Acid strength resonance effects

The ionization constants of a series of hindered polynuclear aromatic acids have been determined by Newman and Boden (1961). Two factors are held to affect the acidic strength of such acids the decrease of solvation and the inhibition of resonance due to steric effects. Solvation of an ion should be an acid-strengthening factor and if steric factors decrease the amount of anion solvation, the acid should be correspondingly weaker. The resonance factor only comes into play with aromatic (or unsaturated) acids. Here resonance would involve transfer of... 

This means that the effect of resonance at any one carbonyl group is diminished and it will remain strongly electrophilic. With an ester, there is only one carbonyl group and so it experiences the full impact of the resonance effect. Therefore, its electrophilic strength will be diminished relative to an acid anhydride. 

The resonance effect makes an acid stronger when the resonance energy introduced with the structural change is greater in the ion than in the acid. One of the most important resonance effects on acid strengths occurs when the structural change introduces a double or triple bond next to the atom from which the proton dissociates. The resulting structure is A—B—C H for the acid and... 

Now all that is necessary is to interpret the effect of the groups on the acid strengths of the azo and hydrazone forms. But the effects of groups on acid strength have been very thoroughly discussed in Chapter VI. It was there shown that both the resonance and the inductive effects of a group on the strength of an aromatic acid could be expressed by... 

These three concepts allow a quick inspection of a simple acid (HX) for its relative acid strength. Experimental analysis of the acids commonly encountered in organic reactions requires knowledge of at least three factors (1) electronic effects, (2) resonance effects, and (3) solvent effects. 

In addition to inductive and field effects, resonance effects can also play an important role in determining the strength of organic acids and bases. For example, the pX of a simple alkyl alcohol is 15, but that of the hydroxyl of tyrosine is 9.11. By analogy, the pX of the hydroxyl of phenol is 9.8 (see Table 2.1). Such may be understood by the realization that once the ionization has occured, the phenoxide anion may be stabilized by electron resonance ... 

The occurrence of steric as well as polar substituent effects in aliphatic systems and orf/io-substituted aromatic systems complicates the devising of correlation equations. A typical situation is shown in Figure 2. There is clearly no simple relationship between the rates of alkaline hydrolysis of the ethyl esters of the alkanoic acids and the strengths of the acids themselves. Little progress was made until the early 1950s, when Taft made an excellent start in developing linear free energy relationships in this area. This topic is sometimes referred to as the separation of polar, steric, and resonance effects and is the subject of Section 3. 

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