A number of acyl radicals, RĊ=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 RĊ=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 RĊ=O radicals some typical values of vC =O are: CH3Ċ=O, 1864 cm-1; (CH3)3CĊ=O, 1848 cm-1; and C6H5Ċ=O, 1828 cm-1, while the corresponding acylperoxyl radicals, RC(O)OO, formed by reaction with oxygen have vC =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, 6.0x104; C6H5SH, 4.8×107; CCl3Br, 2.2×108; Tempo, 1.1×109; and oxygen, 1.8×109. Alkanoyl radicals have a rather similar reactivity to benzoyl. The propanoyl radical reacts with tributyltin deuteride with a rate constant of 3×105 M-1s-1. The hex-5-enoyl radical undergoes a 5-exo-trig cyclization to form the 2-oxocyclopentylmethyl radical with a rate constant of 2.2×105 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.

The combination of laser flash photolysis (LFP) with timeresolved infrared (TRIR) detection provides a new and potentially extremely powerful tool for the measurement of absolute rate constants for fast reactions and for structural studies on reactive intermediates. However, IR bands generally have small extinction coefficients relative to UV bands, and this has restricted the application of the LFP/TRIR technique almost entirely to the study of metal carbonyl systems.4"7 The single exception in organic chemistry is Schuster and co-workers' investigation of 1,2-didehydroazepines.8It seemed important to us to widen the organic applications of this valuable new technique. The /3-scission of alkoxyl radicals in solution should be an ideal reaction to study since ketones (and aldehydes) have rather strong IR absorptions within the 1600-2000-cm"1 range of line-tunable carbon monoxide lasers. There has been only one direct, time-resolved measurement of an alkoxyl /3-scission in solution (vis.,9 C6H5CH2CH20• -C6H5CH2* + CH20). Reliable /3-scission rate data for other systems are required in order to understand why solvents would appear to have such a profound effect on fc^9'13 as well as to correlate k$ with structure.

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