Fix the mistake: Aromatic Compounds

Benzene and Aromaticity

Benzene C₆H₆ is a representative aromatic compound that shows unusual stability because six π electrons are delocalized over a ring of six carbon atoms.

Basic Knowledge

In 1825, Faraday isolated it from coal tar; in 1865, Kekule proposed its ring structure (according to an anecdote, he dreamed of a snake biting its own tail).
Structure: regular hexagon; all C-C bond lengths are 139 pm (between a single bond at 154 pm and a double bond at 133 pm); sp²-hybridized carbon atoms; π electrons are delocalized throughout the ring.
Huckel's rule: A cyclic π-electron system is especially stable when it has (4n+2) π electrons (n=0,1,2..., 2, 6, 10...). Benzene has six electrons when n=1.
The main reactions are substitution reactions, not addition reactions, because aromaticity is preserved:
① Halogenation: C6H6 + Cl2 → C6H5Cl + HCl (FeCl₃ catalyst)
② Nitration: C6H6 + HNO3 → C6H5NO2 + H2O (concentrated H₂SO₄ catalyst)
③ Sulfonation: C6H6 + H2SO4 → C6H5SO3H + H2O (concentrated sulfuric acid)
④ Friedel-Crafts reaction: alkylation (R-Cl) and acylation (RCO-Cl) using an AlCl₃ catalyst.
Addition reactions also occur under special conditions: H₂ (Ni catalyst, high temperature and pressure) → cyclohexane; Cl₂ (ultraviolet light) → benzene hexachloride (insecticide BHC); O₃ → ring cleavage.

Further Study (Principles and Applications)

Orientation: When one substituent is already attached to benzene, the position where a second substituent enters is determined by the existing substituent.
・Ortho/para-directing groups (electron-donating groups): -OH, -NH₂, -OR, -R (alkyl), -X (halogen).
・Meta-directing groups (electron-withdrawing groups): -NO₂, -COOH, -SO₃H, -CHO, -CN, -CF₃.
This is explained by the stability of the carbocation reaction intermediate: during ortho/para attack, the positive charge of the carbocation can be localized on the carbon bearing the substituent. Electron-donating groups stabilize it, whereas electron-withdrawing groups destabilize it, resulting in meta substitution.
Carcinogenicity of benzene: Benzene causes acute myeloid leukemia and is classified as IARC Group 1. It was once widely used as a high-percentage gasoline additive, a dry-cleaning solvent, and an organic synthesis reagent, but it is now strictly regulated.
Polycyclic aromatic hydrocarbons (PAHs): naphthalene (moth repellent), anthracene (dyes), and benzo[a]pyrene (a carcinogen found in tobacco smoke and charred materials).

Points of Caution (Common Confusions and Frequent Errors)

① Benzene mainly undergoes substitution rather than addition because preserving aromaticity is more stable. ② Halogens have a -I effect (electron-withdrawing) but are ortho/para directors because the +M effect of lone-pair electrons predominates; this exceptional case is frequently tested. ③ The FC reaction means Friedel-Crafts reaction and uses an AlCl₃ catalyst. Acylation is easier to control than alkylation because the carbocation is stabilized. ④ In polysubstituted compounds, electron-donating groups increase reactivity and electron-withdrawing groups decrease it (nitrobenzene has low reactivity).

Practice

1. Explain why benzene favors substitution reactions and resists addition reactions from the viewpoint of resonance and aromaticity.
2. Write the equation for the nitration of benzene and state the active species produced in the nitrating mixture.
3. At which position is the second nitration of toluene most likely to occur? Answer from the viewpoint of orientation.