Hyperconjugative isotope effect

C which suggests a hyperconjugative isotope effect. Further, there is a kinetic isotope effect on both the homo- 1,5- and 1,7-hydrogen shifts of 4. 

Reasons can be given for preferring the comparison between acid enolization of cyelohexyl phenyl ketone and basic enolization of cyclopentyl phenyl ketone. Emmons and Hawthorne have also determined the primary isotope effect of deuteration at the a-carbon atom. For these two reactions kjt/kr> = 6.7 and 6.2, respectively, indicating similar extensive bond rupture in the transition state. Acid enolization of cyclopentyl phenyl ketone shows a smaller primary isotope effect, kn/kv = 4.0 it is also much faster, suggesting by Hammond s (137) well-known postulate that the transition state is more reactant-like. It is reasonable to conclude that in this case bond rupture is leas far advanced, and electron de-localization leas complete, so that the full hyperconjugative isotope effect is not developed. 

The view was advanced in Sec. IIIC, 3(b) that a hyperconjugative isotope effect on the stability of radicals should occur, even though that on the stability of fully formed carbonium ions might be negligible. The reason given was that only the difference in Hs C and D C overlap would affect radical stability, whereas carbonium ion stability is affected in the opposite sense by the isotopic electropositivity difference. 

Secondary isotope effects at the position have been especially thoroughly studied in nucleophilic substitution reactions. When carbocations are involved as intermediates, substantial /9-isotope effects are observed. This is because the hyperconjugative stabliliza-... 

Kinetic studies of mercuration have also been used as a test for hyperconjugation. Toluene and toluene-aaa-mercuric acetate (0.5 M) in acetic acid containing water (0.25 M) and perchloric acid (0.050 M) and an isotope effect, ArH/ArD = 1.00 0.03, obtained. This insignificant effect was considered as evidence against the participation of hyperconjugation in electron supply by a methyl group449. 

Solvolytic experiments specifically designed to test Bartell s theory were carried out by Karabatsos et al. (1967), who were primarily interested in an assessment of the relative contributions of hyperconjugation and non-bonded interactions to secondary kinetic isotope effects. Model calculations of the (steric) isotope effect in the reaction 2- 3 were performed, as well as that in the solvolyses of acetyl chloride... 

The calculated isotope effects for the solvolyses of t-butyl chloride-dg (previously estimated by Bartell, 1961a) and acetyl chloride-ds were much weaker than the experimentally observed ones. The authors conclude (Karabatsos et al., 1967) that in ordinary systems where hyperconjugation (from the j8-position) is possible, the effect of non-bonded interactions accounts for only a small part (less than 10%) of the observed isotope effect. 

This corresponds to an isotope effect of approximately 3.5% per deuterium. In comparison, the secondary /3-deuterium KIEs in SN1 reactions are all normal and range from 5% to 15% per deuterium. Because the normal KIEs in SN1 reactions result from the weakening of the C,—L bond by a hyperconjugative interaction with the incipient carbocation in the transition state, the authors concluded that hyperconjugative interactions are present also in the transition state for the insertion reaction. The normal secondary /3-deuterium KIE observed for the insertion reaction is consistent with the dipolar three-centre transition state structure [15] proposed by Seyferth et al. (1970a,b) because the partial positive charge on the a-carbon is stabilized by hyperconjugation. 

It is important to point out once again that explanations (rationalizations) of isotope effects which employ arguments invoking hyperconjugation and/or steric effects are completely equivalent to the standard interpretation of KIE s in terms of isotope independent force constant differences, reactant to transition state. In turn, these force constant differences describe isotope dependent vibrational frequencies and frequency differences which are not the same in reactant and transition states. The vibrational frequencies determine the partition functions and partition function ratios in the two states and thus define KIE. The entire process occurs on an isotope independent potential energy surface. This is not to claim that the... 

Similar conclusions attend the insertions of CCI2 (from the thermolysis of ClsCCOONa at 120 °C) into a-deuteriocumene and cumene in which the primary fen/feo = 2.6, similar to Seyferth s finding with 32, and the p-secondary kinetic isotope effect is 1.20-1.25 for six deuteriums. Here, hyperconjugation at the p-CH (CD) bonds is thought to stabilize the partial cationic charge at the reaction center in transition state 33. 

When reactions were run with substrate deuterated in the ortho position, isotope effects of about 1.22 were obtained.23 It is difficult to account for such high secondary isotope effects in any other way except that an incipient phenyl cation is stabilized by hyperconjugation,24 which is reduced when hydrogen is replaced by deuterium. 

Solvolysis of 15 in 97% trifluoroethanol gave a secondary isotope effect of 1.17, which indicates a vertically stabilized transition state. Thus the highly unsymmetrical dihedral dependence of silicon participation can almost entirely be attributed to the hyperconjugation model with little non-vertical involvement of the silicon nucleophile. 

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