Benzene: structure and combustion

In a nutshell

​Benzene has the molecular formula $$C_6H_6$$. The three evidences for the acceptance of the delocalised model over the Kekulé model are the experimental values of the enthalpy change of hydrogenation of benzene, the carbon to carbon bond lengths and the fact that it doesn't decolourise bromine water. Benzene undergoes incomplete combustion in air. Molecules with a benzene ring are called aromatic compounds.

Equations

This is an equation for a specific chemical reaction.

​$$benzene \ + \ oxygen \rightarrow carbon \ dioxide \ + \ water$$​​

Benzene ring

Benzene is a cyclic structure with the molecular formula $$C_6H_6$$. It has been illustrated using the Kekulé model and now currently represented using the delocalised model.

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Kekulé's model

Friedrich August Kekulé theorised that the benzene ring had the structure of cyclohexa- $$1,3,5$$-triene.

He modified the structure later stating that the benzene was in rapid equilibrium between two isomeric forms - by switching the positions of the double bonds (see diagram below). 

He suggested that this rapid flipping between the isomers meant it could not become halogenated by a bromine molecule as the structure kept changing before a reaction could take place. 

Kekulé's structure suggested that  $$C-C$$ and $$C=C$$ bonds alternated in the molecule. You would expect the molecule to have three bond lengths of $$154 \, pm \ (C-C \ bond)$$ and three bond lengths of $$134 \, pm \ (C=C \ bond)$$. However, years later X-ray diffraction studies revealed that all the bond lengths of the carbon to carbon bonds in benzene were equal with a bond length of $$140 \ pm$$ - between that of​ $$C-C$$​ and $$C=C$$ bonds. This proved that the Kekulé's structure was incorrect.

Delocalised model

The results given by the X-ray diffraction studies led to the development of the delocalised model. Each carbon atom uses three of its electrons to form $$\sigma$$-bonds. The remaining electron is in a p-orbital and is delocalised. The p-orbitals overlap sideways with adjacent p-orbitals above and below the plane of the atoms, forming a ring of $$\pi$$-electrons. 

The delocalisation of the $$\pi$$-electrons extends over the six carbon atoms in the ring and is shown by two rings in the diagram which represents the electron density above and below the plane. The electrons in the ring are classed as delocalised as the electrons do not belong to a particular carbon atom, but shared by all six carbon atoms. The carbon to carbon bonds in this model are of equal length which takes in to account the observations made regarding benzene.

Evidences for the delocalised model

Enthalpy change of hydrogenation

Experimentally determined enthalpy change of hydrogenation of cyclohexene, which has one $$C=C$$ bond is $$-120 \, kJmol^{-1}$$​ . According to Kekulé's structure the expected enthalpy change of benzene is $$-360 \ kJmol^{-1}$$, three times the value of cyclohexene. However, the experimental value of hydrogenation of benzene is less exothermic than expected and far more stable, with a value of $$-208 \ kJmol^{-1}$$. 

Bromination

Alkenes readily decolourises bromine water at room temperature. You would expect the same using Kekulé's structure of benzene. However, benzene is resistant to addition reactions. It only reacts with bromine when the reaction mixture is boiled and in the presence of UV light. 

The difference in reactivity between alkenes and benzene is explained by the delocalised ring of electrons in benzene which contributes to the the stability of the molecule. Instead benzene undergoes electrophilic substitution reactions.

X-ray diffraction 
X-ray diffraction studies revealed that all the bond lengths of the carbon to carbon bonds in benzene were equal with a bond length of $$140 \ pm$$ - between that of​ $$C-C$$​ and $$C=C$$ bonds. This provided further evidence for the delocalised model of benzene.

The structure, resistance to addition reactions and the stability of the benzene molecule is explained by the ring of delocalised $$\pi$$-electrons, supported by the delocalised model.

Combustion of benzene 

Benzene can completely burn in oxygen to give carbon dioxide and water.

​$$benzene \ + \ oxygen \rightarrow carbon \ dioxide \ + \ water\\2C_6H_6 \ + \ 15O_2 \rightarrow 12CO_2 \ + \ 6H_2O$$​​

However, burning benzene in air leads to incomplete combustion due to lack of oxygen. Most of the product formed are carbon particulates of soot. This gives rise to a smoky flame.

Aromatic compounds

Aromatic compounds (arenes) are compounds with a benzene ring. These compounds are either named as substituted benzenes or as a phenyl group attached.

Example

You need to follow a set of rules when naming aromatic compounds which are not monosubstituted. 

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PROCEDURE

1.	If the functional groups are the same, select the first carbon in a way that each group will have the lowest possible carbon number (position). Example ​$$1,3$$-dinitrobenzene​
2.	If the functional groups are different, begin from the carbon which has a suffix. Continue counting in the direction whether that would be clockwise or anti clockwise to ensure the functional groups can be given the lowest possible carbon number (position). Example ​$$2$$-ethylphenol​