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I.Alkyl Halides: Halogen Atom Is Bonded to an Alkyl Group. Ii

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I.Alkyl Halides: Halogen Atom Is Bonded to an Alkyl Group. Ii Haloalkanes and Haloarenes X ORGANO HALOGEN COMPOUNDS SP 3C-X: i.Alkyl halides: Halogen atom is bonded to an alkyl group. 3 ii. Allylic halides: SP C-X connected to carbon-carbon double bond.eg: CH2=CH-CH2X 3 iii. Benzylic halides: SP C-X connected to aromatic ring. Eg: C6H5-CH2-X SP 2C-X: 2 i.Vinyl halides: Halogen atom bonded to an SP hybridised carbon atom. Eg : CH2=CHX ii. Aryl halides: Halogen atom bonded to an SP2 hybridised carbon atom of an aromatic ring. Eg: C6H5-X Same halogen atoms are present on the same carbon atom : Geminal halides Eg: CH3-CHCl2. Same halogen atoms are present on the adjacent carbon atoms : Vicinal halides. Eg : CH2Cl-CH2Cl. 2-Bromo-2-chloro-1,1,1-trifluoroethane was given the name Halothane. Which is widely used in hospitals as anaesthetics. C-F bond is very strong, hence it is very unreactive. C-Cl bond is more reactive than C-F bond. CFCs are responsible for serious damage to the stratospheric ozone layer which filters out most of the sun’s harmful UV radiation before it can reach us on earth. In CFCs, the hemolytic fission of the C-Cl bond results Cl* radicals, which converts O3 to O2 and ClO* radicals. Hydrofluorocarbons, HFCs [Hydrofluoro alkanes,HFAs] replace CFCs. Haloalkanes is boiled with aq. NaOH, then cooled and neutralized with dil.HNO 3. then AgNO3 solution is added. A white precipitate of AgCl indicates a chloroalkane, a pale yellow precipitate indicates a bromo alkane and a yellow precipitate indicates an iodo alkane. The C-X bond is polar. The positively charged carbon atom tends to be attacked by nucleophiles. The Ar-X bond is considerably less reactive than the R-X bond. Optically active isomers related to each other as object and mirror images are called enantiomers. Equimolal mixture of enantiomers are called racemic mixture. Conversion of enantiomers in to racemic mixture are called racemisation. In the presence of a strong base, haloalkanes can undergo elimination reactions, forming alkenes. According to Saytzeff rule,the neutral substrates in elimination reactions predominantly yield the most substituted alkene, i.e., the one carrying the largest number of alkyl substituents. Presence of electron withdrawing group (NO2) at ortho and para- positions increases the reactivity of haloarenes towards replacement reactions. None of the resonating structures bear the negative charge on carbon atom bearing electron withdrawing group at meta- positions. Hence no effect on reactivity. Preparation of R-X : From hydrocarbons: Halogenations of alkanes.: Addition of alkenes with hydrogen halides.: R-CH=CH2 + H-Br →R-CH(Br)-CH3 Mechanism is in accordance with Markownikoff’s rule. From alcohols: With HX and anhydrous ZnCl2 Anhydrous ZnCl2 and con. HCl is called Lucas reagent. With phosphorus halides H r e e n d r a k u m a r : 9 4 9 6 1 0 5 8 5 4 : An Ideal Learner’s School Of Chemistry 1 Haloalkanes and Haloarenes X Better yield of R-X is obtained by using PX3 or PX5 in comparison to HX, because (i).sec- and tert- alcohols tend to dehydrate in presence of HX, and (ii).the intermediate carbocation, can undergo rearrangement in presence of HX. With SOCl2 (Darzen’s method) By halide exchange reaction: R-Cl or R-Br is heated with con.NaI in acetone. [Finkelstein reaction] R-X with mercurous fluoride,Hg2 F2 gives R-F [Swarts reaction] Preparation of Ar-X: Direct halogenations of aromatic hydrocarbons. From aromatic amines: Sandmeyer’s reaction: When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed . Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in t he replacement of the diazonium group by –Cl or –Br. C6H5 N2X + CuCl/HCl C6H5 Cl + CuBr/HBr C6H5 Br + KI C6H5 I From Phenol: C6H5 OH + PCl5 → C6H5 Cl + HCl + POCl3 R-X: Reactions. 1. Nucleophilic Substitution Reactions: : R-X warm with RCOOAg gives esters , RCOOR : R-X heated with KCN in ethanol gives cyanides, RCN : R-X heated under reflux (H2O) gives alcohols, ROH : R-X reflux with aq.NaOH gives alcohols, ROH : Heat under reflux with con.NH3 (aq) gives amines, RNH2, R2NH, etc : R-X react with R’ONa (in ethanol) gives ethers, R’OR H r e e n d r a k u m a r : 9 4 9 6 1 0 5 8 5 4 : An Ideal Learner’s School Of Chemistry 2 Haloalkanes and Haloarenes X There are two mechanisms for nucleophilic substitution. (a) SN1 – substitution nucleophilic unimolecular: The SN1 mechanism involves two steps and an intermediate carbonium ion is formed. rate determining step consist of substrate only. Hence it follows first order kinetics. i.e. Rate, r = k [RX]. (b). SN2- substitution nucleophilic bimolecular: These reactions proceed in one step and is a second order reaction with r = k[RX] [Nu]. SN1 SN2 Follows first order kinetics Follows second order kinetics Nucleophile may attack the central carbon from the front Nucleophile may attack the central carbon from the back or back side. side. When the substrate is optically active, racemic mixture Inversion of configuration occurs. will be the product. Solvents of high polarity favour. Solvents of low polarity favour. Order of reactivity of alkyl halides is 30>20>10>methyl Order of reactivity of alkyl halides is 30<20<10<methyl Tertiary alkyl halides are the common substrates primary alkyl halides are the common substrates Mild nucleophiles favour it. Strong nucleophiles favour it. Allylic and benzylic halides follows the SN1 mechanism. This is due to the stabilisation of intermediate carbocation through resonance. 2. Elimination reactions When a haloalkane with β-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from β-carbon and a halogen atom from the α-carbon atom. As a result, an alkene is formed as a product. Since β- hydrogen atom is involved in elimination, it is often called β-elimination. This reaction takes place in accordance with Saytzeff rule and it states that, the neutral substrates in elimination reactions predominantly yield the most substituted alkene, i.e., the one carrying the largest number of alkyl substituents. Ease of dehydrohalogenation among halides: 3° > 2° > 1° 3. Reduction : C2H5 –Br + H2 → C2H6 + HBr C2H5I + HI → C2H6 + I2 4. Reaction with Metals i. With Mg: R-X react with certain metals to give compounds containing carbon-metal bonds. Such compounds are known as organo- metallic compounds. Alkyl magnesium halide, RMgX, referred as Grignard Reagents, obtained by the reaction of haloalkanes with magnesium metal in dry ether. H r e e n d r a k u m a r : 9 4 9 6 1 0 5 8 5 4 : An Ideal Learner’s School Of Chemistry 3 Haloalkanes and Haloarenes X ii. with Na: Wurtz reaction. R-X react with Na in ether gives alkanes. This reaction is known as Wurtz reaction. R-X + 2Na + X-R → R-R + 2NaX iii. with Ar-X and Na : Wurtz- Fittig reaction. R-X and Ar-X react with Na gives alkyl substituted aromatic hydrocarbons. This reaction is known as Wurtz- Fittig reaction. R-X + 2Na + X-Ar →Ar-R + 2NaX Reactions of Haloarenes: 1. Nucleophilic substitution The Ar-X bond is considerably less reactive than the R-X bond ,due to 1. Resonance effect: C-X bond in Ar-X has some partial double bond character due to resonance. 2. Instability of phenyl cation: The phenyl cation formed as a result of self-ionisation will not be stabilised by resonance. 3. Difference in hybridisation of carbon atom in C—X bond: The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp3 -hybridised carbon in haloalkane with less s-chararcter. Thus, C—Cl bond length in haloalkane is greater than that in haloarene. 4. Because of the possible repulsion between the electron rich nucleophile and electron rich arenes. However, aryl halides having electron withdrawing groups (like – NO2 , -SO3H, etc.) at ortho and para positions undergo nucleophilic substitution reaction easily. Picric acid 2. Electrophilic substitution reactions: Halogens are deactivating but O, p-directing. Thus, chlorination, nitration, sulphonation and Friedel Craft’s reaction give a mixture of o- and P- chloro substituted derivatives. (i) Halogenation: H r e e n d r a k u m a r : 9 4 9 6 1 0 5 8 5 4 : An Ideal Learner’s School Of Chemistry 4 Haloalkanes and Haloarenes X (ii) Nitration: (iii) Sulphonation: (iv) Friedel-Crafts reaction: 3. Reaction with Metals: (i) Wurtz Fittig reaction; (ii) Fitting reaction: (iii) Ullmann reaction: Polyhalogen Compounds: Carbon compounds containing more than one halogen atom are referred to as polyhalogen compounds. Dichloro- methane (Methylene chloride): Dichloromethane (CH2Cl2 ) is widely used as a solvent, as a propellant in aerosols. Direct contact of dichloromethane in humans causes intense burning and mild redness of the skin. Chloroform [Trichloromethane, CHCl3 ]: CHCl3 is widely used in the production of freon refrigerant R-22. On nitration, it gives tear producing insecticide substance chloropicrin. On dehalogination, it gives acetylene. Oxidation of CHCl3 gives poisonous gas phosgene (carbonyl chloride). To avoid this oxidation CHCl3 will stored in dark brown bottles and filled to the brim. 1% ethanol is added to chloroform which converts harmful phosgene gas into diethyl carbonate. 2CHCl3 + O2 → 2COCl2 + 2HCl H r e e n d r a k u m a r : 9 4 9 6 1 0 5 8 5 4 : An Ideal Learner’s School Of Chemistry 5 Haloalkanes and Haloarenes X Iodoform (tri-iodornethane, CHl3 ): Compounds containing either CH3 CO- or CH3 CH(OH) group form yellow colour iodoform with I2 and NaOH.
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