Secondary isotope effects and tunneling in elimination reaction of quaternary ammonium salts

Thesis/Dissertation · Fri Dec 31 04:00:00 EST 1993

OSTI ID:111296

Lin, S

In order to gain more experimental evidence of the tunneling effect on the non-transferred isotopically-substituted hydrogen in the rate determining step and to investigate in more detail concerning the variable nature of the E2 transition state, tritium tracer-labeled {beta}- and/or {alpha}-phenyl substituted ethyltrimethylammonium ions in the elimination reaction and their 2,2-d{sub 2} analogues were studied. The three different substrates are 2-(p-trifluoromethylphenyl)ethyltrimethylammonium bromide (I), 1-phenylethyltrimethylammonium bromide (II) and 1-phenyl- 2-p-chlorophenylethyltrimethylammonium bromide (III). The reactions were found to proceed via a concerted E2 process. The proton is more than one-half transferred to the base at the transition state, especially for case I and III. There is more C{sub beta}-H and less C{sub alpha}-N bond rupture at the transition state when an electron-withdrawing group is introduced on the {beta}-phenyl ring, i.e., more carbonion character in the transition state. The secondary tritium isotope effects were measured and they were found larger than the maximum value. (1.17) for rehybridization. It was found that these values were strongly temperature dependent. They increase as the temperature goes down. The Arrenhius pre-exponential factors were below unity and the exponential factor to convert the isotope effect of D/T to H/T were much greater than predicted for zero-point energy effects alone (3.26). Tunneling turns to be the only plausible explanation. By reviewing this evidence, one may conclude that the contribution of tunneling is indeed a common occurrence in proton transfer processes. For elimination of compound I, the tunneling effect is greatest while it is less for the other two, which are about the same, but the nature of their transition states is quite different. The variable nature of the transition states for the three eliminations are discussed in terms of the More O`Ferral-Jencks diagram.

• Research Organization: Rochester Univ., NY (United States)
• OSTI ID: 111296
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
40 CHEMISTRY
ALUMINIUM COMPOUNDS
ISOTOPE EFFECTS
TEMPERATURE DEPENDENCE
TUNNEL EFFECT