Read the passage carefully and answer the questions.

Carboxylic acids are the organic compounds which comprise of carboxyl functional group, -COOH. These are generally found in nature, some higher members are known as fatty acids as they occur in natural fats as esters of glycerol. They can be prepared from oxidation of alcohols and aldehydes, hydrolysis of nitriles and amides and Grignard reagents. They are stronger acids than simple phenols and alcohols. but weaker than mineral acids. Substituents affect the acidity of aliphatic and aromatic carboxylic acids. They undergo reactions involving cleavage of -OH and C-OH bond. Carboxylic acids having an alpha hydrogen get halogenated in the presence of red phosphorus to form alpha halo carboxylic acids. The reaction is known as Hell Volhard Zelinsky reaction. Besides these aromatic carboxylic acids undergo electrophilic substitution reactions. They are versatile and are employed in the manufacture of various useful compounds. For example, methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Higher fatty acids are used in the manufacture of soaps and detergents. Esters of Benzoic acid are used in perfumery.

Identify the strongest acid

Benzoic acid

The correct answer is Option (1) → Benzoic acid

To identify the strongest acid among the given carboxylic acids, we need to consider acidic strength, which depends on the stability of the conjugate base (i.e., carboxylate ion) formed after loss of a proton (H⁺). The more stable the conjugate base, the stronger the acid.

Let’s analyze each option:

(A) Benzoic acid (C₆H₅–COOH):

• Aromatic ring attached to the –COOH group.

• The phenyl ring is electron-withdrawing through resonance and inductive effect.

• This stabilizes the carboxylate ion, increasing acidic strength.

(B) Phenylacetic acid (C₆H₅–CH₂–COOH):

• The –CH₂ group separates the phenyl ring from the –COOH group.

• Hence, the electron-withdrawing effect of the phenyl ring is weaker.

• Less acidic than benzoic acid.

(C) Acetic acid (CH₃–COOH):

• Methyl group is electron-donating, which destabilizes the carboxylate ion slightly.

• Weaker acid.

(D) Propanoic acid (CH₃CH₂–COOH):

• Ethyl group is also electron-donating, so it's slightly weaker than acetic acid.

Acidic strength order:

Benzoic acid > Phenylacetic acid > Acetic acid > Propanoic acid