Kinetics and Stereochemistry of the Addition of Chlorine to Styrenes

Article abstract of DOI:10.1021/jo01296a011

The chlorination of several ring- and side-chain-substituted styrenes has been studied in anhydrous acetic acid, in both the presence and absence of added perchlorate, chloride, and acetate salts.The reactions give three types of product: the 1,2-dichlorides arising from simple addition, 1-acetoxy-2-chloro compounds from addition followed by solvent incorporation, and β-chlorostyrenes from an addition-elimination process.The reactions are completely regiospecific in the Markownikoff sense, and the addition-elimination products are formed with high stereoselectivity.However, both types of addition product are formed nonstereospecifically.Both the product distribution and stereoselectivity are remarkably insensitive to added perchlorate, chloride, or acetate, and very high concentrations of these salts (ca. 1.0 M) are required to produce any significant change in product composition.These results are explicable in terms of a product-determining intermediate which consists of an intimate ion pair between an open β-chlorobenzyl carbonium ion and a tightly held chloride counterion.However, since (E)- and (Z)-1-phenylpropenes do not give similar product distributions under any conditions used, rotation of the C_α-C_β bond in the carbonium ion must be highly restricted.The rates of reaction have been studied by a combination of potentiometric and stopped-flow spectrophotometric techiques.The reactions are first order in chlorine and first order in olefin and are generally very fast, with most of the rate constants being in the range 10²-10⁵ L mol^-1 s^-1.The rate constants for the ring-substituted styrenes give a good linear correlation against ?⁺ with a β value of -3.22.This value is less negative than expected from a comparison with other electrophilic additions and is discussed in terms of an earlier transition state with less charge development at C_α than in the analogous bromination reaction.The activation parameters obtained for two of the styrenes support this hypothesis.There is no direct evidence for chlorine bridging at either the transition state or the intermediate stage.