Acidity of benzoic acids

Table 19 3 lists the ionization constants of some substituted benzoic acids The largest effects are observed when strongly electron withdrawing substituents are ortho to the carboxyl group An o nitro substituent for example increases the acidity of benzoic acid 100 fold Substituent effects are small at positions meta and para to the carboxyl group In those cases the values are clustered m the range 3 5-4 5... 

Comparison of the gas-phase acidity of benzoic acids with pAT values of the same compounds in aqueous solution provides some interesting relationships. 

Benzoic acid Methyloxonium ion Conjugate acid of benzoic acid Methanol... 

For the aromatic carboxylic acids, substituents on the aromatic ring may also influence the acidity of the acid. Benzoic acid, for example, has = 4.3 x 10 . The placements of various activating groups on the ring decrease the value of the equilibrium constant, and deactivating groups increase the value of the equilibrium constant. Table 12-2 illustrates the influence of a number of para-substituents upon the acidity of benzoic acid. 

All models provide a reasonable account of the effects of remote substituents on the acidity of benzoic acid. The performance of STO-3G and 3-2IG models is comparable to their performance for amine basicities. Also noteworthy is the fact that the 6-3IG basis set is adequate for these types of comparisons, that is, the effect of diffuse functions (in the 6-311+G basis set) largely cancels. Also encouraging (and unexpected), is the excellent account provided by all three semi-empirical models. 

Salicylic acid is the lactic acid of benzoic acid —... 

Organic chemists have studied the influence of substituents on various reactions for the better part of a century. Linear free energy relationships have played an important role in this pursuit by correlating equilibrium and rate processes. One of the earliest examples is now known as the Hammett equation. It emerged from the observation that the acidities of benzoic acids correlated with the rates at which ethyl esters of benzoic acids hydrolyzed. The relationship was expressed as follows in which K represents an equilibrium constant and k is a rate constant. The proportionality constant, m, is the slope of the log-log data plot for the two processes. 

The effea of substituents on ionization has already been discussed in Chapter 2, where the Hammett substituent constant was described, and in Chapter 4 in connection with the acidity ol phenols, These effects influence the relative acidity of benzoic acid derivatives in a similar way. 

Substituents in the 3- and 4-positions of the phenyl ring influence the acidity of benzoic acids in accordance with their ability to donate or withdraw electron density from the carboxyl function. Electron-donating substituents decrease the acidity through their +I and +M effects this effect is quite small, as can be seen from the data in Table 5.1. 

Problem 18.8 There is evidence that certain groups like / -methoxy weaken the acidity of benzoic acids not so much by destabilizing the anion as by stabilizing the acid. Draw structures to show the kind of resonance that might be involved. Why would you exisect such resonance to be more important for the acid than for the anion ... 

Reactions with a positive p-value are accelerated (or the equilibrium constants of analogous equilibria are increased) by substituents with positive a-constants. Since the sign of a was defined so that substituents with a positive a increase the acidity of benzoic acid, such substituents are generally described as attracting electrons away from the aromatic ring. It follows that... 

Electron-withdrawing substituents increase the acidity of benzoic acids and phenols ... 

Thus from the experimentally obtained acidities of benzoic acids with a particular substituent at the meta and para positions, we obtain insight into the inductive and resonance effects for that substituent. 

This means that substituents that can withdraw electrons conjugatively will have a greater effect on the acidity of phenols (reaction 3.9) than on the acidity of benzoic acids. The difference will only be appreciable at the para position for the meta position, no through resonance is possible. If we make a plot of logATA for the acidities of mcta-substituted phenols against Hammett cr constants, we find that the points lie on a straight line, the slope of which gives the p value for the reaction as +2.23... 

Sotomatsu T, Murata Y, Fujita T. Correlation analysis of substituent effects on the acidity of benzoic acids by the AMI method. J Comput Chem 1989 10 94-98. 

The effect of m- and p-Me3Si groups on the acidity of benzoic acids has been studied by several groups38-42. Apparent ionization constants for these compounds are given in... 

From the results summarized in Table I, apparently the Brpnsted relationship will hold for all combinations of nucleophiles and electrophiles. Because, as pointed out previously, the Hammett equation is really a special case of the Brpnsted relationship, all the legion of nucleophile-electrophile, rate-equilibrium Hammett correlations that have been studied also fall under the scope of the Brpnsted relationship. For example, nucleophilicities of ArO , ArS , ArC(CN)2 , and the other families listed in footnote c of Table I have generally been correlated by the Hammett equation, where the acidities of benzoic acids in water are used as a model for substituent interactions with the reaction site (a), and the variable parameter p is used to define the sensitivity of the rate constants to these substituent effects. The Brpnsted equation (equation 3) offers a much more precise relationship of the same kind, because this equation does not depend on an arbitrary model and allows rate and equilibrium constants to be measured in the same solvent. Furthermore, the Brpnsted relationship is also applicable to families of aliphatic bases such as carboxylate ions (GCH2C02 ), alkoxide ions (GCH20 ), and amines (GCH2NH2). In addition, other correlations of a kinetic parameter (log fc, AGf, Ea, etc.) can be included with various thermodynamic parameters (pKfl, AG°, Eox, etc.) under the Brpnsted label. 

Substituent constants have been widely used in organic chemistry for the interpretation of electronic effects on chemical reactivity [79, 80]. However, it is well known that the character of the electronic effects can differ quite substantially between different chemical systems and that it is not possible to find a universal set of substituent constants. In this section we will demonstrate how the electrostatic potential can be used as an alternative and a complement to the traditional constants in analyses of electronic substituent effects. The main advantage with this approach is that we can study systems whose substituent effects are unknown, or systems for which the substituent effects cannot be described by the traditional constants. An analysis of the electrostatic potential can also provide a direct insight to the effects the substituents have on the electronic structures of the molecules. We will both discuss the classical examples of substituent effects on the acidities of benzoic acid and phenol, and a more novel example, i.e. the substituent effects on the 0-H bond dissociation energy in phenols. 

We have also discussed the use of the electrostatic potential for the analysis of substituent effects in aromatic systems. Substituent effects on gas phase and solution acidities of benzoic acids and phenols are dominantly determined by the relative stabilization of the negative charge in the ionized forms of these systems. The oxygen Vmin is an excellent tool for the analysis of this stabilization effect. On the other hand, we have found that the homol5dic O-H bond dissociation energy in phenols depends both on the substituent s ability to stabilize the parent molecule (the phenol) and the radical. The relative stabilization energies of the parent molecule and the radical can be estimated from their computed Vmin and surface maxima in the spin density, respectively. 

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