Question

Which one of the following is correct?
Formic acid has lower $p{k_a}$ than that of $C{H_3}COOH$ because :
A. Formic acid does not dissociate
B. Formic acid does not have an alkyl group
C. Formic acid is smaller in size than acetic acid
D. Formic acid is a strong reducing agent

Answer 1

The $p{k_a}$ value is a metric that indicates how strong an acid is. It is the negative logarithm of the acid dissociation constant, ${k_a}$ . The strength of an acid in a chemical solution is measured by the dissociation constant. It is the equilibrium constant in equilibrium.

Complete answer:
As formic acid is a stronger acid than acetic acid, which lacks an alkyl group, it has a lower $p{k_a}$ than $C{H_3}COOH$ . Because the methyl group in acetic acid is an electron-donating group that can destabilise the conjugate base's negative charge, formic acid is more acidic than acetic acid.
The acidity of acids is affected by the presence of an alkyl group. Alkyl groups have an inductive action, which causes the electron density on the oxygen atom to increase, preventing the breaking of the $OH$ bond. The inductive effect increases as the number of alkyl groups increases, and the dissociation rate of the $OH$ bond decreases. As a result, the acid's ionisation constant, or $p{k_a}$ value, will be larger.
$HCOOH$ is does not have any alkyl group in it so, it will dissociate into ${H^ + }$ ions easily and due to greater ionization, it will have the lowest $p{k_a}$ than $C{H_3}COOH$ .
Hence, the correct option is B. Formic acid does not have an alkyl group.

Note:
Higher ${k_a}$ values, i.e., higher ionisation constant, are associated with lower $p{k_a}$ values. It implies that the acid's acidity will be higher. As a result, substances with low $p{k_a}$ values are more acidic than those with higher $p{k_a}$ values.