KINETIC AND SPECTROSCOPIC STUDIES ON ACYL RADICALS IN SOLUTION BY TIME-RESOLVED INFRARED-SPECTROSCOPY

A number of acyl radicals, RC=O, have been generated in hexane or di-t-butyl peroxide as solvent at room temperature by 308 nm laser flash photolysis, and their spectroscopic and kinetic properties have been examined by time-resolved infrared spectroscopy. The C=O stretching frequencies for the RC=O radicals are found to be higher than those of the corresponding aldehydes, RCHO, by between 108 and 128 cm(-1), an effect attributed to a higher C=O bond order in the radicals. For the RC=O radicals some typical values of nu(C=O) are: CH3C=O, 1864 cm(-1); (CH3)(3)CC=O, 1848 cm(-1); and C6H5C=O, 1828 cm(-1) while the corresponding acylperoxyl radicals, RC(O)OO formed by reaction with oxygen have nu(C=O) values of 1838, 1840 and 1820 cm(-1), respectively. The acyl radicals exhibit a reactivity towards a variety of substrates that is roughly comparable to that of simple alkyl radicals. For reactions of the benzoyl radical some typical rate constants/M(-1) s(-1) are: CCl4, 6.0x10(4); C6H5SH, 4.8x10(7); CCl3Br, 2.2x10(8); Tempo, 1.1x10(9); and oxygen, 1.8x10(9). Alkanoyl radicals have a rather similar reactivity to benzoyl. The propanoyl radical reacts with tributyltin deuteride with a rate constant of 3x10(5) M(-1) s(-1). The hex-5-enoyl radical undergoes a 5-exo-trig cyclization to form the 2-oxocyclopentylmethyl radical with a rate constant of 2.2x10(5) s(-1) a value which is almost identical to that for cyclization of the hex-5-enyl radical. It is hoped that our kinetic data will prove useful in the planning of organic synthetic strategies which involve acyl radical chemistry.