When alcohols react with a hydrogen halide, a substitution occurs, producing an alkyl halide and water:
Scope of ReactionThe order of reactivity of alcohols is 3° > 2° > 1° methyl.
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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, & HI, but they do not react with nonacidic NaCl, NaBr, or NaI. Primary and secondary alcohols can be converted lớn alkyl chlorides và bromides by allowing them lớn react with a mixture of a sodium halide và sulfuric acid:
Mechanisms of the Reactions of Alcohols with HX
Secondary, tertiary, allylic, and benzylic alcohols appear lớn react by a mechanism that involves the formation of a carbocation in an (S_N1) reaction with the protonated alcohol acting as the substrate.
The (S_N1) mechanism is illustrated by the reaction tert-butyl alcohol & aqueous hydrochloric acid ((H_3O^+), (Cl^-) ). The first two steps in this (S_n1) substitution mechanism are protonation of the alcohol to size 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^+)) and (H_2O). Protonation of the alcohol converts a poor leaving group (OH-) khổng lồ a good leaving group ()H_2O\_), which makes the dissociation step of the (S_N1) mechanism more favorable.
In step 3, the carbocation reacts with a nucleophile (a halide ion) khổng lồ complete the substitution.
When we convert an alcohol to lớn an alkyl halide, we perform the reaction in the presence of acid và in the presence of halide ions & not at elevated temperatures. Halide ions are good nucleophiles (they are much stronger nucleophiles than water), & because halide ions are present in a high concentration, most of the carbocations react with an electron pair of a halide ion to size a more stable species, the alkyl halide product. The overall result is an (S_n1) reaction.
Not all acid-catalyzed conversions of alcohols to lớn alkyl halides proceed through the formation of carbocations. Primary alcohols & methanol react to khung alkyl halides under acidic conditions by an SN2 mechanism.
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In these reactions, the function of the acid is to lớn produce a protonated alcohol. The halide ion then displaces a molecule of water (a good leaving group) from carbon; this produces an alkyl halide:
As we might expect, many reactions of alcohols with hydrogen halides, particularly those in which carbocations are formed, are accompanied by rearrangements. The general rule is that if rearrangement CAN OCCUR (to form more stable or equally stable cations), it will! In these reactions, mixtures of products can be formed.