Page 190 - Mechanism and Theory in Organic Chemistry

The Solvent, Substrate, Nucleophile, and Leaving Group 179

It is only more recently that reactions of Charge type 4 have been known
and studied, but the theory proposed by Hughes and Ingold predicted the results
accurately here too, for the reaction of trimethylamine with trimethylsulfonium
ion (Equation 4.12) proceeds more rapidly in nonpolar than in polar solvents.le

In fact, most of the data that have accumulated since 1935 are qualitatively in
accord with their predictions,lg but the predictions should not be used indis-
criminately without considering the effect the solvent may have on nucleophile
and leaving-group reactivity (see pp. 190-194).
Substrate Alkylation
It is a well-established fact_tzi_t S& reactinnssccur less readily in molecules
where the a or 6_c~hnshear_alkyl substituents. For example~~he relative reacti-
vities shown in Table 4.1 are derived from studies of 15 different SN2 reactions in
various solvents. Several explanations have been proposed for this trend. The
first supposes that alkyl groups bonded to saturated carbon are electron-donat-
ing.20 According to this hypothesis, in the ground state the central carbon is
slightly electron-deficient because of the electron-withdrawing ability of the
leaving group, but in the activated complex the positive charge on the carbon
has diminished due to the presence of a second Lewis base with its unshared pair
of electrons. Thus electron-donating groups stabilize the ground state more than
the transition state and thereby increase the activation energy.21
However, recent experiments show that the polar22 influence of a methyl

Table 4.1 AVERAGE RELATIVE SN2 RATES OF ALKYL SYSTEMS
R in R-X Relative Rate
Methyl
Ethyl
n-Propyl
i-Propyl
t-Butyl
Neopentyl
Ally1
Benzyl
SOURCE: Data from A. Streitwieser, Solvo1,ytic Displacement Reactions, McGraw-Hill, New York, 1962,
p. 13. Reproduced by permission of McGraw-Hill.
a This value is not from Streitwieser but is the reactivity of t-butyl bromide to SN2 substitution by
free C1- in DMF at 25°C relative to the reactivity of CH,Br under the same conditions [D. Cook
and A. J. Parker, J. Chem. Soc. B, 142 (1968)l. This value is corrected for the substitution that actu-
ally precedes by an elimination addition mechanism.

la E. D. Hughes and D. J. Whittingham, J. Chem. Soc., 806 (1960).
lB For other examples see: (a) note 15, p. 177; (b) C. K. Ingold, Structure and Mechanism in Organic
Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969, pp. 457-463.
ao (a) C. N. Hinshelwood, K. J. Laidler, and E. W. Timm, J. Chem. Soc., 848 (1938); (b) P. B. D. de
laMare, L. Fowden, E. D. Hughes, C. K. Ingold, and J. D. H. Mackie, J. Chem. Soc., 3200 (1955).
a1 A. Streitwieser, Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962, p. 14.
an The termpolar effect refers to the influence, other than steric, that nonconjugated substituents exert
on reaction rates. It does not define whether the mechanism for its transmission is through bonds
(inductive effect) or through space (field effect).