JEE Chemistry 2024 Question with Solution

Given: List I contains ethanol, phenol, $m$-nitrophenol and $p$-nitrophenol. List II contains the $pK_a$ values $15.9$, $10.0$, $8.3$ and $7.1$.

Find: The correct matching between the compounds and their $pK_a$ values.

The solution states that $pK_a$ is a measure of acidity, and a lower $pK_a$ value indicates stronger acidity.

• Ethanol is a simple alcohol and is weaker as an acid than phenols, so it has the highest $pK_a$ value.
• Phenol is more acidic than ethanol because the phenoxide ion is resonance stabilized.
• $m$-Nitrophenol and $p$-Nitrophenol are more acidic than phenol because the nitro group is electron-withdrawing.
• Among the nitrophenols, the para derivative has lower $pK_a$ and hence stronger acidity.

Therefore, the matching is:

• A. Ethanol $\to$ II. $15.9$
• B. Phenol $\to$ I. $10.0$
• C. $m$-Nitrophenol $\to$ IV. $8.3$
• D. $p$-Nitrophenol $\to$ III. $7.1$

Hence, the correct matching is A-II, B-I, C-IV, D-III. The correct option is D.

Given: The compounds are ethanol, phenol, $m$-nitrophenol and $p$-nitrophenol.

Find: Which option gives the correct compound-$pK_a$ correspondence.

From the extracted solution:

1. Ethanol has $pK_a$ around $15.9$.
2. Phenol has $pK_a$ around $10.0$.
3. $m$-Nitrophenol has $pK_a$ around $8.3$.
4. $p$-Nitrophenol has $pK_a$ around $7.1$.

Since lower $pK_a$ means stronger acidity, the acidity order is:

This matches the numerical order:

So the final correspondence is:

Therefore, the correct option is D.