Effect of solvent and radical traps

Kinetic data for the thermal decomposition of benzoyl peroxide in various solvents are given in Table 70 according to rate law (1). Barnett and Vaughan assumed that, by employing low concentrations of peroxide, the observed rate coefficient was equivalent to the first order coefficient, k. In other instances styrene or iodine was used to trap radicals so that induced decomposition would be suppressed and thus the observed rate coefficient would correspond to kf  

It is seen that k varies little with solvent changes, while larger variations are found with ki. The small variations in k may be due to cross-termination or in instances where radical traps were used, to further induced decomposition derived from these traps. Also the employment of low peroxide concentrations may not completely eliminate induced decomposition. With phenol as the solvent, a large deviation in from other values is observed. This value is suspect. It should be noted that low concentrations of peroxide were used with this solvent and it was assumed that the observed rate coefficient corresponded to k. The activation parameters in terms of induced and unimolecular decomposition are given in Table 71. Activation parameters for the first-order term are not significantly influenced by changes in solvent polarity. 

Substituent effects in the decomposition of benzoyl peroxide have been thoroughly investigated. Data in dioxane solvent with added 3,4-dichlorostyrene to prevent induced decomposition are presented in Table 72. A Hammett plot using the sum of the substituent constants gave p = —0.38 (ref. 339). The observation that electron-releasing substituents increased the rate of decomposition was explained in terms of a dipolar interaction between the two aroyloxy groups , viz. 

ACTIVATION PARAMETERS FOR THE DECOMPOSITION OF BENZOYL PEROXIDE ACCORDING TO RATE LAW (1) 

Solvent Ea (kcal.mole ) A (sec ) Ea (kcal.mole ) A Hmote isec )  

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