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Is Benzene an Alkene?

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Introduction

Is Benzene an Alkene? The molecule benzene was first discovered by a chemist named Michael Faraday in 1825 whilst he was analysing an unknown chemical derived from whale oil. This compound was named benzene, however not much was known about it at this point in time. In 1866, the eventual structure of benzene was discovered by a chemist name Kekul´┐Ż, whose idea of the structure of Benzene appeared to him in a dream. Benzene possesses special properties however, and goes against the suggested trends of other hydrocarbons (namely, alkenes). Benzene has the formula C6H6; a ratio of carbon to hydrogen of 1:1 suggests high reactivity and instability. Benzene however is fairly inert, and fails to undergo reactions that would normally be characterised by other alkenes. This leads to the conclusion that benzene may not be an alkene. An alkene is defined as being an unsaturated chemical compound, with at least one carbon-carbon double bond. As the formula for benzene is C6H6, one would assume that the structure of benzene does contain one of these double bonds, and therefore would be prone to taking part in addition reactions with an electrophile such as Br2. ...read more.

Middle

This is due to a property known as aromaticity. This property also affects the heat of hydrogenation. When a hydrocarbon is unsaturated, it can be hydrogenated by adding hydrogen into the double bond. This requires energy and a catalyst. Returning to the example of cyclohexene, this contains one double bond. The energy required to break the double bond and add hydrogen to it is -28.6 kcal mol-1. One would expect that as benzene has three times the amount of double bonds that it would need 3 times the energy. Therefore the ?H or benzene should be 3(-28.6 kcal mol-1) = -85.6 kcal mol-1. The actual ?H for the hydrogenation of benzene is -49.8 kcal mol-1. This is 36 kcal mol-1 less than 3 times ?H for cyclohexene. Benzene is more stable than expected, suggesting that the structure of benzene may not be as straightforward as what was first thought. The aforementioned chemical property "aromaticity" describes a case of delocalised bonding and is demonstrated when a conjugated hydrocarbon ring containing unsaturated bonds, lone pairs or empty orbitals displays a stabilization that would not have been expected from the stabilization of a conjugation alone. ...read more.

Conclusion

To conclude, it would be incorrect to label benzene as an alkene for a number of different reasons. Firstly, it reacts more like an alkane than an alkene, reacting by substitution rather than addition if at all. We could not label it as an alkane however as it doesn't follow in the same homologous series that alkanes follow (CnH2n+1). Secondly its bond lengths do not differ, all the C-C bonds in benzene are the same, and can therefore form a perfect, planar, hexagonal structure. Whilst the formula C6H6 would suggest double bonds, in the case of benzene there are neither single bonds nor double bonds. The bonds present are intermediate bonds, which all have the same length as one another. This is due to the property aromaticity. It is this property which also affects the heat of hydrogenation, and further alienates benzene from the alkene group. In actual fact, benzene is neither alkane nor alkene, but instead falls under a separate category labelled arenes. Arenes are aromatic hydrocarbons that are based on the benzene molecule, all with properties similar to benzene. Their properties are unique, and therefore are placed in this separate group. ?? ?? ?? ?? ...read more.

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