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 TertiarySecondaryPrimaryMethanol most reactiveleast reactive

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

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

Mechanism of the Reaction of Alcohols with Hydrogen Halides

CarbocationCarbocation 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. C R R R+

CarbocationCarbocation 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. C R R R+

ExampleExample (CH 3 ) 3 COH + HCl (CH 3 ) 3 CCl + H 2 O 25°C Carbocation intermediate is: C H3CH3CH3CH3C CH 3 + tert-Butyl alcohol tert-Butyl chloride tert-Butyl cation

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

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

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