Solvent effect on the rate of decomposition of peroxy compounds

Abstract

Solvent effect on the rate of decomposition of peroxy compounds was described by multiparameter equations based on the linear Gibbs energy relationship. In the decomposition of dialkyl peroxides, the determining factors are the cohesion energy density of solvents and their ability to electrophilic solvation, which accelerate the process. In the case of hydroperoxides, the nonspecific solvation factor should additionally be taken into account. Solvent effects on the rate of heterolytic reactions can be described on the basis of the linear Gibbs energy relationship using multiparameter linear equations. Analysis of the significance of particular solvation terms in these equations could give valuable information on the reaction mechanism [1-3]. The number of analogous studies in the field of homolytic reactions is considerably smaller. A probable reason is relatively low sensitivity of homolytic reactions to solvent effects; moreover, these reactions can involve a number of steps and are characterized by a high probability of side processes. The above stated also applies to decomposition of peroxy compounds in various media. This process usually takes two concurrent pathways: unimolecular homolytic and initiated decomposition, whose rates differently depend on the solvent properties. Therefore, appropriate analysis of the apparent rate constants is impossible.However, inhibition of the initiated decomposition makes it possible to analyze solvent effects on a quantitative level. An almost classical example is the publication by Pincock [4], where decomposition of tertbutyl peroxyformate in the presence of pyridine was studied in 20 solvents [5,6]. Analogous general conclusions can be drawn for the inhibited decomposition of diacyl peroxides: in all cases, the rate of the process is determined only by solvation factors, primarily by nucleophilic solvation but not by self-association of the medium. These data confirm the absence of cage effect (i.e., initiated decomposition) [7].Study of the decomposition of benzoyl peroxide in various solvents showed that the main factor determining initiated decomposition is the cohesion energy density of the medium while the rate of homolytic uninitiated decomposition depends mostly on the solvation ability which originates from the polarity and basicity of the solvent [8]. The effect of self-association becomes significant in the inhibited decomposition of tert-butyl peroxybenzoate and tert-butyl o-phenylsulfanylperoxybenzoate.Up to now, solvent effects on the decomposition of only two groups of peroxy compounds, diacyl peroxides and peroxyacid esters, have been studied on a quantitative level. Moreover, the conclusions drawn on the basis of these studies cannot be regarded as strictly unambiguous. Therefore, we thought it reasonable to perform analogous correlations for other classes of peroxy compounds. As previously [6-9], the correlations were based on the extended Koppel'-Pal'm equation: log k = a 0 + a 1 ──── + a 2 ──── + a 3 B + a 4 E T + a 5 δ 2 + a 6 V M .(1)Here, n and ε ...