Reactions of Aromatic Compounds

Other Aromatic Reactions

Reactions of the Benzylic Carbon

The benzylic carbon is very reactive because of resonance stability of the benzylic position. Radical halogenation will preferentially occur on a benzylic carbon over other carbon atoms.
A benzylic carbon is an sp3 carbon that is directly attached to a benzene ring. The proximity of a radical that forms on the benzylic carbon to the resonance of the benzene ring provides stability that increases the reactivity of the benzylic carbon.
The sp3 carbons that are directly attached to the benzene are called benzylic carbons.
Oxidation of the benzylic position will yield a carboxylic acid as long as there is at least one hydrogen atom on the benzylic position. Any carbons in the chain past the benzylic carbon will not be part of the final structure because the bond is broken before oxidation occurs. Any benzylic carbon attached to the ring (with at least one hydrogen atom) will be converted to a carboxylic acid.

Benzylic Carbon Oxidation

Benzylic carbons are easily oxidized. As long as there is at least one hydrogen on the benzylic carbon, the chain will be oxidized to carboxylic acids. 1,4-dipropylbenzene (C12H18) is oxidized by hot potassium permanganate (KMnO4) and water (H2O) to form 1,4-dibenzoic acid (C8H6O4).
A free radical is an atom, molecule, or ion with one or more unpaired electrons. Free radicals are extremely reactive. For example, ultraviolet (UV) light can break the bond between a diatomic halogen molecule to yield two halogen radicals. These radicals then interact with other molecules. If they interact with other radicals, they form a covalent bond and do not generate more radicals. If they interact with other diatomic halogen molecules, they will form more radicals. Free radicals can also abstract a hydrogen atom from an aromatic compound to form a free radical at the benzylic carbon. Abstraction is the removal of an atom or a group by a free radical. Radical bromination of the benzylic carbon will occur as long as there is at least one hydrogen on the benzylic position. If there are multiple hydrogen atoms, each can be replaced with bromine with excess reagent.

Radical Bromination of Benzylic Carbon

UV light breaks the bond between the bromine atoms in diatomic bromine, causing free radicals. The free radicals interact with the benzylic carbon by adding bromine atoms anywhere there is a free hydrogen atom. This will occur on the benzylic carbon as long as there is one hydrogen atom bonded to the benzylic carbon.
Substitution of the benzylic position is more easily accomplished than other positions because of the stability of the arenium ion that is formed in the benzene ring. Likewise, elimination toward the benzylic position is more likely to occur than elimination away from the benzylic position because of conjugation.

Resonance of the Benzylic Arenium Ion

The arenium ion formed with a benzylic carbon has several resonance forms that stabilize the structure.

Reduction Reactions

Reduction reactions of benzene include catalytic hydrogenation, which forms cyclohexane, and Birch reduction, which forms 1,4-cyclohexadiene.
Catalytic hydrogenation (with a metal catalyst such as Pt, Pd, or Ni) of benzene to cyclohexane can only occur at high temperature and pressure. The high temperature and catalyst are needed to lower the extremely large activation energy for the hydrogenation of benzene. Without high temperatures and pressures, the double bonds of the benzene ring would not be reduced, but an alkene substituent would be reduced if present.

Catalytic Hydrogenation of Benzene

Benzene can be converted into cyclohexane through catalytic hydrogenation with the use of a metal catalyst, high temperature, and high pressure.

Energy Diagram of Catalyzed Reaction

The noncatalyzed hydrogenation of benzene has a very large activation energy, which cannot be overcome under normal conditions. However, hydrogen (H2) in the presence of a catalyst (Pd, Pt, or Ni) lowers the activation energy of the reaction and allows hydrogenation of benzene to occur, but high temperature is still required.
Another reduction reaction that can occur with benzene is the Birch reduction of benzene. The Birch reduction is a reduction of benzene to 1,4-cyclohexadiene using Na, NH3, and methanol. Birch reduction selectively occurs on the carbons with alkyl groups (electron donating groups) substituents not being reduced (staying a double bond) and the carbons with EWG being reduced (turning into a single bond). An electron attacks the benzene, causing the bonds to rearrange and yielding a carbon with a lone pair of electrons, a carbanion, and one carbon with a single lone electron, a radical. The lone pair of the carbanion attacks the alcohol (usually methanol, CH3OH) and abstracts (or removes) a hydrogen atom from the alcohol. A free electron reacts with the radical carbon of the benzene ring to form a new lone pair of electrons. The new lone pair of electrons attacks another alcohol and abstracts a second hydrogen atom. This results in a six-membered ring with two isolated double bonds and two reduced carbons.

Birch Reduction of Benzene

Benzene can be reduced using the Birch method. The Birch method reduces benzene to 1,4-cyclohexadiene with Na, NH3, and methanol.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions can substitute a hydrogen with a functional group by using nucleophiles instead of electrophiles. These reactions are classified as nucleophilic substitution reactions (SNAr) and elimination-addition reactions.

In addition to EAS reactions, benzene will undergo nucleophilic substitution reactions. There are two types of nucleophilic substitution reactions, SNAr and elimination-addition. Nucleophiles can include but are not limited to I, Br, CH3O, CH3S, OH, CN, NH3, and NH2.

Nucleophilic substitutions in organic chemistry have the notation SN1 and SN2, with SN# indicating a nucleophilic substitution and the number indicating the number of molecules. SNAr is similarly named but with Ar indicating an aromatic group. An SNAr reaction is a nucleophilic substitution reaction of an aromatic group that requires a leaving group (Cl, Br, I, …) and a powerful EWG group (NO2{-}{\rm{NO}_2}) to be either ortho or para to each other. The stronger the electron withdrawing group is, the faster the reaction will proceed. A strong nucleophile will cause a substitution reaction to occur via a Meisenheimer complex. The Meisenheimer complex is an intermediate that contains both the nucleophile and the leaving group. It is stable because of its resonance. The lone electron pair on the carbon forms a double bond, and the leaving group leaves. This results in the nucleophile substituting where the leaving group was in the structure.
An SNAr reaction demonstrates a nucleophilic substitution on a benzene ring. The nucleophile attacks the carbon with the leaving group. The intermediate contains a carbon with both the nucleophile and the leaving group. For example, 1-bromo-2,4-dinitrobenzene (C6H3BrN2O4) reacts with sodium hydroxide (NaOH) to form 2,4-dinitrophenol (C6H4N2O4).
An elimination-addition (sometimes called a benzyne reaction) is a reaction that occurs when a leaving group is on a benzene ring and there is not a strong EWG on the ring in the ortho and/or para position. A base is added and will cause a substitution reaction to occur via an elimination-addition mechanism. The base acts as the nucleophile that attacks the hydrogen adjacent to the leaving group. The lone pair of electrons on the carbon forms a triple bond, and the leaving group leaves. This forms a benzyne intermediate, which is a short-lived intermediate of benzene that has a triple bond after the leaving group has left. The nucleophile attacks the triple bond, causing the nucleophile to add at the position where the leaving group originally was located.

Elimination-Addition Reaction via a Benzyne Intermediate

p-chlorotoluene reacts with NaNH2 in an elimination addition reaction. This results in a benzyne intermediate that undergoes nucleophilic attack by the amide ion, NH2-, to form the product.