Chemistry

Alkyl and Aryl halides–1

Session objectives 1.Classification 2.Nature of C — X bond 3.Preparation of alkyl halides 4.Physical and chemical properties of alkyl halides 5.Substitution reaction: S N 1 and S N 2 reactions

Elimination reaction E 1 E 2 E1 c B Ei

E1 Mechanism

Elimination reaction 3° > 2° > 1° > CH 3 Rate = k[RX] 1

E2 Rate = k[RX] [Base] = k[CH 3 CH 2 Cl]

E1 cB carbanion stabilization by the participation of –R (conjugation), –I effect etc. Mechanism

Ei Observed for pyrolytic elimination for xanthates (Tschugaev’s reaction), esters, oxides of amines (Cope’s reaction) which produces alkene. It occurs intramolecularly through the formation of a transition state, when they are heated.

Comparison between E1 and E2

Polyhalogen compounds Dichloromethane (CH 2 Cl 2 ) Colourless sweet smelling volatile liquid. It has low boiling point (313 k) and inflammable; used as a solvent in extraction in food and pharmaceutical industries. Vicinal dihalide When two halogen atoms are attached to adjacent carbon atoms, the compound is called vicinal dihalide. 1, 2-dichloroethane e.g.,

Polyhalogen compounds Geminal dihalide When two halogen atoms are attached to the same carbon atom, the compound is called geminal dihalide e.g. H 3 C – CHCl 2 1,1-dichloroethane

Polyhalogen compounds Chloroform (CHCl 3 ) Methods of preparation (1) By chlorination of methane (2)By reduction of carbon tetra chloride

Polyhalogen compounds (3) From ethyl alcohol

Polyhalogen compounds (4) From acetone

Properties (1)Sweet smelling liquid, boiling point 61° C. (2) Oxidation Stored in dark coloured bottled because on oxidation chloroform gives phosgene gas which is poisonous 1% C 2 H 5 OH is added while storing chloroform

Properties (3) Reduction (4) Chlorination (5) Hydrolysis (6) Nitration

Properties (7)Reaction with acetone (8) Dehalogenation (9) Carbylamine reaction

Iodoform (CHI 3 ) Preparation

properties 1.Yellow solid melting point 119° C. 2.Chemical properties of iodoform are same as that of chloroform. 3.Chlorine and bromine can replace iodine in iodoform reaction to give CHCl 3 and CHBr 3 respectively. The reaction is called haloform reaction.

Carbon tetrachloride Preparation Properties (1) Colourless liquid, boiling point 350 K.

Uses (1) Used as fire extinguisher. Trade name is pyrene. At 773 K it is hydrolysed by water vapours into phosgene gas. Therefore, the room should be well ventilated after using CCl 4 as fire extinguisher. (2) Industrial solvent for oils, fats, resins etc.

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