Coupling of benzene diazonium chloride with 1-naphthol in alkaline medium will give
(a) Option A
(b)

Q: Why do we need an alkaline medium for this reaction?

The alkaline medium removes the proton from the hydroxyl group of 1-naphthol, converting it into a phenoxide-like ion (1-naphthoxide). The negatively charged oxygen strongly donates electron density into the ring via resonance, making it highly nucleophilic and reactive towards the weak diazonium electrophile.

Q: Why doesn't the diazonium ion attack the other ring of naphthol?

The hydroxyl group (or oxide ion) is attached to the first ring, making that specific ring highly electron-rich. Electrophilic aromatic substitution preferentially occurs on the most activated ring.

Q: Why is the para position preferred over the ortho position?

Although both ortho (C2) and para (C4) positions are activated, the diazonium ion is a very bulky electrophile. Attacking the ortho position would result in severe steric repulsion with the adjacent oxygen atom. Thus, it prefers the less hindered para position.

Q: What happens if the para position is already blocked?

If the para position (C4) were already occupied by another substituent, the diazonium ion would be forced to attack the ortho position (C2), despite the steric hindrance, to form the azo dye.

Q: Why is the product colored?

The product is an azo dye. The $-N=N-$ linkage connects two aromatic systems, creating a highly extended conjugated pi-electron system. This extended conjugation lowers the energy gap between molecular orbitals, allowing the molecule to absorb visible light, which gives it a vibrant orange-red color.

Question

Coupling of benzene diazonium chloride with 1-naphthol in alkaline medium will give

Sigma Percentile · Accepted Answer

### The Colorful Chemistry of Azo Coupling

Hello students! Today we are going to explore the fascinating coupling reaction between benzene diazonium chloride and 1-naphthol. This reaction is a classic example of electrophilic aromatic substitution and is widely used in the dye industry to synthesize vibrant azo dyes.

### The Role of the Alkaline Medium

The reaction takes place in an alkaline medium, which plays a crucial role in activating our nucleophile. In the presence of a base like $NaOH$, 1-naphthol loses its slightly acidic hydroxyl proton to form the **1-naphthoxide ion**.

$$ 	ext{Ar-OH} + OH^- ightleftharpoons 	ext{Ar-O}^- + H_2O $$

Notice the negative charge on the oxygen atom. The $-O^-$ group is a strongly activating group. It donates electron density into the aromatic ring via the $+R$ (resonance) effect, making the ring highly nucleophilic and exceptionally reactive towards electrophiles.

### Electrophilic Aromatic Substitution

Now, let's look at the benzene diazonium ion ($Ph-N_2^+$). Because of the positive charge on the nitrogen atom, it acts as an electrophile. However, it is a relatively weak electrophile, which is why it requires a highly activated aromatic ring (like a phenoxide or naphthoxide) to react.

The strongly activating $-O^-$ group directs incoming electrophiles to the **ortho** and **para** positions. In the case of 1-naphthol, the activated positions are C2 (ortho) and C4 (para).

### Steric Hindrance: The Deciding Factor

So, the attack can happen at either C2 or C4. But there is a catch here! The diazonium ion is quite bulky. If it tries to attack the ortho position (C2), it will face severe **steric hindrance** from the adjacent, bulky oxygen atom.

Therefore, to avoid this steric repulsion, the diazonium ion prefers to attack the less hindered para position, which is C4.

### Conclusion

After the electrophilic attack at the para position (C4) and subsequent deprotonation, we get our final product: **4-(phenylazo)-1-naphthol**. This molecule features an extended conjugated pi-electron system spanning across both aromatic rings and the $-N=N-$ linkage, which allows it to absorb visible light and appear as a beautiful orange-red azo dye. Thus, the correct structural representation corresponds to option (c).

Animated Solution for Chemistry - Organic Compounds Containing Nitrogen: Coupling of benzene diazonium chloride with 1-naphthol in alkaline medium will give

Select Answer:

Visualized Solution

$R e a c t i o n S e t u p$

$F or ma t i o n o f N a p h t h o x i d e I o n$

$T h e E l ec t r o p hi l e$

$D i r ec t in g E f f ec t s$

$S t er i c H in d r an ce$

$P a r a A tt a c k$

$F ina l P r o d u c t$

The Sigma Insight: Diazonium Salts

The Colorful Chemistry of Azo Coupling

The Role of the Alkaline Medium

Electrophilic Aromatic Substitution

Steric Hindrance

Conclusion

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