What is a tosylate group?

A tosylate is the anion of tosylic acid. This is tosylic acid (or p-toluenesulfonic acid): so the conjugate base is a tosylate. It is toluene (methylbenzene) with a sulfonate group attached on the 4-position.Related are tosyl leaving groups because they are large and they tend to have a low %pKa% (making them thermodynamically unstable, i.e. making their bonds weak). A common tosyl compound is tosyl chloride. NOTE: These actually have sulfonyl groups, not to be confused with sulfonate groups.