Kinetics of acid-catalyzed hydration of acetylene. Evidence for the presence in the solution phase of unsubstituted vinyl cation

Article abstract of DOI:10.1021/ja00173a016

The rates of acetylene hydration in the convenient range of aqueous sulfuric acid (and those of propyne, tert-butylacetylene, ethylene, propene, and tert-butylethylene, for comparative purposes) have been measured at 25 °C with an NMR technique. The correlation of the kinetic data with the excess acidity function X gives a value of 1.12 for the slope parameter m*, which suggests that the intermediate is protonated acetylene, C₂H₃⁺ (probably as vinyl cation 3 rather than as hydrogen-bridged ion 4). The comparison with the m* value for the hydration of ethylene (1.50) indicates that protonated acetylene possesses stronger susceptibility to solvation than ethylium ion C₂H₅⁺. The deuteration patterns in the products (acetaldehyde and crotonaldehyde) obtained in deuteriosulfuric acid rule out the reversibility of the protonation process and also the conversion between 3 and 4.