Q.Assertion: Aryl iodides can be prepared by reaction of arenes with iodine in the presence of an oxidising agent.
Reason: Oxidising agent oxidises $\mathrm{I_2}$ into HI.

Concept: Electrophilic Aromatic Substitution – Iodination requires an oxidising agent because iodine is the least reactive halogen and the reaction produces HI, which is a reducing agent that would reverse the reaction.

Reasoning:

1. Direct iodination of arenes is reversible and unfavourable because $\mathrm{I_2}$ is a weak electrophile and the by-product HI reduces the product back to the arene.
2. An oxidising agent (e.g., $\mathrm{HNO_3}$ or $\mathrm{H_2O_2}$) oxidises the HI formed back to $\mathrm{I_2}$, driving the equilibrium forward. …

1. Understanding the reaction: Aromatic iodination Direct iodination of benzene (or other arenes) with iodine alone is extremely slow and reversible. The reaction is:

The equilibrium lies far to the left because HI is a strong reducing agent — it can reduce the aryl iodide back to the arene. To make the reaction practical, we need to remove HI as it forms, or prevent its accumulation.

2. Role of the oxidising agent An oxidising agent (like $\mathrm{HNO_3}$, $\mathrm{H_2O_2}$, or $\mathrm{IO_3^-}$) is added to oxidise the $\mathrm{I^-}$ ions (from HI) back to molecular iodine:

This continuously regenerates $\mathrm{I_2}$, shifting the equilibrium to the right and allowing the aryl iodide to form in good yield. The oxidising agent does not oxidise $\mathrm{I_2}$ into HI — that would be chemically backwards (HI is a reduced form of iodine).

3. Why the reason is wrong The reason states: "Oxidising agent oxidises $\mathrm{I_2}$ into HI." That is incorrect because:
   • $\mathrm{I_2}$ is already in a higher oxidation state (0) than HI ($-1$).
   • Oxidising an element means increasing its oxidation number — going from $\mathrm{I_2}$ (0) to HI ($-1$) is actually a reduction, not oxidation.
   • The correct role is the opposite: the oxidising agent oxidises HI (or $\mathrm{I^-}$) back to $\mathrm{I_2}$. …

Method: Assertion–Reason Analysis

Concept: Electrophilic aromatic substitution (iodination) — iodine is the least reactive halogen for direct substitution, so an oxidising agent is needed to generate a better electrophile.

Steps:

1. Check the Assertion

   • Aryl iodides can be prepared by reacting arenes with iodine in the presence of an oxidising agent (e.g., $\ce{HNO3}$ or $\ce{H2O2}$).
   • This is correct — iodine alone is too weak an electrophile; the oxidising agent converts $\ce{I2}$ into a more reactive species like $\ce{I+}$ or $\ce{IOH}$.

2. Check the Reason

   • The reason states: Oxidising agent oxidises $\ce{I2}$ into HI. …

Common Mistakes & How to Avoid Them

Mistake 1: Thinking the Reason is Correct

The error: Students assume that since an oxidising agent is used, it must oxidise $\mathrm{I_2}$ to HI. They don't check the chemistry carefully.

Why it's wrong: An oxidising agent gains electrons — it oxidises something else while itself getting reduced. Here, the oxidising agent (like $\mathrm{HNO_3}$ or $\mathrm{H_2O_2}$) actually oxidises $\mathrm{I_2}$ to $\mathrm{I^+}$ (a more reactive electrophile), not to HI. HI is a reducing agent, not an oxidation product.

How to avoid: Always ask: "What is the actual role of the oxidising agent?" In this reaction:

• $\mathrm{I_2}$ is oxidised to a reactive $\mathrm{I^+}$ species
• The oxidising agent itself gets reduced
• HI would be a reduction product, not an oxidation product

Mistake 2: Confusing the Assertion as Wrong

The error: Some students think aryl iodides cannot be prepared directly, so they mark the assertion as false.

Why it's wrong: Aryl iodides can be prepared by direct iodination of arenes — but only because the oxidising agent makes the reaction possible. Without it, $\mathrm{I_2}$ is too unreactive for electrophilic substitution.

How to avoid: Remember the reactivity order for halogens in electrophilic substitution:

• $\mathrm{F_2}$ > $\mathrm{Cl_2}$ > $\mathrm{Br_2}$ > $\mathrm{I_2}$
• $\mathrm{I_2}$ is the least reactive — it needs an oxidising agent to generate $\mathrm{I^+}$ (a better electrophile)

Mistake 3: Choosing Option (i) — Both Correct, Reason Explains Assertion

The error: Students see both statements as "true" and assume the reason correctly explains the assertion.

Why it's wrong: The reason is factually incorrect (oxidising agent does not produce HI). Even if both were true, the reason given is not the correct explanation.

How to avoid: Always verify the mechanism behind the reason. The correct explanation is:

• Oxidising agent converts $\mathrm{I_2}$ to $\mathrm{I^+}$ (electrophile)
• $\mathrm{I^+}$ attacks the benzene ring
• HI is never formed — instead, $\mathrm{H^+}$ is released as a byproduct

Mistake 4: Misreading the Options …