Alcohol 6 cont:

3-Methyl-1-chlorobutane 2 is the reaction product. Alcohol 1 displaces chloride ion from thionyl chloride to form intermediate 3. This species is deprotonated by the base pyridine to form 4 and pyridinium hydrochloride 5. The use of pyridine assures a sufficient supply of chloride ion (as opposed to HCl in the absence of pyridine) to allow S_N2 displacement to occur on 4 to produce the product 2. Although pyridine hydrochloride is shown as one of the products, it is there in catalytic amounts. The remainder is HCl.

The displacement of ClSO₂^- by chloride ion is facile because the anion is the conjugate base of a strong acid, chlorosulfurous acid (cf. chlorosulfuric acid).