Synthesis and Sources of Aromatic Hydrocarbons

Preparation of Naphthalene

Naphthalene is a fused-ring aromatic hydrocarbon with two benzene rings. It can be prepared via the following methods:

• From Coal Tar: Coal tar, a by-product of coal processing, is a major source of naphthalene. It is extracted and purified using distillation followed by crystallization.
• Synthesis from Benzene: Benzene undergoes a series of reactions to form naphthalene:
  • Alkylation: Benzene reacts with alkyl halides (like methyl chloride) to form methylbenzene (toluene).
  • Cyclization: Toluene undergoes further reaction with chlorine and heat to form naphthalene.

Preparation of Anthracene

Anthracene is a tricyclic aromatic hydrocarbon consisting of three fused benzene rings. It is mainly obtained from coal tar but can also be synthesized.

• From Coal Tar: Coal tar is distilled, and anthracene is separated in the high-boiling-point fraction. It is then purified by recrystallization or chemical treatment.
• Friedel-Crafts Alkylation: Anthracene can also be synthesized by reacting benzene with a dicarboxylic acid derivative (phthalic anhydride) in the presence of AlCl₃, followed by cyclization.

Preparation of Phenanthrene

Phenanthrene is another tricyclic aromatic hydrocarbon with three fused benzene rings but a different arrangement than anthracene.

• From Coal Tar: Phenanthrene is extracted from the high-boiling fraction of coal tar.
• Dehydration of Hydrocarbons: Phenanthrene can also be synthesized by dehydrating certain hydrocarbons that already have a multi-ring structure.

Preparation of Diphenylmethane

Diphenylmethane is an aromatic compound with two benzene rings connected by a single methylene (-CH₂-) group.

• Friedel-Crafts Reaction: Benzene reacts with benzyl chloride (C₆H₅CH₂Cl) in the presence of AlCl₃ as a catalyst to produce diphenylmethane.

Cycloalkanes

Cycloalkanes are saturated hydrocarbons containing carbon atoms arranged in a ring structure, with the general formula C_nH_2n. This unit discusses their preparation methods.

Methods of Preparation of Cycloalkanes

• From Dihalides (Wurtz Reaction): Dihalogenated alkanes (compounds with halogen atoms attached to two ends of a hydrocarbon chain) can react with sodium metal in dry ether to form cycloalkanes.
  Example: Ethylene dibromide reacts with sodium to form cyclopropane:
  BrCH₂CH₂CH₂Br + 2Na → C₃H₆ + 2NaBr
• From Calcium or Sodium Salts of Dicarboxylic Acids: Heating sodium or calcium salts of dicarboxylic acids leads to decarboxylation, resulting in cycloalkanes.
  Example: Adipic acid (a dicarboxylic acid) is heated to form cyclopentanone, which can further be reduced to cyclopentane.