Nucleophilic substitution of vinylic halogens in 3-halomethacrylonitriles. Some observations on olefin stabilities

3-Chloro- and 3-bromomethacrylonitrile (E- and Z-) have been shown to undergo nucleophilic substitution at C-3 with > 95% retention of configuration. The configuration of each product has been established by the use of nuclear magnetic resonance spectroscopy. The thermodynamic position of equilibrium for each pair of E–Z isomers has been determined and the factors which affect this equilibrium are discussed. Some form of cis interactions, non-steric in origin, dominates in determining the equilibrium positions.