Thermodynamic Stability of Substituted Carbonyl Groups

Because of the high-energy LUMO, nucleophilic substitution at the carbonyl is greatly slowed down compared to other X -substituted derivatives [205]. If the geometric constraints of the molecular framework force the nitrogen to be distorted from planarity, the T2N-7t interaction is reduced. As a consequence, reactivity of the carbonyl toward nucleophilic attack is increased [206], as is the nucleophilicity of the nitrogen atom [207]. 

Theoretically derived values were compared to some which could be derived from experimental data. The calculated values agreed quite closely with the experimental values where both were available. We present here a list of the calculated ATf values for the above reaction as G, ATT (kJ/mol), and refer the reader to the original reference for a detailed discussion [208] H, -43 CH3, -39 NH2, +77 OH, +93 F, +67 SiHs, -53 PH2, -16 SH, +23 Cl, +28 CN, -46 CH=CH2, -2 C=CH, -23 and CF3, -52. Thus, electropositive groups (SiH3) and both Z (CN, CF3) and C (CH=CH2, C=CH) substituents prefer attachment to methyl rather than carbonyl, possibly because more stabilization is available to them from 7r-type (hyperconjugative) interactions with the methyl group orbitals than with the carbonyl group orbitals. 

Copyright 2001 John Wiley Sons, Inc. ISBNs 0-471-35833-9 (Hardback) 0-471-22041-8 (Electronic) 

Orbital Interaction Theory of Organic Chemistry, Second Edition. Arvi Rauk 

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