Inductive effects, hydrogen isotope

Both sets of results may also be discussed in terms of inductive differences between hydrogen and deuterium (see Halevi, 1963). Brown et al. (1966) jDoint out that both the inductive and steric explanations qualitatively predict isotope effects in the same direction, but that an inductive effect would be expected to operate from the 3 and 4 positions nearly as effectively as from the 2 position . Furthermore, there is no observable isotope effect on the heat of reaction of 2,6-(dimethyl-de)-pyridine with the relatively small molecule diborane A AH = —20 18 cal mol ), but a significant effect is obtained with the larger molecule boron trifluoride AAH = 230 + 150 cal mol ). 

The inductive effect of the donating C—D bonds to the observed large inverse secondary deuterium isotope effect has not been given proper consideration but treated as a rather minor component superimposed on the important steric component caused by larger amplitudes of vibrations of C—H bonds than those of the C—D bonds. 14C KIE have not been studied in this reaction. The C2, C V. C3 , C3, endo hydrogens are separated only by 2.11 A, substantially less than van der Waals radii (2 x 1.2 A)404. 

The major reaction product formed from the solvolysis of the trans y-silyl ester was cyclohexene formed from the carbocation 138 (Scheme 18) by 1,2-hydrogen migration to give the p-silyl cation 139 followed by loss of the silicon substituent. In contrast to the behavior shown by 136, the cis y-silyl ester 134 exhibited a small inverse p-d4 isotope effect (kH/kn = 0.97) attributed to the inductive effect of the P-deuteriums and implies that there is very little hyperconjugative stabilization of... 

The pattern of deuterium distribution in isotope exchange products of 1,8-nap-hthyridine and its methyl-, hydroxyl-, amino- and nitro derivatives indicates that the rate and pathway of this process depend on the pH of the medium as well as the nature and position of substituent (1988ACH267). In neutral D20 and a D20-DC1 mixture, hydrogen atoms are replaced with deuterons exclusively at positions 2 and 7. In D20 solutions containing NaOD, hydrogen atoms are replaced with deuterons in all positions of the naphthyridine system. In neutral D20, the replacement of the protons in monomethyl-substituted 1,8-naphthyridines occurs predominantly in the ortho positions with respect to the nitrogen atoms (positions 7 and 2, 7). The high reactivity of these positions is determined by the inductive effect of the substituent. 

Secondary /7-deuterium isotope effects are associated with the interaction of the empty p-orbital of the carbenium ion centre with the adjacent tr-carbon-hydrogen/deuterium bond. Small inductive effects are operating in the opposite direction. Release of steric strain is not considered to be associated with isotope effects in these reactions. 

The beta-tritium secondary isotope effect for elimination from the propyl-trimethylammonium ion (8, Table 7) seems most probably explained in terms of reduced acidity of the beta hydrogen caused by the greater inductive effect of the bond to the heavier isotope. A hyperconjugative effect also fits the observed data as in the transition state the developing double bond would be more effectively stabilised by the methyl than the monotritiomethyl group. However, the latter explanation seems less likely as the elimination from the propyl compound is slightly slower than that from the ethyl derivative . 

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