SolutionsAromatic Compounds
1. Aromatic or not?
Which of these compounds are aromatic? a) cyclohexa-1,3-diene; b) toluene; c) pyridine; d) cyclooctatetraene; e) naphthalene; f) cyclohexene.
Toluene, pyridine and naphthalene are aromatic: they have a cyclic, planar, conjugated π system with 6 or 10 electrons (4n + 2). Cyclohexadiene and cyclohexene are not cyclically conjugated (they have sp3 carbons in the ring), and cyclooctatetraene, with 8 π electrons, is not planar and behaves as a polyene.
2. Addition versus substitution
Cyclohexene instantly decolorizes a bromine solution, but benzene does not react with bromine unless FeBr3 is added, and then it gives bromobenzene instead of a dibromide. Explain the difference.
Cyclohexene undergoes an electrophilic addition to the double bond. If benzene added bromine it would lose its aromaticity and the product would be much less stable, so the addition is unfavorable. With a catalyst that makes bromine more electrophilic, benzene undergoes a substitution: the benzenonium ion intermediate loses a proton and the aromatic ring is recovered.
Aromatic compounds prefer reactions that preserve the aromatic ring: electrophilic substitution (SEAr) and, in electron-poor rings, nucleophilic substitution (SNAr).
3. Reagents and products
Write the main product of the reaction of benzene with: a) HNO3/H2SO4; b) Cl2/FeCl3; c) SO3/H2SO4; d) CH3COCl/AlCl3; e) (CH3)3CCl/AlCl3.
- Nitrobenzene.
- Chlorobenzene.
- Benzenesulfonic acid.
- Acetophenone (methyl phenyl ketone).
- tert-Butylbenzene.