Transition states of bimolecular eliminations

In the previous section, three general mechanisms of olefin formation by beta-elimination of the elements HX from adjacent carbon atoms, were outlined. Whereas the El and ElcB mechanisms involve preliminary breaking of one bond, the E2 process is concerted, both the C -H and C -X bonds being partially broken at the transition state. When initially designating 

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. 

The choice of transition state utilised by a particular reaction is greatly dependent on the environmental factors, these being substrate structure, base and solvent. Usually it is safer to assign a reaction to a particular range of the spectrum rather than to a specific type. This approach was followed by 

Bunnett in 1962, when the relevant factors affecting the choice of transition state were summarised. Since that date, kinetic techniques have supported the ideas which are listed below. 

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Although certain experimental observations are more easily interpreted by the merged mechanism than the more conventional transition states of bimolecular elimination and substitution, they do not require it. The unusual effectiveness of weak bases to promote elimination reactions is unconvincing, and Bunnett has stated that equilibrium and kinetic basicity need not parallel... 

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