Benzene and Aromaticity

Structure and Properties

Benzene exists as a resonance hybrid. Because of delocalization of electrons over the π\pi system, benzene is much more stable than other isolated and conjugated polyenes. Because of its stability, benzene does not react in electrophilic addition reactions like other alkenes or conjugated polyenes.

Benzene is a cyclic aromatic molecule with the chemical formula C6H6. In 1825 Michael Faraday isolated benzene from oil lamp residue and, after experimentation, discovered that benzene has the same number of carbons as hydrogens, C6H6. A chemist, August Laurent, proposed the name should be pheno, from the Greek phainein, meaning "to shine." The term phenyl persisted and is used as a synonym for benzene today. The name benzene was derived from a German chemist, Eilhard Mitscherlich, who used the term benzin.

In 1865 August Kekulé, a German scientist, proposed that benzene was a hexagon. However, he thought that benzene was two structures in equilibrium rather than a resonance hybrid. The molecular structures that Kekulé proposed only differ in the arrangement of the electrons rather than the atoms. Structures that only differ in the arrangement of electrons are called resonance structures and are stable because of their resonance. Resonance is the method that organic chemists use to depict the Lewis structures of organic molecules where electrons can be in two or more different places around fixed atoms. Aromatic compounds have two equivalent and significant resonance structures.

An aromatic molecule is cyclic, has a continuous circle of p orbitals, has planar geometry, and has a Hückel number [4n+24n+2] of π\pi electrons. Benzene is the prototypical aromatic compound, with each carbon atom bonded to two other carbon atoms and one hydrogen atom using sp2 orbitals. The p orbitals on each carbon atom overlap, allowing the electrons to form π\pi bonds. The overlapping p orbitals create torus-shaped, or donut-shaped, clouds of π\pi electrons above and below the plane of the flat benzene ring. The delocalization of the electrons in the π\pi cloud makes the molecule more stable. This aromatic stability makes benzene nonreactive, which means benzene does not easily react with other molecules. Benzene does not react in electrophilic addition reactions like other alkenes or conjugated polyenes, which are compounds with multiple alternating single and double bonds. Benzene rings will serve as a source of electrons for strongly acidic or electrophilic reagents, but the reactions lead to substitution, not addition products.

Benzene Pi Electron Cloud

The π\pi orbitals of benzene (C6H6) share delocalized electrons, creating equivalent resonance structures and a very stable structure.