Alkyl benzenes & Diazonium Salts

Question 1

Why does phenol undergo nitration with aqueous HNO₃ alone, whereas toluene requires HNO₃ + H₂SO₄?

Answer 1

• H₂SO₄ activates HNO₃ to form the nitronium ion (NO₂⁺), which is highly reactive and required for toluene nitration.
• Phenol contains an activating –OH group, making nitration easier without the need for harsh conditions.

HNO3 + 2H2504 -> NOz* + 2H50; + H30*

Question 2

What is the role of the nitronium ion in electrophilic substitution?

Answer 2

• It is the active electrophile (NO₂⁺) generated from HNO₃ and H₂SO₄.
• It attacks the electron-rich aromatic ring, especially in toluene, which lacks strong activating groups.

Question 3

Why is Friedel-Crafts alkylation not used for industrial synthesis of toluene?

Answer 3

The methyl group activates the ring towards further substitution at ortho and para positions, causing multiple alkylated products, which reduces yield and purity.

Question 4

How is toluene synthesized industrially? Why isn’t Friedel-Crafts used industrially?

Answer 4

• Industrial route: From methylcyclohexane using Al₂O₃ / Pt catalyst and heat (eliminates 3 H₂).
• Friedel-Crafts alkylation of benzene with MeCl/AlCl₃ produces a mixture of ortho, meta, and para isomers, which is not ideal for large-scale synthesis.

Question 5

What is the effect of the –CH₃ group on the reactivity of toluene?

Answer 5

• —CH₃ is an electron-donating group (EDG)
• It activates the ortho and para positions on the benzene ring, increasing reactivity in electrophilic substitution reactions.

Question 6

What are the nitration products of toluene at different temperatures?

Answer 6

30–40°C: Mononitration gives ortho- and para-nitrotoluene
60°C: Dinitration occurs
110°C: Forms 2,4,6-trinitrotoluene (TNT)
–CH₃ directs substitution to ortho/para, –NO₂ to meta

Question 7

What happens when you oxidize toluene or any alkylbenzene?

Answer 7

• Oxidation occurs at the benzylic position (the carbon directly attached to the ring)
• Reagents like KMnO₄ or K₂Cr₂O₇ with heat convert it to benzoic acid
• Even longer alkyl chains will give benzoic acid

Question 8

What are the two important industrial alkene + benzene reactions?

Answer 8

Benzene + propene → cumene
Catalyst: H₃PO₄ / Al₂O₃ or zeolites
Cumene is then oxidized and cleaved to phenol + acetone
Benzene + ethene → ethylbenzene → styrene
Requires dehydrogenation for final step

Question 9

How are diazonium salts made from nitrobenzene?

Answer 9

-Nitrobenzene is reduced to aniline using Sn/HCl
- Aniline is diazotized using NaNO₂ + HCl at 0°C to form the diazonium salt (PhN₂⁺Cl⁻)

Question 10

What reactions can diazonium salts undergo?

Answer 10

• Substitution with CuX → Cl, Br, CN
• Substitution with NaBF₄ → F
• Reaction with H₃PO₂ → H
• Coupling with phenols or amines → azo dyes

Question 11

What is the mechanism of coupling with diazonium salts?

Answer 11

• It’s an electrophilic substitution reaction
• The diazonium salt acts as the electrophile
• Lone pairs on –OH or –NH₂ groups donate into the ring, activating ortho and para positions
• Substitution occurs at ortho or para only

Question 12

Based on the scheme in the final slide, what are the correct reagents for each step?

Answer 12

• Benzene → Me-benzene: (a) MeCl / AlCl₃
• Me-benzene → NO₂ derivative: (b) HNO₃, H₂SO₄
• NO₂ derivative → NH₂: (c) Sn, HCl
• NH₂ → CN derivative: (d) NaNO₂, HCl, 0°C → CuCN
• NH₂ → azo dye: (e) NaNO₂, HCl, 0°C, PhOH
• Methylbenzene → benzenedicarboxylic acid: (f) KMnO₄, heat