Systematic structural variation

In this last example, the hope is that the para-methyl group will have too weak an electronic or steric effect and, in any case, will be too far away to affect the outcome. It is intended to make nearly as slight a change in the structure as an isotopic label. Many structural investigations have exactly the opposite hope. Some systematic change is made in the structure of the molecule in the expectation of a predictable change in rate. A faster or slower reaction will lead to some definite conclusion about the charge distribution in the transition state. Allylic compounds can react efficiently with nucleophiles by either the SN1 or SN2 mecha nisms. Here are two examples.

The carbon skeleton is the same in both reactions but the leaving groups and the nucleophiles are different. These reactions might both go by SN1 or SN2 or one might go by SN1 and the other by SN2. One way to find out is to make a large change in the electronic nature of the carbon skeleton and see what happens to the rate of each reaction. In these experiments one of the methyl groups was changed for a CF3 group—exchanging a weakly electron-donating group for a strongly electron-withdrawing group. If a cation is an intermediate, as in the SN1 reaction, the fluorinated compound will react much more slowly. Here is the result in the first case.

The fluorinated compound reacts half a million times more slowly so this looks very much like an SN1 mechanism. The slow step in an SN1 mechanism is the formation of a carbocation so any group that destabilizes the positive charge would have (and evidently does have) a large effect on the rate. Rate ratios of several powers of ten often are worth noticing and a rate ratio of nearly 106 is considerable. In the second case the rate difference is much less.

A rate ratio of 11 is not worth noticing. The point is not that the fluorinated compound reacts faster but that the two compounds react at about the same rate. This strongly suggests that no charge is generated in the transition state and an SN1 mechanism is not happening. The SN2 mechanism makes good sense with its concerted bond formation and bond breaking requiring no charge on the carbon skeleton.

The CF₃ group works well here as a mechanistic probe because it is held well out of the way of the reaction site by a rigid π system but is connected electronically by that same allylic system. Steric effects should be minimized and electronic effects clearly seen. This approach is clearly limited by the small number of groups having properties like those of the CF₃ group and the small number of reactions having such favourable carbon skeletons. We will now present the most important serious correlation between structure and reactivity.

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مواضيع ذات صلة

The Hammett relationship

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct

Cross metathesis