Benzene, C6H6, is an aromatic organic molecule (the terms aromatic, arene and aryl compound are used interchangeably to classify benzene and related molecules).
It was first extracted from crude oil in 1825 by Michael Faraday and later from the distillation of coal. Although its molecular formula was well established, its structure was a puzzle to chemists for several decades. In 1865 August Kekulé proposed that benzene was a cyclic molecule with alternating C-C and C=C bonds, and that the two possible structures were in equilibrium with each other.
However, it quickly became apparent that the physical and chemical properties of benzene do not match those predicted for a molecule with the Kekulé structure.
- benzene is considerably more stable, and hence less reactive, than the Kekulé structure would suggest
e.g. cyclohexene and cyclohexa-1,3-diene both react exothermically with hydrogen in the presence of a platinum catalyst to form cyclohexane
C6H10 + H2 ⇾ C6H12 ΔrH⦵ = -120 kJ mol-1
C6H8 + 2H2 ⇾ C6H12 ΔrH⦵ = -232 kJ mol-1
Which would suggest that benzene should react in a similar way releasing ∼ 360 kJ mol-1 of energy …
C6H6 + 3H2 ⇾ C6H12 ΔrH⦵ = -208 kJ mol-1
The difference in the enthalpy of hydrogenation between benzene and a hypothetical molecule of cyclohexa-1,3,5-triene (the Kekulé structure) is 152 kJ mol-1. Clearly we need to put much more energy in to breaking the bonds so that we can add hydrogen atoms than we might expect if we were simply breaking three C=C bonds.
- modern analytical techniques show that all the C-C bonds in benzene are the same length (0.139nm) and the molecule is planar with bond angles of 120°. It is perfectly symmetrical. The Kekulé structure of shorter C=C bonds (0.134 nm) alternating with longer C-C bonds (0.154 nm) would produce a distorted hexagonal shape.
- if the Kekulé structure was correct we would expect benzene to react in a similar way to alkenes, by electrophilic addition, but benzene undergoes electrophilic substitution reactions.
To understand the structure and the reactions of benzene we need to reconsider the bonding in the molecule because it turns out that the pi electrons (those that make up the double bonds in the Kekulé structure) are actually delocalised in a ring system that extends above and below the plane of the carbon backbone of the molecule.
Aromatic molecules
Benzene is not the only aromatic molecule. They may be neutral molecules, carboanions or carbocations but they share three key features – aromatic molecules are cyclic, planar so that the unhybridised p orbitals can overlap and hence they all have a complete delocalised ring of ∏ electrons.
Atoms other than carbon can also be part of the aromatic ring e.g. nitrogen donates its lone pair of electrons to the delocalised ring in a pyrrole and oxygen donates one lone pair of electrons to the delocalised ring in a furan.
Practice questions
- Describe the similarities and differences between the bonding in the structure for benzene that was proposed by August Kekulé and the delocalised ring of electrons model of benzene.
- Describe two pieces of evidence that support the delocalised ring of electrons model of the structure of benzene over the structure proposed by August Kekulé.
Answers
- The similarity is that in both models p orbitals overlap and the pi / ∏ electrons are found both above and below the hexagonal carbon ring.
The difference is that in Kekulé’s structure there are alternating single C-C and double C=C bonds. In a double C=C bond there is a pi bond formed from the overlap of a p orbital between the two carbon atoms only – there are only 2 electrons in this pi bond.
In the delocalised ring model of the structure of benzene the unhybridised p orbital on each carbon atom overlaps to form a ring of 6 delocalised electrons above and below the plane of the hexagonal carbon backbone.
2. Kekulé structure: alternating single C-C and double C=C bonds of differing lengths, distorted hexagonal shape
Delocalised ring model: all C-C bond lengths are the same, planar hexagonal shape
Kekulé structure has alkene functional groups – benzene is less reactive than Kekulé structure would suggest.
Alkenes react by electrophilic addition / easily decolonise bromine water at room temperature; benzene reacts by electrophilic substitution / will only react with bromine in the presence of a halogen carrier catalyst such as FeBr3 or AlBr3.
The enthalpy change of hydrogenation is far less exothermic in benzene compared with the theoretical value for the Kekulé structure – benzene is more stable and less reactive.