Why the reactivity of hi is higher for the reaction of converting roh to rx?
Answers
When alcohols react with a hydrogen halide, a substitution occurs, producing an alkyl halide and water:
Scope of Reaction
The order of reactivity of alcohols is 3° > 2° > 1° methyl.
The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF is generally unreactive).
The reaction is acid catalyzed. Alcohols react with the strongly acidic hydrogen halides HCl, HBr, and HI, but they do not react with nonacidic NaCl, NaBr, or NaI. Primary and secondary alcohols can be converted to alkyl chlorides and bromides by allowing them to react with a mixture of a sodium halide and sulfuric acid:
Mechanisms of the Reactions of Alcohols with HX
Secondary, tertiary, allylic, and benzylic alcohols appear to react by a mechanism that involves the formation of a carbocation in an SN1SN1 reaction with the protonated alcohol acting as the substrate.
The SN1SN1 mechanism is illustrated by the reaction tert-butyl alcohol and aqueous hydrochloric acid ( H3O+H3O+ , Cl−Cl− ). The first two steps in this Sn1Sn1 substitution mechanism are protonation of the alcohol to form an oxonium ion. Although the oxonium ion is formed by protonation of the alcohol, it can also be viewed as a Lewis acid-base complex between the cation ( R+R+ ) and H2OH2O . Protonation of the alcohol converts a poor leaving group (OH-) to a good leaving group (\)H_2O\_), which makes the dissociation step of the SN1SN1 mechanism more favorable.
