2-Chloro-1,3-butadiene (chloroprene) is the monomer from which the elastomer neoprene is prepared. 2-Chloro-1,3-butadiene is the thermodynamically controlled product formed by addition of hydrogen chloride to vinylacetylene (CH2=CHC≡CH). The principal product under conditions of kinetic control is the allenic chloride 4-chloro-1,2-butadiene. Suggest a mechanism to account for the formation of each product.