Conjugate base energies

Procedures to compute acidities are essentially similar to those for the basicities discussed in the previous section. The acidities in the gas phase and in solution can be calculated as the free energy changes AG and AG" upon proton release of the isolated and solvated molecules, respectively. To discuss the relative strengths of acidity in the gas and aqueous solution phases, we only need the magnitude of —AG and — AG" for haloacetic acids relative to those for acetic acids. Thus the free energy calculations for acetic acid, haloacetic acids, and each conjugate base are carried out in the gas phase and in aqueous solution. 

Obtain the energies of the various conjugate bases of ascorbic acid. Which one is the most stable Is it the base which results from deprotonation of the hydrogen you previously assigned as the most acidic ... 

S02—group decreases dramatically the energy level of the HOMO. This low reactivity can be overcome since the conjugated base of the sulphone, which is obviously richer in reactive electrons, can produce the corresponding free neutral radical which is able to dimerize. 

Bordwell129 developed a method of estimating relative bond dissociation energies (BDE) for families of acids, HA, by combining equilibrium acidity constants, pXeA, with the oxidation potential of their conjugated bases, A, both measured in DMSO ... 

The pKi, values of a series of para- and meffl-substituted benzaldoximes and phenyl methyl ketoximes, ArCR=NOH (R=H, Me), have been measured in DMSO. The aldoximes exhibit pK. = 20.05 + 3.21ap. The homolytic bond dissociation energy of the O-H bond has been estimated as 88.3 (aldoximes) and 89.2kcal mol" (ketoximes) by relating the pK to the oxidation potential of the conjugate base (i.e. ox for ArCR=NO- ArCR=NO ). 

Because acidity depends on the difference in energy of the acid and its conjugate base, we must be sure that the stabilization of the enolate anion by electron delocalization represented by 14a and 14b is greater than the analogous stabilization of the neutral enol represented by 15a and 15b ... 

A variation on Scheme 27 and Equation (19) has been utilized by Keeffe and the author to evaluate p fas of alkyl, aryl and alkoxy carbenes.26 For carboca-tions for which the pKA for loss of a proton from a (3-carbon atom is known, combination of this pKA with the experimental or calculated energy difference between alkene and carbene conjugate bases leads to the pAa for protonation of the carbene, provided it can be assumed that the energy difference between alkene and carbene is insensitive to solvent. Where a pKA for loss of a (3-hydrogen of the carbocation is not accessible, for example, for carbenes lacking a (3-hydrogen, p fR can be used instead. Thus the cycle of Scheme 28 relates a pKa for protonation of the carbene to an experimentally measured... 

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