6 of 6 formatting & typo

Comparing acidities of substituted and aromatic carboxylic acids

When comparing the acidities of carboxylic acids, we primarily see the electropositivity of the carboxylic acid carbons, i.e. we see how effectively the negative charge on the carboxylate ion is dispersed upon ionization. Electron withdrawing groups increase the acidity by stabilising the anion via electron withdrawal.

Now consider the following two comparisons:-

  1. Formic acid ($\ce{HCOOH}$) and benzoic acid($\ce{C6H5COOH}$)
  2. Benzoic acid and 3-chloropropanoic acid ($\ce{ClCH2CH2COOH}$)

$$\begin{array}{c|c} \hline \text{Species} & \mathrm{p}K_\mathrm{a} \\ \hline \ce{HCOOH} & 3.75\mathrm{-}3.77 \\ \ce{PhCOOH} & 4.19 \\ \ce{ClCH2CH2COOH} & 3.98 \\ \hline \end{array}$$


My questions:

  1. Why is 3-chloropropanoic acid less acidic than formic acid. The negative inductive effect of chlorine helps in dispersing the negative charge on the carboxylate ion. It is not conjugated and too far to exert positive mesomeric effect. Therefore, the negative inductive effect should increase the acidity by improving the stability of the conjugate base.
  1. Why is benzoic acid weaker than formic acid? Unless I am mistaken, in several other places, the phenylic moiety is treated as slightly electron withdrawing, as in case of alcohol acidity, acidity of hydrogens in base catalysed reactions etc. I am unable to understand the resonance here or any other factor, that might cause this anomaly.