| A kinetic study of the sodium ethoxide promoted elimination reactions of a series of substituted 2-phenylethyl chlorides in ethanol at 45(DEGREES)C has been carried out giving the following results: substituent, relative rate; p-OMe, 0.44; p-H, 1.00; p-Cl, 3.57 and p-CF(,3), > 300. These results agree well with those from a kinetic study of thirteen substituted 2-phenylethyl chlorides under the same solvent/base conditions('1). Marked deviation from linearity was found in the Hammett plot for the thirteen compounds. The (rho) values increased from 1.62 at the EDG end to 3.70 at the EWG end. This indicates an increase in the amount of carbanion character in the transition states as the electron withdrawing ability of the substituent increases. (beta)-Deuterium isotope effects measured for three substituted 2-phenylethyl chlorides('1) (p-H, m-Br and p-NO(,2)) showed little variation as the substituent was varied, indicating little change in the extent of C(,(beta))-H bond rupture.;The principle focus of this study was the measurement of the (alpha)- and (beta)-carbon-14 kinetic isotope effects for the sodium ethoxide promotion elimination reactions of p-trifluoromethyl-, p-chloro-, p-methoxy- and unsubstituted 2-phenylethyl chlorides in ethanol at 45(DEGREES)C. The results are reported here as substituent, ('12)k/('14)k (+OR-) standard deviation for ('(alpha))C, ('(beta))C: CF(,3), 1.0490 (+OR-) 0.0020, 1.0330 (+OR-) 0.0034; Cl, 1.0513 (+OR-) 0.0029, 1.0287 (+OR-) 0.0018; H, 1.0611 (+OR-) 0.0033, 1.0240 (+OR-) 0.0043; OCH(,3), 1.0395 (+OR-) 0.0073, 1.0106 (+OR-) 0.0019.;These results rule out both the E1 and irreversible E1cb mechanisms. For the E1 mechanism no isotope effect is expected at C(,(beta)), while for the irreversible E1cb mechanism no isotope effect is expected at C(,(alpha)). A distinction between the E2 and reversible E1cb mechanisms is not possible from these results. However, neither 2-phenylethyl fluoride('2) nor bromide('3) was found to undergo (beta)-deuterium-exchange in this solvent/base system, thus ruling out the reversible E1cb mechanism for these compounds. Therefore, by interpolation, this mechanism can be ruled out in the present case.;An explanation of the curved Hammett plot and of the variations in the carbon isotope effects and the lack of variation in the (beta)-D(,2) isotope effects with substituent is that the extent of C(,(beta))-H bond rupture remains constant and the extent of C(,(alpha))-Cl bond rupture decreases as the substituent becomes more electron withdrawing. This decrease in C(,(alpha))-Cl bond rupture leads to a decrease in C(,(alpha))-C(,(beta)) double bond character, leaving C(,(beta)) more negative (more E1cb-like transition state). This increase in negative charge and concomitant decrease in bonding at C(,(beta)) results in an increase in the C(,(beta)) isotope effect. At C(,(alpha)), as the substituent becomes more electron withdrawing compensation of decreased C(,(alpha))-Cl bond rupture (which would lower the isotope effect) by decreased double character (which would increase the isotope effect) can account for the maximum observed.;These results are interpreted using E2 transition state diagrams adapted by Fry('4) from the More O'Farrell potential energy surfaces('5).;;References;('1)Kawamura, I. M.S. Thesis; obtained through Yamataka, H., I.S.I.R.Osaka, Japan 1976. ('2)Saunders, W. H.; Schreiber, M. R. Chem.Comm. 1966, 145-146. ('3)Saunders, W. H.; Edison, D. H. J. Am.Chem. Soc. 1960, 82, 138-142. ('4)Fry, A. Chem. Soc. Rev. 1972, 1,163-210. ('5)More O'Ferrall, R. A. J. Chem. Soc. (B) 1970, 274-277. |
