Inductive effects in aliphatic carboxylic acids

Problem 19.12 Draw the products of each acid-base reaction. / NaOH 

Problem 19.14 Rank the labeled protons (Hg-Hc) in mandelic acid, a naturally occurring carboxylic acid in plums and peaches, in order of increasing acidity. Explain in detail why you chose this order. 

The pATa of a carboxylic acid is affected by nearby groups that inductively donate or withdraw electron density. 

The relative acidity of CH3COOH, CICH2COOH, and (CH3)3CCOOH illustrates these principles in the following equations. 

We first learned about inductive effects and acidity in Section 2.5B. 

Summary The relationship between acidity and conjugate base stability for acetic acid, phenol, and ethanol 

The number, electronegativity, and location of substituents also affect acidity. 

---

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. 

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... 

---