Mechanisms of elimination reactions. XXXII. Tritium isotope effects and tunnel effects in the reaction of 2,2-diphenylethyl-2-t derivatives with various bases
Primary tritium isotope effects as a function of temperature have been determined for the E2 reaction of 2,2-diphenylethyl-2-t-tosylate with ethoxide in ethanol and tert-butoxide as well as the anions derived from imidazole, 2-methylimidazole, and 2,4,5-trimethylimidazole in tert-butyl alcohol. The temperature dependence of the tritium isotope effect has also been determined for the E2 reaction of 2,2-diphenylethyl-2-t bromide with ethoxide in ethanol and tert-butoxide in tert-butyl alcohol. Observed k/sub H//k/sub T/ values at 50/sup 0/C ran from 17.6 to 31.3, and the ratios of Arrhenius preexponential factors, A/sub aH//A/sub aT/, from 0.15 to 0.74, indicating moderate tunneling. A temperature dependence calculated from the Bell theory of tunneling was fitted to the observed temperature dependence to obtain tunnel corrections, Q/sub tH//Q/sub tT/, ranging from 1.3 to 2.5 at 50/sup 0/C. Observed isotope effects are larger with tert-butoxide in tert-butyl alcohol than with ethoxide in ethyl alcohol for both the tosylate and with tert-butoxide. There is no marked effect of methyl substitution in the imidazole anion on either the observed isotope effect or the tunnel effect, even with the sterically hindered 2,4,5-trimethylimidazole anion. The semiclassical (without tunneling) isotope effects run approximately 80 to 100% of the maximum values predicted by the treatment of Melander and Westheimer. The proton thus seems to be nearly symmetrically located in the transition states, and the contribution of heavy-atom motion to the reaction coordinates cannot be large.
- Research Organization:
- Univ. of Rochester, NY
- OSTI ID:
- 7011941
- Journal Information:
- J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 102:20; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400302* -- Organic Chemistry-- Isotope Effects-- (-1987)
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
DATA
EXPERIMENTAL DATA
HYDROGEN ISOTOPES
INFORMATION
ISOTOPE EFFECTS
ISOTOPES
LIGHT NUCLEI
NUCLEI
NUMERICAL DATA
ODD-EVEN NUCLEI
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
ORGANIC SULFUR COMPOUNDS
RADIOISOTOPES
TRITIUM
YEARS LIVING RADIOISOTOPES