Aromatic Chemistry
The Benzene Ring
Benzene is a molecule that has the formula C6H6. The chemistry relating to it is called aromatic because many fragrant substances contain this structure. Benzene forms a rings as follows.
The carbon atoms are bonded in a ring with a single hydrogen bonded to each. However, this means that each carbon only has 3 bonds. The fourth bond is the pi bond, which is delocalised in the ring. All of these delocalised electrons act like a glue which make the ring stable.
The stability of benzene can be demonstrated by comparing the hydrogenation of cyclohexa-1,3,5-triene with benzene. We work this out by tripling the hydrogenation of cyclohexane. The results of this investigation are outlined in the diagram below.
If the enthalpy of hydrogenation of cyclohexa-1,3,5-triene is three times that of cyclohexene then the enthalpy is -360kJ/mol. So the hydrogenation of benzene relases 152kJ less energy per mole to reach the same point and hence it is more stable.
The benzene molecule is susceptible to attack by electrophilles (ie. positively charged species). This results in an intermediate where the delocalised ring is disrupted, which then regenerates: resulting in a substitution reaction.
Nitration of Benzene
The nitration of benzene has the above overall reaction. This process is important in producing amines and also explosives. An important feature to note is the generation of the electrophile, as the nitric acid does not directly react with the benzene, rather the ion: NO2+ does. This ion is generated by the reaction of nitric acid with the catalyst: sulphuric acid in the following reactions.
The reaction of this electrophile with benzene is represented by the mechanism below. This is the same mechanism for any electrophillic substitution of benzene, you simply need to replace the nironium ion with the relevant electrophile.
Friedel-Crafts Reactions
The Friedel-Crafts alkylation and acylation are two reactions of electrophilic substitution of benzene that use an AlCl3 catalyst to generate an electrophille from either a haloalkane or acylhalogen. The mechanism with the electrophile is the same as shown above with nitronium; but the following occurs to generate this electrophile.
RCl + AlCl3 ® CH3CH2+ + AlCl4-
RCOCl + AlCl3 ® [RCO]+ + AlCl4-
The molecule ethylbenzene is required in the manufacture of polystyrene. However, using Chloroethane would be too expensive. So first of all, ethene is protonated with HCl in the following reaction.
CH2=CH2 + HCl ® CH3CH2+ + Cl-
The remaining chloride ion then reacts with the aluminium chloride catalyst to make an anion.