Studies on Phenols The Substituent Effect

The radical reactivity of phenols is markedly influenced by substitution. In this paragraph, the substituent effect will be treated in terms of the Hammett equation. All possible substituted phenols are considered except those having t-butyl groups in both ortho positions which are dealt with under IIC. It could be questioned whether phenols containing or(Ao-substituents other than t-butyl groups would not also cause steric 

The separation of steric effects from the gross substituent effect has been one of the problems connected -with the Hammett equation studied for the last two decades (see Shorter, 1970 Laurence and Wojtkowiak, 1970). The probably easier separation of polar and resonance effects could significantly contribute to our understanding of the nature of radical processes. We must agree, however, with DaRooge and Mahoney (1967) that there is an incredible lack of appropriate data in the literature . Consequently, the op treatment has not generally been as careful in radical reactions as in ionic systems. (For a careful treatment in radical reactions, cf. Foldes-Berezhnykh et al., 1969.) 

In our opinion, however, such correlations are useful even if drastic approximations are involved. First, despite the enormous amount of work, the Hammett equation still needs some theoretical foundation. As the apt remark of Shorter (1969) expressed The empirical success of the Hammett equation is thus to some extent a mystery. This mystery is connected with the fact that, while the Hammett equation correlates the rate constant with one structural parameter (cr), the rate constant itself is believed to be dependent on at least two parameters and there are only speculations (Ritchie and Sager, 1964) as to what type of correlations between the latter two parameters may exist. 

On the other hand, even approximate Hammett equations for different radical reactions of phenols permit important comparisons for different systems. In Table 3 data on Hammett equations of substituted phenols in different systems are collected. 

In spite of the large scatter characteristic of some of the earlier data, it is apparent that electron-donor substituents on the phenol always increase the rate constant of reaction (15). The substituent effect was proved in the same sense by substitution in the radical electron-attracting substituents in the radical increased the rate of hydrogen abstraction (Howard and Ingold, 1964c, 1965a, 1965c). 

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