There is a subtle difference between reactions of Thionyl Chloride (SOCl2) and Sulfuryl Chloride (SO2Cl2). Basically, thionyl chloride is preferred as a reagent when we need to convert an alcohol to a haloalkane, while sulfuryl chloride is preferred when we need to convert an alkane to a haloalkane.

The reactivity patterns of SO2Cl2 and SOCl2 are quite different. SOCl2 is a good electrophile, and can be thought of as a source of Cl− ions. These ions can go on to react in their typical nucleophilic fashion. SO2Cl2 however is often a Cl2 source, as it readily decomposes giving off sulfur dioxide. Usually much easier/safer to use this than measuring out (and getting into solution) chlorine gas. The chlorination of simple alkanes by Cl2 gas (or something that makes it in solution) happens by a radical mechanism i.e. Cl⋅ not Cl-

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