Origin of the Acidity of Enols and Carboxylic Acids

Abstract

The origin of the acidity of carboxylic acids and enols has been examined via ab initio MO calculations at the MP2/6-31+G** and Becke3LYP/6-31+G** levels as well as with the CBS-4 and CBS-Q model chemistries. The changes in electron populations that occur on ionization were examined in some detail. In the conversion of vinyl alcohol to its alkoxide ion, the σ electrons of the proton are donated to the oxygen. The repulsion between the increased σ density at oxygen and its π electrons leads to transfer of π electron density to the CC double bond. At the same time, the negative charge at oxygen repels the σ and π electrons of the attached carbon. A part of the negative charge appears at the hydrogens that are in the nodal plane of the π system. With longer chains, σ/π polarization leads to compensating charge shifts so as to minimize the differences between adjacent carbons. When a carbonyl group is attached, as in carboxylic acids and enols, it serves to stabilize the system by accepting some π electron density (∼0.1 e) and by a stabilizing electrostatic interaction between the positively charged carbonyl carbon and the adjacent negatively charged atom. The stabilization is on the order of 40 kcal/mol for carboxylic acids and 28 kcal/mol for enols. The effect of polar solvents on relative acidities also was examined using a new reaction field model, and experimental observations were reproduced.

Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices

“…Until the 90ies, this barrier height has been found [11,14] ranging from 4.7 to 10.0 kcal·mol -1 , the lowest value coming from the work of Frisch et al [13] which emphasizes the role of electron correlation. Consistently, more recent and accurate calculations [15][16][17][18][19][20][21][22][23][24][25][26][27] agree with lower values between 3.1 and 4.6 kcal·mol -1 . Discrepancies originate from the difficulty of describing the complex multidimensional dynamics of this proton tunnelling.…”

The Effect of Lewis Bases on the 13C NMR of Iodoalkynes

“…This observation complements recent descriptions of bonding in carboxylate and enolate ions. 17 In those species, upon proton transfer the σ bonding network undergoes extensive electron-density shifts in response to polarization of the π system; here, the σ and π electron densities shift in response to changes in the adjacent C-I σ bond.The iodoalkynes serve as a reminder that our simplest pictures of Lewis acid-base interactions are incomplete. Iodoalkynes act as electron-pair acceptors because of the polarizability of iodine but also of the alkynyl triple bond, which absorbs much of the additional charge.…”

“…This observation complements recent descriptions of bonding in carboxylate and enolate ions. 17 In those species, upon proton transfer the σ bonding network undergoes extensive electron-density shifts in response to polarization of the π system; here, the σ and π electron densities shift in response to changes in the adjacent C-I σ bond.…”

Substituent Effects on Carbon Acidity in Aqueous Solution and at Enzyme Active Sites

“…14 These estimates reinforce and further define well-established trends. 20 The most striking observation is that the resonance interaction of –OEt (-16.7 kcal/mol) and of –NH 2 (-17.7 kcal/mol) with the carbonyl are similar to one another, and nearly as large as the -20.2 kcal/mol interaction for–O - . This shows that the energetic penalty to the separation of positive and negative charge at zwitterionic resonance structures for ethyl actetate and acetamide (Scheme 6) is small [(2-4)-kcal/mol] in comparison to the 20 kcal/mol stabilization obtained from delocalization of the unit negative charge at the acetate anion.…”