Carboxylic acid aliphatic, inductive effects

Steric effects on the reactivity of benzoic acid derivatives are a little more complicated. The first significant point is that the aromatic carboxylic acids and esters are often some two orders of magnitude less reactive than the corresponding aliphatic compounds. Chapman et al. 7, have explained this difference in terms of three co-operative factors (i) the stabilization of the initial state by delocalization in the case of the aromatic compounds, (ii) inductive electron-withdrawal by the ring, which is significant in esterification... 

Recently, a paper was published in which the acidities of aliphatic carboxylic acids were analysed in terms of a quantum mechanical model. The inductive effect of Taft was shown to be composed of one electrostatic and one polarization term [37]. We will discuss examples of similar approaches based on the hardness and electronegativity as strictly defined concepts of density functional theory in the chapter on amides and peptides, and also discuss the problem of ill-defined terms a bit further in the chapter on carbocations, coming next. 

We don t usually look to aromatic systems for examples of inductive effects, because the pi system of electrons is ripe for resonance effects. However, in analyzing the resonance forms of phenoxide on the next page, it becomes apparent that the negative charge is never distributed on the meta carbons. Meta substituents cannot exert any resonance stabilization or destabilization at the meta position, substituents can exert only an inductive effect. The series of phenols demonstrates this phenomenon, consistent with aliphatic carboxylic acids. 

Acidity of an a-Carboxyi Group (Section 27.2A) An a-COOH pK approximately 2.19) of a protonated amino acid is a considerably stronger acid than acetic acid 4.76) or other low-molecular-weight aliphatic carboxylic acid, owing to the electron-withdrawing inductive effect of the a-NHj+ group. 

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