From Alcohols

Converting the –OH Group into a Halogen

An alcohol's hydroxyl group can be replaced by a halogen atom by treating it with a concentrated halogen acid, a phosphorus halide, or thionyl chloride ($\text{SOCl}_2$).

Reaction with Halogen Acids

Primary and secondary alcohols react with HCl only in the presence of a catalyst — anhydrous zinc chloride ($\text{ZnCl}_2$). Tertiary alcohols react without any catalyst at all: simply shaking the alcohol with concentrated HCl at room temperature is enough, since the more stable carbocation formed from a tertiary alcohol reacts readily.

For alkyl bromides, the alcohol is refluxed with constant-boiling hydrobromic acid (48% HBr).

For alkyl iodides, good yields are obtained by heating the alcohol with sodium iodide or potassium iodide in 95% orthophosphoric acid ($\text{H}_3\text{PO}_4$) rather than using HI directly.

Across a given haloacid, alcohols follow the reactivity order:

Preparing alkyl chloride specifically: either dry hydrogen chloride gas is passed through the alcohol, or the alcohol is heated with concentrated aqueous HCl directly.

Reaction with Phosphorus Halides

Phosphorus tribromide and phosphorus triiodide are rarely used as isolated reagents — they are usually generated in situ (formed directly inside the reaction mixture) by reacting red phosphorus with bromine or iodine respectively.

Reaction with Thionyl Chloride

$\text{R–OH} + \text{SOCl}_2 \longrightarrow \text{R–Cl} + \text{SO}_2\uparrow + \text{HCl}\uparrow$ …