Hydrohalogenation of Ethynylpyridines Involving Nucleophilic Attack of a Halide Ion

Efficient hydrochlorination of 2-ethynylpyridines was achieved without the use of special reagents. Ethynylpyridine readily reacts with hydrochloric acid to form a pyridinium salt. The salt formation considerably enhances the electrophilicity of the ethynyl group and attracts a chloride ion as the counteranion. The spatial proximity facilitates the nucleophilic addition of the halide anion to the ethynyl group, producing 2-(2-chloroethenyl)pyridine in high yields. This protocol could also be applied for hydrobromination and hydroiodination using hydrobromic and hydroiodic acids, respectively. In the case of acetic acid, the reaction did not proceed because of the low acidity and lack of salt formation. This problem was overcome by exchanging the counteranion using silver acetate; the resultant pyridinium acetate underwent hydroacetoxylation.