Darzen's process.

Explanation

This method is used for the preparation of alkyl chloride by treating alcohols with thionyl chloride.
`underset("Alcohol")(ROH)+underset("Thionyl chloride")(SOCl_(2))overset(Delta)(to)underset("Alkyl chloride")(RCl)+SO_(2)+HCl`
This is the best method for the proparation of pure alkyl chlorides because the other two products (`SO_(2)` HCl) being gases, leave the reaction mixture.