Preparation and chemical properties of arenes

Methods for the preparation of arenes

The synthesis of aromatic hydrocarbons is carried out both in industry and in laboratories via various pathways:

• Industrial method. The main source of benzene and toluene is coal. The process is called dry distillation, in which coal is heated without access to oxygen.
• Dehydrogenation of cycloalkanes. Benzene can be synthesized from cyclohexane. This requires a catalyst — platinum ($Pt$), and the reaction releases hydrogen.
• Laboratory and other methods. These include dehydrocyclization of alkanes and trimerization of alkynes. For example, heating propyne in the presence of concentrated sulfuric acid yields symmetrical 1,3,5-trimethylbenzene (mesitylene).
• Decarboxylation of salts. Benzene can be obtained in a single step by fusing solid sodium benzoate and sodium hydroxide. The reaction yields benzene and sodium carbonate.

Oxidation and addition

The aromatic system of benzene is very stable, which is why addition and ring oxidation reactions proceed with difficulty.

Oxidation:

• Combustion: Benzene burns with a smoky flame due to its high carbon content, which leads to incomplete combustion and soot formation.
• Catalytic oxidation: Upon exposure to oxygen and a $V_2O_5$ catalyst, the benzene ring is cleaved to form maleic acid.
• Oxidation of homologs: The action of strong oxidizing agents (e.g., $KMnO_4$ in an acidic medium) cleaves the side chains rather than the ring itself. The methyl group of toluene is oxidized to a carboxyl group, forming benzoic acid. In longer radicals (ethyl, propyl), the bond between the $\alpha$- and $\beta$-carbon atoms is cleaved.

Addition reactions:

• Hydrogenation: Upon strong heating ($180^\circ C$) and in the presence of a nickel catalyst, benzene adds hydrogen, turning into cyclohexane.
• Chlorination (radical): Under UV irradiation, benzene reacts with chlorine to form hexachlorocyclohexane (lindane), a substance previously used as an insecticide but banned due to its high toxicity and slow degradation in soil.

Electrophilic substitution reactions

The most characteristic reactions for arenes are hydrogen substitution reactions proceeding via the electrophilic substitution mechanism ($S_E$). They are used for the synthesis of drugs, dyes, and other substances.

The reaction mechanism using bromination as an example includes three stages:

1. Polarization. The bromine molecule is polarized by the action of a catalyst — a Lewis acid (e.g., $FeBr_3$).
2. Formation of the $\sigma$-complex. The positive $Br^+$ particle attacks the ring. At the same time, the carbon atom transitions to $sp^3$-hybridization, and aromaticity is disrupted.
3. Restoration of aromaticity. A proton ($H^+$) is split off from the $sp^3$-atom, the conjugated system is restored, and the substitution product is formed.

Other important $S_E$ reactions include sulfonation, in which benzene reacts with oleum to form benzenesulfonic acid.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed mechanism of electrophilic substitution
• Specifics of the oxidation of long-chain benzene homologs
• Application of Lewis acids as catalysts
• Environmental aspects of lindane use

Other topics in "Chapter 15. Hydrocarbons"

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