The preparation of 2,4-dichloronitrobenzene from nitrobenzene and chlorine gas involves an electrophilic aromatic substitution reaction. The mechanism can be described in the following steps:1. Formation of electrophile: Chlorine gas Cl2 reacts with a Lewis acid catalyst, such as aluminum chloride AlCl3 or ferric chloride FeCl3 , to form a highly reactive electrophile, the chloronium ion Cl+ .Cl2 + AlCl3 Cl+ + AlCl4-2. Electrophilic attack: The electrophile Cl+ attacks the aromatic ring of nitrobenzene, forming a sigma complex arenium ion with a positive charge on the carbon atom where the electrophile has attacked. The nitro group -NO2 is a meta-directing group, so the electrophilic attack occurs at the meta-position relative to the nitro group.3. Deprotonation: A base, usually the counterion from the Lewis acid catalyst AlCl4- or FeCl4- , abstracts a proton from the carbon with the positive charge, restoring the aromaticity of the ring and forming the product, 2-chloronitrobenzene.4. Second electrophilic substitution: The same mechanism is repeated for the second chlorination, with the electrophile attacking the 4-position of the 2-chloronitrobenzene, followed by deprotonation to form the final product, 2,4-dichloronitrobenzene.Optimum conditions for this reaction are:- Temperature: The reaction is typically carried out at a temperature between 0C and 40C. Lower temperatures help to minimize side reactions and over-chlorination.- Pressure: The reaction can be performed at atmospheric pressure, but slightly elevated pressure 1-2 atm can be used to ensure sufficient contact between the gaseous chlorine and the nitrobenzene.- Concentration: The reaction is usually performed in an inert solvent, such as carbon tetrachloride CCl4 or dichloromethane CH2Cl2 , to ensure good mixing and control the reaction rate. The concentration of the reactants and catalyst can be adjusted to optimize the reaction rate and yield. A typical concentration range for the reactants is 0.1-1.0 M, and for the catalyst, it is 0.05-0.5 M.- Catalyst: As mentioned earlier, a Lewis acid catalyst such as aluminum chloride AlCl3 or ferric chloride FeCl3 is used to facilitate the formation of the electrophile. The catalyst is usually used in a stoichiometric or slightly excess amount relative to chlorine gas.