SUBSTITUENT EFFECTS ON KETENE STRUCTURE AND STABILITY - AN ABINITIO STUDY

Calculated structures and energies of ketenes RCH = C = O (R = H, Li, BeH, BH2, CH3, NH2, OH, F, Na, MgH, AlH2, SiH3, PH2, SH, Cl, c-Pr, CH = CH2, C = CH, CF3, CH = O, CN, NC, N = O, CO2H, CH = C = O) at the 6-31G*/6-31G* level reveal major stabilization by electropositive groups and destabilization by electronegative substituents. There is a good correlation of substituent stabilization of ketenes defined by DELTA-E for the isodesmic reaction RCH = C = O + CH3CH = CH2 --> CH3CH = C = O + RCH = CH2 and group electronegativities chi(BE) reported by Boyd and Edgecombe. Thus the primary stabilizinginfluence of substituents on ketenes is evidently by sigma-electron donation to the electronegative ketene moiety. There is also evidence that substituents stabilize ketenes by pi-withdrawal and destabilize ketenes by n-pi-donation. The calculated substituent effects on ketenes are compared to known experimental results and to the corresponding effects on allenes and on beta-ethyl carbocations.