Question

why SOCl2 is called as best chlorinating agent​

Answer 1

Answer:

Thionyl chloride is preferred for preparing alkyl chlorides from alcohols because the by-products formed in the reaction are SO2 and HCl which are in gaseous form and escape into the atmosphere leaving behind pure alkyl chlorides.

Answer 2

Answer:

SOCl2 is a good electrophile and can be thought of as a source of Cl- ions SO2Cl2 however is often a Cl2 source as it readily decomposes giving of sulpher dioxide. Usually much easier to use this than measuring out ( and getting into solution ) chlorine gas.