Dr. Anil Kumar Ojha, Assistant Professor, Department of Chemistry B.Sc Part -1 ( Honour’s and Subsidiary)
Halogen derivatives
Compounds derived from hydrocarbon by the replacement of one or more hydrogen atom (s) by the halogen atom (s) are known as halogen derivatives. The halogen derivatives of alkanes, alkenes, alkynes and arenes are known as alkyl halides (haloalkanes), alkenyl halides (haloalkenes), alkynyl halides (haloalkynes) and aryl halides (halobenzenes) respectively. Halogen derivatives may be classified as follows :
Organic Compounds Containing Halogen (S)
Alkyl halide Aryl halide
Mono haloalkanes Poly haloalkanes
AlkylPrimary Halides alkyl halide. Secondary alkyl halide Tertiary alkyl halide
(1) Monohaloalkanes : These are halogen-substituted alkanes. The general formula is C n H 2n+1 X , where X is any halogen. Alkyl halides have been further classified, on the basis of nature of the carbon atom to which halogen is attached into following categories :
(i) Primary alkyl halide : The halogen is attached to primary carbon atom, i.e. RCH 2 X . R (ii) Secondary alkyl halide : The halogen is attached to secondary carbon atom, i.e. CHX . R R (iii) Tertiary alkyl halide : The halogen is attached to tertiary carbon atom, i.e. R C X . R Preparation of alkyl halides : (1) By free radical halogenation of alkanes, e.g.
UV light CH 4 + X 2 ⎯ ⎯ ⎯⎯ →CH 3 X + HX (where, X = Cl, Br only) Excess Note: ❑ The formation of di, tri, tetra-halides etc. (as side products) also takes place. ❑ Iodination of alkanes is a reversible process, therefore, formation of iodoalkanes is possible only in the presence of oxidising agents such as HIO 3 , conc. HNO 3 etc., which oxidises the HI produced. Iodination with methane does not take place at all. ❑ Fluorination of alkanes takes place with rupture of C – C bonds in higher alkanes. Therefore alkyl fluorides are generally prepared by halide exchange reactions. Alkyl fluorides are, therefore, prepared indirectly by heating alkyl chlorides with inorganic fluorides e.g. AsF3 , SbF 3 , AgF, Hg 2 F2 etc. (Swarts reaction)
2CH 3 CH 2 Cl + Hg 2 F2 → 2CH 3 CH 2 F + Hg 2 Cl 2 (2) By reaction of halogen acids (HX) to alkenes, e.g. RCH = CH 2 + HX → R C H − CH 3 | X
(3) By reaction of PCl5 or PCl 3 or SOCl 2 with alcohol :
RCH 2 OH + PCl5 → RCH 2 Cl + POCl 3 + HCl ; 3 RCH 2 OH + PCl 3 → 3RCH 2 Cl + H 3 PO3
Pydrine RCH 2 OH + SOCl 2 ⎯ ⎯ ⎯⎯ → RCH 2 Cl + HCl + SO 2
ZnCl 2 (4) By reaction of alcohols with halogen acids: RCH 2 OH + HX ⎯ ⎯ ⎯ → RCH 2 X + H 2 O
(5) By Hunsdiecker reaction: RCOOAg + X 2 → RX + AgX + CO 2 Note: ❑ This reaction proceeds through free radical mechanism.
❑ The yield of alkyl halide is 1o 2 o 3 o . ❑ This reaction is used to reduce the length of carbon chain. ❑ Only bromides are obtained in good yield in this reaction. The chlorides can also be obtained by this reaction but the yield is very low. Iodides however cannot be obtained because these form esters with silver salts.
(Birnbaurn Simonini reaction). 2RCO 2 Ag + I2 → RCO 2 R + CO 2 + 2AgI Properties :
Physical properties: (1) Lower members like CH 3 Cl, CH 3 Br, C 2 H 5 Cl are colourless gas while some of the higher members are colourless sweet smelling liquids. Higher homologues are odourless solids. (2) The boiling point and density is in the order R − F R − Cl R − Br R − I Example CH 3 F CH 3 Cl CH 3 Br CH 3 I
B.P. (o C) –78.4 – 23.8 3.6 42.5
Density (gm ml −1 ) 0.84 0.92 1.73 2.28
(3) They have very low solubilities in water but miscible with non-polar solvents. (4) These have a cumulative toxicity and are carcinogenic. Chemical properties : (1) Alkyl halides undergo nucleophilic substitution reaction :
In general, RCH 2 − X + Y → RCH 2 − Y + X
Where, Y = Cl , Br ,OH , RO , RS ,CH 3 COO ,CN , R , NH 2 , N 3 , NO 2 Similarly alkyl halide reacts with compound with general formula ‘HY’ as below
RX + HY → RY + HX , Where, HY = H 2 O, ROH, RCOOH, NH 3 The general reactions are as follows :
R – OH (Alcohol)
R – OR' (Ether)
R – SH (Thiol)
R – CN (NItrile)
R – X
, Pri or Sec) R – SR' (Thioether) (X = Cl, Br or I)
(Alkyne)
(Ester)
(quaternary ammonium halide)
Note: ❑ Reactivity of halides towards SN 1 –mechanism is 3 o 2 o 1 o and SN 2 –mechanism is 1 o 2 o 3 o . ❑ Polar solvents favour while non polar solvents favour mechanism. ❑ High concentration of nucleophiles favour while low concentration mechanism.
❑ Rate of reaction in mechanism do not depend upon the nature of the attacking nucleophile. However, in , rate depend on the strength of the attacking nucleophile. (2) Alkyl halides undergo dehydrohalogenation which is a -elimination reaction in which halogen is lost from -carbon atom while H is lost from -carbon atom. This reaction is governed by Saytzeff’s rule. Note: ❑ Ease of dehydrohalogenation among halides : .
Br CH 3 | | (3) Isomerization : / anhy. AlCl3 CH 3 CH 2 CH 2 CH 2 Br ⎯ ⎯ ⎯ ⎯ ⎯⎯ →CH 3 CH 2 − CH − CH 3 + CH 3 − C− Br | CH 3 (2) Dihaloalkanes : These have been classified in two categories namely Gem-dihalide in which the two halogens are attached to same carbon atom and vic-dihalide in which the two halogens are attached to
Cl CH 2 Cl adjacent carbon atom, e.g. CH CH ; 3 Cl | Ethylidene chloride (gem-dihalide) CH 2 Cl Ethylene chloride (vic-dihalide) The vic-dihalides are prepared by reaction of halogens to alkene where as gem-dihalide are prepared by
CH 2 Cl reaction of PCl5 to aldehyde. ; CH 3 CH = O + PCl5 → CH 3 CHCl 2 + POCl 3 + HCl CH 2 = CH 2 + Cl 2 → |
CH 2 Cl The gem and vic-dihalides can be distinguished by reaction with aq. KOH :
CH 2 Cl CH 2 OH | + aq. KOH → | ;
CH 2 Cl CH 2 OH diol OH CH CHCl + aq. KOH → CH CH ⎯ ⎯−H⎯2O →CH CHO 3 2 3 OH 3 (3) Trihaloalkanes
(i) Chloroform (CHCl 3 ) : It can be prepared by reaction of acetone or ethyl alcohol with bleaching powder.
2CH 3 CH 2 OH + 8Cl 2 + Ca(OH )2 → 2CHCl 3 + (HCOO )2 Ca + 10 HCl
The important properties of CHCl 3 (sweet smelling liquid) are as follows :
sun light (a) On oxidation it yields phosgene gas which is very poisonous: CHCl 3 + [O] ⎯ ⎯ ⎯⎯ → COCl 2 + HCl air Phosgene For medical purpose, the chloroform is filled in coloured bottles and kept in dark to avoid sunlight. Further the bottles are filled upto neck to avoid oxygen (air) and 1% C 2 H 5 OH is added to convert phosgene into harmless ethyl carbonate.
(b) Upon alkaline hydrolysis with KOH, chloroform gives potassium formate. OH KOH CHCl 3 + 3KOH → CH OH → HCOOH ⎯ ⎯ ⎯ → HCOOK + H 2 O OH (c) With acetone it forms chloretone which is used as a hypnotic. (d) When chloroform is warmed with a primary amine and alc. KOH, it forms isocyanide. The reaction is
called carbylamine reaction, RNH 2 + CHCl 3 + 3KOH → RNC + 3KCl + 3H 2 O (R = alkyl or aryl) The carbene is the reaction intermediate.
(e) When CHCl 3 is treated with phenol in the presence of NaOH, salicyladehyde is formed. The reaction is called Reimer Tiemann reaction.
OH OH OH
CHO
+ CHCl 3 + 3NaOH ⎯⎯ → + 3NaCl + Major product CHO
O || (ii) Iodoform : When methyl ketones or alcohols having following structure reacts with I2 and − C− CH 3 o alkali (aq. NaOH or aq. Na 2 CO 3 ), a yellow solid of melting point 119 C is obtained. The solid is called H iodoform and the reaction is called Iodoformic. | (Where, R = H, alkyl, aryl) R − C − CH 3 | OH As a rule all methyl ketones and alkanol-2 give +ve iodoform test, e.g.
Alcohols and ketones which gives +ve iodoform test –ve Iodoform test (due to absence of − CH 3 gp.) H CH 3 CH 2 OH | H − C − OH | H
CH 3 − CH − CH 3 H | | CH 3 CH 2 C − OH OH | H
C6 H 5 COC 6 H 5 , C6 H 5 COC 6 H 11
The reaction is as follows : C2 H5OH + 4 I2 + 6 NaOH → CHI 3 + HCOONa + 5 NaI + H 2O H | R The mechanism is as follows : R − C − CH 3 + NaOI → C = O + NaI + H 2 O | CH 3 OH
R R C = O + 3NaOI → R C = O + 3NaOH ; C = O + NaOH → CHI + RCOONa CH I C 3 3 3 I3 C Yellow ppt. (4) Tetrahaloalkanes
(i) Freons is trade name of fluorochloromethanes having general formula CFX Cl y , where (x + y = 4).
(ii) The prefix per means that all H atoms of the hydrocarbons are replaced by fluorine atoms, e.g. perfluorooctane CF3 (CF2 )6 CF3 . These are chemically inert due to electrostatic force of attraction between + on C and − on F.
(iii) When CCl 4 instead of CHCl 3 is used in Reimer Tiemann reaction the product is salicylic acid.
OH OH OH
+ CCl 4 + 4 NaOH ⎯⎯ → COOH +
Salicylic acid (Major product) Aryl Halides. COOH
These are organic compounds in which halogen is directly attached to benzene ring, e.g.
Cl These can be prepared by reaction of halogen to aromatic hydrocarbon. Cl
+ + HCl chlorobenzene
(2) By Sandmeyer’s reaction : When acid solution of diazonium salt is added to a solution of cuprous halide dissolved in corresponding halogen acid, aryl halides are obtained, e.g.
Cl I
Cu 2Cl2 ⎯ ⎯ ⎯⎯ → + N 2 ; + KI ⎯⎯ → + N 2 + KCl HCl
Br
+ HBr ⎯ ⎯Cu 2⎯Br⎯2 → + HCl + OH Cl
PCl5 (3) By reaction of phenol with PCl5 , ⎯ ⎯⎯ → + POCl 3 + HCl Properties Physical properties : Aryl halides are generally colourless liquid or crystalline solid. They are soluble in organic solvents but insoluble in water. The boiling points and melting points are in the following order : Aryl iodides > Bromides > Chlorides > Fluorides
Chemical properties : The properties can be discussed under two categories : (1) Properties due to benzene ring : As halogen (s) are ortho para directing, the reactions are as follows :
Cl Cl Cl
⎯ ⎯Cl2 ⎯/ FeCl⎯⎯3 → Cl +
o-dichloro benzene Cl p-dichloro benzene
Cl Cl Cl Cl
+
+ +
(2) Properties due to halogen atom : Unlike alkyl halide, the aryl halides do not easily undergo nucleophile substitution reaction since halogen atom is tightly attached to benzene ring, e.g. Cl OH
o ⎯2 Na⎯, CH⎯⎯3 I ⎯ ⎯ ⎯NaOH⎯ ,⎯300⎯⎯C → + NaCl Wurtz-Fittig's 200 atm reaction Phenol CN
NH ,Cu O ⎯Cu2⎯(CN⎯)2 ⎯ ⎯ ⎯ ⎯3 ⎯2⎯ → Pyridine, 200 o C
When chlorobenzene is heated with chloral in the persence of conc. H 2 SO 4 , D.D.T. is formed. it is non biodegradable. It slowly changes to another compound, DDE by loss of a HCl molecule.
Cl Cl Cl H 2SO 4 CCl 3 CH = O + ⎯ ⎯ ⎯⎯ →CCl 3 CH ⎯ ⎯−HCl⎯ →CCl 2 = C −H 2O Cl Cl Cl p, p-dichlorodiphenyl (D.D.E) trichlloroethane (D.D.T)(insecticide) Alcohols. Alcohols are compounds of the general formula ROH, where R is alkyl or substituted alkyl group. The group may be open chain or cyclic, it may contains double bond, a halogen atom, an aromatic ring or additional hydroxyl groups, e.g.
OH
CH 3 CH 2 CH 2 CH 2 OH ; ; ; ; Butanol -1 Benzyl alcohol Cyclohexanol Cinnamyl alcohol (1-cyclo hexene) methanol
Alcohols can be classified as, ALCOHOLS
Monohydric alcohols Dihydric alcohols Trihydric alcohols contain (1-OH group) contains (2-OH groups) contains (3-OH groups)
e.g.Glycol e.g. Glycerol
Primary Secondary Tertiary
(1) Monohydric alcohols Preparation : (i) By oxymercuration - demercuration reaction of alkene :
− THF NaBH 4 / OH − CH 3 − CH = CH 2 + (CH 3 COO)2 Hg + H 2O ⎯ ⎯⎯ →CH 3 − CH − CH 2 ⎯ ⎯ ⎯ ⎯ ⎯ →CH 3 − CH − CH 3 + Hg + CH 3 COO CH 3COOH | | −1 / 2B2 H6 | OH OH HgOCOCH 3 2-propanol
Note: ❑ It is fast and convenient. The addition of water to an alkene is anti-Markownikoff and free from rearrangement. (ii) Hydroboration of alkene : 1 3CH − CH = CH + B H → (CH − CH − CH ) B ⎯ ⎯3 H⎯2O⎯2 → 3CH − CH − CH − OH + H BO 3 2 2 6 3 2 2 3 − 3 2 2 3 3 Propene 2 Tripropyl boron OH / H 2O 1-propanol Diborane Note: ❑ In this reaction addition of water to an alkene is syn, anti-Markownikoff and free from rearrangement.
dil. H 2SO 4 (iii) Hydrolysis of ethers : C 2 H 5 − O − C 2 H 5 + H 2 O ⎯ ⎯ ⎯ ⎯⎯ → 2C 2 H 5 OH (iv) By reaction of Grignard reagent with formaldehyde/ other aldehydes/ ketones (a) When Grignard reagent reacts with HCHO, it forms primary alcohol. R R | | X H − C = O + RMgX → H − C − OMgX → H − C − OH + Mg | | | OH H H H (b) Oxirane on reaction with Grignard reagent also forms primary alcohol.
O X + RMgX → RCH CH OMgX → RCH CH OH + Mg 2 2 2 2 OH (c) Any aldehyde except formaldehyde when treated with Grignard reagent forms secondary alcohol. R R | | X R − C = O + RMgX → R − C − OMgX → R − C − OH + Mg | | | OH H H H (d) When 2-alkyl oxirane reacts with Grignard reagent, secondary alcohol is formed. The ring is opened from least hindered site.
1 CH 3 CH 3 O | | X + RMgX → RCH 2 − CH − OMgX → RCH 2 − C − OH + Mg | OH 3 2 H (e) When ketone reacts with Grignard reagent, it forms tertiary alcohol. R R R R | R | X C = O + RMgX → C− OMgX → C− OH + Mg R R R OH (f) When 2, 2-dialkyl oxirane reacts with Grignard reagent, it forms tertiary alcohol.
R R O 1 1 | | R1 X + RMgX → RCH 2 − C − OMgX → RCH 2 − C − OH + Mg | | R 2 OH R2 R2 OH (v) By reduction of carbonyl compounds : C = O + 2H → C H
The reduction can be carried out by using H 2 /catalyst like Ni, Pt, Pd, LiAlH 4 , NaBH 4 , NaH, LiH. When the reduction is carried out by using metal/solvent combination it is known as Bouveault blanc recduction. The intermediates are : R − C− OEt ; R − CH − OEt ; R − CH . In case of metal hydride the reaction intermediate | | | O O O is R 2 − C − O | H
(vi) By hydrolysis of ester : R1COOR 2 + KOH → R1COOK + R 2 OH (vii) By reaction of alkyl boride with CO : O || OH LiBH 4 OH R3 B + CO → R 2 − B − C− R ⎯ ⎯ ⎯⎯ → R 2 BCH ⎯ ⎯ ⎯ → RCH 2 OH R H 2O
(viii) By reaction of primary amines with nitrous acid : RNH 2 + HNO 2 → ROH + N 2 + H 2 O Zymase (ix) From glucose : C6 H 12 O6 ⎯ ⎯ ⎯⎯ → 2CH 3 CH 2 OH + 2CO 2 (ix) Industrial method for methanol and ethanol are as follows :
Heat ZnO +CrO3 CH 4 + H 2 O ⎯ ⎯⎯ →CO + 3H 2 ; CO + 2H 2 ⎯ ⎯ ⎯ ⎯⎯ →CH 3 OH 400 o C, 200 atm
H 3 PO4 for ethanol we take ethylene obtained from petroleum : CH 2 = CH 2 + H 2 O(steam) ⎯ ⎯ ⎯⎯ →CH 3 CH 2 OH 300 -350 o C It is an Oxo process. Physical properties: Alcohols are liquid with high boiling point due to intermolecular hydrogen bonding. Methanol, ethanol, both propyl alcohols and tert-butyl alcohol are completely miscible with water. Chemical properties : Chemical properties of alcohols can be discussed under three categories :
(1) Reactions involving RCH2 OH cleavage : The reactivity order : Allyl > benzyl > Tert > Sec > Pri
(i) RCH 2 OH + HX → RCH 2 X + H 2 O ; R may rearrange, Reactivity : HI > HBr > HCl
(ii) RCH 2 OH + PX 3 → RCH 2 X + H 3 PO3 (iii) RCH 2 OH + PCl5 → RCH 2 Cl + HCl + POCl 3
(iv) RCH 2 OH + SOCl 2 → RCH 2 Cl + SO 2 + HCl (v) 3RCH 2 OH + PCl 3 → 3RCH 2 Cl + H 3 PO3
(2) Reactions involving RCH2 – O H cleavege : The order of reactivity :
CH 3 OH > Primary > Secondary > Tertiary (i) Alcohols react with metals like Na to form alkoxide and evolve . This reaction shows acidic nature of 1 alcohols : RCH − O − H + M → R O M + H Where, M = Na , K, Al, Mg, Zn 2 2 2
The relative acidity is : H 2 O ROH RC CH H 2 NH 3 RH The relative basicity is : R N H 2 H RC C R O O H
H (ii) Ester formation : ROH + RCOOH ⎯ ⎯ → RCOOR + H 2 O (3) Miscellaneous : (i) Oxidation : (a) Oxidation of alcohols : Number of -H atoms Oxidation rate of alcohol [O] Product yield e.g. R − CH 2 − OH ⎯ ⎯⎯ → RCHO + H 2 O (Primary alcohol or Alcohol with 2-H )
R R R CHOH ⎯ ⎯[⎯O] → C = O + H O ; R C OH ⎯ ⎯[⎯O] → No oxidation under R R 2 Secondary alcohol R or Tertiary alcohol Alcohol with 1-H or alcohol with no -H atom ordinary conditions (ii) Iodoform test : Alcohols having following structure gives positive iodoform test. H | R − C − CH 3 i.e. As a rule all alkanol-2 gives positive iodoform test. | OH
(iii) Reaction with Lucas reagent (anhydrous ZnCl 2 and conc. HCl) : Lucas reagent has been used to distinguish primary, secondary and tertiary alcohols : (a) Allyl alcohol or tertiary alcohol ⎯ ⎯Lucas⎯ Reagent⎯ ⎯⎯ → White turbidity at once The alkyl halide so formed is insoluble in the medium hence turbidity appears. (b) Secondary alcohol + Lucas reagent → Turbidity after 5 minutes (c) primary alcohol + Lucas reagent → No turbidity Cu (iv) Dehydrogenation : RCH 2 OH ⎯ ⎯⎯ → RCHO + H 2 300 o C R R CH CHOH ⎯ ⎯Cu⎯ → C = O + H ; CH − C − OH ⎯ ⎯Cu⎯ → 3 C = CHCH o 2 3 o 3 R 300 C R 300 C CH 3 CH 3 C2H5
(v) When ethanol is heated with Al 2 O3 , it forms different depending upon temperature, e.g.
CH CH OH ⎯ ⎯Al2O⎯⎯3 →CH = CH + H O ; CH CH OH ⎯ ⎯Al2O⎯⎯3 →CH CH OCH CH + H O 3 2 o 2 2 2 3 2 o 3 2 2 3 2 350 C 200 C Diethyl ether
(vi) Primary, Secondary and Tertiary alcohols can be distinguished by Victor Meyer’s Method :
HI AgNO 2 OHNO NaOH (a) RCH 2 OH ⎯ ⎯⎯ → RCH 2 I ⎯ ⎯ ⎯⎯ → RCH 2 NO 2 ⎯ ⎯ ⎯⎯ → R − C− NO 2 ⎯ ⎯ ⎯⎯ → R − C− NO 2 Primary alcohol || || NOH NONa Nitrolic acid Red sodium salt or nitrolic acid R R R R HI AgNO 2 OHNO NaOH (b) CHOH ⎯ ⎯⎯ → CHI ⎯ ⎯ ⎯⎯ → CH − NO 2 ⎯ ⎯ ⎯⎯ → C − NO 2 ⎯ ⎯ ⎯⎯ → Blue colour R R R R | Secondary alcohol NO Pseudo nitrolic acid R R (c) R C OH ⎯ ⎯⎯HI → R C I ⎯ ⎯HNO⎯⎯2 → No reaction R R
(2) Dihydric alcohol Chemical Properties Glycol (Ethane – 1, 2- di-ol) CH2 − ONa CH 2ONa | | Preparation Na Na CHOH ⎯ ⎯ ⎯ → CH − OH Lab. Method | | CH − OH CH − ONa 3CH2 = CH2 + 2KMnO4 + 4H2O → 2 2 Mono sodium Disod. glycerolate glycerolate CH 2 − OH + 2MnO 2 + 2KOH HCl gas 3 | CH − Cl CH − OH limited 2 2 CH 2 − OH | | (Glycol) CHOH + CHCl | | Chemical Properties CH2OH CH2 − OH HCl gas CH2 − Cl CH2 − Cl CH2ONa excess Na 323K | | | − + − 433 K CH2ONa CH Cl CH OH Disodium glycolate | | CH2 − OH CH2 − Cl HI CH − I CH CH CH2 −Cl 2 2 3 HCl 433K | || | | HI − → ⎯ ⎯ ⎯ → − 473 K CH −Cl CH I CH CH I 2 | | | Ethylene chloride CH2 − I CH2 − I CH2 − I
CH3 CH3 CH2 − I | | HI HI | ⎯ ⎯ ⎯ →CH2 = CH2 ⎯ ⎯ ⎯ → ⎯ ⎯ ⎯ → − −I − CH CH I CH − I 2 I 2 || | CH − OH 2 2 CH CH | . CH − ONO 2 3 Conc HNO3 2 2 CH − OH Isopropyl − | 2 CH2 OH | iodide (Glycol) Conc.H2SO4 CH − ONO CH2 − ONO2 2 2 CH − OH Conc.HNO3 Glycol dinitrate | | Conc.H2SO4 CHONO CH COOH CH − OCOCH CH2 − OH 2 3 2 3 | Glycerol trinitrate | H SO CH2ONO2 2 4 CH2 − OCOCH3 Glycol diacetate CH2 − ONO2 dil HNO3 COOH COOH CH3COOH | + | | oxidation Glycerol trinitrate CH2 −OH COOH CHONO2 Glycollic acid Oxallic acid | CH2ONO2 HIO4 2 + + oxidation HCHO H2O HIO3 Dehydration KHSO4 or P2O5 CH2 = CH − CHO Acrolein or Acroldehyde CH3CHO CH 2O ]p- toulene Sulphuric | CH − CH3 acid CH 2O Cyclic acetal KMnO4 oxidation CH COCH 3 3 COOH +CO + H O CH − O CH | 2 2 p- toulene 2 3 | C COOH sulphuric acid CH2 − O CH3 cyclic ketal CH −OH FeSO4 /H2O2 CHO 2 oxidation | | CHOH + = Fenton’s reagent C O | | CH2 − OH CH2 −OH Glyceraldehyde Dihydroxyacetone
dil HNO3 COOH COOH | | oxidation CHOH + CHOH | | CH2 − OH COOH Glyceric acid Tartonic acid
Glycerose
(Glycerol)
Phenols. Phenols are the organic compounds in which − OH group is directly linked to aromatic ring system. The simplest formula of such compound is phenol(C6 H 5 OH ). Structure of some common phenols is as follows : OH OH OH OH
; ; ;
Phenol o-Cresol m-Cresol
p-Cresol OH OH OH OH Catechol OH Resorcinol OH Quinol Preparation : (1) Phenols can be prepared by reaction of benzene sulphonic acid with NaOH.
OH
(i) NaOH + NaOH ⎯⎯ → + H 2O ⎯ ⎯ ⎯ ⎯ → + Na 2 SO 3 (ii) HCl (2) Phenols can also be obtained by warming diazonium salt solution with hot dilute acid solution.
OH
+ HX + N2
Cl OH
300 o C (3) Phenols can be prepared from aryl halides, + NaOH ⎯ ⎯ ⎯⎯ → + NaCl 200 atm
OH OH (4) By reaction of phenolic acid with soda lime. + NaOH (CaO) → + Na2CO3 COOH
Physical properties : (1) Phenols are colourless liquid or crystaline solids. They turn pink due to oxidation. (2) Phenols are weak acid and have high boiling point due to intermolecular hydrogen bonding. Chemical properties : Due to benzene ring
similar to alcohols Chemical Due to – OH group different from alcohols Miscellaneous
(1) Properties due to benzene ring : We have already discussed that − OH group is o-p directing. The reactions due to benzene ring are briefly as follows :
OH OH NO2 dil. H2SO4, 200 C + HNO3 p- notrophenol o- notrophenol NO2
OH
O2N NO2 conc. NHO3 H2SO4,
NO2 2, 4, 6 – trinitrophenol (Picric acid)
OH OH
SO3H H2SO4 + 20oC
o- phenol sulphonic acid (major product) SO3H
OH OH
SO3H H2SO4 + 100oC
OH SO3H major product
Note: ❑ It is important to note that Br2 in water yields tri-bromo derivative but in CCl 4 only monobromoderivative. It is because of greater ionisation of in water in comparison to that of in which the ionisation is very small. (2) Reactions due to – OH groups which are similar to alcohols (i) Reactions with acids : Ester formation occurs. OH OOCR
+RCOOH +H2O OH OOC. C6H5 Esterification can also be carried out with acid chloride. +C6H5COCl Benzoyl chloride Phenyl benzoate The reaction is known as Schotten-Baumann reaction.
Further, when phenyl esters are treated with anhyd. AlCl 3 they undergo Fries rearrangement which involves migration of acyl group to o– or –p– positions of the ring with respect to − OH group, to form phenolic ketones.
OOC.CH3 OH OH
COCH3 +
Steam volatile COCH3
o & p – hydroxy acetophenone
OH OH Cl NH2 (ii) +HCl + POCl3 (iii) + H2 O
Chlorobenzene OH AnilineOR
(iv) Phenols with alkyl halide forms ether. +HX
(3) Reactions in which phenol differs from alcohols
(i) Phenols gives colour with FeCl 3 due to formation of the complex, the exact nature of which is not known, but iron is hexavalent. The colours are as follows : Phenol, Resorcinol, Salicylicacid : Violet colour p-Hydroxy benzoic acid Catechol, o-cresol : Green m-cresol, p-cresol : Blue or blue-violet Quinol : Red OH (ii) Phenols are acidic in nature.
(iii) When phenol is distillet with Zn, − OH group is removed. + ZnO
Reimer Tiemann Reaction : When phenol is refluxed with chloroform and NaOH, it forms salicylic aldehyde as major product. OH OH OH
CHO + Salicylic acid (Major product) CHO
However, if CCl 4 / NaOH is used the major product is salicylic acid. OH OH OH COOH +
Salicylic aldehyde (Major Product) COOH The reaction intermediates are as follows : ⊝ ⊝ ⊝ O O O O
H CHCl2 ; CCl2 ; ; ⊝
CHCl2 Salicylic acid is of great importance since it has been used to prepare some compounds.
OH OH OCOCH3
COOCH3 COOH Methyl salicylate (Oil of winter green) Acetyl salicylic acid (aspirin) (used as analgesic)
o Kolbe reaction or Kolbe-Schmidt reaction : When sodium phenoxide reacts with CO 2 at 120 C under 5-7 atmosphere followed by acidification with H , it forms salicylic acid.
ONa OH OH
120 o C COONa H COOH + CO 2 ⎯ ⎯ ⎯⎯ → ⎯ ⎯ → 5 −7 atm
Salilylic acid The reaction intermediate (s) are : ⊝ ⊝ O O OH ⊝ ⊝ COO COO ; ;
Phthalein reaction : OH OH OH OH
+
O ||C C O O C C ||O ||O Phenolphthalein Phthalic anhydrolic note that H para to OH is removed. Ledrer-Manasse reaction : When phenol reacts with formaldehyde in the presence of acid or alkali, it forms bakelite, a thermostat polymer. OH OH OH OH
–H2O
CH2OH CH2 OH CH Bakelite + 2
OH Liebermann’s nitroso reaction
Phenol ⎯ ⎯NaNO⎯ 2⎯/ conc.⎯ H⎯2SO⎯⎯4 → deep green or blue colour ⎯ ⎯H 2⎯O → Red ⎯ ⎯NaOH⎯⎯ → green or blue colour The reactions are as follows : OH OH O
NO NOH p-nitrosophenol quinone monoxime
; H2SO4
OH O ⊕ OH O
N
N– OH N– OH
OH OH
H Nitrosation : + HNO 2 ⎯ ⎯⎯ → 10 o C NO p-nitrosophenol
OH OH
Ni Hydrogenation : + 3H 2 ⎯ ⎯⎯ → 150 −200 o C Cyclohexanol Oxidation : Phenols turn pink or red or brown when exposed to air and light due to slow oxidation. The exact nature of these oxidation product is not known; but probable products are quinones and phenoquinones.
OH O O ………..
+
O O ……….. quinone Phenoquinone When phenols is oxidised with potassium persulphate in alkaline solution, it forms quinol and the reaction is known as Elbs persulphate oxidation. OH OH
O quinol Some important Points. Halogen derivatives (1) Many of alkyl halides burn with green edged flame.
(2) C 2 H 5 Cl is used as a local anaesthetic.
(3) C 2 H 5 SH (Ethyl mercaptan) is added to LPG (household cooking gas) to detect leakage. The compound has a typical smell.
(4) CCl 4 is used to extinguish fire under the name pyrene. (5) resist hydrolysis with boiling water due to non availability of d-orbitals in C.
(6) CCl 4 is an antihelminthic medicine against hookworm.
(7) C2Cl6 is a solid and is known as artificial camphor.
(8) Freon-14 is CF4 , Freon-13 is CF3 Cl , Freon-12 is CF2 Cl 2 and Freon-11 is CFCl 3 . All of these are used as refrigerant.
(9) Lindane or Gammaxane or 666 is an isomer of BHC (C6 H 6 Cl6 ) and is used as insecticide against
termites. It is obtained by reaction of Cl 2 / hv with benzene. Cl
Cl Cl hv + 3Cl 2 ⎯ ⎯ → Cl Cl
Cl BHC (10) Westron (acetylene tetrachloride) is used as a good industrial solvent for oil, fats,, paints ansd varnishes etc. (11) Iodoform has antiseptic properties because on coming in contact with organic matter of skin it decomposes to give free iodine which is an antiseptic. (12) Chloretone is a sleep inducing medicine (hypnotic). Phosgene is harmful poisonous gas used in warfare. Chloropicrin is a tear gas and insecticide. Alcohols
(1) Wood spirit : Methyl alcohol (CH 3OH ) is called wood spirit. It is obtained by distructive distillation of wood. Drinking of methanol causes blindness.
(2) Grain alcohol. Ethyl alcohol (C2 H5OH ) is called grain alcohol. It is used in preparation of various beverages, by using different percentages. (3) Proof, over proof and under proof spirit. An alcohol-water mixture containing 57.1% alcohol by volume or 49.3% by weight is called proof spirit. A sample having higher percentage of ethyl alcohol in comparison to proof spirit is called over proof spirit (O.P.) and the one having lower percentage is called under proof spirit (U.P.). e.g., 10 O.P. means 100 ml of given sample contains alcohol as 110 ml of proof spirit. 10 U.P. means 100 ml of given sample contains as much alcohol as present in 90 ml of proof spirit. (4) Methylated spirit of denatured spirit. Ethyl alcohol containing 5 to 10% methyl alcohol is called methylated spirit. It is unfit for drinking purposes. Widespread deaths due to liquor poisoning occur mainly due to the presence of methyl alcohol. It is also called denatured spirit. Denaturing can also be done by adding 0.5% pyridine, petroleum naptha, rubber distillate (caoutchoucine) or CuSO 4 . (5) Power alcohol. A mixture of 80% petrol, 20% ethyl alcohol with co-solvent benzene is called power alcohol. It is used to run automobiles. (6) Absolute alcohol. 100% ethanol is called absolute alcohol. It is generally prepared by azeotropic distillation. (7) Rectified spirit. Ethyl alcohol (95.87%) + water (4.13%) mixture is known as rectified spirit. (8) Pyroligneous acid contains acetic acid (10%), methyl alcohol (2.5%) and acetone (0.5%). (9) Fusel oil is mixture of n-propyl alcohol, n-butyl alcohol, n-amyl alcohol and isoamyl alcohol. (10) Argol is potassium hydrogen tartarate. It is used to manufacture tartaric acid. (11) Tincture of iodine is 2-3% alcoholic solution of iodine.
(12) In the oxidation of alcohol by acidified K 2 Cr2 O7 , its colour changes from orange to green due to the formation of Cr2 (SO 4 )3 .
(13) 3 o alcohols are resistant to oxidation due to lack of -hydrogen. (14) 1o alcohols on dehydrogenation with red hot copper form aldehydes, 2 o alcohols form ketones and alcohols form alkenes by dehydration. dry HCl (15) Fischer-Speier esterification, CH 3 COOH + HOC 2 H 5 ⎯ ⎯ ⎯⎯ →CH 3 COOC 2 H 5 + H 2 O Acetic acid Ethyl alcohol Ethyl acetate (16) The rates of esterification of three types of alcohols (RO − H) are quite different and can be employed in their distinction. The percentages of esters obtained by , and alcohols with acetic acid are 45.7%, 5.4% and 1.4% respectively (because reactivity order is 1o 2 o 3 o ). (17) In the esterification reaction, the water formed is removed by Dean and Stark apparatus. It should be noted that, bulkier the acid or alcohol, slower is the rate (due to stearic hinderance), Decreasing order of rate of reaction of differnet alcohols and acids is, CH 3 OH C 2 H 5 OH (CH 3 )2 CHOH (CH 3 )3 C − OH
HCOOH CH 3 COOH (CH 3 )2 CH − COOH (CH 3 )3 C − COOH
(18) Alcohols cannot be dehydrated using anhydrous CaCl 2 because it forms an addition compound,
CaCl 2 .4CH 3 OH . (19) The name fermentation has been derived from Latin word ferver meaning to boil, as during this process there is lot of frothing due to evolution of CO 2 and this gives the appearance of boiling liquid. Favourable conditions for fermentation are (i) Optimum temperature (25 − 30 o C), (ii) A low concentration of solution, (iii) Presence of certain inorganic compounds (like (NH 4 )2 SO 4 , phosphate etc.) which acts as food for the ferment cells. (20) Pinacol-Pinacolone rearrangement. The reaction involves dehydration of diols through the formation of carbocation intermediate which rearranges to more stable compound.
OH OH CH 3 | | | H + CH 3 − C − C − CH 3 ⎯ ⎯⎯ →CH 3 − C − C − CH 3 | | −H 2O || | CH 3 CH 3 O CH 3 Pinacol Pinacolone o o o (21) Order of acidity : H 2 O ROH (1 ) ROH (2 ) ROH (3 ) RC CH RCH 3
(22) Hydroxylation of alkene can be carried out either by cold alkaline KMnO 4 or peroxy formic acid
(HCO 2 OH ). Hydroxylation by either reagent is stereoselective and stereospecific. Cold alkaline gives syn addition and peroxy acid gives anti addition. (23) The reaction of glycol with ketone is used in the protection of keto group in the reactions where it is to be protected. (24) Oxidation of glycol with lead tetracetate give formaldehyde.
CH 2 OH | + (CH 3 COO)4 Pb → 2HCHO + 2CH 3 COOH + (CH 3 COO)2 Pb CH 2 OH (25) Glycol is used as an antifreeze for automobile radiators and as a coolant for aeroplane aviation petrol under the name prestone. (26) Glyceryl trinitrate is an inorganic ester. (27) Glyceryl trinitrate is colourless, oily liquid insoluble in water and is called Nobel’s oil.
(28) On detonating it explodes violently giving CO 2 , N 2 , O 2 as gaseous products.
4C 3 H 5 (ONO 2 )3 12CO 2 + 10 H 2 O + 6 N 2 + O 2 (29) It is a safer explosive when absorbed on keiselguhr and is known as dynamite. (30) Its mixture with cellulose nitrate is known as blasting galetin or geilgnite. (31) Its mixture with cellulose nitrate (gun cotton) and vaseline is called cordite. It is a smokeless powder. (32) Nobel’s oil is also used in the treatment of angina pectoris and asthma.
(33) Dunstan’s test for glycerol. A drop of phenolphthalein is added to approx. 5 ml. of borax solution. The pink colour appears. On adding 2-3 drops of glycerol, the pink colour disppears. The pink colour reappears on heating and disappears on cooling again. Phenols (1) Aqeous solution of phenol is also called carbolic acid. (2) 2, 4, 6-Trinitrophenol is called picric acid. (3) Phenol is stronger acid as compared to ethanol but weaker acid as compared to acetic acid. (4) Groups with +M effect and +I effect decrease the acid strength. (5) Groups with +M effect and –I effect decrease the acidity at p-position, but at m-position, the acidity is increased due to –I effect. (6) Benzene-1, 3, 5-triol is also called phloroglucinol. (7) In general, acid strength increases as Cresols << Phenol << p-Chlorophenol << m-Nitrophenol << o-Nitrophenol < p-Nitrophenol << Picric acid (8) Ph-O bond acquires double bond character due to resonance and is less susceptible to cleavage. O (9) Fries rearrangement : || O – C – CH3 OH OH
COCH3 +
Phenyl acetate o-Hydroxyacetophone COCH3 p-Hydroxyacetophone The p-isomer is formed predominantly at low temperature while high temperature favours the formation of o
(10) Phenol on condensation with resorcinol in the presence of conc. H 2 SO 4 give fluorescein.
(11) In the Reimer-Tiemann reaction, the electriophile used is : CCl 2 (dichloromethylene).
NO2
(12)NO 2 OH is also called as 2, 4, 6-trinitroresorcinol or styphinic acid. OH NO2 (13) The solubilities of o, m and p-nitrophenol in water increases in the order o m p . In fact o- nitrophenol contains intramolecular H-bonds. (14) Phenols cannot be converted into esters by direct reaction with carboxylic acids whereas alcohols can be. This is due to the fact that the esterification reaction is exothermic for alcohols but slightly endothermic for phenols. (15) In Elb’s persulphate oxidation the second − OH group is mainly introduced at the p-position but if this position is occupied than at o-positions.