Theoretical Study of the Reactivity of Ketene with Free Radicals

The structures and energies for the addition of free radicals R• (R = H, CH 3, OH, F, SiH 3, Cl) to CH 2 C O to give the radicals RCH 2 Ċ O, ĊH 2 (C O)R, CH 2 ĊOR have been calculated by ab initio and B3LYP-DFT methods, and the latter method gives good agreement with available experimental energies. Product radicals ĊH 2 C( O)R for groups R which possess electron lone pairs are stabilized and have predominant spin density on carbon, and this is attributed to conjugation of the carbonyl group in the product with substituents OH, F, and Cl at the α-position. Additions of H and SiH 3 have lower barriers to form the more stable product RCH 2 Ċ O, which for the latter is favored due to hyperconjugative stabilization by the β-SiH 3 . For CH 3 attack at both carbons is competitive, while for OH, F, and Cl, the barriers are low for attack at either carbon, although attack at C α gives much more stable products. Initial complexes between ketene and the CH 3, OH, SiH 3, and Cl radicals are detected, and for Cl using B3LYP this species has the structure of a π-complex with the C C double bond that is stabilized by 16.2 kcal/mol relative to the reactants and forms ĊH 2 C( O)Cl with a barrier of 2.8 kcal/mol. For F no barriers for addition to either carbon were found, but for B3LYP there is a barrier of 27.6 kcal/mol for conversion of FCH 2 Ċ O to ĊH 2 C( O)F, which is more stable by 19.1 kcal/mol. The corresponding rearrangement of ClCH 2 Ċ O has a barrier of 4.6 kcal/mol, and the predicted preference for initial attack at C β to give the less stable product agrees with experiment.