Q.The reaction of toluene with chlorine in the presence of iron and in the absence of light yields ____________.

The key idea is electrophilic aromatic substitution — in the presence of iron (which generates $\ce{Cl+}$ from $\ce{Cl2}$) and in the absence of light, chlorine attacks the aromatic ring, not the side chain. The methyl group is an ortho/para director, so substitution occurs at the ortho and para positions.

1. Iron reacts with chlorine to form $\ce{FeCl3}$, which polarises $\ce{Cl2}$ to produce the electrophile $\ce{Cl+}$. …

The key here is to recognise that the reaction conditions — chlorine gas, iron (which generates FeCl₃ in situ), and no light — are the classic setup for electrophilic aromatic substitution (EAS) on an aromatic ring.

If light were present, you’d get free-radical substitution at the benzylic position (giving benzyl chloride). But without light, the iron catalyst activates chlorine to form an electrophile, and the reaction proceeds on the ring.

Toluene has a methyl group attached to the benzene ring. The methyl group is an activating group and an ortho/para director in EAS reactions. This is because the methyl group donates electron density into the ring via hyperconjugation and the inductive effect, stabilising the intermediate carbocation (arenium ion) when substitution occurs at the ortho or para positions.

So, when chlorine attacks the ring, it will preferentially go to the ortho and para positions relative to the methyl group. Both ortho and para products are formed, and they are not separated under typical reaction conditions — you get a mixture.

Let’s walk through the reasoning step by step:

1. Identify the reaction type.

   Given: toluene + Cl₂, in presence of iron (Fe), in absence of light.

   Iron reacts with Cl₂ to form FeCl₃, a Lewis acid. This is the classic catalyst for electrophilic aromatic chlorination. No light means no free radicals are generated, so the mechanism is ionic, not radical.

2. Formation of the electrophile.

   FeCl₃ polarises the Cl–Cl bond, making one chlorine more electrophilic:

The actual attacking species is effectively $\text{Cl}^+$ (or a chlorine atom with strong positive character).

3. Attack on the aromatic ring. …

Concept: Electrophilic Aromatic Substitution (EAS) vs. Free Radical Substitution

The key idea is that the reagent and conditions determine the mechanism — and therefore the product.

• Chlorine ($Cl_2$) + Iron ($Fe$) → generates $FeCl_3$ in situ, which acts as a Lewis acid catalyst for electrophilic aromatic substitution.
• Absence of light → prevents the free radical pathway (which would give side-chain substitution, i.e., benzyl chloride).

So, under these conditions, the reaction is electrophilic aromatic substitution on the toluene ring.

Method: Electrophilic Aromatic Substitution (EAS) — Chlorination of Toluene

Steps:

1. Catalyst generation

   $2Fe + 3Cl_2 \rightarrow 2FeCl_3$

   $FeCl_3$ polarises $Cl_2$ to form the electrophile:

   $Cl_2 + FeCl_3 \rightarrow Cl^\delta+ \cdots FeCl_4^-$

2. Electrophilic attack

   The methyl group ($-CH_3$) on toluene is an activating and ortho/para-directing group. …

Here’s a breakdown of the common mistakes students make on this question and how to avoid each.

Common Mistake #1: Confusing the catalyst/condition with free radical chlorination

The mistake:

Students see “chlorine” and “toluene” and immediately think of free radical substitution (which gives benzyl chloride, $\mathrm{C_6H_5CH_2Cl}$). They pick option (i).

Why it happens:

In school, the reaction of toluene with $\mathrm{Cl_2}$ in sunlight or UV light (or at high temperature) is drilled as:

So the presence of chlorine + toluene triggers that memory.

How to avoid:

• Read the condition carefully. The question says:

  “in the presence of iron and in the absence of light”

• Iron ($\mathrm{Fe}$) acts as a Lewis acid (forms $\mathrm{FeCl_3}$ in situ) — this is an electrophilic aromatic substitution catalyst, not a free radical initiator.
• Rule of thumb:
  • Light / heat → free radical → side chain chlorination (benzyl chloride)
  • $\mathrm{Fe}$ / $\mathrm{FeCl_3}$ / dark → electrophilic substitution → ring chlorination

Common Mistake #2: Forgetting that the methyl group is ortho/para directing

The mistake:

Students correctly identify that ring substitution occurs, but they pick only one isomer — either (ii) or (iii) — instead of the mixture.

Why it happens:

They remember that $\mathrm{-CH_3}$ is an activating group, but forget that it directs both ortho and para positions.

How to avoid:

• Recall the directing effect:
  • Alkyl groups (like $\mathrm{-CH_3}$) are ortho/para directing (due to hyperconjugation and +I effect).
  • So chlorination of toluene in the presence of $\mathrm{FeCl_3}$ gives both ortho and para products.
• The correct answer is therefore a mixture of o-chlorotoluene and p-chlorotoluene → option (iv).

Common Mistake #3: Ignoring the “absence of light” clause

The mistake: …