Competition between alkyl radical addition to carbonyl bonds and H-atom abstraction reactions
The activation energies for the methyl radical addition to CH2O, CH3CHO, and (CH3)2CO, are small and around 28, 29, and 40 kJ mol−1, respectively. For the addition of primary alkyl radicals to formaldehyde and higher aldehyde homologues, we find significantly lower activation barriers of (20 ± 4) kJ mol−1 and (27 ± 2) kJ mol−1, respectively. An almost negligible activation barrier of about 5 kJ mol−1 was found for the secondary radical addition to formaldehyde. In the case of addition of a tertiary radical to formaldehyde no activation barrier could be identified. Generally, methyl addition reactions to carbonyls will compete with hydrogen abstraction reactions. For larger alkyl radicals addition to carbonyls dominates over the direct H-atom abstraction. A self-consistent set of standard enthalpies of formation ΔfH° for saturated alkoxy (CnH2n+1O˙, where n ⩽ 5) radicals was obtained by the modified Gaussian-3 (G3(MP2)//B3LYP) ab initio molecular orbital theory. The calculated heats of formations are in good agreement with literature values estimated from O–H bond dissociation energies. The expected maximum error of the calculated ΔfH° is less than 4 kJ mol−1. The primary, secondary, and tertiary alkoxy group values (GV) were evaluated from the ab initio ΔfH°-s with a standard deviation of 1.2 kJ mol−1.