EFFECTS OF DEUTERIUM SUBSTITUTION ON RATES OF ORGANIC REACTIONS .11. ALPHA- AND BETA-DEUTERIUM EFFECTS ON SOLVOLYSIS RATES OF A SERIES OF SUBSTITUTED 1-PHENYLETHYL HALIDES

The rates of solvolysis of eight different meta- and para-substituted 1-phenylethyl chlorides, three 1-phenylethyl bromides, and their α-d and β-d% analogs have been measured in a series of acetone-water and ethanol-water mixtures with a precision of 0.1 % or better using conductance techniques. The fact that 1-phenylethyl chloride and all of its more reactive derivatives, including the p-methoxy compound, have closely similar α-deuterium effects (kH/kD) = 1.15) is interpreted in terms of a limiting carbonium ion mechanism applying for all of these compounds. The smaller α effects for the m-bromo (1.13) and p-nitro (1.11) derivatives are believed to indicate that these solvolyses involve appreciable nucleophilic character. The fact that the three bromides examined show α-d effects which are about 3 % smaller than those of the corresponding chlorides is believed to be due to the HCBr bending force constant being smaller than the HCC1 bending force constant in the initial state. The similarity in the influence of substituents indicates that the solvolytic mechanisms for bromides and chlorides are similar. Increasingly electron-releasing substituents in the ring were found to be associated with increasingly smaller β-d effects for all compounds more reactive than 1-phenylethyl chloride. The substituents and approximate kH/Kβ effects were H (1.22), m-CH3 (1.22), p-F (1.21), p-C33 (1.20), p-phenoxy (1.16), and p-methoxy (1.11). This trend has been attributed to the electron-releasing substituents raising electron density at the reaction center and reducing the demand for hyperconjugation from the β-CH3 (or CD3) group. Thus, this result confirms earlier conclusions that the β-deuterium effect on the rate of carbonium ion forming reactions depends in major part on a hyperconjugative mechanism of interaction between the reaction center and the site of isotopie substitution. The smaller than otherwise expected β-d3 effects for the m-bromo (1.22) and p-nitro (1.20) a-phenylethyl chlorides is in accord with the conclusion that these compounds react in part at least by a nucleophilic mechanism. The β-deuterium effects for the a-phenylethyl bromides are very near those for the corresponding chlorides in agreement with the conclusion that chlorides and bromides react by very similar mechanisms and that the difference in α effects is due primarily to the HCX bending force constant difference in the initial state. © 1968, American Chemical Society. All rights reserved.