Proper, IUPAC names for substituted benzene compounds use numbers to indicate relative positions of substituents. When only two substituents are present the terms ortho, meta, and para are commonly used instead.
Were benzene simply cyclohexatriene, it would be expected that reduction would release three times the energy released upon reduction of cyclohexene, or 3 x –28.6 = –85.8. However, the observed value is 49.3 and thus benzene is some 36 kcal/mole more stable than predicted for cyclohexatriene.
The molecular orbitals of the pi system of benzene are derived by combination of the six p-orbitals on the six carbon atoms. All six molecular orbitals have a nodal plane that includes the carbon atoms as well as the hydrogen atoms. The lowest lying orbital has no other nodes whereas the next two, equal energy orbitals have one additional node. The nodes of these two orbitals are at right angle to each other. These three lowest lying orbitals constitute the bonding molecular orbitals and hold the six-pi electrons of benzene. The relative energies of orbitals of cyclic systems with sp2 atoms can be simply predicted by drawing the polygon of the structure with one atom at the bottom. The vertices of the polygon are then located so as to represent the energy levels of the orbitals.
The infrared spectrum of benzene compounds with two or more substituents show characteristic patterns that can be used to determine where the substituents are on the ring. However, analysis by proton and carbon nmr is now generally used for this purpose. In the nmr experiment, the cyclic electron of the pi system of benzene create a magnetic field that is shielding above and below the ring and deshielding outside the ring. As a result, protons on the ring resonate downfield from those of simple alkenes.
Multiple aromatic rings can be present in a molecule. Examples are biphenyl, where two benzene rings are linked by a C—C sigma bond, and naphthalene, where two benzene rings are fused to each other and share two carbons between them. Naphthalene does not have twice the stabilization energy of benzene even thought it formally has two such rings. From heats of hydrogenation, naphthalene is 61 kcal/mole more stable than if the pi electrons were contained in simple double bonds.
Molecules with rings of all sp2-hybridized atoms are aromatic if the have 2, 6, 10 etc electrons in the pi system. A simple rule, called Huckel's rule, states that the required number of electrons for a compound to be aromatic is 4n + 2, where n is an integer.
Cyclic collections of sp2 atoms have a single lowest lying orbital. Orbitals then are in pairs of identical energy. For such a system to be aromatic, there must be sufficient electrons to fill a level. Thus, 2, or 6, or 10 etc. Huckel's rule summarizes this with the formula: 4n + 2, where n is an integer. The atoms involved need not be carbon, as in pyrrole, and the molecule can be charged, as in the anion derived by deprotonation of cyclopentadiene.
Powerful electrophiles react with aromatic compounds where 2 of the pi electrons are used to form a bond to the electrophile producing a cyclic pentadienyl cation. Loss of the proton from the carbon bearing the electrophile restores two electrons to the pi system and restores aroaticity.
A Lewis acid is required to activated molecular halogens for reaction with benzene. Thus, FeBr2 + Br2 results in bromination of benzene and AlCl3 + Cl2 results in chlorination. A halogen substituent on benzene reduces the reactivity and thus further halogenation is slower than the first.
The combination of HNO3 and H2SO4 produces the nitronium ion [O=N=O]+ which reacts with benzene to produce nitrobenzene. Fuming sulfuric acid, a combination of SO3 and H2SO4, results in sulfonation, producing benzene sulfonic acid. Both the nitro and sulfonic acid groups are strongly deactivating and thus further reaction is much slower than is the first.
Reaction of benzene with an alkyl halide in the presence of AlCl3 results in alkylation. With primary alkyl halides, the reactive species is a complex between the alkyl halide and the Lewis acid whereas with secondary and tertiary alkyl halides, this same complex produces secondary and tertiary cations which then react with benzene.
The presence of an alkyl group increases reactivity and thus alkylation is difficult to control. Further, rearrangements are common.
Carboxylic chlorides react with AlCl3 to form a complex very similar to that derived from alkyl chlorides. The C—Cl bond in this complex breaks, producing an acylium ion. The acylium ion is stabilized by resonance and therefore does not undergo rearrangement. Acylium ions react with benzene to produce ketones. The ketone products react irreversibly with the Lewis acid producing a complex that is ultimately broken by the addition of water. Because the initially formed ketone reacts with the Lewis acid, the acid must be used in equimolar amounts and is not a catalyst.