6.4. Reaction with hydrogen halide

  Alcohols, Phenols and Ethers

\[R---OH + {\text{ }}HX\xrightarrow{{}}R---X{\text{ }} + {\text{ }}{H_2}O\]

The reactivity of the hydrogen halide is $HI > HBr > HCl$ ($HF$ is generally unreactive) and the order of reactivity of alcohols is $3^\circ > 2^\circ > 1^\circ < C{H_3}$

The least reactive hydrogen halide i.e. $HCl$ generally requires the presence of $ZnC{1_2}$ for reaction with 1° and 2° alcohols, however the highly reactive 3° alcohol do not require $ZnC{1_2}$

$$C{H_3}---C{H_2}---C{H_2}---OHC{H_3}---C{H_2}---C{H_2}---Br$$

The chloride ion is a weaker nucleophile than bromide and iodide ion. $HCl$ does not react with less reactive $1^\circ $ and 2° alcohol unless some good Lewis acid like $ZnC{l_2}$ is added to a reaction mixture.

Primary alcohol being least reactive requires some heat in addition to $ZnC{l_2}$. Zinc chloride a good Lewis acid for a complex with alcohol. This complex provides a better leaving group for the reaction than ${H_2}O$

Except with most primary alcohols rearrangement of the alkyl group occurs.