Role of structural imperfections in the decomposition of dibenzoyl peroxide

Abstract

By employing optical microscopic and goniometric techniques the morphology of solution grown dibenzoyl peroxide has been characterised and the slip systems responsible for emergent dislocations at growth and cleavage surfaces identified. Topographical changes taking place at thermally and photochemically decomposed single crystal surfaces have been correlated with the abnormal structure and stereochemistry of molecules in the vicinity of defective regions.

The kinetics of the isothermal decomposition of single-crystal dibenzoyl peroxide have been studied by micro-gravimetry in high vacua. In all experiments the mass continuously decreases with time in a deceleratory manner, after first traversing a minor acceleratory stage preceded by a very short induction time. The initial nucleation process obeys the Avrami–Erofeeve equation and the bulk reaction, occurring with a deceleratory rate, satisfies a contracting envelope model. Values of the activation enthalpies were found to be 45 kJ mol^–1 and 72.0 kJ mol^–1 for the nucleation stage and the main reaction respectively, for both as-grown and deliberately deformed crystals. The value of 45 kJ mol^–1 for the nucleation process is attributed to the energy required to break the O—O bond of molecules situated at defective regions, and the value of 72 kJ mol^–1 for the main reaction assigned to the induced decomposition of peroxide molecules in perfect regions of the crystal as a result of the attack of phenyl radicals generated in the nucleation process. Phenyl benzoate, the main product of the reaction, is believed to form in a topochemical manner within the dibenzoyl peroxide matrix, the b axis of both structures appear to coincide.