why is pyridine used as a catalyst in the prepn of haloalkanes from alcohols by action of thionyl chloride?

Solution-

The reaction of thionyl chloride with alcohol in presence / absence of pyridine is called as Darzen's reaction.

Here, the pyridine molecule acts as a catalyst which is used to neutralize the HCl -produced in the reaction.

CH₃CH₂OH + SOCl₂ $\stackrel{\mathrm{Pyridine}}{\to }$ CH₃CH₂Cl + SO₂ ↑+ HCl ↑

Here, pyridine acts as neucleophiles because, in the reaction a proton is added to the alcoholic oxygen due to the strong electronegative SOCl₂ group,and this proton is removed by the pyridine and   reforms the S=O, and also the Cl^- ion is removed from SOCl₂.

This  Cl^- ion reacts with electrophilic carbon of alcohol and displaces SO₂ and releases another Cl^- ion which makes the alkyl chloride.