Benzene homologues: structure, preparation, and chemical properties

Structure and nomenclature of benzene homologs

Benzene homologs are hydrocarbons that retain a six-carbon benzene ring in sp²-hybridization, while the carbon skeleton is extended via side chains. The general formula of the homologous series is C_nH_2n–6. The three π-bonds and the sigma framework of the ring are preserved, while side substituents exert an electronic influence, broadening the spectrum of available reactions.

Names are formed by adding the word "benzene" to the substituent name: methylbenzene (toluene), 1,2-dimethylbenzene, isopropylbenzene (cumene, or 2-phenylpropane), vinylbenzene (styrene). The C₆H₅– radical is called phenyl.

For disubstituted derivatives, positional isomerism is introduced:

• ortho- (1,2-) — substituents are adjacent;
• meta- (1,3-) — separated by one carbon atom;
• para- (1,4-) — opposite each other.

Thus, the three isomers of xylene are distinguished as o-xylene, m-xylene, and p-xylene.

Methods of preparation

• Dehydrocyclization of alkanes: heptane over Cr₂O₃ upon heating yields toluene and 4H₂ (loss of 8 hydrogen atoms during the transition from C_nH_2n+2 to C_nH_2n–6).
• Wurtz-Fittig reaction: chlorobenzene reacts with a haloalkane and metallic sodium to form a benzene homolog (e.g., toluene from C₆H₅Cl and CH₃Cl).
• Trimerization of alkynes: trimerization of propyne yields 1,3,5-trimethylbenzene (mesitylene).
• Alkylation of benzene in the presence of AlCl₃: with haloalkanes (CH₃Cl → toluene + HCl), with alkenes (ethylene → ethylbenzene; the double bond disappears in the process), with alcohols (with elimination of water). When working with alkenes and alcohols, H₃PO₄ is often mentioned; in problem sets, it is safer to write "cat." or AlCl₃.

Electronic effects of substituents

Substituents are divided into two classes depending on their mesomeric and inductive effects on the ring.

• First-class substituents (–CH₃, –OH, –NH₂, alkyls, vinyl) are electron donors; they increase electron density in the ring and direct electrophilic substitution to the ortho- and para-positions. Examples of compounds: toluene, phenol, aniline, ethylbenzene.
• Second-class substituents (–COOH, –CHO, –NO₂) are electron acceptors; they withdraw electron density and direct substitution to the meta-position. Examples: benzoic acid, benzaldehyde, nitrobenzene.

The benzene ring is prone to electrophilic substitution, and the direction of attack is determined precisely by the electronic effect of the existing substituent.

Chemical properties of toluene

Halogenation with AlCl₃ is electrophilic substitution into the ring at the ortho- and para-positions. When writing the product, three variants are possible: a general equation with substitution at all three positions simultaneously, separate equations for o- and p-isomers, and the most frequent exam variant — para-chlorotoluene (the predominant product due to steric hindrance at the ortho-position).

Radical substitution proceeds under the action of Cl₂ in the presence of light (UV) without AlCl₂. Under these conditions, the halogen goes into the side chain: C₆H₅–CH₂Cl is formed. Fundamentally: the choice of catalyst determines the product.

Nitration proceeds with a nitrating mixture of HNO₃ + H₂SO₄ (sulfuric acid as a catalyst), also at the ortho- and para-positions. The trisubstituted product — 2,4,6-trinitrotoluene (trotyl, TNT) — clearly demonstrates the explosive nature of benzene homologs.

Vigorous oxidation with KMnO₄ in an acidic medium destroys the side chain down to a carboxyl group: toluene → benzoic acid + MnSO₄ + K₂SO₄ + H₂O. In a neutral or alkaline medium, the corresponding benzoate is formed.

Chemical properties of styrene

Styrene (vinylbenzene) combines the properties of an aromatic ring and a double bond in the side chain; therefore, reactions involving the ethylene fragment are added to the reactions characteristic of the benzene series.

• Electrophilic addition across the double bond: halogenation, hydrohalogenation without AlCl₃. In the presence of AlCl₃, halogenation will proceed into the ring at the ortho- and para-positions.
• Hydration according to Markovnikov's rule: H attaches to the more hydrogenated carbon atom, and OH to the less hydrogenated one.
• Hydrogenation: under mild conditions, the double bond is reduced to ethylbenzene; in the presence of excess H₂ and an active catalyst, further reduction of the ring to ethylcyclohexane is possible (an additional 3 moles of H₂).
• Mild oxidation (Wagner's test): KMnO₄ in a neutral medium yields the dihydric alcohol C₆H₅–CH(OH)–CH₂OH, MnO₂, and KOH.
• Vigorous oxidation: KMnO₄/H₂SO₄ cleaves the double bond, the single-carbon fragment is oxidized to CO₂, and the remaining fragment to benzoic acid.
• Polymerization: n molecules of styrene upon heating in the presence of a catalyst form polystyrene. The reaction is written using the coefficient n in front of the monomer and square brackets around the repeating unit with the index n.

Key points

• Benzene homologs have the formula C_nH_2n–6; chain extension occurs via side radicals rather than the ring.
• Positional isomerism of disubstituted derivatives: ortho- (1,2), meta- (1,3), para- (1,4).
• Main preparation methods: dehydrocyclization of alkanes, Wurtz-Fittig reaction, alkyne trimerization, benzene alkylation (with haloalkanes, alkenes, alcohols).
• First-class substituents direct electrophilic substitution to the ortho- and para-positions, while second-class substituents direct it to the meta-position.
• Chlorination of toluene in the presence of AlCl₃ proceeds in the ring, whereas in the presence of light (UV) it proceeds in the side chain.
• Vigorous oxidation of toluene and styrene with KMnO₄/H₂SO₄ leads to benzoic acid.
• Trinitrotoluene is a practically significant product of toluene nitration (the explosive TNT).
• Styrene is capable of addition reactions, mild oxidation via Wagner's reaction, and polymerization into polystyrene.