The Enhanced Acidity of Carboxylic Acids and Enols Relative to Alcohols

4 The enhanced acidity of carboxylic acids and enols relative to alcohols 

In this early application of the TURTLE program, the ability to restrict the wavefunction is used to ascertain the relative importance of 7t-electron delocalisation and induction to the enhanced acidity of carboxylic acids and enols compared to alcohols. These generic classes of molecules are represented by formic acid, vinyl alcohol and ethanol respectively. 

The enhanced acidity of carboxylic acids and enols relative to alcohols has long been attributed to the stabilisation of the carboxylate and enolate anions by delocalisation of their n electrons (see 1 and 2 below). Alkoxide anions, as saturated systems, are not subject to resonance stabilisation. 

The parent acids and alcohols, on the other hand, are not expected to display any significant mesomeric stabilisation, because this would involve the participation of some rather unreasonable Lewis structures with separated positive and negative charges. As a consequence, the tt-delocalisation in 1 and 2 is a factor that lowers the deprotonation energy of carboxylic acids and enols, thus reinforcing their acidity, according to standard organic-chemistry textbooks. 

Since the molecules are planar, there is a strict separation between the a-and the 7t-system. In the calculations, the o-system was handled with the usual delocalised orbitals. For the 7i-system two different models were defined. 

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Hiberty, P. C. Byrman, C. P. Role of x-electron delocalization in the enhanced acidity of carboxylic acids and enols relative to alcohols, 7. Am. Chem. Soc. 1995, 777, 9875-9880. 

Delocalization in the Enhanced Acidity of Carboxylic Acids and Enols Relative to Alcohols. 

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