Aromatic Compounds

 
 

Benzene

Benzene has six carbon atoms joined in a ring and a series of alternate single and double carbon bonds. The pi electrons in the three double bonds are delocalised and spread out over the whole ring system – they don’t belong to any one carbon atom.

Benzene has the formula C6H6 and was originally represented using the Kekule model. The Kekule model shows benzene as a ring of alternating single and double bonds with the pi electrons localised between certain carbon atoms.

It has now been replaced with the delocalised model, showing how the pi electrons are spread over the whole ring.

Three pieces of evidence suggested that the Kekule model wasn’t right:

  1. All carbon-carbon bonds in benzene are the same length, in-between that of a single and a double bond. Double bonds are shorter than single bonds, so the Kekule model suggests that benzene should consist of carbon-carbon bonds of varying lengths.

  2. Benzene is less reactive than alkenes – alkenes can take part in electrophilic addition reactions to form halogenoalkanes at room temperature and pressure. Benzene can only undergo these kinds of reactions in the presence of a ‘halogen carrier’ catalyst.

  3. The enthalpy change of hydrogenation is lower than expected – if the Kekule structure was correct, benzene should have 3x the enthalpy of hydrogenation as cyclohexene since it has 3x double bonds. Its actual enthalpy of hydrogenation is much less exothermic, suggesting that the structure is more stable than the structure proposed by Kekule.

In benzene, the p-orbitals of the carbon atoms overlap to form a pi-system. The sideways overlap of p electrons forms two regions of electron density above and below the plane of the carbon atoms, with the electrons delocalised over the whole ring. The delocalised system is represented as a circle inside the hexagonal ring.

 
 

Aromatic compounds

Aromatic compounds (also known as arenes) are compounds that contain a benzene ring. Here are some examples:

 
 

Electrophilic addition vs electrophilic substitution

Aromatic compounds such as benzene are less reactive (and more stable) than alkenes, since their pi electrons are spread out over a larger area, rather than being concentrated between two carbon atoms. In alkenes, there is a high region of electron density because the pi electrons are found in a relatively small space, making them more reactive. They react with halogens in electrophilic addition reactions, in which the double bond breaks and the halogen is added to the carbons of the double bond, forming an alkane.

 
 

Aromatic compounds cannot undergo these types of reactions. They can only react with halogens in electrophilic substitution reactions.

The reaction involves breaking the delocalised pi system and the formation of a positively charged intermediate. The final step involves the loss of hydrogen and the delocalised system is reformed.

 
 

Adding a halogen atom onto benzene requires the presence of a halogen carrier catalyst. Examples of halogen carriers include aluminium chloride, AlCl3 and iron (III) bromide, FeBr3. Technically it is not strictly a catalyst, because it is permanently changed during the reaction. It works by polarising the halogen and making it a stronger electrophile.

 
 

Friedel-Crafts reactions

Friedel-Crafts reactions are a type of electrophilic substitution reaction where carbon-containing groups are added to the aromatic ring. It is useful for forming carbon-carbon bonds in organic synthesis. These reactions are carried out by refluxing the arene with a halogen carrier and either an acyl chloride (acylation) or a haloalkane (alkylation).

 

Acylation

An acyl chloride is basically an aldehyde group except there’s a chlorine in place of the hydrogen. The acyl chloride is refluxed with benzene in the presence of a halogen carrier to form phenylketones and hydrogen chloride.

 
 

Alkylation

Benzene is refluxed with a haloalkane (e.g. chloromethane) in the presence of a halogen carrier. The alkyl group is substituted for hydrogen, forming methylbenzene and hydrogen chloride.

 
 

Nitration

Nitration is yet another type of electrophilic substitution, in which a nitronium ion (NO2+) electrophile is substituted for a hydrogen atom on the benzene ring. It requires:

  • Concentrated nitric acid — as a source of the electrophile.
  • Concentrated sulfuric acid — to act as a catalyst.
 
 

Here's how H2SO4 acts as a catalyst to generate the NO2+ electrophile:

 
 

The hydrogen ion that is produced after the nitronium ion has substituted in the ring will react with HSO4- to regenerate H2SO4, leaving it unchanged by the end of the reaction.

Mononitration occurs at temperatures below 55oC, so if you want to form nitrobenzene, temperatures need to be kept cool. Multiple substitutions will occur at higher temperatures.

Phenol and electron-directing effects

Phenol is benzene with a hydroxyl (-OH) group attached. It has the formula C6H5OH. The addition of the –OH group makes phenol more reactive than benzene, since the lone pairs of electrons on the oxygen contribute to the delocalised pi system. This means phenol has a higher electron density than benzene, giving it a greater negative charge and making it more susceptible to attack by electrophiles.

The presence of an –OH group, or other functional groups, on benzene can affect the position at which other groups attach in substitution reactions.

  • Hydroxyl (-OH) and amine (-NH2) groups are electron-donating — they push their electrons towards the delocalised ring. This increases the electron density on some carbon atoms more than others – electron density is increased at the 2, 4 and 6 positions. Electrophiles are more likely to attack these carbons.
  • Nitro (-NO2) groups have the opposite effect and pull their electrons away from the delocalised system – it is said to be electron-withdrawing. NO2 decreases electron density from the 2, 4 and 6 positions so substitution is more likely to occur at the 3 and 5 positions.

Reactions of phenol

Phenol is weakly acidic and reacts with alkalis to form a salt and water. The salt is a metal phenoxide in which the hydrogen in the –OH group is replaced with a metal ion (from the alkali).

 
 

Phenol reacts with bromine in electrophilic substitution reactions.

The hydroxyl group has a 2,4,6-directing effect so substitution takes place at these positions. 2,4,6-tribromophenol is insoluble so it precipitates out of the solution. You’ll observe a colour change from orange to colourless as the bromine water reacts. Hydrogen bromide is formed as the second product.

 
 

Phenol also reacts with dilute nitric acid to form nitrophenol and water.

 
 

Because –OH is 2,4-directing, two structural isomers are formed: 2-nitrophenol and 4-nitrophenol.