Electrophilic Substitution Reaction of Anilines
An electrophile refers to an electron seeking specie. Thus, electrophilic substitution reaction refers to the reaction in which an electrophile substitutes another electrophile in an organic compound. Anilines undergo the usual electrophilic reactions such as halogenation, nitration and sulphonation. We will discuss them one-by-one, but let’s first understand the behaviour of anilines towards the attack of an electrophile:
- The functional group (-NH2) associated with aniline is electron donating group and hence is very activating towards the electrophilic substitution reaction.
- Due to its various resonating structures, there’s an excess of electron or negative charge over ortho- and para- positions of the benzene ring than the meta- position. Thus, anilines are o- and p- directive towards electrophilic substitution reaction.
In the above reaction when aniline comes in the vicinity of bromine water, then the bromine molecule develops a polarity within itself and bromine with a slightly positive charge acts as an electrophile (electron seeking) and attacks the electron rich ortho and para positions of aniline. A white coloured precipitate of 2,4,6-tribromoaniline is obtained at room temperature.
In this reaction, alongside para isomer, the meta isomer is also observed. It is because the aniline molecule gets protonated in acidic medium to become anilinium ion which is meta directing.
Sulphuric acid reacts vigorously with aniline to form anilinium hydrogen sulphate which on heating produces sulphanilic acid which in turn also has a resonating structure with zwitter ion as shown in the above figure. Zwitter ion basically refers to a dipolar ion in which both positive and negative charge exist and the molecule as a whole is neutral. It is also called as inner salt sometimes. It differs from amphoteric ion in the sense that it has negative and positive charge simultaneously while amphoteric ions are either cationic or anionic at a time.
Anilines don’t undergo Freidel crafts reaction because they react with ferric chloride of the reaction mixture which acts as catalyst for the reaction.
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