The correct order of decreasing acidity of the following aliphatic acids is:
A
$(\text{CH}_3)_3\text{CCOOH} > (\text{CH}_3)_2\text{CHCOOH} > \text{CH}_3\text{COOH} > \text{HCOOH}$
B
$\text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} > (\text{CH}_3)_3\text{CCOOH} > \text{HCOOH}$
C
$\text{HCOOH} > \text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} > (\text{CH}_3)_3\text{CCOOH}$
D
$\text{HCOOH} > (\text{CH}_3)_3\text{CCOOH} > (\text{CH}_3)_2\text{CHCOOH} > \text{CH}_3\text{COOH}$
The answer is C.
Solution
Alkyl groups are electron-donating ($+I$) which destabilize the carboxylate anion, decreasing acid strength: $\text{HCOOH} > \text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} > (\text{CH}_3)_3\text{CCOOH}$.
Acidity of carboxylic acids depends on the stability of the conjugate carboxylate anion ($\text{RCOO}^-$). Electron-donating inductive groups ($+I$) intensify the negative charge on the carboxylate group and destabilize it, thereby reducing acid strength. Formic acid (HCOOH) has no alkyl group and is the strongest acid. Acetic acid has one $+I$ methyl group, isobutyric acid has an isopropyl group (two methyls), and pivalic acid has a tert-butyl group (three methyls), making it the weakest acid. The decreasing order of acidity is $\text{HCOOH} > \text{CH}_3\text{COOH} > (\text{CH}_3)_2\text{CHCOOH} > (\text{CH}_3)_3\text{CCOOH}$.
ConceptInductive Effect on Carboxylic Acid Acidity
Acidity of Carboxylic Acids in past papers
3 questions from this chapter have appeared across 3 exam years.
