tert-Butyl 4-methylbenzenesulfonate, commonly known as tert-butyl tosylate, is an organosulfonate ester used as an alkylating agent in organic synthesis. It serves as a source of the electrophilic tert-butyl cation, enabling the introduction of the tert-butyl group onto nucleophiles including enolates for C-alkylation, alcohols and phenols for O-alkylation to form tert-butyl ethers, and amines for N-alkylation. The tosylate acts as an excellent leaving group, facilitating reactions via SN1 or SN2 mechanisms under mild conditions. This reagent is particularly valuable in multi-step syntheses of pharmaceuticals, natural products, and fine chemicals, where selective tert-butylation is required. It is also utilized in mechanistic studies of carbocation chemistry and solvolysis.