Transition state reactantlike

Figure 5-7. Reaction coordinate diagram for Fig. 5-6. The final state is more stable than the initial state, and the transition state is reactantlike.

This valence bond description leads to an interesting conclusion. Because the transition state occurs at the point where the initial and final state VB configurations cross, the transition state receives equal contributions from each. This is so whether the transition state is early or late. Thus, the nucleophile Y and the leaving group X possess about equal charge densities in the transition state. This conclusion means that an early transition state is not (in this sense) reactantlike , for a reactantlike transition state should have most of the charge on Y. Similarly, a late transition state is not necessarily productlike. This view is at variance with other interpretations. 

If a monoarylacetylene (ArC = CH) is taken as a model for a transition state of an arenediazonium ion with a nucleophile Nu, two types of transition state can be visualized the first, 7.13, leads to the (Z)-azo compound 7.14, whereas the second, 7.15, results in the (E )-isomer 7.16 (Scheme 7-3). If the transition state is reactantlike (i.e., early on the reaction coordinate), repulsive interaction between the nucleophile and the aryl nucleus is small because the distance Nu-Np is still large. Therefore, the repulsion between the lone pair on Np and the aryl nucleus becomes the decisive factor. It favors an (E )-configuration of the Np lone pair with respect to the aryl nucleus (obviously it is energetically dominant compared with the repulsion between the lone pairs on Na and Np) therefore, transition state 7.13 is at a lower energy level, and Nu attacks NB in the (Z)-configuration. 

Similarly, from Fig. 15, the activation energy for hydrogen abstraction by H can be determined to be about 10-12 kcal/mol. This value again compares very well with the available experimental value of 12.5 kcal/mol (Kerr and Drew, 1987), As is also evident from Figs. 14 and 15, both reactions proceed via tight and early transition states, i.e., the interatomic distances remain within bonding distances and the geometry of the transition state exhibits reactantlike features. 

However, even if we put 0Sn2 = 1, the transition states are still too reactantlike. Hence the plausible choice of 0.66 for the limiting value for is probably wrong. If one takes a value of 0.69, which is the value for L30+, then one would obtain /x = 0.41. 

Q Use the Hammond postulate to predict whether a transition state will be reactantlike or product-like, and explain how this distinction affects the selectivity of a reaction. 

As pointed out earlier, addition of organocuprates to enones followed by alkylation of the resultant enolates generates two carbon-carbon bonds in a single reaction. Alkylation of an enolate proceeds via an early, hence a reactantlike, transition state. Thus, steric factors in the ground state play an important role. 

Negative charge accumulates on in both the carbanion-like E2 and reactantlike E2 transition states and positive charge develops on C in the carbanion ion-like E2 and reactant-like E2 transition states. Bond breaking can be small or extensive in three of the transition states but is well-developed always in the product-like E2 transition state, which alone has considerable double-bond character. For the remaining transition states, double-bond character approximates to the least stretched of the bonds undergoing fragmentation. 

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