El/E2/ElcB spectrum

One way to determine just where a given reaction stands on the El-E2-ElcB spectrum is to study isotope effects, which ought to tell something about the behavior of bonds in the transition state. For example, CH3CH2NMe3 showed a nitrogen isotope effect of 1.017, while PhCH2CH2NMe3" gave a corres-... 

As in the El-E2-ElcB mechanistic spectrum, Bredt s rule applies and if a double bond is present, a conjugated system will be preferred, if sterically possible. Apart from these considerations, the following statements can be made for Ei eliminations ... 

The factors that decide which elimination path will occur are listed in Figure 4.22. For the El and the ElcB, solvation of the ionic intermediate is very important. The El and ElcB should be considered the two extremes of a spectrum of elimination mechanisms in which the E2 is in the center. 

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

Fig. 26 Energy diagrams showing the mechanistic spectrum of elimination reactions (a) concerted E2, (b) concerted E2 with El character, (c) stepwise El, (d) concerted E2 with ElcB character, (e) stepwise ElcB. (f) concerted E2C and (g) stepwise E2C-I...

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