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Discovery of benzene. Benzene was first isolated in 1825 by Michael Faraday, who deduced that its empirical formula was CH.
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Discovery of benzene Benzene was first isolated in 1825 by Michael Faraday, who deduced that its empirical formula was CH. In 1834, the German chemist Eilhard Mitscherlich determined that benzene’s Mr was 78, and its formula was C6H6. However, it was not until 1931 that benzene’s structure was fully resolved. Benzene’s molecular formula suggests it is a highly unsaturated compound. But unlike alkenes, it does not readily undergo addition reactions. This suggests that its structure and bonding is fundamentally different.
The benzene ring Benzene undergoes hydrogenation in the presence of a nickel catalyst to form cyclohexane: This suggests that benzene also has a cyclic structure. In order to fit with the molecular formula of C6H6, the ring would have to contain three double bonds and three single bonds.
Kekulé structure of benzene In 1865, the French chemist Friederich Kekulé proposed a cyclic structure for benzene, consisting of alternating single and double bonds. Kekulé’s structure for benzene was the first time it had been proposed that a hydrocarbon chain formed a ring (he later claimed that his inspiration came from a dream of a snake eating its own tail). There were, however, a number of problems with Kekulé’s structure in that it didn’t fully explain the physical and chemical properties of benzene.
Problems with the Kekulé structure: 1 Firstly, the three double bonds in the structure suggest that benzene should readily undergo electrophilic addition reactions, similar to other unsaturated compounds. However, unlike alkenes, benzene (left) does not decolourize bromine water. It also does not easily take part in other electrophilic addition reactions. Benzene therefore has lower chemical reactivity than would be predicted by Kekulé’s structure.
Problems with the Kekulé structure: 2 A second problem for Kekulé’s model arises when the isomers of dibromobenzene are studied. According to Kekulé’s structure, there should be four different isomers: 1,6 1,4 1,2 1,3 However, it was discovered that only three isomers of dibromobenzene are formed: the 1,6-isomer is not distinguishable from the 1,2-isomer.
The equilibrium model Kekulé tried to resolve these problems by suggesting that there were two forms of benzene that rapidly interconverted (the rapid equilibrium model). He proposed that this model could explain the low reactivity of benzene, as the structure was in such rapid flux that the location of the double bonds would change before any attracted molecules had time to react with them. This later evolved into the idea ofresonance between the two Kekulé structures of benzene.
0.154nm 0.133nm Bond lengths in benzene The final problem with the Kekulé structure of benzene was identified by X-ray crystallography. As double bonds are shorter than single bonds, the Kekulé structure would be asymmetrical. Based on the alkanes and alkenes, the two different bond lengths could be predicted to be 0.154nm and 0.133nm. However, X-ray studies revealed the structure of benzene to be a perfect hexagon: all internal bond angles were 120° and all bonds were of an equal length – 0.140nm; somewhere in between that which would be expected for a single and a double bond.
Evolution of scientific knowledge The history of the structure of benzene exemplifies how scientific knowledge evolves dynamically between different ideas as new data emerges. C6H6 Each successive model can be seen as a working hypothesis that best explains current observations, but which is only tentative in nature, as new information may require revision or even replacement of the current model.
What are arenes? Arenes are aromatic hydrocarbons that contain one or more benzene rings. The name aromatic derives from the fact that many strong-smelling substances found in nature, such as plant fragrances, contain benzene rings. For example,vanillin(right) isthe fragrant compound found invanilla plants. However, more recently it has been found that the presence of a benzene ring has little to do with the smell of a substance. Many compounds containing benzene rings have no odour.
hexane benzene Mr Boiling point (°C) Melting point (°C) Physical properties of arenes Benzene is a colourless liquid at room temperature. The boiling point of benzene is comparable to hexane but its melting point is much higher. 86.0 78.0 69.0 80.0 -95.0 5.5 This is due to the ability of the flat benzene rings to pack closely together when solid, increasing the strength of intermolecular forces. As carbon and hydrogen are similar in their electronegativity (2.6 and 2.2, respectively), benzene is a non-polar molecule and is therefore immiscible with water.
Combustion of arenes Arenes burn in air to give characteristically sooty flames. The soot is unburnt carbon, a result of the relatively high proportion of carbon that arenes contain, compared with more saturated compounds. However, long-chained alkanes also burn with a sooty flame due to their increased percentage carbon content (not due to unsaturation).
Naming aromatic compounds Benzene derivatives are named in a similar fashion to other organic compounds, with benzene forming the main part of the name. The presence of other groups is denoted by the use of a prefix. methylbenzene chlorobenzene nitrobenzene
Delocalization and reactivity of the arenes Due to the stability of the delocalized ring structure, benzene and its derivates do not readily take part in the typical reactions of the alkenes. • For example, under normal conditions they do not decolourize bromine water, react with strong acids, or react with other halogens. • Aromatics usually take part in substitution rather than additionreactions – this allows the product to retain the stability of the benzene ring. • In typical reactions, the benzene ring reacts with an electrophile to undergo electrophilic substitution.
Nitrated arenes As well as being precursors to amines, nitrated arenes are also useful as explosives. TNT (trinitrotoluene), or 2,4,6-trinitromethylbenzene, is formed by nitrating methylbenzene (also known as toluene) at a high temperature, leading to the substitution of three hydrogen atoms. TNT has a low melting point, and is fairly stable to shock and friction, making it safer to handle than other types of explosive.
Friedel–Crafts reactions Friedel–Crafts reactions are substitution reactions used to attach extra carbon atoms to a benzene ring. There are two main types of Friedel–Crafts reaction: alkylation is used to add alkyl groups (―R), and acylation is used to add acyl groups (―COR). Friedel–Crafts alkylation Friedel–Crafts acylation Friedel–Crafts reactions are important in industry; for example, in the production of plastics, detergents and petrol.
Reaction of phenol with bromine Unlike benzene, phenol reacts instantly with bromine in the absence of a catalyst, and at room temperature, to form a precipitate of 2,4,6-tribromophenol. The increased reactivity of phenol is due to the fact that, in solution, the lone pair of electrons from the oxygen atom are drawn into the delocalized ring, increasing its overall electron density.
