Preparation of Benzene
Synthesize the fundamental aromatic ring. Master the Decarboxylation of Benzoate, Phenol reduction with Zinc, and the highly-tested Diazonium salt pathways.
Module Focus: Entering the Aromatic World
Benzene ($C_6H_6$) is the simplest and most vital aromatic hydrocarbon. While it can be isolated commercially from coal tar or petroleum, laboratory synthesis requires specific reagents to strip functional groups off substituted benzene rings. In NEET, you are frequently tested on the reagents required to remove $-OH$, $-COOH$, or $-N_2^+Cl^-$ groups to yield pristine benzene, along with their associated by-products.
1. Cyclic Polymerization of Ethyne (Review)
When Ethyne ($HC\equiv CH$) is passed through a Red Hot Iron (or Copper) tube at 873 K, three molecules trimerize to form the aromatic benzene ring. This is a crucial reaction for converting aliphatic compounds into aromatic compounds.
2. Decarboxylation of Aromatic Acids
Sodium salt of benzoic acid (Sodium Benzoate) is strongly heated with Soda-Lime to yield benzene.
It is a mixture of Sodium Hydroxide ($NaOH$) and Calcium Oxide ($CaO$) in a 3:1 ratio.
Role of CaO (Quicklime):
- $NaOH$ is highly hygroscopic (absorbs water). $CaO$ keeps it dry.
- It prevents the mixture from fusing at lower temperatures, making it easier to handle.
The carboxylate group ($-COONa$) is completely removed as Sodium Carbonate ($Na_2CO_3$).
3. Reduction of Phenol
Phenol ($C_6H_5OH$) can be reduced directly to Benzene by passing its vapors over heated Zinc dust. This is a classic oxidation-reduction reaction where Zinc strips the oxygen from the aromatic ring.
Benzene can also be prepared by the hydrolysis of Phenylmagnesium bromide ($C_6H_5MgBr$) in an acidic medium.
4. Reduction of Diazonium Salts
Benzene diazonium chloride ($C_6H_5N_2^+Cl^-$) is a highly reactive, incredibly useful intermediate (prepared from Aniline via diazotization). It can be reduced to pure benzene using mild reducing agents. This is one of the most frequently tested conversions in NEET.
Reacting the diazonium salt with $H_3PO_2$ (also called phosphinic acid) in the presence of water.
Oxidation By-product: Phosphorous Acid ($H_3PO_3$)
Reacting the diazonium salt directly with Ethanol. Ethanol acts as the reducing agent.
Oxidation By-product: Ethanal / Acetaldehyde ($CH_3CHO$)
NEET Grand Test: Benzene Prep
15 High-Yield Questions testing reagents, redox by-products, and conversion pathways.
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