~-Deuterium Isotope Effect in Solvolysis of l ,2-Dimethyl-exo-2--norhornyl p-Nitrohenzoate

K Humski
Secondary (:\-deuterium isotope effects have been shown to be remarkably constant with change of solvent1. The ratio, kH/kD, , was 1.22 in solvolysis of 1-phenylethyl chloride in ethanol-water mixtures ranging from 50 vol 0 /o to 80 vol O/o of ethanol. It was proposed that the transition state is very nearly the same in all the solvents studied. The yields of styrene (as an elimination product) were between 1 and 3 percent. It was also found 2 that the (:\-isotope effect on solvolysis of
more » ... olvolysis of t-butyl-d 0 chloride was 2.62 in 97 wt O/o trifluoroethanol-water and 2.49 in 70 wt O/o trifluoroethanol. In 970/o trifluoroethanol 300/o of olefin was obtained but only 170/o in 700/o trifluoroethanol. One possible interpretation given for the discrepancy in the effects is based on ion pair return competing with rate-determining elimination. The (:\-hydrogen isotope effect and product composition in the solvolysis of 1,2-dimethyl-ex o-2-norbornyl p-nitrobenzoate (I) in the dioxane-water mixtures, ranging from 50 vol 0 /o to 70 vol O/o of dioxane, has been measured and the results are presented in the Table. Compound I was synthesized as described previously 3 , while 1,2-dimethyl-exo-2-norbornyl-3,3-d2 p-nitrobenzoate (II) was obtained from 1-methyl-2-norbornanone-3,3-d23. The deuterated ketone was prepared by the hydrogen-deuterium exchange of the ketone in heavy water and deuterated trifluoro-acetic acid and contained 1.78 atoms of deuterium per molecule (according to mass spectra). The titrimetric rates were obtained employing the automatic potentio-metric titration method maintaining a constant pH of 6.8. The rate constants were calculated from the standard integrated first-order rate law using the non-linear least square program*. No trend w as observed in the constants between 200/o· to 80 6 /o of reaction completion. The product study was conducted in the usual way using the ampoule technique in the presence of a slight excess (200/o to 300/o) of 2,6-lutidine (to neutralize the acid produced by solvolysis). 4 The products were found to be stable under the reaction conditions. From the Table it is evident that the measured isotope effect, kH/kD, is directly related to the amount of elimination product formed. This indicates that the elimination is rate-determining and in competition with the substitution and internal return, as shown in the following scheme.