Electron-directing and steric effects solvents

2 FACTORS AFFECTING INHIBITOR EFFICIENCY 2.2.1 Electron directing and steric effects solvents 

In many respects, the behaviour of amino and phenolic inhibitors can be predicted on the same grounds, because of the fact that the peroxy radical is an electron acceptor and prefers to react with a centre of high electron density [41]. As a result, electron-releasing substituents, such as alkoxy or alkyl groups, where they do not sterically hinder the reaction, will improve antioxidant performance. Bulky ortho substituents, which involve steric influences, may retard the rate of reaction [42], although this will obviously depend on individual systems. The effect of phenolic inhibitors has been dealt with elsewhere [1], but it is of interest to consider some factors that throw light on the mechanism and kinetics of amino inhibition. 

Attempts to relate the efficiency of inhibition with the Hammett— Taft equation were soon abandoned in favour of more accurate correlations with Brown s electrophilic substituent constants, o+, as given by [43] 

Brown s o factor are very similar [2], and it was possible for Boozer and his coworkers [8] to relate the action of phenols and amines in these simpler terms. 

To reinforce the point that a similar electron directing effect operates, results published by Emanuel [32] for the autoxidation of a mixture of hydrocarbons in the presence of substituted 7V,7V -dialkyl-p-phenylene-diamines show that substituents raising the electron density at the inhibitor reaction centre increase the efficiency of the antioxidant, while substituents that decrease electron density decrease inhibitory powers. No attempt was made to quantify these observations in terms of a or a factors. 

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