Chlorination of Nitro benzene:
Chlorination of Nitro benzene is an electrophilic substitution reaction. In this reaction of electrophile is generated, the electrophile is a positively charged species and it is attracted towards electrons thus it is named electrophile.
Chlorination of Nitrobenzene is carried out by reacting Nitrobenzene with chlorine gas in the presence of a halogen carrier, Aluminium Chloride or Ferric Chloride.
The overall reaction for chlorination of Nitrobenzene is as below
In the first step, chlorine gas reacts with the halogen carrier and heterolytic fission occurs on the chlorine molecule to produce chloronium ion. This ion is responsible for the chlorination reaction.
In Nitrobenzene, there are three available positions for the incoming chloronium ion to join the arene ring these are ortho, meta, and para positions respectively on the Nitrobenzene ring as shown in the figure.
The chloronium ion approaches all the three available positions and the corresponding resonating forms a possible on the Nitrobenzene ring as shown in the figure below.
In the above resonating forms structure 1 and structure 2 (Marked with *) are especially unstable structures because the positive charge is present on the carbon atom which is directly attached to an electron-withdrawing group (nitrogen is more electronegative than carbon and – I effect of nitro group is withdrawing electron density from the benzene ring).
But in the case of meta attack, the resonating forms do not contain an unstable structure thus relative stability of meta attack is higher than the corresponding ortho and para attack.
As a result of the higher stability of meta attack chlorine joins that meta position. So chlorination of Nitrobenzene gives meta Chloro Nitrobenzene a primary product.
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