tosylate
/ TOSS-il-ate /
An alcohol's -OH is a terrible leaving group — kick it out and you would have to expel hydroxide, which clings to its electrons too tightly. The tosylate trick solves this: convert the stubborn -OH into a tosylate ester, and suddenly the oxygen leaves easily, opening the alcohol up to substitution and elimination reactions it could never do on its own.
You make a tosylate by treating the alcohol with tosyl chloride (TsCl, p-toluenesulfonyl chloride) and a base like pyridine. The reaction replaces the O-H hydrogen with a tosyl group, turning R-OH into R-O-Ts, where Ts is the -SO2-C6H4-CH3 (sulfonyl attached to a methyl-bearing benzene ring) piece. Crucially, the C-O bond of the original alcohol stays intact, so the carbon's stereochemistry is untouched. Now the leaving group is a tosylate anion (TsO-), whose negative charge is spread over three oxygens of the sulfonyl group by resonance, making it remarkably stable — a leaving group as good as or better than a halide.
Tosylates are prized because they convert an alcohol into a reactive substrate without disturbing the stereocenter, so you can plan SN2 reactions with clean inversion or E2 eliminations with confidence. They also offer a milder, more controllable alternative to harsh reagents like HBr or SOCl2. Related sulfonate leaving groups — mesylates (OMs) and triflates (OTf) — work the same way, with triflate being one of the best leaving groups known.
(R)-2-butanol + TsCl/pyridine gives (R)-2-butyl tosylate with the stereocenter untouched. Then cyanide does SN2: CN(-) displaces OTs with inversion, giving (S)-2-methylbutanenitrile.
Tosylation leaves the C-O bond alone, so the carbon's configuration is preserved until the later step.
Because tosylation does not break the alcohol's C-O bond, it never inverts the original stereocenter — the inversion (if any) happens in the later substitution step, not when the tosylate is formed.