Addition to unsaturated carbonyl compounds. Part I. The addition of hydrogen halides to propiolic acid

Abstract

The kinetics, products, and stereochemistry of the addition of hydrogen chloride, bromide, and iodide to propiolic acid in water have been studied. The addition is predominantly trans to give the cis-3-halogenoacrylic acid. Both the rate of addition and the selectivity giving trans-addition increase with the nucleophilicity of the halide in water, i.e., I^– > Br^– > Cl^–. The reaction is of the first order in propiolic acid and the halide anion, whilst the dependence on acidity appears to be a function of H₀. Solvent isotope, salt, and solvent effects have been observed. Addition appears to involve two mechanistic pathways: a major trans-addition via a transient carbanion formed with specific geometry and a minor cis-addition process.