Formation of tosylates from alcohols
Description: Treatment of alcohols with p-toluenesulfonyl chloride (TsCl) results in the formation of tosylates, which is an excellent leaving group.
Notes: Sometimes a base such as pyridine is added to this reaction to neutralize the HCl that is formed. The tosylates formed can be used in subsequent substitution or elimination reactions.
Notes: The fourth example shows pyridine (a base) being used.
Mechanism: Tosylation of alcohols begins with nucleophilic addition to the sulfur (Step 1, arrows A and B). The Cl is then expelled in a 1,2-elimination reaction (Step 2, arrows C and D). Finally the positively charged oxygen is deprotonated (Step 3, arrows E and F).
Notes: It’s also reasonable to show a proton transfer after step 1, followed by elimination. Alternatively, another equivalent of alcohol could act as the base in step 3 instead of Cl.