Acidity resonance / delocalization effects

The following groups of compounds illustrate the profound effect that resonance delocalization has on the stability of anions and hence the acidity of the conjugate acids. To compare the acidities of these acids, the conjugate bases can be ranked according to their resonance stabilization and that ranking of anion stabilization is predictive of the acidity orders. 

The intrinsic delocalized (resonance) electrical-effect parameter. It represents the delocalized electrical effect in a system with zero electronic demand. cTg The electronic demand sensitivity parameter. It adjusts the delocalized effect of a group to meet the electronic demand of the system. cTj) A composite delocalized electrical-effect parameter which is a function of and cTg.Examples of constants are the ctr and ctr constants.The ctr constants, where k designates the value of the electronic demand rj, are also examples of Ojy constants. ctr a composite delocalized electrical-effect parameter of the ctd type with rj equal to 0.380. It is derived from 4-substituted benzoic acid pK values. 

A more complete discussion of acidity and electronic effects can be found is Appendix 2.) A few words about the two types of electronic effects induction and resonance. Inductive effects are a result of polarized a bonds, usually because of electronegative atom substituents. Resonance effects work through n systems, requiring overlap of p orbitals to delocalize electrons. 

Exner and Carsky suggested that conclusions about the relative importance of resonance/delocalization versus Coulombic/inductive effects on the acidity of carboxylic acids in the gas phase should not be generalized implicitly to discussions of acidities in solution. They reported calculations indicating that both the inductive effect and the resonance effect of the carbonyl group are attenuated in aqueous solution but that the inductive effect is reduced to a greater extent. ° As Exner and Carsky noted, however. 

According to the resonance effect, certain groups, such as nitro groups, make phenols more acidic (especially at the ortho and para positions) because they can stabilize phenoxide anions through additional resonance delocalization of the negative charge. 

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

Note that m-nitrophenol has pATa 8.4, and is a lot less acidic than o-nitrophenol or p-nitrophenol. We can draw no additional resonance structures here, and the nitro group cannot participate in further electron delocalization. The increased acidity compared with phenol can be ascribed to stabilization of resonance structures with the charge on a ring carbon through the nitro group s inductive effect. 

As we move to A-methylaniline, we see only a modest change in pK ,. This is undoubtedly due to the electron-donating effect of the methyl group, and this would be expected to stabilized the conjugate acid, increasing observed basicity. There is a modest increase in basicity, but it is apparent that the resonance effect, as in aniline, is also paramount here, and this compound is also a weak base. However, diphenylamine (A-phenylaniline) is an extremely weak base this can be ascribed to the resonance effect allowing electron delocalization into two rings. 

The model of the polymer derived from magnetic resonance and fluorescence spectroscopy studies has proved very useful in suggesting other types or reactions besides hydrolytic ones in which catalytic effects might be achieved. For example, it has been shown by Kemp and Paul46-47 that the decarboxylation of certain benzisoxazole carboxylic acids is very markedly accelerated in apolar, aprotic solvents, in contrast to water. Such an apolar solvent apparently lowers the energy of the charge-delocalized transition state in this decarboxylation reaction. Since... 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

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