Q.How will you carry out the following conversions?

(Concept: Diazonium Salt Reactions / Sandmeyer Reaction) The key idea is that converting an aryl-diazonium salt to a carboxylic acid while KEEPING the ring's existing methyl group intact requires replacing $-\text{N}_2^+$ with $-\text{CN}$ first (a Sandmeyer-type reaction), then hydrolysing the nitrile — not oxidising the methyl group, which would destroy it.

Reasoning steps:

1. Toluene → p-toluidine: Nitrate toluene to get a mixture of o- and p-nitrotoluene (the methyl group is ortho/para-directing). Separate the p-isomer, then reduce the nitro group ($-\text{NO}_2$) to an amino group ($-\text{NH}_2$) using $\text{Sn}/\text{HCl}$ or $\text{Fe}/\text{HCl}$, giving p-toluidine. …

Let’s break down each conversion with the reasoning behind every step.

(i) Toluene → p-Toluidine

Concept: The methyl group is an activating ortho/para director.

When you do an electrophilic substitution on toluene, the methyl group pushes electron density into the ring, making the ortho and para positions more reactive than the meta position. The para product is usually the major one because it’s less sterically hindered than the ortho product (which also forms but in smaller amount). So, to get p-toluidine, we first introduce a nitro group at the para position, then reduce it.

1. Nitration of toluene Treat toluene with a mixture of concentrated nitric acid and concentrated sulfuric acid (nitrating mixture). The nitronium ion ($\text{NO}_2^+$) attacks the para position preferentially.

The product is p-nitrotoluene (along with some o-nitrotoluene, which can be separated by distillation or crystallization).

2. Reduction of the nitro group Reduce the nitro group to an amino group. A common laboratory method is catalytic hydrogenation (using $\text{H}_2$ gas with a palladium or nickel catalyst) or chemical reduction using tin and hydrochloric acid (Sn/HCl).

The product is p-toluidine.

(ii) p-Toluidine Diazonium Chloride → p-Toluic Acid

Concept: The diazonium group is a superb leaving group that can be replaced by many nucleophiles.

Here, we want to replace the $\text{-N}_2^+$ group with a carboxyl group ($\text{-COOH}$). The standard route is to first replace the diazonium group with a cyano group ($\text{-CN}$) via a Sandmeyer reaction, then hydrolyze the nitrile to a carboxylic acid.

1. Formation of diazonium chloride Treat p-toluidine with sodium nitrite ($\text{NaNO}_2$) and excess hydrochloric acid at 0–5°C. This gives the diazonium salt. $p\text{-H}_2\text{N-C}_6\text{H}_4\text{CH}_3 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{0-5^\circ\text{C}} p\text{-CH}_3\text{-C}_6\text{H}_4\text{-N}_2^+\text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O}$ …

Method: Diazotisation followed by Sandmeyer-type Hydrolysis

This is a two-step sequence that uses the diazonium group as a temporary handle to replace an amino group with a carboxyl group.

(i) Toluene → p-Toluidine

Method: Nitration followed by reduction

Steps:

1. Nitration

   Treat toluene with a mixture of concentrated $\text{HNO}_3$ and concentrated $\text{H}_2\text{SO}_4$ (nitrating mixture).

   • The methyl group is ortho/para-directing.
   • By controlling temperature (≈ 30°C), the major product is p-nitrotoluene.

2. Reduction

   Reduce p-nitrotoluene to p-toluidine using:

   • $\text{Sn} + \text{conc. HCl}$ (tin and hydrochloric acid), or
   • $\text{Fe} + \text{dil. HCl}$, or
   • Catalytic hydrogenation ($\text{H}_2$/Pd-C).

   Overall:

(ii) p-Toluidine diazonium chloride → p-Toluic acid

Method: Diazotisation followed by hydrolysis (Sandmeyer-type)

Steps:

1. Diazotisation Treat p-toluidine with $\text{NaNO}_2$ and excess dilute $\text{HCl}$ at 0–5°C to form the diazonium salt:

2. Hydrolysis (replacement of –N$_2^+$ by –OH)

   Warm the diazonium salt solution with dilute $\text{H}_2\text{SO}_4$ or simply heat it in aqueous acidic medium.

   • The diazonium group is replaced by a hydroxyl group (–OH), giving p-cresol as an intermediate.

3. Oxidation of –CH$_3$ to –COOH …

Common Mistakes in Oxidation Reactions (Toluene → p-Toluidine → p-Toluic Acid)

Students often struggle with these conversions because they mix up reaction sequences and reagent selectivity. Here are the most frequent errors and how to avoid them.

Mistake 1: Trying to oxidise toluene directly to p-toluidine

The error: Students think they can replace a methyl group with an amino group in one step using an oxidising agent.

Why it's wrong:

Oxidation of toluene always attacks the methyl group — it converts -CH₃ to -COOH (benzoic acid), not to -NH₂. To introduce an amino group, you need nitration followed by reduction, not oxidation.

How to avoid:

Remember:

• Oxidation changes alkyl groups to carboxylic acids.
• Amino groups come from reducing nitro groups (-NO₂ → -NH₂).

Correct sequence for (i):

1. Nitration of toluene → p-nitrotoluene (using conc. HNO₃ / conc. H₂SO₄, at low temperature to favour para product).
2. Reduction of p-nitrotoluene → p-toluidine (using Sn/HCl or Fe/HCl).

Mistake 2: Using the wrong oxidising agent for (ii)

The error: Students try to oxidise the diazonium salt directly with KMnO₄ or K₂Cr₂O₇.

Why it's wrong:

Diazonium salts are unstable and decompose under strong oxidising conditions. You must first replace the diazonium group with a group that can be oxidised.

How to avoid:

Use the Sandmeyer reaction or hydrolysis to convert the diazonium group into a nitrile (-CN) or methyl group, then oxidise.

Correct sequence for (ii):

1. Sandmeyer reaction: p-toluidine diazonium chloride + CuCN → p-tolunitrile.
2. Hydrolysis (acidic or basic): p-tolunitrile → p-toluic acid.

Alternative (if you want to use oxidation directly):

• Replace -N₂⁺ with -CH₃ (using CH₃OH / Cu), then oxidise the methyl group to -COOH. But this is less common.

Mistake 3: Forgetting to protect the amino group during oxidation

The error: Students try to oxidise p-toluidine directly to p-toluic acid.

Why it's wrong:

The -NH₂ group is easily oxidised itself (it forms coloured impurities or gets destroyed). Strong oxidants like KMnO₄ will attack the amino group before the methyl group.

How to avoid:

Always protect the amino group by acetylation before oxidation:

1. Acetylate p-toluidine → p-acetotoluidide (using (CH₃CO)₂O / pyridine).
2. Oxidise the methyl group to -COOH (using KMnO₄ / H⁺).
3. Hydrolyse the amide back to -NH₂ (using dil. HCl or NaOH).

Mistake 4: Ignoring para selectivity in nitration

The error: Students assume nitration of toluene gives only the para product.

Why it's wrong:

Toluene gives a mixture of ortho and para products. The ortho isomer is also formed (about 60% ortho, 40% para at room temperature).

How to avoid:

• Use low temperature (0°C) to favour para product slightly more.
• Separate the isomers by fractional distillation or crystallisation (p-nitrotoluene has a higher melting point).
• In exam problems, assume the para isomer is the desired product unless stated otherwise.

--- …