Page 191 - Mechanism and Theory in Organic Chemistry

group when bound to sp3 carbon is not electron-donating. Alkyl groups, due to
their greater polarizability than hydrogen, appear to be either electron-with-
drawing or electron-donating depending on the electronic demands of the neigh-
boring atomsz3 (see Section 3.4). In any case the difference between the polar
effect of methyl and that of hydrogen is very small-much too small to account
for the large retardation observed with increasing substit~tion.~~ This is especially
obvious in the neopentyl system, where the branching is one carbon atom re-
moved from the reaction site but the rate of substitution is ca. times slower
than in the ethyl system.
An explanation standing on firmer experimental ground is that the decrease
in rate with increasing substitution is caused by nonbonding interactions in the
transition state. This explanation was first proposed by Dostrovsky, Hughes, and
Ingoldz5 and was later refined in a series of eight papersz6 that were then
summarized by Ing01d.~~
In the ground state all H-C-H and H-C--X bond angles in CH3X are,
of course, approximately 109.5". The activated complex of an S,2 reaction on this
substrate is more crowded: the H-C-X bond angles have decreased to 90°,
whereas the H-C--H bond angles increase to 120" and an'additional atom or
group of atoms (Y:) is included which also forms an angle of only 90" with the
protons. Ingold et al. calculated that in CH3X, because the protons are small,
there is little, if any, increase of nonbonding interaction between X and Y and
the protons in going from the ground state to the transition state. However, when
one of the H's is replaced by the much larger CH, group (i.e., when CH3CHzX
is the substrate), the interference between X and Y and the methyl group does
increase as the angle between them decreases. This leads to (1) compressions of
the covalent bonds to lengths shorter than normal and (2) a corresponding in-
crease in potential energy.z8 Thus the activated complex of the ethyl system has
a higher potential energy relative to the starting material than that of the methyl
system.
A closely related argument for the decrease in rates with increasing substi-
tution, put forward by Bauer, Ivanoff, and Magat,z9 is that it is not the enthalpy
(through the potential energy), but the entropy of activation that is affected.
They propose that the greater number of atoms associated with the transition
state as compared to the ground state restricts the motion of these atoms; this

23 (a) H. Kwart and L. J. Miller, J. Amer. Chem. Soc., 83, 4552 (1961); (b) H. Kwart and T. Take-
shita, J. Amer. Chm. Soc., 86, 1161 (1964); (c) R. C. Fort and P. v. R. Schleyer, J. Amer. Chem. Soc.,
86, 4194 (1964); (d) V. W. Laurie and J. S. Muenter, J. Amer. Chem. Soc., 88, 2883 (1966); (e)
J. I. Brauman and L. K. Blair, J. Amer. Chem. Soc., 90, 6561 (1968); (f) N. C. Baird, Can. J. Chem.,
47, 2306 (1969).
24 (a) H. D. Holtz and L. M. Stock, J. Amer. Chem. Soc., 87, 2404 (1965); (b) H. J. Hinze, M. A.
Whitehead, and H. H. Jaff.4, J. Amer. Chem. Soc., 85, 148 (1963).
25 I. Dostrovsky, E. D. Hughes, and C. K. Ingold, J. Chem. Soc., 173 (1946).
26 F. B. D. de la Mare, L. Fowden, E. D. Hughes, C. K. Ingold, and J. D. H. Mackie, J. Chem. Soc.,
3169 (1955) et seq. See also C. K. Ingold, Structure and Mechanism in Organic Chemistry, pp. 544ff.
27 C. K. Ingold, Quart. Rev. (London), 11, 1 (1957); see also D. Cook and A. J. Parker, J. Chem. Soc.
B, 142 (1968).
aa See Figure 2.0.
N. Ivanoff and M. Magat, J. Chim. Phys., 47, 914 (1950); E. Bauer and M. Magat, J. Chim. Phys.,
47, 922 (1950).