Identify the correct \(pK_a\) value order

A. Phenol

B. p-Cresol

C. Benzoic acid

D. 4-Methyl benzoic acid

E. 4-Nitrobenzoic acid

Choose the correct answer from the options given below:

E < C < D < A < B

The correct answer is option 1. E < C < D < A < B.

Let us dive into each compound’s acidity and \( pK_a \) values to understand the correct order.

A. Phenol 

Phenol has a hydroxyl group (-OH) attached to a benzene ring.

The acidic strength of phenol is influenced by the ability of the phenoxide ion (the conjugate base) to stabilize the negative charge. The resonance in the phenoxide ion helps in stabilizing the charge, but the acidic nature is not very strong compared to carboxylic acids.

\( pK_a \) Value: Around 10. Phenol is less acidic compared to carboxylic acids but more acidic than aliphatic alcohols.

B. p-Cresol

p-Cresol is similar to phenol but has a methyl group (-CH₃) at the para position relative to the hydroxyl group.

The methyl group is an electron-donating group through inductive effects, which increases the electron density on the aromatic ring and the hydroxyl group. This reduces the tendency of the hydroxyl group to release a proton, making p-cresol less acidic than phenol.

\( pK_a \) Value: Higher than phenol, indicating a weaker acid.

C. Benzoic Acid

Benzoic acid has a carboxyl group (-COOH) attached to a benzene ring.

The carboxyl group is a strong acid due to the resonance stabilization of the carboxylate anion formed after deprotonation. This resonance stabilization is more significant compared to phenol, making benzoic acid much more acidic.

\( pK_a \) Value: Around 4.2. Benzoic acid is significantly more acidic than phenol and p-cresol.

D. 4-Methylbenzoic Acid

This is benzoic acid with a methyl group at the para position.

The methyl group is an electron-donating group, which slightly decreases the acidity of the carboxyl group compared to benzoic acid. This reduces the stability of the carboxylate ion slightly.

\( pK_a \) Value: Higher than benzoic acid but lower than phenol. This indicates it is less acidic than benzoic acid but more acidic than phenol.

E. 4-Nitrobenzoic Acid 

4-Nitrobenzoic acid has a nitro group (-NO₂) at the para position relative to the carboxyl group.

The nitro group is a strong electron-withdrawing group, which increases the acidity of the carboxyl group. The electron-withdrawing effect stabilizes the negative charge on the carboxylate anion more effectively, making 4-nitrobenzoic acid the strongest acid among the listed compounds.

\( pK_a \) Value: Lower than benzoic acid, indicating higher acidity.

Thus, the correct order of increasing \( pK_a \) values (which corresponds to decreasing acidity) is: E < C < D < A < B

So, the correct option is: 1. E < C < D < A < B