CHM 247 CH.18
• Aromatic substances differ from others in their chemical behaviour.
• Aromaticity: The special characteristics of cyclic conjugated molecules, including
unusual stability and a tendency to undergo substitution rather than addition reactions
on treatment with electrophiles. Aromatic molecules are planar, cyclic, conjugated
species with 4n+2 π electrons.
15.1 Sources and Names of Aromatic Compounds
• Above are some of the common names for some aromatic molecules.
• Monosubstituted benzenes are names with benzene as the parent name.
• Arenes: An alkyl-substituted benzene.
• Phenyl: The name for the –C H6u5it when the benzene ring is considered a substituent. A
phenyl group is abbreviated as –Ph.
• If the alkyl substituent is longer than the ring, it is the parent.
• Benzyl: The C 6 5H - 2roup.
• Ortho, o-: A naming prefix used for 1,2-disubstituted benzenes.
• Meta, m-: A naming prefix used for 1,3-disubstituted benzenes.
• Para, p-: A naming prefix used for 1,4-disubstituted benzenes.
• Numbers should be the lowest possible combination.
15.2 Structure and Stability of Benzene
• Benzene reacts with Br 2o give the addition product, but never the substitution product.
• Benzene is a planar molecule with all the carbon atoms being sp -hybridized and with
each p orbital being perpendicular to the plane.
• Each p orbital overlaps its neighbour’s p orbital leading the electrons to be able to move
freely around the entire rings. This gives benzene a lot of stability from resonance. 15.3 Aromaticity and the Hückel 4n+2 Rule
• Benzene is cyclic and conjugated, stable, planar, sp hybridized, undergoes substitution