Page 257 - Mechanism and Theory in Organic Chemistry

Application of the Reacting Bond Rules .
It may be useful at this point to analyze the reaction coordinate in greater detail
than we have done so far. We shall be making use of the reacting bond rules
(Section 2.6, p. 104). Briefly summarized, the rules predict that (1) the effect of
structural changes on location of the transition state with respect to the reaction
coordinate should follow the Hammond postulate, that is, the easier a process the
less advanced it will be at the transition state; and (2) the effect of structural
change on location of the transition state with respect to a bound vibration will
be opposite to the Hammond behavior, that is, the easier the process the more
advanced it will be at the transition state. The discussion to follow is based on
that of Thornton,'15 but is extended through reaction coordinate diagrams of the
kind proposed by More O'Ferrall1l6 and developed by Jencks.l17
We want to consider a simplified nucleophilic substitution scheme consisting
of the entering nucleophile, N, the carbon undergoing substitution, C, and the
leaving group, X. An SN2 reaction will have a transition state with both N and
X partially bonded to C (29). The transition state may be tighter (30) or looser
(31) ; it may also be unsymmetrical, with bond making to N more advanced (32)
N...C...X N..C..X N....C ....X
29 30 31
or less advanced (33) than bond breaking. An SN1 reaction will have the C-X

bond partly broken, but no bonding to N, at the transition state of the ionization
step (34). We shall represent the solvated ion pair that is the intermediate first
N C-...X N C+ X-
34 35
formed on ionization of C-X by 35; here N would be a solvating solvent mole-
cule. We shall suppose that N does not move appreciably closer to C during the
ionization, although we know this assumption to be an oversimplification, as is
our neglect of other kinds of ion pairs.
There are two structural parameters of interest in this scheme: the N-C
distance and the C-X distance. We construct in Figure 5.9 a diagram of the
potential energy surface as a function of these two parameters for an SN2 process.
The horizontal plane represents the two coordinates, so that at the back left-
hand corner are the reactants, N + C-X. Coming forward to the front of the
diagram represents increasing the C-X distance, so that the left front corner is
ion pair 35, with N still in its original position but the C-X bond broken.
Going from left to right represents decreasing the N-C distance, so that at the
right front is product, +N-C + X-. At the back right is a hypothetical penta-
coordinate state, with both N and X bonded to C. The height above the plane at
each point represents the free energy for that particular combination of C-X

115 E. R. Thornton, J. Amer. Chem. Soc., 89, 2915 (1967).
lla R. A. More O'Ferrall, J. Chem. Soc. B, 274 (1970).
"' W. P. Jencks, Chem. Rev., 72, 705 (1972).