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.
Benzene C6H6 (colorless aromatic liquid, carcinogenic, distinctive odor)
Aromaticity (cyclic, planar, (4n+2) π electrons; Huckel's rule)
Resonance (stabilization by delocalization of π electrons)
Electrophilic substitution reaction (H+ and other cationic species are substituted; a typical reaction of benzene)
Friedel-Crafts reaction (AlCl3 catalyst; introduction of hydrocarbons into benzene)
Orientation (position selectivity toward ortho/para or meta positions caused by an existing substituent)
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).
- Benzene is C6H6, a regular hexagon, and is stabilized by resonance.
- It is aromatic because of Huckel's rule: (4n+2) π electrons.
- Substitution reactions, rather than addition reactions, are the main reactions.
- Four representative reactions: halogenation, nitration, sulfonation, and the FC reaction.
- Orientation: -OH/-NH2 direct to ortho/para; -NO2/-COOH direct to meta.
- Benzene is carcinogenic and can cause leukemia.
- Many polycyclic PAHs are also carcinogenic.
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
- Explain why benzene favors substitution reactions and resists addition reactions from the viewpoint of resonance and aromaticity.
- Write the equation for the nitration of benzene and state the active species produced in the nitrating mixture.
- At which position is the second nitration of toluene most likely to occur? Answer from the viewpoint of orientation.