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

✍ Scribed by Minh Tho Nguyen; Greet Raspoet; Luc G. Vanquickenborne

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 207 KB
• Volume: 13
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(200001)13:1<46::aid-poc195>3.0.co;2-4

✦ Synopsis

The protonation and hydration of carbon suboxide (O=C=C=C=O) were studied by ab initio molecular orbital methods. While the geometries of the stationary points were optimized using MP2/6-31G(d,p) calculations, relative energies were estimated using QCISD(T)/6-31G(d,p) and 6-311G(d,p) ZPE. The behaviour of carbon suboxide was compared with that of carbon dioxide and ketene. The protonation at the b-carbon is consistently favoured over that at the oxygen; the proton affinities (PA) are estimated to be PA(C 3 O 2 ) = 775 AE 15 and PA(H 2 CCO) = 820 AE 10 kJ mol À1 (experimental: 817 AE 3 kJ mol À1 ). The PAs at oxygen amount to 654, 641 and 542 kJ mol À1 (experimental: 548 kJ mol À1 ) for C 3 O 2 , H 2 CCO and CO 2 , respectively. Using the approach of one and two water molecules to model the hydration reaction, the calculated 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 is about 31 kJ mol À1 smaller than that of C=O addition. The C=C addition in C 3 O 2 is inherently favoured owing to a smaller energetic cost for the molecular distortion at the transition state, and a higher thermodynamic stability of the acid product. Molecular deformation of carbon suboxide is in fact a fairly facile process. A similar trend was observed for the addition of H 2 , HF and HCl on C 3 O 2 . In all three cases, the C=C addition is favoured, HCl having the lowest energy barrier amongst them. These preferential reaction mechanisms could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks.