Mechanisms of elimination reactions. 30. The contributions of tunneling and heavy-atom motion in the reaction coordinate to deuterium kinetic isotope effects in eliminations from 2-phenylethyl derivatives

Journal Article · Tue Dec 04 23:00:00 EST 1979 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00519a021· OSTI ID:5738182

Kaldor, S B; Saunders, Jr, W H

The temperature dependence of the rates of elimination of 2-phenylethyltrimethylammonium (1) and 2-phenylethyl-2,2-d/sub 2/-trimethylammonium (1-2,2-d/sub 2/) bromides with hydroxide ion in mixtures of water and dimethyl sulfoxide and with ethoxide ion in ethanol reveals abnormalities in the Arrhenius parameters that are characteristic of tunneling. Fitting a temperature dependence calculated from the Bell theory of tunneling to the observed temperature dependence permits the evaluation of the tunnel correction to the isotope effect, Q/sub tH//Q/sub tD/, and the semiclassical isotope effect, the effect in the absence of tunneling, (k/sub H//k/sub D/)/sub s/. The tunnel corrections are modest but by no means negligible, and the semiclassical isotope effects are well below the values predicted for a transition state with a symmetrically located proton, even though k/sub H//k/sub D/ and (k/sub H//k/sub D/)/sub s/ show maximum values in 40% dimethyl sulfoxide. It is suggested that the contribution of heavy-atom motion to the motion along the reaction coordinate keeps the semiclassical isotope effect small. Examination of selected data from the literature reveals a similar pattern of moderate tunnel corrections and below-maximum semiclassical isotope effects. The implications of these results for the interpretation of deuterium kinetic isotope effects in slow proton transfers are discussed. 5 tables.

• Research Organization: Univ. of Rochester, NY
• OSTI ID: 5738182
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 101:25; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400302* -- Organic Chemistry-- Isotope Effects-- (-1987)
ALCOHOLS
AMINES
AMMONIUM COMPOUNDS
ARRHENIUS EQUATION
BELL THEOREM
BROMIDES
BROMINE COMPOUNDS
DEUTERIUM
DMSO
ETHANOL
HALIDES
HALOGEN COMPOUNDS
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
HYDROXIDES
HYDROXY COMPOUNDS
ISOTOPE EFFECTS
ISOTOPES
LIGHT NUCLEI
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
ORGANIC SULFUR COMPOUNDS
OXYGEN COMPOUNDS
QUATERNARY COMPOUNDS
STABLE ISOTOPES
SULFOXIDES
TEMPERATURE DEPENDENCE
TUNNELING
WATER