Page 295 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

276 reaction will be traveled in the reverse direction, because it affords the lowest energy
barrier for either process. Thus, information about the nature of a transition state or
CHAPTER 3 intermediate deduced by a study of a forward reaction is applicable to the reverse
Structural Effects on process occurring under the same conditions.
Stability and Reactivity
The overall rate of the reaction is controlled by the rate-determining step—the
step that has the highest energy requirement and is the bottleneck for the overall
process. In general, steps that occur after the rate-determining step do not enter into
the overall rate equation for the reaction. In the example illustrated in Figure 3.2b, the
first step is rate determining and the overall rate of the reaction would be given by the
rate expression for the first step. In example (c), the second step is rate-determining.

3.2.2.2. Reaction Energy Diagrams with Two or More Dimensions. The reaction
energy diagrams in Figure 3.2 depict reaction progress in terms of rupture and
formation of the affected bonds. The reaction coordinate dimension represents the
extent of completion of the bond-breaking/bond-making process. For example, in the
S 2 reaction shown in Figure 3.2a, the reaction coordinate corresponds to the trajectory
N
for approach of the nucleophile and departure of the leaving group, but also implicitly
includes the inversion of the carbon atom. The reaction coordinate can be expressed in
terms of the Nu−C bond order, which begins as 0 at the left and proceeds to 1 when
the reaction is complete.

Nu: – C X

Many reactions can usefully be described in terms of two-dimensional energy
diagrams in which change in two different bonds are represented. For example, a
nucleophilic substitution reaction at some element X (e.g., silicon) might occur by any
of three general mechanisms:

+
(a) Dissociative mechanism H X −Z → H X + Z −
n n
+
−
H X + Y → H X −Y
n n
(b) − − −
Associative mechanism H X −Z+ Y → H X −Z → H X −Y+Z
n
n
n

Y
−
(c) Concerted mechanism: H X−Z+ Y → Y —H X—Z → H X−Y+Z −
−
n n n
The two-step mechanisms [(a) and (b)] represent paths along the edge of the
two-dimensional reaction energy diagram, differing in the order of the two steps
(Figure 3.3). The single-step (concerted) mechanism involves a more or less diagonal
path with simultaneous bond breaking and bond making, as indicated by path C in
Figure 3.3. These two-dimensional diagrams are often referred to as Albery-More