The Mechanism of Diacyl Peroxide Decomposition

Abstract

Various substituted 4-X-diphenylacetyl Y-benzoyl peroxides have been prepared and characterized in situ by low temperature NMR and IR spectroscopy. These peroxides decompose in non-nucleophilic solvents below -20°C, with net retention(13%, X^H^, Y=N02 in methylene chloride), formingester in high yields. No evidence of radical in­volvement, such as solvent-derived products, radicalcoupling products or CIDNP, was observed in thisdecomposition. The corresponding carboxy inversion compounds (benzhydryl4-nitrobenzoic carbonic carboxylic anhydrides) were not obtained as products of, nor were they intermediates in, diacyl peroxidedecomposition.Three 4-X-benzhydryl 4-nitrobenzoic carboniccarboxylic: anhydrides (X=C1, H, CH^) were preparedand characterized. These compounds decompose with net retention (29%; X=CH^; methylene chloride) in nonnucleophilic solvents to afford ester in high yield. Decomposition in chloroform at 26°C occurs with a Hammett -a+ p = -3.84 and innitromethane-dg with p = -4.36iiR eproduced with permission of the copyright owner. The activation parameters for these decompositionsin chloroform-d,- were calculated for X=H and X=C1:6 *AEact 13-14 Kcal/mol, Asl6 = 37-39 cal/°mol.The decomposition of these diacyl peroxidesin 90$ aqueous acetone afforded only alcohol,acid and ester. The fraction ester (R) producedwas similar to that obtained in the solvolysis ofsubstituted benzhydryl-N-nitrosoamides, indicatinga similar spectrum of ion pair intermediates. TheR value could be increased by the addition ofcommon ion (.Jj-NOg-benzoate) from R=0.54 to R=0.95.This common ion effect, coupled with the formationof alcohol as a major product, is conclusiveevidence for the intermediacy of ion pairs in thisdecomposition. Preliminary results obtained in thephotodecomposition of diphenylacetyl 4-nitrobenzoylperoxide in methanol-tetrahydrofuran suggests somedecomposition is occurring via a. radical pathway.The mixed carbonic carboxylic anhydrides under­went two modes of decomposition in 90$ aqueousacetone: 1) decomposition to ion pair intermediateswhich gave alcohol, acid, and ester as products;and 2) decomposition by nucleophilic attack at the carbonyl giving alcohol and 4-nitrobenzoic