Explain why is thionyl chloride method preferred for preparing alkyl chlorides from alcohol ?

Explanation

The byproducts of reaction i.e., `SO_2` and HCl being gases escape into the atmosphere leaving behind alkyl chlorides in almost pure state e.g.
` CH_3 CH_2 -OH +SOCl_2 overset("Pyridine ")to CH_3 CH_2 -Cl +SO_2 (uarr) +HCl ( uarr)`