Q.Assertion: N-Ethylbenzene sulphonamide is soluble in alkali.
Reason: Hydrogen attached to nitrogen in sulphonamide is strongly acidic.

Concept: Inductive Effect on Acidity — the electron-withdrawing sulphonyl group ($-\text{SO}_2-$) makes the N–H bond in sulphonamides strongly acidic.

Reasoning:

1. In N-ethylbenzene sulphonamide, the sulphur atom is in a high oxidation state and strongly pulls electron density away from the nitrogen via the inductive effect.
2. This weakens the N–H bond, making the hydrogen fairly acidic (pKa ~ 10–12) — comparable to phenols.
3. Hence, it reacts with a strong base like NaOH to form a water-soluble salt, confirming the assertion. …

1. Understanding the Assertion: Solubility in Alkali

   N-Ethylbenzene sulphonamide has the structure $\text{C}_6\text{H}_5-\text{SO}_2-\text{NH}-\text{C}_2\text{H}_5$. The assertion says it is soluble in alkali (like aqueous NaOH). For an organic compound to dissolve in a dilute aqueous alkali, it must be able to react with the base to form a water-soluble salt. This typically requires the compound to have an acidic hydrogen — one that can be removed by the hydroxide ion ($\text{OH}^-$).

2. The Reason: Acidity of the N–H Hydrogen

   The reason states that the hydrogen attached to nitrogen in a sulphonamide is strongly acidic. Is this true? In a simple amine ($\text{R}-\text{NH}_2$), the N–H hydrogen is very weakly acidic (pKa ~ 35–40) — it does not react with NaOH. But in a sulphonamide, the nitrogen is directly bonded to a sulphonyl group ($-\text{SO}_2-$). This group is powerfully electron-withdrawing due to the high oxidation state of sulfur and the presence of two strongly electronegative oxygen atoms.

3. The Inductive Effect in Action

   The sulphonyl group pulls electron density away from the nitrogen through the sigma bonds (inductive effect). This makes the N–H bond more polarised: the hydrogen becomes more $\delta^+$ and thus more easily lost as a proton. The resulting conjugate base (the sulphonamide anion) is stabilised by resonance delocalisation of the negative charge onto the oxygen atoms of the sulphonyl group.

   The resonance stabilisation of the sulphonamide anion:

   $\text{R-SO}_2-\text{NH}^- \longleftrightarrow \text{R-SO}_2^-=\text{NH}$

   This delocalisation makes the anion much more stable than a typical amide or amine anion, which is why the N–H hydrogen in a sulphonamide is acidic enough to react with NaOH.

4. Connecting to the Assertion

   When N-ethylbenzene sulphonamide is treated with aqueous NaOH, the following acid-base reaction occurs:

   $\text{C}_6\text{H}_5-\text{SO}_2-\text{NH}-\text{C}_2\text{H}_5 + \text{NaOH} \rightarrow \text{C}_6\text{H}_5-\text{SO}_2-\text{N}^-(\text{Na}^+)-\text{C}_2\text{H}_5 + \text{H}_2\text{O}$ …

Concept: Acidity of Sulphonamides

Method: Structure–Acidity Analysis

Step 1: Understand the structure of N-Ethylbenzene sulphonamide

• General sulphonamide structure: $\ce{R-SO2-NH-R'}$
• Here, $\ce{R = C6H5-}$ (benzene ring), $\ce{R' = C2H5-}$ (ethyl group)
• The key functional group is the $\ce{-SO2-NH-}$ linkage.

Step 2: Analyse the acidity of the N–H bond

• The sulphonyl group ($\ce{-SO2-}$) is strongly electron-withdrawing due to the high oxidation state of sulphur and the two oxygen atoms.
• This withdrawal of electron density makes the $\ce{N-H}$ bond polarised, and the hydrogen becomes weakly acidic (pKa ~ 10–11).
• However, "strongly acidic" is incorrect — it is only weakly acidic (comparable to phenol, not to mineral acids).

Step 3: Check solubility in alkali

• A compound is soluble in alkali if it can donate a proton to form a salt.
• N-Ethylbenzene sulphonamide can lose the $\ce{N-H}$ proton in the presence of a strong base like $\ce{NaOH}$, forming a water-soluble sodium salt.
• Hence, assertion is correct.

Step 4: Evaluate the reason …

Let’s break this down step-by-step, focusing on why students get confused and how to avoid those mistakes.

🧪 The Core Concept: Sulphonamide Acidity

Sulphonamides have the general structure:

The hydrogen attached to nitrogen is weakly acidic (pKa ~10–11), not strongly acidic.

This is because the $-SO_2-$ group withdraws electron density, but not enough to make the N–H bond as acidic as, say, a carboxylic acid (pKa ~5) or mineral acid.

✗ Common Mistake #1: Thinking the N–H hydrogen is “strongly acidic”

Why students make this mistake:

They see the sulphonamide group and recall that the N–H bond can be deprotonated by strong bases like NaOH. They then overgeneralize and call it “strongly acidic.”

How to avoid:

Remember: “Acidic” in organic chemistry is relative.

• Sulphonamide N–H is weakly acidic (pKa ~10–11).
• It can react with strong alkali (NaOH), but that does not make it strongly acidic.
• Compare: Acetic acid (pKa ~4.8) is stronger. Sulphonamide is weaker.

Key fact: The reason statement says “strongly acidic” — this is wrong.

✗ Common Mistake #2: Assuming solubility in alkali always means strong acidity

Why students make this mistake:

They think: “If it dissolves in NaOH, the H must be very acidic.”

But many weakly acidic compounds (e.g., phenols, pKa ~10) also dissolve in strong alkali.

How to avoid:

Solubility in alkali only means the compound is more acidic than water (pKa ~15.7). It does not mean strong acidity.

• Sulphonamides are weakly acidic — they still dissolve in NaOH because NaOH is a strong base.

✗ Common Mistake #3: Confusing N-Ethylbenzene sulphonamide with primary sulphonamide

Why students make this mistake:

Primary sulphonamides ($R-SO_2-NH_2$) have two hydrogens on nitrogen, and are more acidic.

N-Ethylbenzene sulphonamide is secondary ($R-SO_2-NH-R'$), with only one N–H.

How to avoid:

Check the structure carefully.

• Primary: $R-SO_2-NH_2$ — two acidic H’s.
• Secondary: $R-SO_2-NH-R'$ — one acidic H, slightly less acidic than primary. …