Electronic Substituent Effects on Reaction Intermediates

Curtin, Rec. Chem. Prog., 15, 111 (1954) E. L. Eliel, Stereochemistry of Carbon Compounds, McGraw-Hill, New York, 1962, pp. 151-152, 237-238. 

Eor a more complete discussion of the relationship between conformational equilibria and reactivity, see J. 1. Seeman, Chem. Rev., 83, 83 (1983). 

The halogens are an interesting group. They act as electron donors by resonance involving the unshared pairs, but this is opposed by a polar effect resulting from their 

Alkoxy, Acyloxy, Amino, Acvlamido RO, RC02,R2N,RC0NH, etc ERG EWG 

Carbocations have a vacant orbital that bears a positive charge. Within the standard hybridization framework, there are five possible hybridization types if all the electrons are paired. 

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Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

While tocopherylacetic aicd (51), the lower Crhomologue of 3-(5-tocopheryl)-propionic acid (50) showed a changed redox behavior (see Section 6.5.1), compound 50 displayed the usual redox behavior of tocopherol derivatives, that is, formation of both ortho- and para-quinoid oxidation intermediates and products depending on the respective reaction conditions. Evidently, the electronic substituent effects that... 

The reverse reaction is determined by the nature of R. The generating [Pt(PR3)2] moiety is a rather stable intermediate. A comparison of the results obtained with the NMR data (22) of substituted phenylacetylenes led to the conclusion that there is a relation between equilibrium constant, first-order rate constants, and chemical shifts of the acetylenic proton they all depend on substituent effects on the electron density in the triple bond. 

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