Chapter 14: Aromatic Compounds

Nomenclature of Benzene Derivatives

Aromatic compounds are a fundamental class of organic molecules, with benzene as the prototypical example. The nomenclature of benzene derivatives follows specific rules to ensure clarity and consistency.

• Monosubstituted Benzenes: For some derivatives, benzene is the parent name and the substituent is added as a prefix (e.g., fluorobenzene, chlorobenzene, bromobenzene, nitrobenzene).

• Special Parent Names: Certain substituents result in a new parent name (e.g., toluene for methylbenzene, phenol for hydroxybenzene, aniline for aminobenzene, benzoic acid, benzene sulfonic acid, acetophenone, anisole).

Example: Chlorobenzene (benzene ring with a chlorine substituent), Toluene (benzene ring with a methyl group).

Disubstituted Benzenes

• When two substituents are present, their positions are indicated by the prefixes ortho (o-, 1,2-), meta (m-, 1,3-), and para (p-, 1,4-), or by numerical positions.

• Example: 1,2-dibromobenzene (ortho), 1,3-dibromobenzene (meta), 1,4-dibromobenzene (para).

• Dimethyl-substituted benzenes are called xylenes (e.g., o-xylene, m-xylene, p-xylene).

Polysubstituted Benzenes

• Numbers must be used as locants when more than two substituents are present.

• Rules:

  • Assign the lowest possible set of numbers to the substituents.

  • List substituents in alphabetical order.

  • If a substituent defines a parent other than benzene, it is position 1.

• Example: 1,2,3-trichlorobenzene, 1,2,4-tribromobenzene, 3,5-dinitrobenzoic acid, 2,4-difluorobenzenesulfonic acid.

Special Benzene Substituent Groups

• The C6H5- group is called phenyl when it is a substituent. Abbreviated as Ph or Φ.

• The phenylmethyl group is called a benzyl (abbreviated Bz).

• When naming, the larger structural unit is chosen as the parent; if the chain is unsaturated, it is the parent and benzene is the substituent.

Example: Butylbenzene (alkyl chain attached to benzene), benzyl chloride (benzene-CH2Cl).

Reactions of Benzene

Benzene is highly unsaturated but does not undergo typical alkene reactions such as addition or oxidation under normal conditions.

• No Addition or Oxidation: Benzene does not react with Br2/CCl4, KMnO4/H2O, or H2/Ni under standard conditions.

• Substitution with Bromine: In the presence of a Lewis acid catalyst (e.g., FeBr3), benzene undergoes substitution, not addition.

Example Reaction:

• (Observed)

• (Not observed)

This indicates all six C-H bonds in benzene are equivalent.

The Kekulé Structure for Benzene

The Kekulé structure was the first reasonable representation of benzene, suggesting alternating double and single carbon-carbon bonds.

• Based on this structure, two different 1,2-dibromobenzenes would be expected, but only one is observed.

• Kekulé proposed rapid equilibrium between the two forms, but no such equilibrium exists.

The Stability of Benzene

Benzene is much more stable than expected for a hypothetical cyclohexatriene, as shown by its heat of hydrogenation.

• Predicted heat of hydrogenation for cyclohexatriene:

• Experimental heat of hydrogenation for benzene:

• Resonance energy: (extra stability due to resonance)

Modern Theories of the Structure of Benzene

The Resonance Explanation

Benzene is best described as a resonance hybrid of two equivalent structures, with delocalized electrons.

• Each C-C bond length is 1.39 Å, intermediate between a single (1.47 Å) and double bond (1.33 Å).

• The resonance hybrid is often represented by a hexagon with a circle inside.

The Molecular Orbital (MO) Explanation

All carbons in benzene are sp2 hybridized with unhybridized p orbitals that overlap to form a delocalized π system.

• Six p orbitals combine to form six π molecular orbitals (three bonding, three antibonding).

• The six π electrons fill the three lowest (bonding) molecular orbitals, resulting in a stable, delocalized electron cloud above and below the ring.

Diagram: (Not shown here, but typically includes MO energy levels and electron filling.) *Additional info: The MO explanation accounts for the unique stability and reactivity of benzene compared to non-aromatic compounds.*