Kinetic and Spectroscopic Studies on Acyl Radicals in Solution by Time-Resolved Infrared Spectroscopy

Abstract

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 v_{C =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 v_{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, 6.0x10⁴; 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.