Conversion of Alcohols to Alkyl Halides

Alcohols react with hydrogen halides to generate corresponding alkyl halides.

In the mechanism, the first step is the protonation of the hydroxyl group. Thereafter, while tertiary alcohols interact via the S_N1 mechanism, primary alcohols prefer an S_N2 route. Secondary alcohols can proceed via either mechanism, where the preference is dictated by the reaction conditions.

Recall that an S_N1 mechanism occurs sequentially: a loss of the leaving group, followed by the nucleophilic attack. Since this reaction proceeds via the carbocation intermediate, it is suitable for the tertiary carbocation, which is stabilized by hyperconjugation.

For primary alcohols, the protonation of the hydroxyl group leads to the S_N2 mechanism. However, while the S_N2 mechanism is straightforward with hydrogen bromide, hydrogen chloride needs an additional catalyst, zinc chloride.

Zinc chloride, being ionic, is limited to water-soluble alcohols. It converts their hydroxyl group into a better leaving group, enabling the subsequent S_N2 process.

A more common process for primary alcohols uses thionyl chloride. A primary alcohol interacts with thionyl chloride in the presence of pyridine or a tertiary amine, which