Nomenclature
Monosubstituted benzenes are named by adding the substituent prefix to the word benzene (chlorobenzene). For disubstituted benzenes, the prefixes ortho, meta, and para are used.
Benzene Aromaticity
Benzene is particularly stable due to the resonance delocalization of the π electrons that form the double bonds. Hückel's rule establishes the conditions that a compound must meet to be aromatic: planar, with cyclic conjugation, and 4n+2 π electrons with n = 0, 1, 2, 3..........
Benzene Reactivity
Electrophilic substitution of benzene takes place by addition of the electrophile to the ring followed by the loss of a proton, which allows the recovery of aromaticity. Sulfonation is a reversible reaction that allows protecting a position on the ring. Acylation and alkylation reactions introduce acyl and alkyl groups onto benzene. The nitric sulfuric mix generates the NO2+ cation, which acts as an electrophile in the nitration of benzene.
Activation and Deactivation of the Ring.
Alkyl groups (methyl, ethyl) donate charge to benzene by inductive effect, weakly activating it and thereby increasing its reactivity in electrophilic substitution. These groups are called weak activators and orient to ortho and para positions.
Groups with lone pairs donate charge to benzene by resonance effect, are strong activators, and also orient to ortho and para.
Groups with multiple bonds (aldehydes, ketones, esters, amides) steal charge from benzene by resonance effect, decreasing its reactivity and orienting to the meta position.
Nucleophilic Aromatic Substitution.
The π cloud of benzene prevents the attack of nucleophiles, hence no nucleophilic addition reactions are observed. However, there is an exception when there are strong deactivating groups at the ortho and para positions relative to a halogen, in this case, the substitution of the halogen by the corresponding nucleophile occurs.
Benzylic Position
The position adjacent to the aromatic ring is called the benzylic position. It has reactivity very similar to the allylic position, generating carbocations, carbanions, and radicals of great stability. On benzylic positions, SN1 mechanisms can occur even with primary haloalkanes.
Oxidation of Side Chains
Potassium permanganate and potassium dichromate allow the oxidation of alkyl groups on benzene. The chain breaks at the benzylic position regardless of its length, generating benzoic acid.
Birch Reduction
It is a partial hydrogenation of benzene that is carried out with sodium, liquid ammonia, and ethanol. The result is a cyclohexadiene with double bonds in positions 1,4.