Question: 70. The correct order of decreasing acidity of the following aliphatic acids is:
(1) \left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCOOH}>\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}>\mathrm{CH}_{3}\mathrm{COOH}>\mathrm{HCOOH}
(2) \mathrm{CH}_{3}\mathrm{COOH}>\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}>\left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCOOH}>\mathrm{HCOOH}
(3) \mathrm{HCOOH}>\mathrm{CH}_{3}\mathrm{COOH}>\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}>\left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCOOH}
(4) \mathrm{HCOOH}>\left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCOOH}>\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}>\mathrm{CH}_{3}\mathrm{COOH}
Answer: Option (3)
Explanation:
Acidity of carboxylic acids depends on the stability of the conjugate base formed after loss of a proton.
Alkyl groups show a +I effect, which increases electron density on the carboxylate ion and decreases its stability.
As the number of alkyl groups attached to the carbon chain increases, the +I effect increases and acidity decreases.
Formic acid \mathrm{HCOOH} has no alkyl group, so it shows maximum acidity.
Acetic acid \mathrm{CH}_{3}\mathrm{COOH} has one methyl group, causing a slight +I effect.
Isobutyric acid \left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH} has two methyl groups, increasing the +I effect further.
Tert-butyric acid \left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCOOH} has three methyl groups, showing the maximum +I effect and hence the least acidity.
Therefore, the correct decreasing order of acidity is:
\mathrm{HCOOH}>\mathrm{CH}_{3}\mathrm{COOH}>\left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCOOH}>\left(\mathrm{CH}_{3}\right)_{3}\mathrm{CCOOH}