Preparation of Alkyl Halides from Alcohols and Hydrogen

Preparation of Alkyl Halides from Alcohols and Hydrogen Halides ROH + HX RX + H 2 O

Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH Hydrogen halide reactivity HI HBr HCl HF most reactiveleast reactive

Reaction of Alcohols with Hydrogen Halides ROH + HX RX + HOH Alcohol reactivity R 3 COH R 2 CHOH RCH 2 OH CH 3 OH Tertiary. Secondary. Primary. Methanol most reactiveleast reactive

Preparation of Alkyl Halides (CH 3)3 COH + HCl 25°C (CH 3)3 CCl + H 2 O 78 -88% OH + HBr 80 -100°C Br + H 2 O 73% CH 3(CH 2)5 CH 2 OH + HBr 120°C CH 3(CH 2)5 CH 2 Br + H 2 O 87 -90%

Preparation of Alkyl Halides A mixture of sodium bromide and sulfuric a cid may be used in place of HBr. CH 3 CH 2 CH 2 OH Na. Br H 2 SO 4 heat CH 3 CH 2 CH 2 Br 70 -83%

Mechanism of the Reaction of Alcohols with Hydrogen Halides

Carbocation R + C R R The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. A carbocation is a cation in which carbon has 6 valence electrons and a positive charge.

Carbocation R + C R R The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation. The overall reaction mechanism involves three elementary steps; the first two steps lead to the carbocation intermediate, the third step is the conversion of this carbocation to the alkyl halide.

Example 25°C (CH 3)3 COH + HCl tert-Butyl alcohol (CH 3)3 CCl + H 2 O tert-Butyl chloride Carbocation intermediate is: H 3 C + C CH 3 tert-Butyl cation

Mechanism Step 1: Proton transfer from HCl to tert-butyl alcohol. . (CH 3)3 C. . O : + H Cl: . . H fast, bimolecular H + (CH 3)3 C O : + H tert-Butyloxonium ion . . – : Cl: . .

Mechanism Step 2: Dissociation of tert-butyloxonium ion H + (CH 3)3 C O : H slow, unimolecular H + (CH 3)3 C + tert-Butyl cation : O: H

Mechanism Step 3: Capture of tert-butyl cation by chloride ion. + (CH 3)3 C + . . – : Cl: . . fast, bimolecular (CH 3)3 C . . Cl. . : tert-Butyl chloride