Structure and Bonding of Benzene

Benzene, \( C_6H_6 \), is a planar, cyclic hydrocarbon made of six carbon atoms joined in a ring, each bonded to one hydrogen atom. It is the parent compound of the arenes (aromatic hydrocarbons). Understanding its structure and bonding explains why benzene behaves so differently from alkenes, even though early chemists first drew it with alternating double bonds.

The original model, proposed by Kekule, shows benzene as a hexagonal ring with alternating single and double carbon to carbon bonds, cyclohexa-1,3,5-triene. This structure predicts two different carbon to carbon bond lengths (a shorter double bond and a longer single bond) and predicts that benzene should readily undergo addition reactions, just like an alkene.

Three key pieces of evidence show that the Kekule structure is not correct.

Bond lengths. X-ray diffraction shows all six carbon to carbon bonds in benzene are identical, about 139 picometres long, which is between a typical carbon to carbon single bond (154 pm) and a carbon to carbon double bond (134 pm). This is inconsistent with three distinct double bonds.

Resistance to addition. Benzene does not readily decolourise bromine water, unlike alkenes, showing it does not contain isolated reactive double bonds.

Enthalpy of hydrogenation. Hydrogenating cyclohexene (one double bond) releases about 120 kJ/mol. If benzene really had three separate double bonds, hydrogenating it fully should release roughly three times this, around 360 kJ/mol. The experimental value is only about 208 kJ/mol, some 152 kJ/mol less exothermic than predicted. This missing energy is the extra stability, often called the delocalisation (or resonance) energy, of the real benzene ring.

Each carbon atom in the ring is \(sp^2\) hybridised, forming three sigma bonds (two to neighbouring carbons and one to hydrogen) that lie in the same plane, giving benzene its flat hexagonal shape. Each carbon also has one unhybridised p-orbital, oriented perpendicular to the ring, containing one electron. These six p-orbitals overlap sideways all the way around the ring, merging into one continuous pi system above and below the plane of the ring, rather than three separate double bonds.

This spreads, or delocalises, the six pi electrons evenly over all six carbon atoms, so every carbon to carbon bond has identical bond order and identical length. Benzene is usually drawn as a hexagon with a circle inside it to represent this delocalised ring of electrons.

The delocalised ring gives benzene extra stability compared with three isolated double bonds. Any reaction that broke up this ring, such as electrophilic addition, would destroy the delocalisation and be energetically costly. Instead, benzene reacts by electrophilic substitution, where an electrophile replaces a hydrogen atom, keeping the stable delocalised ring intact.

The delocalised pi electrons make the ring an area of high electron density, which attracts electrophiles. The mechanism has three key stages.

1. The electron-rich ring attacks an electrophile, \(E^+\), forming a carbon to electrophile bond. This uses two of the six delocalised electrons, leaving the ring as a non-aromatic, positively charged intermediate (the arenium ion) with the remaining four electrons delocalised over five carbon atoms.
2. A base (often the counter-ion generated with the electrophile) removes the hydrogen atom from the carbon now bonded to both hydrogen and the electrophile.
3. The pair of electrons from this carbon to hydrogen bond returns to the ring, restoring the full delocalised system of six pi electrons and regenerating the stable aromatic ring.

Nitration

Benzene reacts with concentrated nitric acid, using concentrated sulfuric acid as a catalyst, at around 50 degrees Celsius. The sulfuric acid generates the electrophile, the nitronium ion \(NO_2^+\), which substitutes for a hydrogen atom to form nitrobenzene. This reaction is the starting point for making aromatic amines; the nitro group can later be reduced to an amine, a route explored further in amines, amides and amino acids.

Halogenation

Benzene reacts with chlorine or bromine only in the presence of a halogen carrier catalyst, such as \(AlCl_3\) or \(FeBr_3\), which polarises the halogen molecule to generate a strong enough electrophile, \(Cl^+\) or \(Br^+\). The product is a halogenoarene, for example chlorobenzene, with hydrogen chloride or hydrogen bromide given off as a by-product.

Friedel-Crafts acylation and alkylation

With an acyl chloride and an aluminium chloride catalyst, benzene forms an acylium ion electrophile and is converted into a phenyl ketone; the acyl chloride reagent connects this chemistry to carboxylic acids, acyl chlorides and esters. With a haloalkane and the same type of catalyst, an alkyl carbocation electrophile forms instead, attaching an alkyl side chain to the ring. This is a valuable way of extending a carbon skeleton, alongside methods such as those covered in increasing carbon chain length using Grignard reagents.

Benzene's ring is best described by the delocalised model: a planar hexagon of \(sp^2\) carbons with a continuous pi system of six delocalised electrons above and below the ring, giving all six carbon to carbon bonds equal length and extra stability. This stability explains why benzene undergoes electrophilic substitution, preserving the ring, rather than the addition reactions typical of alkenes.

Related lessons

• Uk A Level Chemistry