The core chemical property of aromatic hydrocarbons is their aromaticity. Overall, they exhibit structural stability, readily undergo electrophilic substitution reactions, and are less prone to addition and oxidation reactions. Specific properties are as follows:
Core Characteristic: Electrophilic Substitution Reactions
This is the most typical reaction of aromatic hydrocarbons. During the reaction, a stable benzene ring structure is preserved. Common types include:
Nitration: Benzene reacts with concentrated nitric acid under concentrated sulfuric acid catalysis and heating to produce nitrobenzene. This is a characteristic substitution reaction of aromatic hydrocarbons.
Sulfonation: Reaction with concentrated sulfuric acid, where hydrogen atoms in the benzene ring are replaced by sulfonic acid groups (-SO₃H). The reaction is reversible and can be used for the localization protection of the benzene ring.
Halogenation: Under the catalysis of Lewis acids (such as FeCl₃), hydrogen atoms in the benzene ring are replaced by halogens to produce haloaromatic hydrocarbons.
French-Crafts Reaction: Including alkylation and acylation, introducing alkyl or acyl groups into the benzene ring under AlCl₃ catalysis is an important method for preparing aromatic ketones and alkylbenzenes. However, alkylation is prone to rearrangement.
Directing Effect: Existing substituents on the benzene ring influence the position and reactivity of subsequent substitutions: Electron-donating groups (such as methyl and hydroxyl) activate the benzene ring, preferentially causing substitution at the ortho/para position; electron-withdrawing groups (such as nitro and carboxyl) deactivate the benzene ring, preferentially causing substitution at the meta position; halogens are a special case, deactivating the benzene ring but still resulting in ortho/para-directed substitution.
Addition Reactions: The conjugated systems of aromatic hydrocarbons are stable, requiring stringent conditions for addition, and the addition process often disrupts the aromatic structure.
Catalytic Hydrogenation: Benzene, under high temperature and pressure and with a catalyst, can completely add hydrogen to form cyclohexane; condensed-ring aromatic hydrocarbons such as naphthalene and anthracene are more prone to addition than benzene, preferentially reacting at the more reactive position.
Photochemical Halogenation: Benzene can add chlorine under light irradiation to form hexachlorocyclohexane, rather than undergoing a substitution reaction.