Tosyl

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A tosyl group (abbreviated Ts) combines the toluene and sulfonyl functional groups. The sulfonyl group consists of a hexavalent sulfur atom double bonded to two oxygen atoms and, in the tosylate group, an aromatic ring; an alkyl substituent on the R group forms a sulfonate ester. Thus, the tosylate group is the ester of toluenesulfonic acid. The para orientation illustrated (p-toluenesulfonyl) is most common, and by convention tosyl refers to the p-toluenesulfonate ester.

A tosylate ester has only a limited shelf life if it is stored outside of a dessicator as the free tosyl is readily hydrolysed by water in the presence of light. The tosyl group is electron-withdrawing. Hence, it is an excellent leaving group.

The tosyl group is a protecting group for alcohols, prepared by combining the alcohol with toluenesulfonyl chloride in an aprotic solvent. Toluenesulfonyl chlorides activate alcohols for nucleophilic attack or elimination (dehydration).

Similarly, the brosyl (Bs) group or brosylate is a p-bromobenzenesulfonyl group with the methyl group of toluene replaced by a bromine atom. Nosyl groups in Nosylates (Ns) are 4-nitrobenzenesulfonyl groups with a nitro group in the para position.