According to the book "Chlorosulfonic acid: a versatile reagent" by Richard James William Cremlyn, chlorosulfonation can be promoted by using a mixture of chlorosulfonic acid and thionyl chloride. Sometimes it is preferable to react the substrate with chlorosulfonic acid alone first and add the thionyl chloride later. This modification allows the amount of chlorosulfonic acid to be reduced sometimes to just one molar equivalent, and may convert the intermediate sulfonic acid to sulfonyl chloride where the conversion does not occur effectively in the absence of thionyl chloride - an example is a substrate containing a substituent capable of hydrogen bonding with the sulfonic acid group, such as an amino group.
Could anybody describe for me the mechanism by which thionyl chloride works in the chlorosulfonation reaction? I know that using thionyl chloride can result in a higher yield, but I am struggling to understand why. Any help is greatly appreciated.