Page 241 - Mechanism and Theory in Organic Chemistry

the small rate differences, make it unclear whether alkyl groups are electron-
donating or -withdrawing compared to hydrogen in these compounds.
The Hammett o-p correlation has proved useful in studying substituent
effects of aryl-substituted systems.55 In Section 6.1 we consider applications of
this linear free-energy relationship to solvolysis with rearrangement.
Isotopic substitution will also affect rates.56 Most of the isotope effects ob-
served in SN1 substitutions are for H-D substitution; the isotopically substituted
bond is not broken in the reaction, and the observed secondary isotope effect
ratios (Section 2.7, p. 109) kH/kD are less than 1.5. Substitution of H by D on the
carbon to which the leaving group is attached leads to the a-isotope effect, with
kH/kD ratios of between 1.22 and 1.25 for systems with sulfonate leaving groups
which appear to react by a limiting SN1 mechanism, and somewhat lower
(C1 z 1.15, Br 2 1.13) for limiting reactions with halide leaving groups.57
As we have seen in Section 2.7, the origin of the rate change is in the out of plane
bending vibration, which decreases in frequency on going from the sp3-hybrid-
ized starting 'material to the transition state, where hybridization is approaching
sp2. The presence of the leaving group and an entering nucleophile nearby
stiffens the bond and makes the frequency change smaller; hence SN2 reactions
show little or no rate change on isotopic substitution. The a-isotope effect is thus a
measure of the degree of participation by nucleophile at the transition state.58
Substitution of D for H at the kcarbon produces effects typically around kH/kD
1.07 per deuterium, but as high as 1.44 for favorable conformations. These effects
are thought to reflect delocalization of the positive charge to the p=C-H
bonds, a point we shall consider further in Section 10.2, and thus to measure
the degree of charge development at the transition state.59 This interpretation
has, however, been que~tioned.~~

The Entering Group
The limiting SN1 mechanism predicts that an added nucleophile, unless it is
the common ion, will have no effect on reaction rate. We have seen in Section
5.1 how the inclusion in the mechanism of ion pairs accounts for the observed
deviations from this principle; the point of interest here is the product distribu-
tion obtained when more than one nucleophile is present.
If none of the products re-ionize to a detectable extent during the time the
system is under observation, their relative amounts will be kinetically controlled

56 (a) A. Streitwieser, Jr., H. A. Hammond, R. H. Jagow, R. M. Williams, R. G. Jesaitis, C. J.
Chang, and R. Wolf, J. Amer. Chem. Soc., 92, 5141 (1970); (b) Streitwieser, Solvolytic Displacement
Reactions, pp. 179-180.
66 For discussions see: (a) H. Simon and D. Palm, Angew. Chem. Int. Ed., 5,920 (1966); (b) A. Streit-
wieser, Solvolytic Displacement Reactions, pp. 172-1 74 (a effects) ; pp. 98-101 (,G effects).
57 (a) J. M. Harris, R. E. Hall, and P. v. R. Schleyer, J. Amer. Chem. Soc., 93, 2551 (1971); (b) V. J.
Shiner, Jr., and R. D. Fisher, J. Amer. Chem. Soc., 93, 2553 (1971); (c) T. W. Bentley, S. H. Liggero,
M. A. Imhoff, and P. v. R. Schleyer, J. Amer. Chem. Soc., 96, 1970 (1974); (d) E. A. Halevi, Pros.
Phys. Org. Chem., 1, 109 (1963); (e) V. J. Shiner, Jr., W. E. Buddenbaum, B. L. Murr, and G.
Lamaty, J. Amer. Chem. Soc., 90, 418 (1968); (f) A. Streitwieser, Jr. and G. A. Dafforn, Tetrahedron
Lett., 1263 (1969); (g) G. A. Dafforn and A. Streitwieser, Jr., Tetrahedron Lett., 3159 (1970).
58 See note 56.
59 See note 56 and (a) V. J. Shiner, Jr., J. Amer. Chem. Soc., 82, 2655 (1960); (b) V. J. Shiner
and J. G. Jewett, J. Amer. Chem. Soc., 86, 945 (1964).
60 L. S. Bartell, Tetrahedron Lett., No. 6, 13 (1960).