Arene Notes

Including:BenzenePhenolReactions- Nitration and HalogenationThe uses of Phenol

Arenes and Stuff

Arenes

Aromatic hydrocarbons containing one or more benzene ring Benzene has the molecular formula C₆H₆. Any compound with a benzene ring is an aromatic compound Not everything that has a benzene ring generates a smell for example aspirin Arenes occur naturally in materials such as crude oil and coal so therefore are produced in refineries from crude oil

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Caption: : Benzene ring

Benzene

A starting material for the synthesis of many other aromatic materials such as ethylbenzene, phenol and styrene Smaller quantities can be used to make a wide range of materials such as detergents, explosives, pharmaceuticals and dyestuff Benzene is classified as a carcinogen it is believed to cause cancer in humans such as leukaemia A colourless liquid with a sweet odour that is highly flammable and formed in natural processes and human activities. Natural sources include volcanoes and forest fires. It is also a component of crude oil, petrol and cigarette smoke

History of the Structure of Benzene

1825, Michael Faraday isolated benzene as a by-product in the manufacture of gas. He determined it had am empirical formula of CH 1834, Elihard Mitscherlich found the relative formula mass to be 78 and the molecular formula to be C₆H₆ 1845, Charles Mansfield introduced the first industrial product of benzene from coal tar Structure was widely speculated- suspected to have several double bonds Experiments were carried out it was discovered that it was highly unreactive despite being believed to be highly unsaturated Benzene did not react in the same manner as alkenes 1890, Friedrich August Kekulé von Stradonitz discovered the ring shape of benzene

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Caption: : Kekulé's Benzene

Benzene's low reactivity

The Kekulé structure fitted with the molecular formula but it failed to explain the chemical and physical properties of benzene fully. Kekulé's structure failed to explain the low reactivity: If C=C double bonds were present the Benzene would react in a similar way to alkenes Each C=C double bonds would be expected to decolourise bromine water This does not happen because Benzene is the problem child of chemistry Benzene does not take part in electrophilic addition reactions Kekulé then decided that had two different forms differing by the position of the double bonds. He suggested that these two forms were in such a rapid equilibrium that a bromine molecule could not be attracted to a double bond before the structure changed, hence the unreactiveness

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Carbon-Carbon Bond Lengths in Bezene

The Kekulé equilibriumd model was represented as symmetrical but C-C symmetrical bonds and C=C double bonds have different lengths X-ray crystallography revealed that all six of the carbon-carbon bond lengths in benzene are the same length: 0.139 nm This is in between the C-C bond length of 0.153 nm and the C=C bond length of 0.134 nm This was evidence that Kekulé's structure was wrong

Hydrogenation of Benzene

When an alkene reacts with hydrogen the energy change is called the enthalpy change of hydrogenation With cyclohexene with one C=C bond the enthalpy change of hydrogenation is -120 kJ mol-1 Kekulé's benzene would be expected to be three times that at -360 kJ mol-1 Actual benzene has an enthalpy change of hydrogenation of -208 kJ mol-1 So benzene is 152 kJ mol-1 more stable this is known as the delocalisation energy of benzene

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Caption: : Enthalpy changes

The Delocalised Model of Benzene

Benzene is a cyclic hydrocarbon with six carbon atoms and six hydrogen atoms The six carbon atoms are arranged in a planar hexagonal ring. Each carbon atom is bonded to two other carbon atoms and one hydrogen atom The shape around each carbon atom is trigonal planar and has a bond angle of 120 degrees Each carbon atom has four outer shell electrons. Three of these electrons bond to two other carbon atoms and one hydrogen atom. The three bonds in this plane are called sigma bonds. This leaves a fourth outer shell electron in a 2p orbital above and below the plane of the carbon atoms. The electron in a p-orbital of a carbon atom overlaps with the electrons in the p-orbitals of the carbon atoms on either side. This results in a ring of electron density above and below the plane of the carbon atoms The overlap produce a system of pi-bonds which spread over all six carbon atoms there fore the p electrons are now said to be delocalised

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Caption: : Delocalised Benzene Ring

The Delocalised Model and Chemical Reactivity

The increased stability provided by the benzene ring means that benzene struggles to take part in addition reactions which are they typical reactions of alkenes.Under normal conditions benzene does not: Decolourise bromine water React with strong acids React with halogens; chlorine, bromine or iodine Addition reaction: electrons from the delocalised system would need to bond to the atom or group of atoms being added. The product would be less stable and the reaction would not be energetically favourable, therefore addition reactions would disrupt the delocalisation of the ring structure.Benzene and its derivatives typically take part in substitution reactions- one of the hydrogen atoms is replaced by another atom or a group of atoms. The organic product retains the delocalisation and the stability of the benzene ring. They are the most common type of reaction with benzene.

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Caption: : Localised pi bond

Reactivity in Benzene

The region of high electron density above and below the plane of the carbon atoms attracts electrophiles. It takes part in substitution reactions in order to retain stability. Therefore the benzene ring reacts with electrophiles and takes part in electrophilic substitution.

Electrophilic Substitution by Nitration