Electrophilic Substitution Reactions
We have discussed in chemical properties of haloarenes that electrophilic substitution reactions are common in haloarenes and that haloarenes are o,p-directing groups.
We have already studied some electrophilic substitution reactions of benzene such as halogenation, sulphonation, nitration, friedel-crafts reactions in reactions of arenes . Let us see how haloarenes undergo these reactions :
In halogenation of haloarenes, ortho and para substitution occurs when dihalogen reacts with haloarenes in the presence of lewis acids such as Fe, FeCl3, AlCl3. The following reaction is an example of chlorination :
Nitration of haloarenes requires a mixture of conc. HNO3 and conc. H2SO4. The mixture is used to generate nitronium ion (NO2+), which acts as electrophile.
In sulphonation, sulphur trioxide (SO3) acts as electrophile. SO3 electrophile can be prepared as shown below :
Generation of SO3 electrophile
Sulphonation reaction :
Friedel-Crafts reactions are the reactions of the benzene derivatives with alkyl halides or acyl halides in the presence of lewis acid catalysts (anhydrous AlCl3 for example).
Haloarenes and suitable alkyl halides in the presence of a Lewis acid catalyst form alkyl derivatives of benzene.
In Friedel-Crafts acylation, benzene derivatives are treated with acid chlorides(−COCl) or acid anhydrides ((−CO2)O) to produce aromatic ketones.
Question : Haloalkanes undergo nucleophilic substitutions whereas haloarenes undergo electrophilic substitutions. Explain why?
Answer : The C-X bond in haloalkanes is more polar than the C-X bond of haloarenes. Consequently, the carbon atom attached to the halogen in haloalkanes is more positively charged than that in haloarenes. As a result, haloalkanes preferentially undergo nucleophilic substitution reactions.
In contrast, haloarenes contain a benzene ring which typically undergoes electrophilic substitution reactions. Due to the presence of benzene ring in haloarenes, they also undergo electrophilic substitution reaction.
You should be able to solve the following conversions provided you have studied the previous tutorials.
Convert benzene to bromonitrobenzene.
Convert chlorobenzene to p-nitrophenol.