Energy diagrams competing reactions

The Hammond Postulate implies that the transition stah of a fast exothermic reaction resembles the reactants (se( reaction energy diagram at left). This means that it wil be hard to predict the selectivity of competing exothermi( reactions both barriers may be small and similar even i one reaction is more exothermic than the other. 

Figure 6.13 Energy diagrams for two similar competing reactions. In (a), the faster reaction yields the more stable intermediate. In (b), the slower reaction yields the more stable intermediate. The curves shown in (a) represent the typical situation.

Figure 14.5 An energy diagram for two competing reactions in which the less stable product B forms faster than the more stable product C.

Figure 2.24 The energy diagrams for two competing reactions. Reaction E to F has the lower barrier to reaction, whereas reaction E to G produces the more stable product. As the reaction becomes more exothermic on the right, it becomes less reversible as the return barriers increase.

In Chapter 2, we discussed conformational equilibria of organic molecules. At this point, let us consider how conformational equilibria can affect chemical reactivity. Under what circumstances can the position of the conformational equilibrium for a reactant determine which of two competing reaction paths will be followed A potential energy diagram is shown in Figure 3.17. It pertains to a situation where one conformation of a reactant would be expected to give product A and another product B. This might occur, for example, in a stereospecific anti elimination. 

The simplest supramolecular species capable of performing such type of process are covalently-linked three-component systems ("triads"). Two possible schemes for charge separating triads are shown in Fig. 5. Although the scheme in Fig. 5b is reminiscent of the natural photosynthetic reaction center, that of Fig. 5a seems to be more popular in the field of artificial triad systems. The functioning principles are shown in an orbital-type energy diagram in the lower part of Fig. 5. In both cases, excitation of a chromophoric component (1) is followed by a primary photoinduced electron transfer to a primary acceptor (2). This is followed by a secondary thermal electron transfer process (3) electron transifer from a donor component to the oxidized chromophoric component (case a), or electron transfer from the primary acceptor to a secondary acceptor component (case b). The primary process competes with excited-state... 

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