Page 192 - Mechanism and Theory in Organic Chemistry

The Solvent, Substrate, Nucleophile, and Leaving Group 181

causes a decrease in the entropy of the system, which in turn raises the free energy
of the transition state.
Streitwieser points out that these enthalpy and entropy effects are inter-
related.,O If, in the transition state of a direct displacement reaction of Y: on
CH,CH2X, the C-X and C-Y bonding distances were identical to those in a
similar substitution on CH,X, the nonbonding interactions would be so great as
to cause a large decrease in the freedom of the CH, group and thus a large de-
crease in entropy. If, however, the distances between C and X and Y were so
great at the transition state that no decrease in entropy occurred, there would
be little bonding between C and X and between C and Y; in such a case the
potential energy and thus the enthalpy of the transition state would be raised.
What probably happens is that a compromise is achieved and the system adopts
that configuration for the transition state that has the smallest increase in free
energy.
When studying Table 4.1 it may be surprising to see that neopentyl systems,
in which all the substituents are in the P position, are substituted more slowly than
t-butyl systems in which the substituents are directly on the reaction site. Ap-
parently, steric hindrance is less important when the larger substituents are a,
because in the activated complex they all lie in a plane perpendicular to X and Y
and thus are fairly well out of the way; only one P substituent can lie in this plane,
as shown in Figure 4.4. If there are only one or two P substituents, it is still
possible to rotate them out of the way of entering and leaving groups. However,
when there are three, as in the neopentyl group, it is not possible, and substantial
steric hindrance occurs in the transition state.31
Heteroatoms in the Substrate
If the suggestion mentioned on p. 179, that there is less positive charge on the
central carbon in the transition state than in the ground state, is correct, then
electron-withdrawing substituents should decrease the rate of SN2 substitutions.
A variety of experiments testing this suggestion have been carried out, giving
inconsistent results. Sometimes electron-withdrawing substituents accelerate and
sometimes they decelerate SN2 reaction^.,^
For example, the rate of displacement of bromide by thiophenoxide ion in
I-bromo-2-chloroethane is slowed down by a factor of 5 compared to substitution
in the structurally similar 1-bromopropane (Equations 4.13 and 4. 14).33 But the

Y
Figure 4.4 Transition state for SN2 substitution in the neopentyl system.

30 A. Streitwieser, Solvent Displacement Reactions, p. 23.
S1 C. K. Ingold, Structure and Mechanism in Organic Chemistry, pp. 547ff.
For a summary see Streitwieser, Solvent Displacement Reactions, pp. 16-20.
33 J. Hine and W. H. Brader, Jr., J. Amer. Chem. Soc., 75, 3964 (1953).