Q.Assertion: Only a small amount of HCl is required in the reduction of nitro compounds with iron scrap and HCl in the presence of steam.
Reason: $FeCl_2$ formed gets hydrolysed to release HCl during the reaction.

The key idea is that the reduction of nitro compounds with iron and HCl is catalytic in acid — the HCl is regenerated via hydrolysis of $FeCl_2$, so only a small initial amount is needed.

Reasoning:

1. The reduction proceeds as: $RNO_2 + 3Fe + 6HCl \rightarrow RNH_2 + 3FeCl_2 + 2H_2O$.
2. $FeCl_2$ formed undergoes hydrolysis with steam: $FeCl_2 + 2H_2O \rightarrow Fe(OH)_2 + 2HCl$.
3. The regenerated HCl re-enters the reduction cycle, so a small initial charge suffices. …

Why This Question Tests Your Understanding of Catalytic Cycles

This isn't just a memorisation problem — it's about recognising a self-sustaining reaction system. The key insight is that the acid (HCl) is not consumed in the net reaction; it acts as a catalyst. Let's see why.

1. The overall reaction The reduction of a nitro compound (say nitrobenzene) to an amine (aniline) using iron and HCl is:

At first glance, this suggests you need 6 moles of HCl per mole of nitro compound. But that's misleading.

2. What actually happens in the presence of steam The reaction is carried out with steam (water vapour) at around 100°C. Under these conditions, the $FeCl_2$ formed undergoes hydrolysis:

The HCl released here goes back into the reduction cycle. So the net consumption of HCl is far less than the stoichiometric amount.

3. The catalytic role of HCl Only a small initial charge of HCl is needed to start the reaction. Once the reaction begins, the hydrolysis of $FeCl_2$ continuously regenerates HCl. The iron metal is consumed (it gets oxidised to $Fe^{2+}$ and eventually to $Fe^{3+}$), but the acid is recycled.

4. Why the reason is the correct explanation The assertion says "only a small amount of HCl is required." The reason says "$FeCl_2$ hydrolyses to release HCl." These are directly linked: because $FeCl_2$ hydrolyses, the HCl is regenerated, so you don't need to add it in large quantity. The reason is not just a true statement — it is the mechanism that makes the assertion true. …

Method: Conceptual Analysis of Reaction Mechanism & Catalytic Role

This is a cause-effect reasoning problem. The method is to:

1. Understand the role of HCl in the reaction.
2. Check if the reason correctly explains why only a small amount is needed.

Step 1: Understand the Assertion

• Reduction of nitro compounds ($\ce{Ar-NO2}$) to anilines ($\ce{Ar-NH2}$) using iron scrap + HCl + steam is a classic method.
• Only a small amount of HCl is used initially — not a large excess.

Step 2: Understand the Reason

• The reaction proceeds as:

• But $\ce{FeCl2}$ formed hydrolyses in the presence of steam (water vapour):

• The HCl is regenerated and can be reused. Hence, only a catalytic amount of HCl is needed to start the reaction.

Step 3: Match Assertion and Reason …

Common Mistakes Students Make on This Question (Inductive Effect & Acidity Context)

This question is actually about reduction of nitro compounds (not directly about inductive effect on acidity). However, students often confuse concepts across organic chemistry. Here are the common mistakes:

1. Misidentifying the Topic

• Mistake: Thinking this is about inductive effect on acidity (due to the heading).
• Avoid: Read the question carefully — it's about the reduction of nitrobenzene to aniline using Fe/HCl.
• Key fact: The reaction is: $\ce{C6H5NO2 + 3Fe + 6HCl -> C6H5NH2 + 3FeCl2 + 2H2O}$

2. Not Knowing the Role of FeCl₂ Hydrolysis

• Mistake: Assuming $\ce{FeCl2}$ is inert and doesn't regenerate HCl.
• Avoid: Remember that $\ce{FeCl2}$ undergoes hydrolysis with steam:

This recycles HCl, so only a small initial amount is needed.

3. Confusing "Small Amount" with "Catalytic Amount"

• Mistake: Thinking HCl is a catalyst (it is not — it is consumed and regenerated).
• Avoid: Understand that HCl is regenerated via hydrolysis, so it acts like a catalyst in net effect, but technically it participates in the reaction.

4. Ignoring the Role of Steam

• Mistake: Overlooking "in the presence of steam" — this is crucial for hydrolysis.
• Avoid: Steam provides the water needed for $\ce{FeCl2}$ hydrolysis. Without steam, HCl would not be regenerated.

5. Wrong Answer Selection …