30 Hydrocarbons

30.1 Arenes:

1. Chemistry of Arenes:
   1. Arenes are aromatic hydrocarbons that contain a benzene ring (C₆H₆) or other aromatic rings. Benzene and methylbenzene (toluene) are commonly used as examples.
2. Substitution Reactions of Benzene and Methylbenzene:

(a) Halogenation: Benzene and methylbenzene undergo substitution reactions with halogens (Cl₂ and Br₂) in the presence of a catalyst such as AlCl₃ or AlBr₃ to form halogenoarenes (aryl halides).

(b) Nitration: Benzene and methylbenzene react with a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at a temperature between 25°C and 60°C to undergo nitration, resulting in the introduction of a nitro group (-NO₂).

(c) Friedel-Crafts Alkylation: Benzene and methylbenzene can undergo alkylation reactions using alkyl halides (e.g., CH₃Cl) in the presence of a Lewis acid catalyst such as AlCl₃ and heat. This leads to the addition of an alkyl group (-R) to the benzene ring.

(d) Friedel-Crafts Acylation: Benzene and methylbenzene can undergo acylation reactions using acyl chlorides (e.g., CH₃COCl) in the presence of a Lewis acid catalyst such as AlCl₃ and heat. This results in the addition of an acyl group (-COR) to the benzene ring.

(e) Side-Chain Oxidation: The side chain of methylbenzene can be completely oxidized using hot alkaline potassium permanganate (KMnO₄) followed by dilute acid, leading to the formation of a carboxylic acid group (-COOH) on the benzene ring.

(f) Hydrogenation: The benzene ring of benzene or methylbenzene can be hydrogenated using hydrogen gas (H₂) and a catalyst such as platinum (Pt) or nickel (Ni) at high temperature, resulting in the formation of a cyclohexane ring.

1. Mechanism of Electrophilic Substitution in Arenes:
   1. Electrophilic substitution is the main type of reaction in arenes.
   2. The mechanism involves the formation of a reactive electrophile and the attack of the electrophile on the electron-rich benzene ring.
   3. The delocalization (aromatic stabilization) of electrons in the benzene ring makes it highly stable, resulting in a predominance of substitution reactions over addition reactions.
2. Side-Chain vs. Aromatic Ring Halogenation:
   1. The position of halogenation (substitution) in arenes depends on reaction conditions.
   2. Under normal conditions, halogenation occurs in the aromatic ring, resulting in the substitution of a hydrogen atom.
   3. However, under specific reaction conditions, such as using a strong electrophile or high temperature, halogenation can occur in the side chain of the arene.