Energy and Stability of Protonated Ketenes: Inductive and Resonance Effects

Abstract

The proton affinities ( PA) of electronegatively-substituted ketenes RCH = C = O (R = H, CH3, NH2, OH and F) at various sites have been assessed by CBS-QB3 calculations. The most favorable protonation site was found to be the CH carbon atom to produce the acylium ions RCH2C+ = O. The PA values, relative to that of CH2 = C = O, can be interpreted in terms of destabilizing effects of the R group in RCH = C = O [ J. Am. Chem. Soc. 113, 6021 (1991)] and by positive or negative inductive effects of R in RCH2C+ = O. For HOCH = C = O the two destabilizing effects are of similar magnitude and this rationalizes that its PA (823 kJ mol−1) is virtually the same as that for CH2 = C = O (820 kJ mol−1). For all ketenes (except for R = H), protonation leads to significant activation of the C=C bond. In the extreme case, protonation of H2NCH = C = O at CH ( PA = 917 kJ mol−1) leads to the weakly bonded complex H2NCH2+••••C = O with a C–C length of 2.92 Å and which only needs 15 kJ mol−1 to dissociate to H2NCH2+ + CO. In fact, the covalently-bonded species, H2NCH2C+ = O, does not exist, the most stable configuration being CH2 = N(H)–H+••••C = O. When NH4+ ( PA[NH3] = 854 kJ mol−1) approaches the NH2 group of H2NCH = C = O ( PA at N = 845 kJ mol−1), the stable hydrogen-bridged cation H3N–H+••••NH2–CH = C = O is produced. The NH4+ group can move over to the CH group or it can attack the CH group directly. After passing a transition state, dissociation follows to NH3 + CH3NH3+ + CO. For neutral methylketene, CBS-QB3 calculates a heat of formation of −63 kJ mol−1, in good agreement with an experimental value (–67 ± 5 kJ mol−1) but not with another experimental number (–95 ± 5 kJ mol−1) and these matters are discussed. Suggestions for further experimental work are proposed to address this discrepancy.