Question

Which of the following statements is not true?
A.Acetic acid is stronger than chloro acetic acid.
B.Formic acid is the strongest of all aliphatic monocarboxylic acids.
C.Benzoic acid is a stronger acid than acetic acid.
D.Propanoic acid is weaker than acetic acid.

Answer 1

Acidity is basically the ability of a chemical species to donate protons or $\,{H^ + }\,$ ions to a medium or according to Lewis theory, it's the ability to accept lone pairs. The more stable the conjugate base of the acid, the stronger the acid will be.

Complete step by step answer:
Let us analyze each option one by one;
-Acetic acid is weaker than chloroacetic acid. Here this statement is not correct because, in Chloroacetic acid, due to the presence of chloro group (- I effect) stabilization of carboxylate ion increases. Hence it is more acidic than acetic acid.
-Formic acid is stronger among all aliphatic monocarboxylic acids, because it has an electron-withdrawing hydrogen atom near the carboxyl group.
-Benzoic acid is stronger than acetic acid because the alkyl group's electron-donating inductive effect (+ I) on acetic acid destabilised the acetic acid conjugate base (i.e., the acetate ion). In comparison, benzene rings are considered to be electron dense and draw electrons away from groups attached to them.
-Propanoic acid is weaker than acetic acid is also true. Here we have to consider the phenomena of solvation. Since propanoic acid has a greater alkyl chain, lower solvation of the anion is required relative to acetic acid, which in turn contributes to lower stabilisation of this anion (in the case of water solution, by hydrogen bonds).Lower stabilisation represents a change in the balance of the dissociation reaction and consequently produces a higher $pKA$ value for propanoic acid.
Hence, the correct option to this question is Option A.

Note: The strength of weak acids is measured on the $pKA$ scale. The smaller the value of $pKA$ , the stronger is the acid. Factors that influence the ionization of acid are the strength of the breaking of bonds and the stability of ions being formed.