Mechanism of reactions of unsaturated ethers and thioethers. I: Add catalyzed addition of water to acetylenic thioethers

The acid catalyzed addition of water to ethylthioethyne has been studied in aqueous solution. The reaction is general acid catalyzed and it is faster in H2O than in D2O. Substitution of a tertiary butyl group for the ethyl group results in a rate enhancement. The entropies of activation ΔS≠ for the hydration of ethylthioethyne and tertiary butylthioethyne are ‐ 6.1 and ‐ 0.6 cal. mole−1, deg.−1, respectively. These facts strongly suggest that protonation of the alkylthioethyne is the rate controlling step of the reaction.