) (i) > (ii) > (iii) > (iv) (A) monohaloarene (B) dihaloarene (B) (iv) > (iii) > (ii) > (i) (C) trihaloarene (D) polyhaloarene 55
Chemistry Vol ‐ 2.2 (Med. and Engg.) 208. IUPAC name of symmetrical triiodobenzene (C) strong reducing agent is ______. (D) mild reducing agent (A) 1,2,3-Triiodobenzene 216. Direct fluorination of benzene ring ______. (B) 2,3,4-Triiodobenzene (A) can be carried out in presence of strong (C) 1,3,5-Triiodobenzene oxidising agent (D) 1,3,4-Triiodobenzene (B) can be carried out if excess of fluorine is used 10.10 Nature of C X bond in haloarenes (C) can be carried out in presence of Lewis acid at ordinary temperatures, in dark 209. Aryl halide is less reactive than alkyl halide (D) cannot be carried out as the reaction is towards nucleophilic substitution because violent and uncontrollable ______. [RPMT 2002] 217. Identify the compound Y in the following reaction: (A) of less stable carbonium ion (B) of large C-X bond energy NH + – 2 N2Cl (C) of inductive effect NaNO2 HCl Cu22 Cl 273 278K Y + N2 (D) all of these 210. The C Cl bond in chlorobenzene as [NCERT Exemplar] Cl compared with C Cl bond in methyl chloride is ______. [MP PMT 1995] (A) (B) (A) longer and weaker Cl (B) shorter and weaker (C) shorter and stronger Cl (D) longer and stronger (C) (D)
211. Chlorobenzene is ______. (A) less reactive than benzyl chloride Cl Cl
(B) more reactive than benzyl chloride Cl (C) nearly as reactive as benzyl chloride (D) more reactive than any alkyl halide 218. Diazonium salts + Cu Cl + HCl , 2 2 10.11 Preparation of haloarenes the reaction is known as ______. [Kerala (Med.) 2002] 212. At the time of preparation of chlorobenzene (A) Williamson’s synthesis from benzene, which of the following can be (B) Sandmeyer’s reaction used as a halogen carrier? (C) Wurtz Fittig reaction (A) FeCl3 (B) BCl3 (D) Friedel Craft’s alkylation (C) AlCl (D) All of these 3 219. The iodobenzene is generally obtained from a 213. Chlorobenzene is formed by reaction of chlorine diazonium salt by reacting it with ______. with benzene in the presence of AlCl3. Which of [MP PMT 2000] the following species attacks the benzene ring in (A) KI (B) I2 this reaction? [NCERT Exemplar] (C) PI3 (D) all of these
(A) Cl– (B) Cl+ – 10.12 Physical and chemical (C) AlCl3 (D) [AlCl4] properties of haloarenes 214. Direct bromination of benzene with excess 220. Arrange the following compounds in the reagent results in the formation of ______. increasing order of their densities. (A) mixture of ortho and meta products Cl (B) mixture of para and meta products (C) mixture of ortho and para products (a) (b) (D) monosubstituted product only
Cl Br 215. Direct iodination of benzene ring (which is a reversible reaction) can proceed in forward (c) (d) direction in presence of ______. Cl (A) strong oxidising agent Cl (B) mild oxidising agent [NCERT Exemplar]
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Chapter 10: Haloalkanes and Haloarenes
(A) (a) < (b) < (c) < (d) (C) NO2 donates electrons at meta position
(B) (a) < (c) < (d) < (b) (D) NO2 withdraws electrons from ortho (C) (d) < (c) < (b) < (a) and para positions
(D) (b) < (d) < (c) < (a) 227. Arrange the compounds in increasing order of 221. The C X bond cleavage in aryl halide is rate of reaction towards nucleophilic more difficult than the C X bond cleavage in substitution. Cl alkyl halide due to ______. Cl
(A) electromeric effect (a) (b) (B) positive inductive effect (C) negative inductive effect
(D) resonance effect NO2
222. % s-character is more in ______. Cl (A) ethyl chloride (B) isobutyl bromide O N NO (C) methyl iodide (D) chlorobenzene 2 2 (c) 223. Which of the following is FALSE? (A) Lesser polarity of aryl halides results in
lesser reactivity compared to alkyl halides. NO2 (B) Electron rich arenes repel the attacking [NCERT Exemplar] nucleophile. (A) (c) < (b) < (a) (B) (b) < (c) < (a) (C) Under drastic conditions, aryl halides (C) (a) < (c) < (b) (D) (a) < (b) < (c) undergo nucleophilic substitution reactions. (D) Phenyl cation formed from self ionization 228. Arrange the compounds in increasing order of of aryl halide is stabilized by resonance. rate of reaction towards nucleophilic substitution. 224. Chlorobenzene on fusing with solid NaOH Cl gives ______. [DPMT 1981; CPMT 1990] Cl (A) benzene (B) benzoic acid NO2 (C) phenol (D) benzyl alcohol (a) (b) 225. What is the product expected in the following reaction? Cl CH3
Cl Cu23 O,NH (c) 473K, Pressure NO2 CH3 CH3 [NCERT Exemplar] NH2 (A) (a) < (b) < (c) (B) (c) < (b) < (a) (A) (B) (C) (a) < (c) < (b) (D) (c) < (a) < (b)
NH2 229. Chlorobenzene reacts with chlorine in CH3 presence of anhydrous FeCl3 to give ______. NH2 NH2 (A) mixture of o-dichlorobenzene and m- (C) (D) All of these dichlorobenzene (B) mixture of p-dichlorobenzene and m-
NH2 dichlorobenzene 226. Replacement of Cl of chlorobenzene to give (C) mixture of ortho and para phenol requires drastic conditions, but Cl of dicholorobenzene 2,4-dinitrochlorobenzene is readily replaced. (D) m-dichlorobenzene This is because ______. [CBSE PMT 1997; KCET 2016] 230. Nitrating mixture is ______.
(A) NO2 group makes the ring electron rich (A) conc.HNO3 + conc.H2SO4 at ortho and para positions (B) conc.HNO3 + conc.HCl (B) NO2 group withdraws electrons from (C) conc.HCl + conc.H2SO4 meta position (D) conc.HNO3 + conc. NaOH 57
Chemistry Vol ‐ 2.2 (Med. and Engg.) 231. Chlorobenzene undergoes nitration on (C) 1,4-dipropyl benzene treatment with conc. HNO and conc. H SO . (D) isopropyl benzene 3 2 4 The product will be ______. (A) mixture of o-chloronitrobenzene and m- 237. Cl dryether chloronitrobenzene + 2Na + CH3Cl (B) mixture of p-chloronitrobenzene and m- CH3 chloronitrobenzene + 2NaCl (C) only m-chloronitrobenzene (D) a mixture of o-chloronitrobenzene and This reaction is known as ______. p-chloronitrobenzene [AP EAMCET (Engg.) 2016] (A) Wurtz-Fittig reaction 232. Nitration is easiest in case of ______. (B) Wurtz reaction (A) benzenesulphonic acid (C) Fittig reaction (B) bromobenzene (D) Friedel-Crafts reaction (C) nitrobenzene (D) toluene 238. Reaction of bromobenzene with Na metal in dry ether gives ______. 233. Br (A) methyl benzene (B) diphenyl (C) ethyl benzene (D) benzene + H2SO4 A + B + H2O 10.13 Uses and environmental effects of Bromobenzene (Conc.) (minor (major some haloalkanes and haloarenes product) product) A and B in the above reaction are ______. 239. Molecular formula of dichloromethane is (A) 2-bromobenzene sulphonic acid and ______. 4-bromobenzene sulphonic acid (A) CHCl3 (B) CCl4 respectively (C) CH2Cl2 (D) CH3Cl (B) 4-bromobenzene sulphonic acid and 240. Dichloromethane is ______. 2-bromobenzene sulphonic acid (A) derivative of methane respectively (B) colorless, sweet smelling liquid (C) 2-bromobenzene sulphonic acid and (C) a liquid having low boiling point 3-bromobenzene sulphonic acid (D) all of these respectively (D) 4-bromobenzene sulphonic acid and 241. Dichloromethane is used as ______. 3-bromobenzene sulphonic acid (A) an organic solvent respectively (B) a paint remover, refrigerant and dewaxing agent 234. Which of the following reagents does NOT (C) a fumigant pesticide and propellent in represent the example of electrophilic aerosols substitution reaction? (D) all of these (A) Cl2 and Fe powder (B) Conc. H2SO4 242. Dichloromethane has harmful effects on (C) Nitrating mixture ______. (D) Anhydrous CuCN and NaCN (A) circulatory system (B) excretory system 235. Introduction of an alkyl group or an acyl (C) CNS group in a/an ______is called as Friedel (D) all of these Craft’s reaction. (A) benzene ring (B) aryl halide 243. Exposure to high level of CH2Cl2 causes (C) chloroethane (D) both (A) and (B) ______. (A) nausea (B) impaired hearing 236. In presence of AlCl3, benzene and iso-propyl (C) impaired vision (D) all of these chloride react by Friedel-Craft's reaction to form ______. [MP PMT 1991] 244. The IUPAC name of chloroform is ______. (A) n-propyl benzene (A) chloromethane (B) halomethane (B) 1,2-dipropyl benzene (C) trichloromethane (D) dichloromethane
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Chapter 10: Haloalkanes and Haloarenes 245. Which of the following statements about 252. ______was used as a fire extinguisher under chloroform is FALSE? [Manipal MEE 1995] the name pyrene. [DPMT 1985] (A) It is a colourless, volatile liquid. (A) CO2 (B) CCl4 (B) It is almost insoluble in water. (C) CS2 (D) CHCl3 (C) It is highly inflammable. 253. At normal temperature, iodoform is ______. (D) It’s inhalation for long time affects CNS. [MP PET 2004] 246. Chloroform is slowly oxidised by air in the (A) thick viscous liquid presence of light to form ______. (B) colourless gas [MH CET 1999; UPSEAT 2001, 02; (C) volatile liquid (D) yellow crystalline solid RPMT 2003; CPMT 2004] (A) chloropicrin (B) phosgene 254. Iodoform can be used as an ______. (C) dichloromethane (D) freon [NCERT 1981] (A) anaesthetic (B) antiseptic 247. When chloroform is exposed to air and (C) analgesic (D) antipyretic sunlight, it gives ______. [NCERT 1984; CPMT 1978, 87; 255. Which of the following is known as freon CBSE PMT 1990; EAMCET 1993; which is used as a refrigerant? MNR 1994; MP PET 1997, 2000; [DPMT 1982; CPMT 1979, 81, 89; BHU 2001; AFMC 2002] AFMC 1995; Manipal MEE 1995; (A) carbon tetrachloride MP PET 1995, 2004] (B) carbonyl chloride (A) CCl2F2 (B) CHCl3 (C) mustard gas (C) CH2F2 (D) CF4
(D) methylene dichloride 256. Freon-12 is ______. 248. Why is chloroform put into dark coloured (A) CCl3F (B) CHClF2 bottles? [MP PET 2002] (C) CCl2F2 (D) CHCl2F (A) To prevent evaporation. 257. Depletion of ozone layer is caused by (B) To prevent from moisture. (C) To prevent it from oxidation to form ______. [RPMT 2002] phosgene. (A) freon (B) alkane (D) To prevent its reaction with glass. (C) Grignard reagent (D) carboxylic acids
249. Two percent of ethanol is added to chloroform 258. Full name of DDT is ______. [KCET 1993] to stop the formation of carbonyl chloride. In (A) 2,2-bis(p-chlorophenyl)-1,1,1-trichloro this reaction ethanol acts as a/an ______. [Pb. CET 2001] ethane (A) auto catalyst (B) 1,1-Dichloro-2,2-diphenyl (B) catalytic inhibitor trimethylethane (C) positive catalyst (C) 1,1-Dichloro-2,2-diphenyl (D) none of these trichloroethane 250. The molecular formulae for phosgene and tear (D) 1,1,1-Trichloroethane gas are ______and ______respectively. 259. The molecular formula of DDT has ______[GUJ CET 2015] chlorine atoms. [MP PMT 1997] (A) SOCl and CCl NO 2 2 2 (A) 5 (B) 4 (C) 3 (D) 2 (B) COCl2 and CCl2NO2 (C) COCl2 and CCl3NO2 260. DDT is used as ______. (D) SOCl2 and CCl3NO2 (A) pesticide (B) insecticide (C) fungicide (D) disinfectant 251. Carbon tetrachloride is used as ______. (A) solvent in production of refrigerants 261. Which one of the following is a non- (B) solvent for oil, fats, waxes biodegradable compound? (C) pesticide and insecticide (A) CHCl3 (B) CHI3 (D) all of these (C) CH2Cl2 (D) DDT 59
Chemistry Vol ‐ 2.2 (Med. and Engg.) 269. What is ‘A’ in the following reaction? Miscellaneous CH2 – CH = CH2 262. Ethylidene chloride is a /an ______. [NCERT Exemplar]
(A) vic-dihalide (B) gem-dihalide + HCl A
(C) allylic halide (D) vinylic halide [NCERT Exemplar] 263. The position of –Br in the compound CH – CH = CH 2 2 CH3CH = CHC(Br)(CH3)2 can be classified as ______. [NCERT Exemplar] (A) Cl (A) Allyl (B) Aryl (C) Vinyl (D) Secondary CH2 – CH2 – CH2 – Cl 264. The IUPAC name of
Br CH3 (B) CH3 C C CH2 COOH is ______. CH2 – CH – CH 3
C2H5 C2H5 Cl (C) (A) 2-Bromo-2,3-diethyl-3-methylhexanoic
acid Cl (B) 4-Bromo-3,4-diethyl-3-methylpentanoic acid CH – CH2 – CH3 (C) 4-Bromo-3-ethyl-3,4-dimethylhexanoic acid (D) (D) 3-Ethyl-3,4-dimethyl-4-bromopentanoic
acid 270. In the reaction with HCl, an alkene reacts in 265. Which reagent will you use for the following accordance with the Markownikoff’s rule, to reaction? give the product 1-chloro-1-methylcyclohexane. CH3CH2CH2CH3 CH3CH2CH2CH2Cl + The possible alkene is ______. CH CH CHClCH 3 2 3 [AIPMT RE-TEST 2015] [NCERT Exemplar] CH (A) Cl2 / UV light (A) 2 (B) CH3 (B) NaCl + H2SO4 (C) Cl2 gas in dark (D) Cl gas in the presence of iron in dark 2
UVlight CH 266. C38 H Cl 2 C 37 H Cl HCl is an (C) Both(A) and (B) (D) 3 example of which of the following types of
reactions? [AFMC 1997; CPMT 1999]
(A) Substitution (B) Elimination
(C) Addition (D) Rearrangement 271. Assertion: Addition of HCl to propene in the 267. In the preparation of C2H5I from C2H6 ______. presence of benzoyl peroxide gives (A) HgO acts as an oxidizing agent 2-chloropropane as the major product. (B) HI formed acts as a reducing agent Reason: Addition follows anti- Markownikoff’s rule due to peroxide effect. (C) HIO3 acts as an oxidizing agent (A) Assertion and Reason both are true. Reason (D) all of these is a correct explanation of Assertion. 268. The starting material for the preparation of (B) Assertion and Reason both are true. Reason CH3I is ______. is not a correct explanation of Assertion. (A) CH3OH (B) C2H5OH (C) Assertion is true. Reason is false. (C) CH3CHO (D) (CH3)2CO (D) Assertion is false. Reason is true.
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Chapter 10: Haloalkanes and Haloarenes 272. Toluene reacts with a halogen in the presence (C) Sandmeyer reaction of iron (III) chloride giving ortho and para (D) Wurtz reaction halo compounds. The raction is ______. 279. Preparation of alkyl halides in laboratory is [NCERT Exemplar] least preferred by ______. (A) electrophilic elimination reaction [Delhi PMT 2000] (B) electrophilic substitution reaction (A) treatment of alcohols (C) free radical addition reaction (B) addition of hydrogen halides to alkenes (D) nucleophilic substitution reaction (C) halide exchange 273. The reaction of toluene with chlorine in the (D) direct halogenation of alkanes presence of iron and in the absence of light 280. In the preparation of iodoalkanes from alkane, yields ______. [NCERT Exemplar] the ratio of HIO3 to iodine molecule is CH Cl 2 ______. (A) (A) 1 : 1 (B) 2 : 5
(C) 1 : 2 (D) 5 : 2
CH3 281. Both methane and ethane can be prepared in Cl one step from ______. (B) (A) C2H4 (B) CH3OH (C) CH3Br (D) CH3CH2OH
(C) H 3C Cl 282. Benzene hexachloride (BHC) is used a/an ______. [MP PMT 1994; KCET 1999] (D) Mixture of (B) and (C) (A) dye 274. When an alkyl halide is heated with dry Ag2O, (B) antimalarial drug it produces ______. (C) antibiotic (D) insecticide [CPMT 1997; BHU 2004] (A) cyanide (B) ether 283. The product obtained by the addition of HBr (C) ketone (D) alcohol to hex-1-ene in presence of a peroxide is 275. Which of the following haloalkanes is most ______. [Assam CEE 2015] reactive? [KCET 2005] (A) 2-bromohexane (A) 1-Chloropropane (B) 1-Bromopropane (B) 1-bromohexane (C) 2-Chloropropane (D) 2-Bromopropane (C) 1, 2-dibromohexane (D) 2,2-dibromohexane 276. Reaction of an aryl halide with acyl chloride in presence anhydrous AlCl3 is called as ______. 284. Gammexane is the name given to ______. (A) sulphonation (A) C6H3Cl3 (B) C6H4Cl2
(B) Sandmeyer’s reaction (C) C6H6Cl6 (D) (C6H5)2CHCCl3 (C) nitration (D) Friedel-Craft’s acylation reaction 285. Phosgene is the common name for ______. [DPMT 1983; CPMT 1993; MP PMT 1994; 277. Wurtz-Fittig reaction uses the reactants Kurukshetra CEE 1998; RPMT 2000, 02] ______in ethereal solution. (A) thionyl chloride (A) alkyl halide + sodium metal (B) phosphoryl chloride (B) aryl halide + alkyl halide + sodium metal (C) carbonyl chloride (C) aryl halide + sodium metal (D) carbon tetrachloride (D) aryl halide + alkyl halide
286. Choose the FALSE statement about CHCl3 278. Identify the name of the following reaction: from the following. dry acetone CH3Br + NaI CH3I + NaBr (A) It was used as an anaesthetic in earlier days. [AP EAMCET (Med.) 2016] (B) It is used as a solvent. (A) Finkelstein reaction (C) It is stored in transparent bottles. (B) Gattermann reaction (D) If inhaled for long time, it affects CNS. 61
Chemistry Vol ‐ 2.2 (Med. and Engg.) 287. Which one of the following is the CORRECT 292. Which of the carbon atoms present in the formula for dichlorodiphenyltrichloroethane? molecule given below are asymmetric?
H Cl [AIIMS 1982] OH H (A) HO C C Cl a b c d O C – C – C – C Cl O H HOH H Cl [NCERT Exemplar] (B) Cl C C Cl (A) a, b, c, d (B) b, c
(C) a, d (D) a, b, c Cl
293. Molecules whose mirror image is non- Cl superimposable over them are known as chiral. Which of the following molecules is H Cl chiral in nature? [NCERT Exemplar] (C) C C Cl (A) 2-Bromobutane (B) 1-Bromobutane Cl (C) 2-Bromopropane Cl (D) 2-Bromopropan-2-ol Cl Cl Cl 294. Which of the following biphenyls is optically active? [NEET P-I 2016] Cl C C Cl (D) I CH 3 Cl (A) (B)
Cl I CH3
288. n-Alkyl halide contains halogen atom attached O2N (C) (D) Br Br to ______.
(A) primary carbon atoms (B) either primary or secondary carbon atom I I I (C) either primary or tertiary carbon atom (D) either tertiary or quaternary carbon atom 295. For a given optically active substance, the amount of rotation of plane of polarized light 289. R – OH + HX R – X + H2O depends upon ______. In the above reaction, the reactivity of (A) the number of molecules different alcohols is ______. [CPMT 1997] (A) tertiary > secondary > primary (B) the nature of beam of light (B) tertiary < secondary < primary (C) specific rotation (C) tertiary < secondary > primary (D) number of asymmetric ‘C’ atoms in the (D) secondary < primary < tertiary molecule of a substance
290. Which of the following is NOT a halogen 296. Which one of the following statements is derivative of alkane? WRONG? (A) Freon-12 (B) DDT (A) Lower alkyl halides are either colourless gases or volatile liquids. (C) Iodoform (D) Vinyl chloride (B) Alkyl halides are very soluble in water. 291. Number of - bonds present in BHC (C) Alkyl halides burn easily with green (Benzene hexachloride) molecule are edge of flame. ______. [RPMT 1999] (D) The higher alkyl halides are colourless (A) 6 (B) 0 (C) 3 (D) 12 solids.
62
Chapter 10: Haloalkanes and Haloarenes 297. Which is the CORRECT increasing order of 301. Which of the following compounds will give boiling points of the following compounds? racemic mixture on nucleophilic substitution by OH– ion? 1-Bromoethane, 1-Bromopropane, Br 1-Bromobutane, Bromobenzene (a) CH – CH – Br (b) [NCERT Exemplar] 3 CH3 – C – CH3
(A) Bromobenzene < 1-Bromobutane < C H 2 5 C2H5 1-Bromopropane < 1-Bromoethane (c) CH3 – CH – CH2Br (B) Bromobenzene < 1-Bromoethane <
1-Bromopropane < 1-Bromobutane C2H5 (C) 1-Bromopropane < 1-Bromobutane < [NCERT Exemplar] 1-Bromoethane < Bromobenzene (A) (a) (B) (a), (b), (c) (C) (b), (c) (D) (a), (c) (D) 1-Bromoethane < 1-Bromopropane < 1-Bromobutane < Bromobenzene 302. Excess of alkyl halide on heating with alc. NH3 in a sealed tube results into formation of 298. An organic compound ‘A’(C4H9Cl) on reaction with Na/diethyl ether gives a ______. [Orissa JEE 2002] hydrocarbon, which on monochlorination (A) 1° amine (B) 2° amine (C) 3° amine (D) all of these gives only one chloro derivative. A is ______. [Kerala PMT 2004] 303. Haloalkanes in the presence of alcoholic KOH (A) tert-butyl chloride (B) sec-butyl chloride undergoes ______. (C) isobutyl chloride (D) n-butyl chloride [KCET (Engg/Med.) 2002]
299. ‘X’ is an optically active alkane having lowest (A) elimination (B) polymerisation (C) dimerisation (D) substitution molecular mass. Predict the structure of the major product obtained on monochlorination 304. Identify the compound ‘D’ in the following of ‘X’. [MH CET 2014] series of reactions.
CH3 CH3 Alc.KOH CH3CHCH2CH2Br A (A) CH3 CH2 CH2 C CH2 CH3
(i) Conc. H SO (ii) H O, 2 4 2 Cl B + C (Major (Minor CH3 product) product) HI, (B) CH3 CH2 CH2 CH CH CH3 B D + E HO2 (Major (Minor [MH CET 2014] Cl product) product)
CH3 CH3
(C) CH CH CH CH CH CH Cl (A) CH3 CH CH CH3 3 2 2 2 2
CH3 I CH3
(D) Cl CH2 CH2 CH2 CH CH2 CH3 (B) CH3 C CH2 CH3 300. An alkyl halide may be converted into an alcohol by ______. I [EAMCET 1980; CBSE PMT 1997; CH3 BHU 1999; AIIMS 2001] (A) addition (C) CH3 CH CH2 CH2 I (B) substitution CH3 (C) dehydrohalogenation (D) all of these (D) CH3 CH2 CH CH2 I 63
Chemistry Vol ‐ 2.2 (Med. and Engg.)
305. Assertion: Sulphonyl chloride is preferred 312. A + 2Na + B Dryether Isobutane + 2NaCl over phosphorus chlorides (tri and penta) for A and B in the above reaction can be ______. the preparation of alkyl chlorides. [KCET 1988] Reason: The byproducts of the reaction (A) methyl chloride and n-propyl chloride carried out using sulphonyl chloride are non- (B) methyl chloride and ethyl chloride volatile liquids. (C) isopropyl chloride and methyl chloride (A) Assertion and Reason both are true. Reason (D) isopropyl chloride and ethyl chloride is a correct explanation of Assertion. (B) Assertion and Reason both are true. Reason 313. Alkyl halides are used for the preparation of is not a correct explanation of Assertion. ______. (C) Assertion is true. Reason is false. (A) primary amines (B) alkenes (D) Assertion is false. Reason is true. (C) alcohols (D) all of these
306. When ethyl chloride and alcoholic KOH are 314. A Grignard's reagent may be made by reacting heated, the compound obtained is ______. magnesium with ______. [MH CET 2003] [CPMT 1973, 83, 84] (A) methyl amine (B) diethyl ether (A) C2H4 (B) C2H2 (C) CH (D) CH (C) ethyl iodide (D) ethyl alcohol 6 6 2 6 307. Which of the following is NOT CORRECT? 315. The order of reactivities of methyl halides in the formation of Grignard reagent is ______. (A) But-1-ene + HBr peroxide 1–Bromobutane [KCET 2003] (B) Propene + HCl 2Chloropropane (A) CH3I > CH3Br > CH3Cl (C) But-1-ene +HCl peroxide 2Chlorobutane (B) CH3Cl > CH3Br > CH3I (D) But-1-ene + HBr 1–Bromobutane (C) CH3Br > CH3Cl > CH3I 308. Which of the following types of reaction (D) CH3Br > CH3I > CH3Cl occurs, during the formation of an unsaturated 316. What is Z in the following sequence of reactions? compound from a saturated compound? 2-Methyl-2-bromopropane Mg XZHO2 (A) Photolysis (B) Addition dryether (C) Substitution (D) Elimination [AP EAMCET (Engg.) 2016] (A) propane (B) 2-methylpropene 309. Propan-2-ol reacts with sulphuric acid at 373 K (C) 2-methylpropane (D) 2-methybutane and gives an alkene, which reacts with bromine to give dibromoalkane. Excess of alcoholic 317. Consider an alkyl halide as the substrate. The potassium hydroxide on reaction with dibromo reagents and the products are given below. product gives X. The structure of X is ______. Mark the FALSE choice. Reagent Product (A) CH2 = CH – CH3 (B) CH – C = CHBr (A) Mg in dry ether Grignard reagent 3 (B) Alcoholic solution Organic acid | of silver salt of a Br carboxylic acid (C) CH – C CH 3 (C) Aqueous alkali Alcohol (D) CH3 C = CH2 (D) Sodium alkoxide Ether | 318. If CH I and C H I are mixed in equal proportion Br 3 2 5 and the mixture is treated with metallic sodium 310. Which metal is used in Wurtz synthesis? in presence of dry ether, the number of possible [CPMT 1986; DPMT 1979; MP PET 2002] organic products are ______. (A) Ba (B) Al (C) Na (D) Fe (A) 1 (B) 2 (C) 3 (D) 4
311. Ethyl iodide on heating with alc. AgCN gives 319. In chlorination of chlorobenzene, the major major product as ______. product formed is ______. (A) ethyl cyanide (A) ortho dichlorobenzene (B) ethyl isocyanide (B) para dichlorobenzene (C) nitroethane (C) meta dichlorobenzene (D) acetic acid (D) both (A) and (B)
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Chapter 10: Haloalkanes and Haloarenes
320. Reaction of ethyl chloride with sodium leads CH3 CH3 to the formation of ______. [NCERT 1984] (B) Br Br (A) ethane (B) propane and (C) n-butane (D) n-pentane CH3 CH3 321. Grignard reagent is useful in preparation of ______. CH (A) alkane (B) ethers (C) CH3 3 (C) alcohols (D) all of these Br and 322. Which of the following statement is WRONG? CH (A) When light is passed through (d)-2 CH3 3 chlorobutane, the angle of rotation is Br measurable. CH3 CH3 (B) 2chlorobutane exhibits optical activity. (D) (C) When light is passed through a Nicol and
prism, the resultant wave vibrates in a CH3 CH plane perpendicular to the direction of Br 3 Br propagation of light. (D) Glucose exhibits optical activity. 326. Assertion: Iodination of benzene is carried
323. 50% of the reagent is used for out in the presence of a strong oxidising agent. Reason: Oxidising agent controls the dehydrohalogenation of 6.45 g CH3CH2Cl. What will be the weight of the main product obtained? violent reaction of benzene and iodine. [Atomic mass of H, C and Cl are 1, 12 and (A) Assertion and Reason both are true. Reason 35.5 g/mol respectively.] [GUJ CET 2015] is a correct explanation of Assertion. (A) 0.7 g (B) 1.4 g (B) Assertion and Reason both are true. Reason (C) 2.8 g (D) 5.6 g is not a correct explanation of Assertion. 324. Assertion: The presence of nitro group at m- (C) Assertion is true. Reason is false. position (with respect to halogen atom) greatly (D) Assertion is false. Reason is true. activates the haloarenes towards nucleophilic substitution reactions. 327. A nucleophile is a ______. Reason: In the case of m-substituted (A) Lewis acid haloarenes, there is no negative charge at m- (B) Lewis acid and also a Lewis base position in the resonating structures. (A) Assertion and Reason both are true. Reason (C) Lewis base (D) neither a Lewis acid nor a Lewis base is a correct explanation of Assertion. (B) Assertion and Reason both are true. Reason 328. Identify the CORRECT order of reactivity of the is not a correct explanation of Assertion. following towards the nucleophilic substitution. (C) Assertion is true. Reason is false. NO2 (D) Assertion is false. Reason is true. NO2 325. What products are formed when the following O2N Cl compound is treated with Br2 in the presence Cl of FeBr3? [AIPMT 2014] CH3 NO2 (1)
NO2 (2)
CH3 O2N Cl Cl CH CH 3 3 (3) (4) (A) Br [AP EAMCET (Med.) 2016] an d (A) 1 > 3 > 2 > 4 (B) 4 > 3 > 2 > 1 CH3 CH 3 Br (C) 3 > 1 > 4 > 2 (D) 2 > 3 > 4 > 1 65
Chemistry Vol ‐ 2.2 (Med. and Engg.) 329. Arrange the compounds in increasing order of 334. For the following reactions:
rate of reaction towards nucleophilic substitution. (a) CH3CH2CH2Br + KOH Cl Cl CH3CH = CH2 + KBr + H2O CH3 (b) H3C CH3 H C CH (a) (b) 3 3 + KOH + KBr Cl Br OH
Br (c) (c) + Br2
CH3 [NCERT Exemplar] Br (A) (a) < (b) < (c) (B) (a) < (c) < (b) which of the following statements is (C) (c) < (b) < (a) (D) (b) < (c) < (a) CORRECT? [NEET P-I 2016] 330. Arrange the compounds in increasing order of (A) (a) is elimination, (b) and (c) are rate of reaction towards nucleophilic substitution. substitution reactions (a) Cl (b) Cl (c) Cl (B) (a) is substitution, (b) and (c) are CH3 addition reactions (C) (a) and (b) are elimination reactions and (c) is addition reaction
CH3 CH3 (D) (a) is elimination, (b) is substitution and [NCERT Exemplar] (c) is addition reaction (A) (a) < (b) < (c) (B) (b) < (a) < (c) 335. Study the following energy profile diagram (C) (c) < (b) < (a) (D) (a) < (c) < (b) and mark the CORRECT statement. 331. Phosgene is ______. [BCECE 2014]
(A) C6H5NC (B) CCl4 T.S.1 (C) COCl2 (D) C2H5NH2 T.S.2 332. Which of the following is halogen exchange Eact2 reaction? [NCERT Exemplar] Eact1 (A) RX + NaI RI + NaX
(B) C = C + HX C – C
Reactant ΔH H X ZnCl2 Potential energy Product (C) R – OH + HX R – X + H2O
CH3 CH3 Reaction co-ordinates (D) (A) The reaction follows one step concerted + X Fe 2 dark mechanism. X (B) The diagram explains the mechanism of
CH3 hydrolysis of methyl bromide.
+ (C) The reaction is exothermic. 2 X (D) The diagram explains SN reaction mechanism. 333. The absolute configuration of 336. Consider the reaction : CO2H CH3CH2CH2Br + NaCN CH3CH2CH2CN H OH + NaBr H Cl This reaction will be the fastest in ______. [NEET P-II 2016]
(A) water CH3 is ______. [JEE (MAIN) 2016] (B) ethanol (A) (2R, 3S) (B) (2S, 3R) (C) methanol (C) (2S, 3S) (D) (2R, 3R) (D) N,N-dimethylformamide (DMF)
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Chapter 10: Haloalkanes and Haloarenes
Answers to MCQs
1. (B) 2. (C) 3. (A) 4. (B) 5. (C) 6. (A) 7. (C) 8. (B) 9. (A) 10. (B) 11. (C) 12. (B) 13. (D) 14. (D) 15. (B) 16. (D) 17. (B) 18. (A) 19. (A) 20. (D) 21. (B) 22. (A) 23. (B) 24. (B) 25. (B) 26. (C) 27. (D) 28. (D) 29. (B) 30. (C) 31. (B) 32. (C) 33. (A) 34. (B) 35. (D) 36. (A) 37. (B) 38. (C) 39. (D) 40. (C) 41. (B) 42. (A) 43. (D) 44. (A) 45. (B) 46. (A) 47. (B) 48. (B) 49. (B) 50. (D) 51. (D) 52. (B) 53. (A) 54. (C) 55. (A) 56. (D) 57. (A) 58. (A) 59. (B) 60. (B) 61. (D) 62. (C) 63. (B) 64. (A) 65. (C) 66. (D) 67. (C) 68. (A) 69. (A) 70. (B) 71. (B) 72. (C) 73. (B) 74. (A) 75. (C) 76. (B) 77. (D) 78. (B) 79. (B) 80. (D) 81. (D) 82. (B) 83. (B) 84. (D) 85. (C) 86. (D) 87. (A) 88. (C) 89. (C) 90. (D) 91. (A) 92. (D) 93. (C) 94. (A) 95. (B) 96. (A) 97. (C) 98. (B) 99. (A) 100. (C) 101. (C) 102. (A) 103. (B) 104. (C) 105. (B) 106. (D) 107. (C) 108. (C) 109. (C) 110. (D) 111. (C) 112. (C) 113. (D) 114. (D) 115. (B) 116. (B) 117. (C) 118. (A) 119. (B) 120. (D) 121. (A) 122. (D) 123. (C) 124. (A) 125. (B) 126. (B) 127. (B) 128. (D) 129. (A) 130. (D) 131. (B) 132. (D) 133. (B) 134. (C) 135. (B) 136. (C) 137. (C) 138. (C) 139. (C) 140. (C) 141. (B) 142. (A) 143. (D) 144. (C) 145. (B) 146. (C) 147. (C) 148. (B) 149. (C) 150. (C) 151. (C) 152. (C) 153. (B) 154. (A) 155. (A) 156. (D) 157. (C) 158. (B) 159. (A) 160. (A) 161. (B) 162. (B) 163. (B) 164. (B) 165. (B) 166. (A) 167. (B) 168. (B) 169. (B) 170. (A) 171. (B) 172. (C) 173. (C) 174. (C) 175. (A) 176. (B) 177. (D) 178. (D) 179. (B) 180. (B) 181. (D) 182. (C) 183. (C) 184. (B) 185. (A) 186. (C) 187. (A) 188. (A) 189. (B) 190. (A) 191. (B) 192. (D) 193. (C) 194. (D) 195. (D) 196. (A) 197. (D) 198. (C) 199. (C) 200. (A) 201. (A) 202. (C) 203. (B) 204. (C) 205. (B) 206. (A) 207. (D) 208. (C) 209. (B) 210. (C) 211. (A) 212. (D) 213. (B) 214. (C) 215. (A) 216. (D) 217. (A) 218. (B) 219. (A) 220. (A) 221. (D) 222. (D) 223. (D) 224. (C) 225. (A) 226. (D) 227. (D) 228. (C) 229. (C) 230. (A) 231. (D) 232. (D) 233. (A) 234. (D) 235. (D) 236. (D) 237. (A) 238. (B) 239. (C) 240. (D) 241. (D) 242. (C) 243. (A) 244. (C) 245. (C) 246. (B) 247. (B) 248. (C) 249. (B) 250. (C) 251. (D) 252. (B) 253. (D) 254. (B) 255. (A) 256. (C) 257. (A) 258. (A) 259. (A) 260. (B) 261. (D) 262. (B) 263. (A) 264. (C) 265. (A) 266. (A) 267. (D) 268. (A) 269. (C) 270. (C) 271. (C) 272. (B) 273. (D) 274. (B) 275. (D) 276. (D) 277. (B) 278. (A) 279. (D) 280. (C) 281. (C) 282. (D) 283. (B) 284. (C) 285. (C) 286. (C) 287. (B) 288. (A) 289. (A) 290. (D) 291. (B) 292. (B) 293. (A) 294. (D) 295. (A) 296. (B) 297. (D) 298. (A) 299. (A) 300. (B) 301. (A) 302. (D) 303. (A) 304. (B) 305. (C) 306. (A) 307. (D) 308. (D) 309. (C) 310. (C) 311. (B) 312. (C) 313. (D) 314. (C) 315. (A) 316. (C) 317. (B) 318. (C) 319. (B) 320. (C) 321. (D) 322. (A) 323. (B) 324. (D) 325. (C) 326. (C) 327. (C) 328. (A) 329. (D) 330. (C) 331. (C) 332. (A) 333. (D) 334. (D) 335. (C) 336. (D)
Hints to MCQs
4. R-X is a general representation of alkyl halides whereas CnH2n+1X is the general molecular formula of monohalogen derivatives of alkanes.
10. In vic-dihalides, two halogen atoms are present on the adjacent carbon atoms.
11. Cl Cl
Ethylene dichloride : CH2 CH2 ; Ethylidene chloride : CH3CHCl2 Since the two differ only in the position of the chlorine atoms, they are position isomers of each other.
13. CnH2n2X4 is the general formula of tetrahalogen derivatives while CnH2n+1X, CnH2nX2 and CnH2n1X3 are general formulae of monohalogen, dihalogen and trihalogen derivatives respectively. 67
Chemistry Vol ‐ 2.2 (Med. and Engg.) 14. In allylic halide, the halogen atom is attached to sp3 carbon atom next to a carbon-carbon double bond. Br Br
CH2 – Cl CH – CH3 Cl Benzyl chloride (1-Bromoethyl)benzene 1-Bromobenzene 3-Chlorocyclohex-1-ene 15. Halogen atom in isopropyl halide is attached to secondary carbon atom, hence it is a secondary alkyl halide. 1 2 1 H3C CH CH3
X Isopropyl halide 16. Neohexyl chloride is a primary alkyl halide as Cl-atom is attached to a primary carbon. CH3
1 4 2 1 Isopropyl iodide and sec-Butyl iodide are secondary alkyl halides while CH 3 C CH2 CH2Cl tert-Butyl bromide is a tertiary alkyl halide. 1 CH3 Neohexyl chloride 17. Compounds having the molecular formula C5H11Cl can form three different structures of secondary alkyl halides as follows: Cl Cl Cl
| | |
H C – CH – CH – CH – CH H C – CH – CH – CH – CH H C – CH – CH – CH 3 2 2 3 3 2 2 3 3 3 2-Chloropentane 3-Chloropentane |
CH3 2-Chloro-3-methylbutane 18. A tertiary (3) carbon atom is the carbon atom which is attached directly to three other carbon atoms. In (A), the halogen is attached to a 3 carbon atom. (B) is a trihalogen derivative, (C) and (D) are 2 alkyl halides. 19. Cl Cl H3C C CH3 H3C C CH3 CH3 H tert-Butyl chloride Isopropyl chloride (Simplest tertiary alkyl halide) (Simplest secondary alkyl halide) (Number of carbon atoms = 4) (Number of carbon atoms = 3) 20. In C4H9Br, all three types of C-atoms are present i.e., 1, 2 and 3 because of position isomerism. 1 CH3 1 2 2 1 1 2 2 1 1 3 Br CH CH CH CH CH3 CH CH2 CH3 2 2 2 3 H3C C Br 1-Bromobutane 1 Br CH3 2-Bromobutane 2-Bromo-2-methylpropane 1 1 CH CH 3 3 1 1 21. 3 4 2 1 1 2 2 1 1 2 1 CH C Cl CH3 C CH2 CH2 Cl CH CH CH 3 CH3 CH2 CH CH3 3 2 2 1 1 CH CH3 Cl 3 Cl tert-Butyl chloride Neohexyl chloride sec-Butyl chloride n-Propyl chloride 23. CH3 CH2 CH CH3 | Cl (Chlorine is attached to a carbon atom which is attached to two other carbon atoms). sec-Butyl chloride
68
Chapter 10: Haloalkanes and Haloarenes 24. H
Br C C2H5
C2H5 IUPAC name- 3-Bromopentane
32 1 27. CH 3 – CH – CH – CH3 | | 4 CH2 Cl | 5 CH3 2-Chloro-3-methylpentane
CH3 Cl 28. 4 3 2 1 CH C CH CH CH 3 2 3 6 5 CH2 CH3
2-Chloro-4,4-dimethylhexane
C 2 H 5 Cl CH 3 | | | 1 2 3 4 5 6 7 8 29. H3C CH2 CH CH CH CH2 CH2 CH3 4-Chloro-3-ethyl-5-methyloctane
30. Ethylene dichloride (vicinal dichloride): Cl CH2 CH2 Cl 1,2-Dichloroethane
Cl H
Ethylidene chloride (geminal dichloride): H C C H Cl H 1,1-Dichloroethane
31. Cl Br 4 3 | 2 | 1 H3C – C – C – CH3 | | CH CH 3 3 2-Bromo-3-chloro-2,3-dimethylbutane When two or more than two halogen atoms are present in the haloalkane molecule, the numbering is done in such a way that alphabetically first halogen group gets the lowest locant.
32. Allyl chloride is Cl CH2 CH = CH2. Its IUPAC name is 3-Chloroprop-1-ene/3-Chloropropene. H H H3C H 33. 4 3 2 1 H H3C C = C CH2 Br H Br 1-Bromobut-2-ene H
36. CH3F > CH3Cl > CH3Br > CH3I The sp3 orbital of carbon cannot penetrate into larger p-orbitals (3rd, 4th, 5th orbitals) sufficiently to form strong bonds. Hence, CF bond will be the strongest and CI the weakest. 69
Chemistry Vol ‐ 2.2 (Med. and Engg.) 37. C–X Bond Enthalpy Halomethane (kJ/mol)
CH3 F 452
CH3 Cl 351
CH3 Br 293 234 CH3 I Thus, the C–X bond energy (bond enthalpy) increases in the order: CH3I < CH3Br < CH3Cl < CH3F 3 38. The C X bond polarity order is CH3Cl > CH3Br > CH3I as the 2sp orbital of carbon cannot penetrate into the larger p-orbitals sufficiently to form strong bonds. 39. Order of C X bond strength: CH3 F > CH3 Cl > CH3 Br > CH3 I Dipole moment in Debye: 1.847 1.860 1.830 1.636
H H H H H H H H H Diffused 1 2 1 sunlight 1 2 1 1 2 1 41. H C C C H + Cl2 H C C C H + H C C C Cl + HCl
H HH H Cl H H H H Propane 2-Chloropropane 1-Chloropropane (Major product) (Minor product)
2-Chloropropane is formed as the major product, as the displacement of H-atom from hydrocarbon during halogenation follows the order 3 H-atom > 2 H-atom > 1 H-atom.
42. Since there are only two types of replaceable H-atoms in n-butane, only two of types of products will be formed. CH3 CH2 CH2 CH2 – Cl Cl2 1-Chlorobutane CH3 CH2 CH2 CH3 n-Butane CH3 CH CH2 CH3 |
Cl 2-Chlorobutane
CH3 CH3
43. H3C CH CH2 CH2 CH2 CH CH3 2,6-Dimethylheptane
2,6-Dimethylheptane is a symmetrical alkane which produces 4 types of monochlorinated derivatives which are as follows:
CH3 CH3 i. Cl CH3 CH3 ii.
H3C C CH2 CH2 CH2 CH CH3 H2C CH CH2 CH2 CH2 CH CH3 Cl 1-Chloro-2,6-dimethylheptane 2-Chloro-2,6-dimethylheptane
CH Cl CH CH3 3 CH3 3 iii. iv. H3C C CH CH2 CH2 CH CH3 H3C CH CH2 CH CH2 CH CH3
H Cl 4-Chloro-2,6-dimethylheptane 3-Chloro-2,6-dimethylheptane
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Chapter 10: Haloalkanes and Haloarenes 44. H Cl
h H3C C CH3 H3C C CH3 + H3C CH CH2 Cl Cl 2 CH CH CH 3 3 3 2-Methylpropane 2-Chloro-2- methylpropane 1-Chloro-2- methylpropane
(Major product) (Minor product) The displacement of H-atom from hydrocarbon during halogenation follows the order: Benzylic allylic > 3 H-atom > 2 H-atom > 1 H-atom > H-atom of methane > vinylic arylic With Cl2, the relative rate of substitution of 1, 2, and 3 hydrogens at 298 K is 1 : 3.8 : 5. Cl 1 1 1 1 1 1 3 Sunlight 3 3 45. CH3 CH CH3 + Cl2 CH3 C CH3 + CH3 CH CH2 Cl + HCl 1 1 CH3 CH3 CH3 Isobutane tert-Butyl chloride Isobutyl chloride (Major product) (Minor product) The H-atom linked to the 3 carbon atom in isobutane is more reactive than the other nine primary (1) H-atoms, thus tert-butyl chloride [(CH ) CCl] will be formed in excess. 3 3 46. Neopentane has four identical methyl groups and hence forms only one monochloro product. A branched molecule cannot be properly packed in the crystal lattice. Hence, the melting and boiling points of a neoalkane is always less than that of the normal isomer. Since neopentane does not have an asymmetric carbon atom, it cannot exhibit optical isomerism.
CH3 CH3 CH C CH Cl2 3 3 CH3 C CH2Cl
CH3 CH3 Neopentane (2,2-Dimethylpropane) 1-Chloro-2,2-dimethylpropane
CH2 47. 2 2 H2C 2 CH2 2 2 H2C CH2 Cyclopentane Since, all the H-atoms in cyclopentane are 2 H-atoms, cyclopentane yields only one type of monosubstituted chloroalkane on monochlorination. Propane, n-butane and isobutane yield two types of monosubstituted chloroalkanes each, as they contain 1 as well as 2 H-atoms.
Anhydrous AlBr3 48. R H + Br2 R Br + HBr Alkane Alkyl bromide 49. Alkanes can be iodinated by I2, but the reaction becomes reversible due to the formation of HI, which is a strong reducing agent. Hence certain oxidizing agents are used, which react with HI, thus preventing the backward reaction.
CH + I C H I + HI n 2n+2 2 n 2n+1 Oxidation 51. R – CH3 + X2 R– CH2X + HX Alkane Halogen Alkyl Hydrogen halide halide
Reduction 71
Chemistry Vol ‐ 2.2 (Med. and Engg.) Thus, halogenation of alkanes is a redox process as the alkane undergoes oxidation to form alkyl halide and the halogen undergoes reduction to form corresponding hydrogen halide. 52. Addition of HCl to pent-1-ene is governed by Markownikoff’s rule. Cl |
H 3C–CH2–CH2–CH = CH2 + HCl H3C–CH2–CH2–CH–CH3 + H3C–CH2–CH2–CH2–CH2–Cl Pent-1-ene 2-Chloropentane 1-Chloropentane (Major product) (Minor product) Markownikoff'srule 55. CH3 CH = CH2 + HBr CH3 CH CH3 + CH3 CH2 CH2 Br | Br Propene Isopropyl bromide n-Propyl bromide (Major product) (Minor product)
Br
Markownikoff's rule 56. CH3 C = CH2 + HBr CH3 C CH3 + CH3 – CH – CH2 – Br
CH3 CH3 CH3 2-Methylpropene 2-Bromo-2-methylpropane 1-Bromo-2-methylpropane (Major product) (Minor product)
57. But-2-ene is a symmetrical alkene and thus, does not follow Markownikoff’s rule. 58. Both of the doubly bonded carbon atoms carry equal number of hydrogen atoms. However, benzyl carbocation is more stable than 2 alkyl carbocation.
C6H5CH = CHCH3 + HBr C6H5 CH CH CH3
Br H Br
59. electrophilic addition of H HBr CH3 C CH + HBr from HBr CH3 C = CH2 CH3 C CH3 Propyne (Methyl acetylene) Br Br 2-Bromopropene 2,2-Dibromopropane 61. The reaction of but-1-ene with HBr gives 2-bromobutane as the major product as the reaction proceeds according to Markownikoff’s rule. 63. The product formed in option (B) (CH3 – CH2 – CH2 – Br) is the major product while the product formed in option (A) (CH3CHBrCH3) is the minor product of Anti-Markownikoff’s addition of HBr to CH3 – CH = CH2. Addition of HCl and HI to an unsymmetrical alkene does not follow anti-Markownikoff’s rule as the peroxide effect (anti-Markownikoff’s rule) is shown only by hydrogen bromide. Hydrogen chloride and hydrogen iodide always add to an unsymmetrical alkene according to Markownikoff’s rule. 65. Reaction of primary and secondary alcohols with conc. HCl in order to obtain corresponding chloroalkanes as the product requires presence of anhydrous ZnCl2 as catalyst whereas tertiary alcohols readily react (simply by shaking) with concentrated HCl, even in absence of anhydrous ZnCl2. 66. 3 alcohols are most reactive. Hence, they readily react with conc. HCl at room temperature.
67. i. Primary and secondary alcohols react with concentrated HCl and anhydrous ZnCl2 to give the corresponding alkyl chlorides. Anhydrous ZnCl2 eg. a. CH3 CH2 OH + HCl CH3 CH2 Cl + H2O Ethanol (conc.) Ethyl chloride CH3 CH3 Anhydrous ZnCl b. CH3 C OH + HCl 2 CH3 C Cl + H2O Room temperature H H Propan-2-ol (conc.) 2-Chloropropane
72
Chapter 10: Haloalkanes and Haloarenes ii. Tertiary alcohols readily react (simply by shaking) with concentrated HCl even in the absence of anhydrous ZnCl . 2 CH eg. CH3 3 Room temperature CH3 C OH + HCl CH3 C Cl + H2O CH3 (conc.) CH3 2-Methylpropan-2-ol 2-Chloro-2-methylpropane Therefore, among the given reactions, (I), (III) and (IV) can be used for the preparation of alkyl halides.
68. 2KBr + conc. H SO K SO + 2HBr 2 4 2 4 Reflux C2H5OH + HBr C2H5Br + H2O Ethanol Ethyl bromide
70. CH3 – CH – CH2 – CH3 + HI CH3 – CH – CH2 – CH3 + H2O | | OH Hydroiodic I Butan-2-ol acid (conc.) 2-Iodobutane (57%) (secondary alcohol)
71. The order of reactivity of alcohols towards halogen acids is 3 > 2 > 1
72. CH3 – CH2 – OH + PCl5 CH3 – CH2 – Cl + POCl3 + HCl Ethyl alcohol Ethyl chloride Phosphorus oxychloride
73. 3R–OH + PCl3 3R–Cl + H3PO3 Alcohol Phosphorus Alkyl Phosphorus trichloride chloride acid
red P Br2 74. 3C2H5OH + PBr3 3C2H5Br + H3PO3 Ethanol Bromoethane (Monohalogen derivative)
red P Br 2 75. 3CH3–CH2–CH2–CH2–OH + PBr3 3CH3–CH2–CH2–CH2–Br + H3PO3 Butan-1-ol 1–Bromobutane
Anhydrous ZnBr2 is not capable of bringing about bromination. Bromine in water is an oxidizing agent and oxidises aldehydes to acids. Concentrated sulphuric acid brings about dehydration of the alcohol at suitable temperature. Hence, among the given options the best reagent which is used in the conversion of butan-1-ol to 1-bromobutane is PBr . 3
red P I 76. R OH + PI 2 3R I + H PO 3 3 3 Alcohol Alkyl iodide Phosphorus acid Pyridine 77. C2H5OH + SOCl2 Reflux C2H5Cl + SO2 + HCl Ethyl Ethyl alcohol chloride
79. Pyridine (C H N) is the solvent used in the preparation of alkyl chloride from alcohol by using SOCl . 5 5 2 Pyridine 80. C2H5OH + SOCl2 reflux C2H5Cl + SO2 + HCl Ethyl Ethyl alcohol chloride
CH OH + HCl AnhydrousZnCl2 C H Cl + H O 2 5 2 5 2 Ethyl Ethyl alcohol chloride 73
Chemistry Vol ‐ 2.2 (Med. and Engg.)
81. Alcohol reacts with PCl3 / PCl5 /HCl to give corresponding alkyl chloride, but conversion of alcohol into alkyl chloride using thionyl chloride (SOCl2) is considered as best method. The byproducts (SO2 and HCl) formed are gaseous which escape out from the reaction mixture leaving behind pure alkyl chloride.
84. Alkyl fluorides can be prepared by the action of mercurous fluoride (Hg2F2), silver fluoride (AgF), cobalt fluoride (CoF2) or antimony trifluoride (SbF3) on alkyl chloride or bromide. This reaction is known as “Swarts Reaction”. eg. 2CH3 Cl + Hg2F2 2CH3 F + Hg2Cl2 Methyl Mercurous Methyl chloride fluoride fluoride
85. Preparation of alkyl fluorides by heating alkyl bromides or chlorides in the presence of metallic fluorides is known as Swarts reaction.
HO22 86. H2C = CH – CH2 –CH3 + HBr Br – CH2 – CH2 – CH2 CH3 Anti- Markownikoff's rule But-1-ene 1-Bromobutane
Dryacetone Br CH2 – CH2 – CH2 – CH3 + NaI I CH2 – CH2 – CH2 – CH3 + NaBr 1-Bromobutane 1-Iodobutane
87. Methyl iodide is a colourless liquid at room temperature while methyl chloride, methyl bromide and ethyl chloride are gases.
88. Solubility of organic compound in water is because of H – bond formation. Alkyl halides are very slightly soluble in water because they do not form hydrogen bonds with water.
89. Density of haloalkane size of halogen atom 1 Density of haloalkane sizeof thealkylgroup Since all are iodides, n-butyl group has largest size and hence lowest density.
90. Density of an alkyl halide increases as the size of halogen atom increases. Thus, for a given alkyl group the densities of the halides follow the order RF < RCl < RBr < RI.
91. Boiling points of alkyl halides are greater than that of corresponding hydrocarbons. For the same alkyl group, the boiling points of alkyl chlorides, bromides and iodides follow the order: RCl < RBr < RI.
92. Physical properties depend upon the nature of an alkyl chain, if the halogen atom is same. For the same halogen atom, boiling point increases as the molecular weight increases. Hence, CH3Br has the lowest boiling point.
93. Boiling point of isomeric alkyl halides increases with the decrease in branching.
94. a. Molecules with higher molecular weights generally have higher boiling points. Therefore, for alkyl halides having same alkyl group but different halide group, boiling point decreases as follows, RI > RBr > RCl > RF. b. Further, the boiling point increases as the number of halogen atoms increases. Therefore, among given four compounds, bromoform having three bromine atoms has the highest boiling point. c. So the boiling points of four compounds increase in the order: Chloromethane (CH3Cl) < Bromomethane (CH3Br) < Dibromomethane (CH2Br2) < Bromoform (CHBr ). 3 100. Ag2O + H2O 2AgOH ; C2H5Br + AgOH C2H5OH + AgBr Silver Silver Ethyl Ethyl oxide hydroxide bromide alcohol
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Chapter 10: Haloalkanes and Haloarenes
boil 102. C3H7Br + KCN C3H7CN + KBr Bromopropane Butanenitrile In IUPAC system the carbon of functional group is also taken in numbering. So CHCN is butanenitrile. 37 103. R–X + KCN boil R–CN + KX Alkyl (alc.) Alkyl halide cyanide The product (alkyl cyanide) formed in the above reaction has one carbon atom more than that of the reactant (alkyl halide). The reaction is a good method for increasing the length of (step up) carbon chain.
104. Heating of an alkyl halide with alcoholic AgCN gives corresponding alkyl isocyanide as the product; here nucleophilic attack takes place through nitrogen atom of cyanide (because of the presence of silver salt). Heating of an alkyl halide with alcoholic KCN gives corresponding alkyl cyanide as the product; here the nucleophilic attack takes place through carbon atom of cyanide. Heating of an alkyl halide with aq. KOH or with moist silver oxide gives corresponding alcohol as the product.
105. C2H5Cl + AgCN C2H5 N = C + AgCl Ethyl (alc.) Ethyl isocyanide chloride (X) The functional isomer of ethyl isocyanide is ethyl cyanide C2H5 – C N.
,under 106. pressure CH25 Cl CH25 Cl C2H5 Cl + NH3 HCl C2H5 NH2 ,under pressure (C2H5)2NH ,under pressure (C2H5)3N HCl HCl Ethyl Ethylamine Diethylamine Triethylamine chloride (1 amine) (2 amine) (3 amine)
, under pressure C H Cl 2 5 + [(C2H5)4N] Cl Tetraethyl ammonium chloride (Quaternary salt)
107. Alkyl halides undergo ammonolysis to give corresponding primary amines. Cl NH2
NH3 +HCl EtOH Br Br
108. When an alkyl halide is treated with excess of ammonia, primary amine is obtained as a major product. CH3 – Cl + NH3 CH3 – NH2 + HCl Under pressure Chloromethane (alc.) Methanamine (Excess) (Major product) 109. Br NH2
Alc.NH3 , CH C CH CH3 C CH3 HBr 3 3
CH3 CH3
2-Bromo-2-methylpropane 2-Amino-2-methylpropane
NaBr 110. C2H5ONa + BrC2H5 H5C2 O C2H5 (Williamson’s synthesis) Sodium salt Ethyl bromide Diethyl ether of ethanol (Ethoxy ethane)
111. C3H7Br + CH3ONa H3C O – C3H7 + NaBr 1-Bromopropane 1-Methoxypropane Sodium bromide
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Chemistry Vol ‐ 2.2 (Med. and Engg.) 112. Dry silver oxide should be completely dry. If it contains traces of moisture then it behaves as silver hydroxide and results in the production of monohydric alcohol (instead of the desired symmetrical ether product) when treated with respective alkyl halide.
O O || || 113. CH3Cl + Ag–O–C–CH CH – C– O CH + AgCl 3 CHOH25 33 Methyl Silver acetate Methyl acetate Silver chloride chloride
O || 114. C2H5 – Br + Ag – O C C2H5 C2H5 – C – O – C2H5 + AgBr || CHOH25
O Ethyl bromide Silver propionate Ethyl propionate 120. Ease of dehydrohalogenation in case of haloalkanes follows the order: tertiary > secondary > primary (when same halogen group is present).
121. CH3CH2CH2CH2 – Cl + KOH(alc.) CH3CH2 – CH = CH2 + KCl + H2O 1-Chlorobutane But-1-ene
122. When 1-bromopropane is heated with alcoholic KOH, propene is obtained. CH3CH2CH2Br + KOH CH3CH = CH2 + KBr + H2O 1-Bromopropane (Alc.) Propene Molar mass of CH3CH2CH2Br = 123 g/mol and molar mass of propene (CH3CH = CH2) = 42 g/mol 12.3 g Number of moles of CH3CH2CH2Br = = 0.1 mol 123 g/mol
0.1 mol of CH3CH2CH2Br will give 0.1 mol of propene. Mass of propene obtained will be 0.1 42 = 4.2 g
However, yield obtained is 50%. 50 Mass of propene obtained = 4.2 = 2.1 g 100 123. H CH2 – CH3 CH – CH – CH – CH – CH + KOH C = C 3 | 2 2 3 (alc.) CH3 H Br Trans-pent-2-ene 2-Bromopentane (Main product)
When alkyl halide reacts with alc. KOH, then it favours elimination reaction (dehydrohalogenation). The main product is trans pent-2-ene, which is more stable than its cis isomer. 124. 1-Phenyl-2-chloropropane, on heating with alcoholic KOH, gives mainly 1-phenylpropene (i.e., the product having more substituted double bond). H Cl
C H C C CH + KOH C H CH = CH CH + KCl + H O 6 5 3 6 5 3 2 (Alc.) 1-Phenylpropene H H
1-Phenyl-2-chloropropane
125. Ethanol cannot undergo dehydrohalogenation as it does not contain any halogen atom. I
126. Alc.KOH 2CH3 CH CH2 CH3 CH3 CH = CH CH3 + H2C = CH – CH2 – CH3 + 2KI + 2H2O 2-Iodobutane But-2-ene But-1-ene (Major product) (Minor product)
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Chapter 10: Haloalkanes and Haloarenes
Dry ether 128. CH3 – CH2 – CH2 – CH – Br + 2Na + Br – CH – CH2 – CH 2 – CH 3 | | CH2 CH2 | | CH3 CH3 3-Bromohexane 3-Bromohexane (X) (X) 1 2 3 4 5 6 7 8 CH3 – CH2 – CH2 – CH – CH – CH2 – CH2 – CH3 + 2NaBr | | CH2 CH2 | |
CH3 CH3 4,5-Diethyloctane Dry ether 129. 2C25 H F 2Na No reaction Ethyl fluoride Bond dissociation energy of C2H5F is comparitively higher than the to other ethyl halides. Thus, ethyl fluoride is least reactive and does not undergo Wurtz reaction.
130. The reaction of alkyl halide with sodium metal in the presence of dry ether is known as Wurtz synthesis. In this reaction, the product formed always contains more number of carbon atoms than reactants. Hence, ethane cannot be formed.
131. In Wurtz reaction, total carbon atoms in product is sum of carbon atoms in alkyl halides used. Thus, n-hexane (6 carbon atoms) can never be formed from isopropyl bromide (3 carbon atoms) and ethyl bromide (2 carbon atoms) in Wurtz synthesis. Also, the alkane formed from isopropyl bromide will not contain normal carbon chain of 6 carbon atoms. Dryether 3C2H5–Br + 6Na + 3Br–CH–CH3 C2H5–C2H5 + CH3–CH–CH–CH3 + CH3–CH2–CH–CH3 + 6NaBr | Ethyl n-Butane || | bromide CH3 CH3 CH3 CH3 (Bromoethane) Isopropyl bromide 2,3-Dimethylbutane 2-Methylbutane (2-Bromopropane) (Isopentane)
132. Grignard reagent is represented as R–Mg–X; where, R = Alkyl group, Mg = Magnesium and X = Halogen. 133. Grignard’s reagent can be prepared by reaction of alkyl halide with pure and dry magnesium in the presence of dry ether. RX + Mg Dry ether R – Mg – X Alkyl halide Grignard’s reagent 134. The compounds in which the metal atom is directly bonded to the carbon atom are known as organometallic compounds. In CH3ONa and C2H5SNa, the sodium atom is not directly bonded to the carbon atom. However, in CH3MgI, the magnesium atom is directly bonded to the carbon atom. 136. Br + Mg Dry ether MgBr HO2 H
Bromocyclohexane Cyclohexyl Cyclohexane magnesium ‘B’ bromide ‘A’ The Grignard reagent (formed as product ‘A’) reacts with any source of proton to give corresponding hydrocarbon. 143. Racemic mixture is optically inactive due to external compensation. When the enantiomers are mixed together, the rotation caused by one enantiomer is exactly cancelled by equal and opposite rotation caused by the other enantiomer. This is known as external compensation. The resulting racemic mixture is therefore optically inactive. 77
Chemistry Vol ‐ 2.2 (Med. and Engg.) 144. Cl * CH3 C CH2 CH3
H 2-Chlorobutane 2-Chlorobutane has one asymmetric carbon atom (marked *).
145. Asymmetric (chiral) carbon atom is the carbon which is attached to four different atoms or groups.
Cl Cl Cl | | | * * 146. CH3 – C – C – C CH3 (Carbon atoms with asterisk (*) are asymmetric.) | | | H H H
2,3,4-Trichloropentane
149. According to Vant-Hoff Le Bel theory, sp3 hybridized C-atom having all the four valencies satisfied by different atoms or groups of atoms is called asymmetric carbon atom.
151. Number of optical isomers for a molecule with ‘n’ asymmetric carbon atom is given by the formula 2n. 2n = 64; 26 = 64; n = 6.
153. H * H C C COOH 3 OH Lactic acid IUPAC name: 2-Hydroxypropanoic acid
154. CH CH 3 3 –OH replaced by –H H C* OH H C H
COOH COOH Lactic acid (asymmetric, Propanoic acid optically active) (symmetric, optically 1 chiral carbon inactive) no chiral carbon 156. Enantiomers are optical isomers which have same molecular and structural formula but different configuration (3D arrangement in space).
157. d and l forms of an optically active compound differ in their specific rotation. d form rotates the plane of polarised light to the right and l– form to the left. Both have similar physical properties like relative density, boiling points and refractive index.
158. Enantiomers rotate the plane of plane polarized light through the same angle but in opposite direction. Thus, if one of the if one of the enantiomer has optical rotation of 30, then its mirror image shows optical rotation of – 30o.
X
| X2 * 159. CH3 – CH2 – CH2 – CH3 Monohalogenation CH3 – C – CH2 – CH3 n-Butane H 2-Halobutane (1 asymmetric carbon atom, optically active)
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Chapter 10: Haloalkanes and Haloarenes 160. Enantiomers are non-superimposable mirror images of each other. Option (A) is the non-superimposable mirror image of the given molecule and the configuration of two groups, i.e., CH3 and C2H5 in it is reversed at the chiral carbon. H H H C2H5 CH3 H3C C C C H3C H5C2 C2H5 Br Br Br Option (A) Given molecule Mirror 180
165. In R, S configuration, the order of priority is I > Br > Cl > SO3H > F > OCOCH3 > OH > NO2 > NH2 > COOCH > COOH > CONH > COCH > CHO > CH OH > CN > C H > C H > CH > D > H. 3 2 3 2 6 5 2 5 3 1 Cl
C* 4 3 168. H CH3 2 H5C6 S-1-Chloro-1-phenylethane
169. Transition state is the state in between initial and final state of a reaction. It has high energy content and is very unstable.
170. When an alkyl halide reacts with aqueous alkali; corresponding alcohol is formed. The reaction is classified as nucleophilic substitution. R X + NaOH R OH + NaX Alkyl halide (aq) Alcohol
173. The given reaction follows one step concerted mechanism i.e., SN2 mechanism. The reaction is called as SN2 i.e., ‘Nucleophilic substitution Bimolecular Reaction’, because the rate of the reaction depends on the concentration of both reactants.
174. Alkaline hydrolysis of 1-Bromopropane takes place according to SN2 mechanism. Rate is given by:
dc 1 1 Rate i.e., = K [C3H7Br] [OH ] dt From the above rate expression, if we double the concentration of any one of the reactant (i.e., either alkyl halide or alkali), the rate will be doubled. It is an exothermic reaction.
176. In SN2 reactions, inversion of configuration occurs. Attack of the nucleophile from the same side of the leaving group is not possible. Therefore, backside attack occurs with the formation of product with opposite sign of optical rotation than the reactant.
177. Because R I bond has least bond dissociation energy among these; it is the most reactive alkyl halide. Therefore, the order is RI > RBr > RCl > RF.
179. The reactivity of alkyl halides in SN2 mechanism decreases in the following order: primary alkyl halide > secondary alkyl halide > tertiary alkyl halide The correct order of reactivity of the given iodides in SN2reaction is CH3CH2CH2CH2I > CH3CH2CHCH3 > (CH3)3CI 1 alkyl halide 3 alkyl halide I
2 alkyl halide 2 180. Reactivity of an alkyl halide in SN mechanism is in the following order: CH3X > 1 alkyl halide > 2 alkyl halide > 3 alkyl halide. 79
Chemistry Vol ‐ 2.2 (Med. and Engg.) 181. In SN1 mechanism, the reactivity of the halide, R-X, follows order: R–I > R–Br > R – Cl > R–F; because as size of atom increases, the bond dissociation energy decreases. Therefore, among the given alkyl halides, (CH ) C – I will undergo SN1 reaction most readily. 3 3 182. There is only one peak in energy profile diagram of SN2 reactions, corresponding to transition state (T.S.)
183. In SN2 reaction, the rate is proportional to concentration of the reactants, i.e., substrate and nucleophilic reagent.
185. (CH3)3C Br + NaOH(aq) (CH3)3C OH + NaBr
Rate [(CH3)3C Br]
Rate = k [(CH3)3C Br] Hence, with respect to the nucleophile (OH), the order of the reaction is zero.
186. Alkaline hydrolysis of tert-Butyl bromide follows SN1 mechanism. Rate = k [(CH ) C – Br] 3 3 187. Alkaline hydrolysis of tertiary butyl bromide follows SN1 mechanism.
Rate of reaction [(CH3)3C – Br] Rate is doubled if the concentration of tertiary butyl bromide is doubled. In SN1 mechanism, reaction rate does not depend on the concentration of nucleophile.
+ 188 (CH3)3C Br (CH3)3C + Br tert –Butyl bromide Carbonium ion
189. Alkaline hydrolysis of tertiary alkyl halide follows SN1 mechanism. It involves two steps (i) formation of carbonium ion and (ii) attack of nucleophile resulting in the formation of the compound.
192. Electron deficient chemical species act as electron acceptor hence termed as Lewis acid or an electrophile. Nucleophile is a negatively charged chemical species or carrying at least one lone pair of electrons.
193. In SN1 reaction, the incoming nucleophile can attack from the back and front sides because it is a 2 step process, in 1st step carbocation is formed and in 2nd step nucleophile attacks, for which both sides are available for attack. Hence there is inversion as well as retention of configuration.
194. The given reaction follows SN1 mechanism. SbCl5 abstracts chloride ion from (+)-2-chloro-2-phenylethane to give the intermediate carbocation (which is planar) in the rate determining step. Front and backside attack of Cl gives a racemic mixture.
195. The given reaction follows SN1 mechanism. Hence, the racemic mixture is formed i.e., (±)butan-2-ol. 196. 1-Bromo-1-phenylethane follows SN1 mechanism. H H
* C* Br + KOH C OH + KBr (aq)
CH3 CH3 1-Bromo-1-phenylethane 1-Phenylethanol Racemic mixture
197. Stability order of carbocation is tertiary > secondary > primary. + + ++ H 3 C C CH3 > H3C CH > H3C CH2 > CH3
CH3 CH3 198. Reactivity is directly related to the rates of formation of carbocations. Rate of formation of carbocation is
3 > 2 > 1 > CH3. Greater the number of alkyl groups, the more stable the carbocation. This is due to the (+I) inductive effect of the alkyl group.
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Chapter 10: Haloalkanes and Haloarenes 199. Tertiary alkyl halides prefer SN1 mechanism which results in the formation of racemic mixture (product).
200. Benzylic halides show high reactivity towards the SN1 reaction, because the carbocation formed undergoes stabilization through resonance.
201. Methyl chloride and ethyl chloride are primary halides and preferably follow SN2 mechanism. Isopropyl chloride is a secondary halide and may follow SN1 or SN2 mechanism depending upon the reaction conditions. Benzyl chloride is a type of benzylic halide which exclusively follow SN1 mechanism.
202. In SN1reaction, the order of reactivity of alkyl/aryl halides is 3 > 2 > 1. The compounds in (ii) and (iii) are secondary halides. But the carbocation intermediate obtained from (ii) is more stable than that obtained from (iii) because it is stabilised by two phenyl groups due to resonance. Hence, (ii) is more reactive than (iii) in SN1reactions.
203. In SN1 mechanism, formation of carbonium ion is rate determining step. So increase in concentration of nucleophilic reagent will not affect the rate of reaction. Thus, rate depends only on the substrate.
204. Benzylic and allylic halides forms stable carbocation through resonance and hence undergo SN1 mechanism. The rate of the SN1 reaction is determined by how fast the alkyl halide ionizes, and hence depends only upon the concentration of alkyl halide. Thus, SN1 reaction follows first order kinetics.
205. An alkyl halide (having -hydrogen atoms) when reacts with a base or nucleophile has two competing routes: substitution (SN1 and SN2) and elimination. I
208. Structure of symmetrical triiodobenzene is I I 1,3,5-Triiodobenzene
209. In the aryl halides, the partial double bond character (acquired due to resonance) results in large amount of energy being required to break C–X bond. As a result, aryl halides are less reactive than alkyl halides. 210. In chlorobenzene, C – Cl bond acquires partial double bond character due to resonating structures. Partial double bond character makes C Cl bond in chlorobenzene shorter and stronger than C Cl bond in methyl chloride. 211. Chlorobenzene is less reactive than benzyl chloride. CH2 Cl
Benzyl chloride In chlorobenzene, the lone pair present on Cl atom get involved in resonance with electrons of benzene due to which C Cl bond acquires double bond character. Hence, reactivity decreases. + Cl + Cl + Cl Cl
212. Lewis acids like FeCl , BCl , AlCl ,etc. can be used in electrophilic substitution of arenes. 3 3 3 213. The reaction of chlorine with benzene in the presence of AlCl3 to form chlorobenzene is electrophilic + substitution reaction. Lewis acid i.e., AlCl3 generates the electrophile Cl , which attacks the benzene ring.
+ – Cl – Cl + AlCl3 Cl + [AlCl4] Chloronium ion H Cl + Cl + Cl + H -complex Chlorobenzene 81
Chemistry Vol ‐ 2.2 (Med. and Engg.)
214. In direct bromination of benzene; if excess reagent (Br2) is used, then the second halogen atom is introduced at ortho or para position with respect to the first halogen. This is because halogens are o and p directing groups. Hence direct bromination of benzene with excess reagent results in the formation of mixture of ortho and para products.
215. HI formed as a byproduct during direct iodination of benzene ring is a strong reducing agent and reduces back
iodobenzene to benzene. Strong oxidising agents like HIO3, HgO prevent the backward reaction.
217. This is Sandmeyer’s reaction and the product ‘Y’ is chlorobenzene. N NCl Cl Cu22 Cl /HCl HCl + N2
Benzene Chlorobenzene diazonium chloride 219. Diazonium salt on treatment with KI gives iodobenzene. eg. N N Cl I + KI + N2 + KCl
Benzene Iodobenzene diazonium chloride 220. Density increases with increase in the number of carbon atoms, halogen atoms and atomic mass of halogen atoms.
222. In alkyl halide, carbon atom in C X bond is sp3 hybridized, whereas in aryl halides it is sp2 hybridized. Thus, % s-character in chlorobenzene (which is an aryl halide) is more than in ethyl chloride, isobutyl bromide and methyl iodide (which are alkyl halides).
223. Phenyl cation formed from self ionization of aryl halide does not get stabilized by resonance.
Cl ONa OH 200atm 300atm, 224. 623K Hydrolysis HCl HCl, NaCl + NaOH
Chlorobenzene Sodium phenoxide Phenol The reaction is known as Dow’s process. 226. It has been found that presence of electron withdrawing groups like NO2, COOH, CN at o – and/or p – position with respect to halogen atom greatly activates haloarenes to undergo nucleophilic displacement reactions. 227. The presence of electron withdrawing groups at o- and p-positions with respect to halogen atom activates the aryl halides towards nucleophilic substitution reactions. As the number of electron withdrawing groups at o- and p-positions increases, the reactivity of aryl halides also increases. 228. The presence of electron withdrawing groups at o- and p-positions with respect to halogen atom activates the aryl halides towards nucleophilic substitution reactions. However, in case of m-substituted aryl halides, there is no charge at m-position in the resonating stuctures. Therefore, the presence of electron withdrawing group at m-position has no effect on reactivity. Thus, compound (b) is more reactive towards nucleophilic substitution than compound (c). Although the presence of –NO2 group at m-position does not stabilise the carbanion by resonance effect, it stabilises the carbanion somewhat by the inductive effect as compared to the carbanion resulting from chlorobenzene itself. Therefore, m-nitrochlorobenzene is more reactive than chlorobenzene towards nucleophilic substitution reactions. 229. Chlorination of chlorobenzene in the presence of anhydrous FeCl3 yields p-dichlorobenzene as major product and o-dichlorobenzene as minor product. 232. In toluene, CH3 group is electron donating group and thus, it activates benzene ring for electrophilic substitution.
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Chapter 10: Haloalkanes and Haloarenes 233. Halogens are ortho and para directing groups. Due to steric hindrance, para substituted product predominates, hence 2-Bromobenzene sulphonic acid is formed as a minor product and 4-Bromobenzene sulphonic acid is formed as a major product.
234. Anhydrous CuCN and NaCN produces a nucleophile CN and hence is a reagent for nucleophilic substitution reaction. (A) (B) and (C) represents reagents for electrophilic substitution reaction.
CH3 CH CH3 236. AlCl3 + CH3 CH CH3 + HCl Benzene Cl Isopropyl benzene Isopropyl chloride
Br 238. 2 + 2Na dryether + 2NaBr
Bromobenzene Diphenyl This reaction is called “Fittig reaction”.
243. Exposure to low level of CH2Cl2 causes impaired hearing and vision whereas exposure to high level of CH Cl causes nausea, dizziness and numbness in fingers and toes. 2 2 245. Under ordinary conditions, chloroform is not inflammable.
1 Sunlight 247. CHCl3 + O2 COCl2 + HCl 2 Phosgene or Chloroform carbonyl chloride
249. Any substance which when added to a chemical reaction, inhibits it is called catalytic inhibitor. In CHCl3 when two percent ethanol is added, it stops the formation of carbonyl chloride. So ethanol acts as a catalytic inhibitor.
254. Iodoform is used as an antiseptic for dressing wounds. When it comes in contact with skin (organic matter), iodine is set free which is responsible for antiseptic action.
259. Structure of DDT is Cl
(5 chlorine atoms) Cl 3 C CH
Cl
263. In allylic halide, the halogen atom is attached to sp3 carbon atom next to a carbon-carbon double bond.
Br CH3 4 3 2 1 264. CH3 C C CH2 COOH 5 CH C H 2 2 5 6CH 3 4-Bromo-3-ethyl-3,4-dimethylhexanoic acid
265. Chlorination of alkanes i.e., replacement of 1 and 2 hydrogens of alkanes by chlorine occurs in the presence of light. UVlight 266. C3H8 + Cl2 C3H7Cl + HCl Propane Propyl chloride
267. HI formed during iodination, reduces C2H5I back to ethane because it is a good reducing agent. But a strong oxidizing agent like HIO3 or HgO destroys the hydroiodic acid and prevents the reverse reaction. 83
Chemistry Vol ‐ 2.2 (Med. and Engg.)
red P + I 2 268. 3CH3 OH + PI3 3CH3 I + H3PO3 OR CH3 OH + HI CH3I + H2O Methyl Methyl Methyl Hydroiodic Methyl alcohol iodide alcohol acid iodide (57% conc.) 269. In the given reaction, the addition of HCl takes place according to Markownikoff’s rule. CH2 – CH = CH2 CH2 – CH – CH3
Cl + HCl
270. According to Markownikoff’s rule, when an unsymmetrical reagent (like HCl) adds to an unsymmetrical alkene, the negative part of the reagent gets attached to that unsaturated carbon atom which carries lesser number of hydrogen atoms.
CH2 CH3 (A) H3C Cl (B) H3C Cl
HCl HCl
1-Chloro-1-methylcyclohexane 1-Chloro-1-methylcyclohexane 271. Peroxide effect is observed only in case of HBr. Addition of HI and HCl to an unsymmetrical alkene follows Markownikoff’s rule even in the presence of peroxide. Cl Cl HCl/ Benzoylperoxide H3C CH = CH2 Markownikoff ’s rule H3C CH CH3 + CH3 CH2 CH2 2-Chloropropane 1-Chloropropane Propene (Major product) (Minor product) 272. The reaction of toluene with a halogen in the presence of FeCl3 is electrophilic substitution reaction and it follows the mechanism as given below: FeCl3 Cl – Cl (Lewisacid) FeCl + Cl+ 4 Chlorine Electrophile CH CH3 3 CH3 CH3 CH3 H Cl Cl + Cl+ O R FeCl4 OR + HCl + FeCl3 Toluene H Cl o-Chlorotoluene Cl
p-Chlorotoluene 273. Methyl group is an activating and ortho-, para-directing group. Hence, the electrophilic substitution occurs at both ortho- and para-positions. CH CH 3 CH3 3
Cl
+ Cl Fe + 2 dark
Cl Toluene o-Chlorotoluene p-Chlorotoluene 274. Alkyl halide on reaction with dry silver oxide yields symmetrical ethers. 2R X + Ag2O R O R + 2AgX Alkyl halide Dry Ether
silver oxide 275. The alkyl halides are highly reactive; the order of reactivity is tertiary > secondary > primary (for a given halogen atom) and iodide > bromide > chloride (for a given alkyl group). Thus 2-bromopropane is the most reactive compound amongst the given options.
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Chapter 10: Haloalkanes and Haloarenes 279. Preparation of alkyl halides in laboratory is least preferred by direct halogenation of alkanes. This method gives mixture of mono, di, tri derivatives which is difficult to separate and it gives lower yield of any one component. Hence, this is the least preferred method.
Zn Cu coupleandalcohol 281. CH3Br + 2[H] CH4 + HBr Methyl Methane bromide
dryether CH3Br + 2Na + BrCH3 CH3 CH3 + 2NaBr Methyl Methyl Ethane bromide bromide
Peroxide 283. H 3C CH2 CH2 CH2 CH = CH2 + HBr Anti Markownikoff 's H3C CH2 CH2 CH2 CH2 CH2 Br Hex-1-ene addition 1-Bromohexane 284. When benzene reacts with chlorine in sunlight it gives Benzene hexachloride. Cl
bright Cl Cl C H + 3Cl sunlight C H Cl or 6 6 2 or UV light 6 6 6 Benzene Benzene hexachloride Cl Cl
Cl Benzene hexachloride (BHC) exists in eight isomeric forms, of which the isomer is the most active and used an insecticide under the name gammexane. 285. COCl carbonyl chloride is commonly called as phosgene. 2 286. In the presence of sunlight and air, CHCl3 is oxidized to poisonous phosgene. In order to prevent oxidation, CHCl3 is stored in dark amber coloured bottles. 288. n-Alkyl halide contains halogen atom attached to primary carbon atoms. Cl 1 2 2 1 H3C CH2 CH2 CH2
n-Butyl chloride 289. R – OH + HX R – X + H2O Alcohol Alkyl halide Reactivity order of alcohols for this reaction is 321ooo. Reactivity order of halogen acids is HI > HBr > HCl > HF. 290. Freon-12(CCl2F2), DDT [2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane] and Iodoform (CHI3) are derivatives of alkane. Vinyl chloride (CH2 = CH Cl) is a derivative of alkene. 291. BHC is a derivative of cyclohexane, it’s a saturated compound and hence does not contain any bonds.
292. (b) and (c) carbon atoms are asymmetric as they are attached to four different atoms or groups.
293. Compounds containing chiral or asymmetric carbon atom i.e., carbon attached to four different atoms or groups, are called as chiral compounds. Among the given options, only 2- bromobutane has an asymmetric carbon atom and hence, it is chiral in nature. H H OH * H 3C – CH2 – C – CH3 H3C – CH2 – CH2 – CH2 – Br H3C – C – CH3 H3C – C – CH3
Br Br Br 2-Bromobutane 1-Bromobutane 2-Bromopropane 2-Bromopropan-2-ol 294. In case of biphenyls, the bulky ortho-substituents restrict the free rotation and the rings remain perpendicular to each other. When these substituents are different as in the given option (D), then the biphenyls become optically active (i.e., the molecule does not have any elements of symmetry). 85
Chemistry Vol ‐ 2.2 (Med. and Engg.) 295. Amount of rotation is directly proportional to the concentration of an optically active compound.
296. Alkyl halides are slightly soluble in water, but readily soluble in organic solvents like methanol, acetone, etc.
297. Boiling point increases with increase in the molecular mass of the compound. In the given halocompounds, the number of carbon atoms in hydrocarbon part of the molecule increases in the order: ethane < propane < butane < benzene. Therefore, the correct increasing order of boiling points is: 1-Bromoethane < 1-Bromopropane < 1-Bromobutane < Bromobenzene
298. CH3 CH3 CH3 CH3 | Dry diethylether CH3 C Cl + 2Na + Cl C – CH3 CH3 C C CH3 + 2NaCl | CH3 CH3 CH3 CH3 tert-Butyl 2,2,3,3-Tetramethylbutane chloride (A) Cl2 h, UV light or
Cl – CH2 CH3
CH3 C C CH3 CH3 CH3 1-Chloro-2,2,3,3-tetramethylbutane
299. CH3 CH3
Cl2 CH3 CH2 CH2 CH CH2 CH3 hv CH3 (CH2)2 C CH2 CH3
(X) Cl
Addition reaction Not shown by alkyl halide
(saturated compound) aq.KOH or moist Ag2O 300. RCHCHOHHX RCHCHX22 Substitution 22 Alkyl halide Alcohol Dehydrohalogenation RCHCHHX Alc.KOH 2 Alkene 301. In compound (a), Br is directly attached to chiral carbon atom. Therefore, it will give a racemic mixture on nucleophilic substitution by OH– ion (SN1). H H
* – H3C – C – Br OH H3C – C – OH Br
C2H5 C2H5 2-Bromobutane Butan-2-ol (Chiral) (Racemic mixture)
302. In ammonolysis reaction, when an alkyl halide is used in excess, mixture of primary amine, secondary amine, tertiary amine and quaternary ammonium salt is obtained. ,under R X + NH pressure R NH RX R NH RX R N RX [R N]+X 3 HX 2 ,under pressure, 2 ,under pressure, 3 ,under pressure 4 HX HX Quaternary Alkyl 1 amine 2 amine 3 amine ammonium halide salt
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Chapter 10: Haloalkanes and Haloarenes
CH3 CH3
Alc.KOH 304. CH3 CH CH2 CH2 Br CH3 CH CH = CH2 (A)
CH3 CH3 (i) Conc. H2SO4 CH3 CH CH CH3 + CH3 CH CH2 CH2 OH (ii) H2O, OH (B) (C) (Major product) (Minor product)
CH3 CH3
HI, CH3 CH CH CH3 CH3 C CH2 CH3 HO2
OH I (B) (D)
305. By using sulphonyl chloride, pure product is isolated as both the byproducts of reaction (SO2 and HCl) are gases which escape easily leaving behind pure alkyl chloride.
306. Ethyl chloride on heating with alcoholic KOH, undergoes dehydrohalogenation to give ethene (C2H4). alc.KOH CH3CH2Cl CH2 = CH2 + HCl
Ethyl chloride Ethene 307. The correct reaction for option (D) is as follows: Br
| H3C – CH2 – CH = CH2 + HBr H3C CH2 – CH – CH3 But-1-ene 2–Bromobutane In the absence of peroxide, the reaction of HBr follows Markownikoff’s rule. peroxide But-1-ene + HCl 2-Chlorobutane This reaction is correct because peroxide effect is seen only with HBr.
308. Photolysis is cleavage of bonds in presence of light. Addition reactions are necessarily limited to compounds that contain atoms sharing more than one pair of electrons, that is to compounds that contain multiple bonds. Substitution is a characteristic reaction of saturated compounds. Elimination is a type of reaction used for removal of atoms or groups that leads to the formation of double bond in drastic conditions. alcoholic KOH eg. CH3CH2Br CH2 = CH2 Ethyl Ethene bromide CH CH CH HSO24 CH CH = CH Br2 H C CH CH alc.KOH CH C CH 309. 3 3 373K 3 2 3 2 3 Methyl OH Br Br acetylene Propan-2-ol Propene 1,2-Dibromopropane (X)
311. C2H5 I + AgCN C2H5NC + AgI Ethyl iodide (alc.) Ethyl isocyanide
Dryether 312. CH3 – CH – Cl + 2Na + Cl – CH3 CH3 – CH – CH3 + 2NaCl | Methyl chloride | (B) CH3 CH3 Isopropyl chloride Isobutane (A) However, isobutane cannot be suitably obtained by this method. This is because if mixture of two different alkyl halides is used in Wurtz synthesis, mixture of three different alkanes will be produced.
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Chemistry Vol ‐ 2.2 (Med. and Engg.)
Dry ether 314. C2H5I + Mg C2H5 – Mg – I Ethyl Ethyl magnesium iodide iodide
316. Br MgBr H Br
dryether HO2 H3C C CH3 + Mg H3C C CH3 H3C C CH3 + Mg
CH CH3 OH 3 CH3 2-Methyl-2-bromopropane Grignard reagent 2-Methylpropane (X) (Z)
317. When an alkyl halide is treated with alcoholic solution of silver salt of carboxylic acid, it gives ester.
R1 X + R COOAg R COOR1 + AgX. Alkyl Ester halide
318. 3CH3 I + 6Na + 3CH3CH2I C2H6 + C4H6 + C3H8 + 6NaI Ethane Butane Propane 319. Cl Cl Cl Cl anhydrousFeCl3 + + Cl2 or sunlight + HCl
Cl 1,4-Dichlorobenzene Chlorobenzene 1,2-Dichlorobenzene (para dichlorobenzene) (ortho dichlorobenzene) (major product) (minor product)
Dry Ether 320. C2H5Cl + 2Na + ClC2H5 C2H5 C2H5 + 2NaCl Ethyl Ethyl n-Butane chloride chloride
322. In the normal light there are numerous planes of vibration and it is not possible to measure angle of rotation. But when plane polarised light is passed through d-2–chlorobutane, we can measure an angle of rotation.
323. Dehydrohalogenation of CH3CH2Cl gives ethene as the main product. CH3CH2Cl + KOH H2C = CH2 + KCl + H2O
Ethyl (Alc.) Ethene chloride Molar mass of CH3CH2Cl = 64.5 g/mol and Molar mass of ethene (C2H4) = 28 g/mol 6.45g Number of moles of CH3CH2Cl = = 0.1 mol 64.5g/mol
0.1 mol of CH3CH2Cl will give 0.1 mol of ethene. However, only 50% of the reagent is used. So, only 50% of product will be formed, i.e., 0.05 mol of ethene. Weight of main product (ethene) obtained = 0.05 mol 28 g/mol = 1.4 g
324. The presence of electron withdrawing groups like –NO2, – COOH, – CN at m-position (with respect to halogen atom) has no effect on the reactivity of the haloarenes towards nucleophilic substitution reactions.
325. CH CH3 CH3 3 Br FeBr3 2 + 2Br2 + + 2HBr
CH3 CH 3 CH3 Br
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Chapter 10: Haloalkanes and Haloarenes 326. Direct iodination of benzene is a reversible reaction due to HI (strong reducing agent) which is formed as a byproduct. The reaction can proceed in the forward direction, when it is carried out in the presence of a strong oxidising agent (like nitric acid or iodic acid or mercuric oxide).
327. A nucleophile is rich in electrons. Hence it acts as a Lewis base. A Lewis acid is a species, which is deficient in electrons and is an electron acceptor. A Lewis base is an electron rich species and is an electron donor.
328. The presence of electron withdrawing groups at o- and p-positions with respect to halogen atom activates the aryl halides towards nucleophilic substitution reactions. As the number of electron withdrawing groups at o- and p-positions increases, the reactivity of aryl halides also increases.
329. The presence of electron releasing group at o- or p-position with respect to halogen deactivates aryl halides towards nucleophilic substitution reactions. The effect is less in case of m-substituted aryl halides. Hence, the rate of reaction towards nucleophilic substitution is highest in chlorobenzene as there is no electron releasing group and is lowest in o-chlorotoluene.
330. The presence of electron releasing group at o- or p-position with respect to halogen deactivates aryl halides towards nucleophilic substituton reactions. As the number of electron releasing groups at o- and p-positions increases, the reactivity of aryl halides decreases.
332. In option (A), halogen (–X) is replaced by iodine and the reaction is Finkelstein reaction. In option (B), the reaction is addition of HX to alkene. In options (C) and (D), halogen replaces alcoholic group and hydrogen of benzene ring respectively.
333. The given example contains two chiral carbons C 2 and C 3 (Fig. a). 1 COOH 2COOH
2 * 4 * 1 H OH H 2 OH 3 4 3 H * Cl H * 1Cl
4 3 CH3 CH 3 (a) (b) From figure (b), it can be seen that, the four substituents attached to C 2 can be arranged in order of priority |
as OH (1), COOH (2), HC3 CH Cl (3) and H(4). Similarly, the four substituents attached to C 3 | can be arranged in order of priority as Cl(1), HOOC CH OH (2), CH3(3) and H(4). In case of both C2 and C3, the order of priority is in anticlockwise direction. But the lowest priority group (H) is on the horizontal line i.e., projects above the plane of paper. Therefore, the actual configuration is opposite to that of the observed configuration. Thus, C2 and C3 are assigned R-configuration i.e., (2R, 3R).
335. The given diagram is an energy profile diagram for SN1 mechanism. SN1 mechanism involves two steps. Methyl bromide undergoes hydrolysis by SN2 mechanism.
336. CH3 CH2 CH2 Br + NaCN CH3 CH2 CH2 CN + NaBr This is SN2 reaction for which polar aprotic solvent is suitable for faster rate of reaction. Among the given options, DMF is a polar aprotic solvent.
CH3 H C N CH3 O N,N-Dimethylformamide
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Chemistry Vol ‐ 2.2 (Med. and Engg.)
Topic Test
1. Alkyl fluorides can be best prepared by ______. CH3 (A) direct halogenation | (B) reaction of alcohols with hydrogen fluoride (C) CH3 CH CH2 Cl (C) halogen exchange CH (D) all of these 3 | 2. Which of the following is Grignard’s reagent? (D) H3C C Cl (A) Alkyl calcium halide | (B) Alkyl calcium hydride CH3 (C) Alkyl magnesium halide 8. Which of the following compounds can occur (D) Alkyl magnesium hydride in enantiomeric forms? 3. Alcohols react with conc. halogen acids to give H C H alkyl halides. The reactivities of the reactants 3 (A) follow the order ______. C
(A) HCl > HBr > HI ; primary alcohol > H3C COOH secondary alcohol > tertiary alcohol (B) HCl > HBr > HI ; tertiary alcohol > H secondary alcohol > primary alcohol H3C (B) (C) HI > HBr > HCl ; primary alcohol > C
secondary alcohol > tertiary alcohol H C COOH (D) HI > HBr > HCl ; tertiary alcohol > 5 2
secondary alcohol > primary alcohol H H C 4. Which of the following reactions will convert (C) 3 aniline into chlorobenzene? C
(A) Hydrolysis H COOH (B) Chlorination H (C) Monochlorination H5C2 (D) Diazotization followed by action of (D) C Cu2Cl2/HCl H COOH 5. Formation of an alkyl halide from another alkyl halide with change in halogen atom is nothing 9. Which one of the following compounds is NOT but a/an ______reaction. a vicinal dihalide? (A) addition Cl Cl (B) elimination | | (C) nucleophilic substitution (A) CH2 CH CH3
(D) electrophilic substitution Cl 6. Chlorine gas on exposure to UV light gives | (B) CH CH CH CH Cl ______. 3 2 2 (A) free radicals (C) ClCH2 CH2Cl (B) anions Cl (C) cations | (D) cations and anions (D) CH CH CH CH Cl 3 2 2 7. Which of the following has the highest boiling 10. Bromomethane on treatment with KOH and point? water undergoes ______. (A) CH3 CH2 CH2 CH2 Cl (A) SN1 mechanism 2 CH3 (B) SN mechanism | (C) SE1 mechanism (B) CH3 CH2 CH Cl (D) SE2 mechanism
90
Chapter 10: Haloalkanes and Haloarenes
CH3 I 17. Haloalkanes contain halogen atom(s) attached to the sp3 hydridised carbon atom of an alkyl
11. IUPAC name of H3C CH CH2 CH CH3 is group. Identify haloalkane from the following ______. compounds. (A) 2-Iodo-4,4-dimethylbutane (A) Isobutyl chloride (B) 4-Iodo-2-methylpentane (B) Vinyl chloride (chloroethene) (C) 3-Iodo-4-methylhexane (C) 2-Chloroacetophenone (D) 2-Iodo-4-methylpentane (D) 3-Chlorotoluene
18. Which of the following is secondary alkyl 12. When C2H5Br is treated with excess of bromide? alcoholic NH3, the major product obtained is ______. (A) (CH3)3CBr (A) ethylamine (B) (CH3)3CCH2Br (B) diethylamine (C) CH3CH(Br)CH2CH3 (D) (CH) CBrCH CH (C) triethylamine 3 2 2 3 (D) tetraethyl ammonium bromide 19. Which of the following compounds is haloarene? PBr3 aq.KOH 13. (CH3)2CHOH A B (A) C6H13 – C(Br)3 In the above reaction B is ______. (B) p-CH3CHCl(C6H4)CH2CH3 (A) propan-2-ol (B) propan-1-ol (C) o-BrH2C(C6H4)CH3 (D) p-Br(C H )CH Br (C) propene (D) propane 6 4 2 20. A mixture of 1-bromopropane and 2- 14. Match the compounds given in column I with bromopropane on heating with alcoholic the uses given in column II. solution of sodium hydroxide gives ______. Column I Column II (A) propene i. Dichloromethane a. Insecticide (B) n-propyl alcohol ii. Chloroform b. Antiseptic (C) isopropyl alcohol iii. Triiodomethane c. Metal cleaning (D) a mixture of n-propyl alcohol and and finishing isopropyl alcohol solvent Answers to Topic Test d. Preparation of freon 1. (C) 2. (C) 3. (D) 4. (D) refrigerant R- 5. (C) 6. (A) 7. (A) 8. (B) 22 9. (D) 10. (B) 11. (D) 12. (A) 13. (A) 14. (B) 15. (A) 16. (B) (A) (i) – (d), (ii) – (b), (iii) – (a). 17. (A) 18. (C) 19. (D) 20. (A) (B) (i) – (c), (ii) – (d), (iii) – (b). (C) (i) – (d), (ii) – (a), (iii) – (b). (D) (i) – (a), (ii) – (b), (iii) – (d).
15. For a given alkyl group, the boiling points of alkyl halides follow the order: (A) RI > RBr > RCl (B) RCl > RBr > RI (C) RI > RCl > RBr (D) RBr > RI > RCl
16. Among the isomers of C5H11Cl, which one has a chiral centre? (A) 1Chloro2,2dimethylpropane (B) 2Chloropentane (C) 2Chloro2methylbutane (D) 3Chloropentane 91