Contrasting mechanism of the hydration of carbon suboxide and ketene. A theoretical study

The protonation and hydration of carbon suboxide (O=C=C=C=O) were studied b y ab initio molecular orbital methods. While the geometries of the stationa ry points were optimized using MP2/6-31G(d,p) calculations, relative energi es were estimated using QCISD(T)/6-31G(d,p) and 6-311++G(d,p)+ZPE. The beha viour of carbon suboxide was compared with that of carbon dioxide and keten e. The protonation at the beta-carbon is consistently favoured over that at the oxygen; the proton affinities (PA) are estimated to be PA(C3O2) = 775 +/- 15 and PA(H2CCO) = 820 +/- 10 kJ mol(-1) (experimental: 817 +/- 3 kJ mo l(-1)). The PAs at oxygen amount to 654, 641 and 542 kJ mol(-1) (experiment al: 548 kJ mol(-1)) for C3O2, H2CCO and CO2, respectively. Using the approa ch of one and two water molecules to model the hydration reaction, the calc ulated results consistently show that the addition of water across the C=O bond of ketene, giving a 1,1-ethenediol intermediate, is favoured over the C=C addition giving directly a carboxylic acid. A reverse situation occurs in carbon suboxide. In the latter, the energy barrier of the C-C addition i s about 31 kJ mol(-1) smaller than that of C=O addition. The C=C addition i n C3O2 is inherently favoured owing to a smaller energetic cost for the mol ecular distortion at the transition state, and a higher thermodynamic stabi lity of the acid product. Molecular deformation of carbon suboxide is in fa ct a fairly facile process. A similar trend was observed for the addition o f H-2, HF and HCl on C3O2. In all three cases, the C=C addition is favoured , HCl having the lowest energy barrier amongst them. These preferential rea ction mechanisms could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks, Copyright (C) 2000 John Wiley & So ns, Ltd.