Q.Which of the following compound will not undergo azo coupling reaction with benzene diazonium chloride?

The key idea is that azo coupling is an electrophilic aromatic substitution reaction. The diazonium cation acts as a weak electrophile, so it only reacts with highly activated aromatic rings.

Step 1: The benzene diazonium ion ($\ce{C6H5N2+}$) is a weak electrophile. It requires the aromatic ring to be strongly electron-rich (activated) for the reaction to occur.

Step 2: Aniline ($\ce{-NH2}$), phenol ($\ce{-OH}$), and anisole ($\ce{-OCH3}$) all have strong electron-donating groups that activate the ring toward electrophilic attack. They undergo azo coupling readily at the para position (or ortho if para is blocked). …

Concept First: Why Azo Coupling Works

Azo coupling is an electrophilic aromatic substitution (EAS) reaction. The benzene diazonium cation ($\ce{C6H5-N2+}$) acts as a weak electrophile. For this weak electrophile to attack an aromatic ring, the ring must be strongly activated — that is, it must have a high electron density. This happens when the ring carries an electron-donating group (EDG) like –OH, –NH₂, or –OCH₃. These groups donate electrons through resonance or induction, making the ring electron-rich and ready to attack the diazonium ion.

If the ring is deactivated (carries an electron-withdrawing group like –NO₂, –CN, –COOH), the electron density drops, and the weak diazonium electrophile cannot react. That’s the key: azo coupling only works on activated rings.

Step-by-Step Analysis

1. Identify the electrophile. Benzene diazonium chloride gives the diazonium cation $\ce{C6H5-N2+}$. This is a weak electrophile because the positive charge is delocalised over the two nitrogen atoms and the phenyl ring.

2. Check each compound’s ring activation.

   • (i) Aniline ($\ce{C6H5NH2}$): The –NH₂ group is a strong EDG. It donates electrons by resonance (lone pair on nitrogen delocalises into the ring). The ring is highly activated, especially at the ortho and para positions. Aniline readily undergoes azo coupling to give azo dyes.

   • (ii) Phenol ($\ce{C6H5OH}$): The –OH group is also a strong EDG. It donates electrons by resonance (lone pairs on oxygen). Phenol is strongly activated and undergoes azo coupling easily — in fact, it’s one of the classic examples. …

Concept: Azo Coupling Reaction

Azo coupling is an electrophilic aromatic substitution reaction. The benzene diazonium ion ($C_6H_5N_2^+$) acts as a weak electrophile. It attacks only those aromatic rings that are highly activated — i.e., rings with strong electron-donating groups ($-$OH, $-$NH$_2$, $-$NHR, $-$OR, etc.).

Method: Activation Check for Electrophilic Substitution

Steps:

1. Identify the substituent on each aromatic compound.
2. Determine its electronic effect:
   • Strongly activating (ortho/para directing): $-$OH, $-$NH$_2$, $-$OCH$_3$, $-$NHR → will undergo azo coupling.
   • Weakly activating or deactivating (meta directing): $-$NO$_2$, $-$CN, $-$COOH, $-$SO$_3$H → will NOT undergo azo coupling.
3. Apply the rule: Azo coupling requires the ring to be more reactive than benzene toward electrophiles. Only strongly activated rings qualify.

Applying to the options:

| Compound | Substituent | Effect | Azo coupling? |

|----------|-------------|--------|---------------| …

Let’s break this down step-by-step.

🧪 Concept: Azo Coupling Reaction

Azo coupling is an electrophilic aromatic substitution reaction.

The active species is the benzenediazonium cation ($C_6H_5N_2^+$), which acts as a weak electrophile.

For the reaction to occur, the aromatic ring must be strongly activated — i.e., it must have an electron-donating group (EDG) like $-\text{OH}$, $-\text{NH}_2$, $-\text{OCH}_3$, etc.

✗ Common Mistakes & How to Avoid Them

1. ✗ Mistake: Thinking all aromatic compounds undergo azo coupling

Why it’s wrong:

Only aromatic rings with strong activating groups (like $-\text{OH}$, $-\text{NH}_2$, $-\text{OCH}_3$) can react.

The benzenediazonium ion is a weak electrophile — it cannot attack deactivated or weakly activated rings.

✓ How to avoid:

Always check the substituent on the ring first.

• Strong EDGs → coupling occurs.
• Weak EDGs (e.g., alkyl) → may or may not react (depends on conditions).
• EWGs (e.g., $-\text{NO}_2$, $-\text{CN}$, $-\text{COOH}$) → no coupling.

2. ✗ Mistake: Forgetting that anisole ($\text{OCH}_3$) is activating

Why it’s wrong:

Some students think $-\text{OCH}_3$ is deactivating because it has an oxygen atom.

In reality, the lone pair on oxygen donates into the ring via resonance, making it a strong activator.

✓ How to avoid:

Remember the resonance effect dominates over the inductive effect for $-\text{OCH}_3$.

Anisole does undergo azo coupling (at the para position).

3. ✗ Mistake: Assuming nitrobenzene will react because it has a benzene ring

Why it’s wrong:

Nitrobenzene has a strong electron-withdrawing group ($-\text{NO}_2$).

This deactivates the ring so much that the weak electrophile ($C_6H_5N_2^+$) cannot attack.

✓ How to avoid:

If the substituent is an EWG (especially $-\text{NO}_2$, $-\text{CN}$, $-\text{SO}_3H$), the ring is too deactivated for azo coupling.

4. ✗ Mistake: Confusing azo coupling with diazotization

Why it’s wrong: …