a) Explain Henry reaction using aliphatic and aromatic aldehydes.
b) How will you prepare 1,4-dinitrobenzene by starting with benzenamine?
The Henry Reaction is a base-catalyzed C-C bond-forming reaction between nitroalkanes and aldehydes or ketones. It is similar to the Aldol Addition, and also referred to as the Nitro Aldol Reaction.
If acidic protons are available (i.e. when R = H), the products tend to eliminate water to give nitroalkenes. Therefore, only small amounts of base should be used if the isolation of the β-hydroxy nitrocompounds is desired.
Mechanism of the Henry Reaction:
The main products of the Henri reaction are β-hydroxy nitrocompounds, while the side products are nitroalkenes (especially in the case of using aromatic aldehydes). Primary nitroalkanes can also react with two moles of a carbonyl compound, therefore the ratio of the reactants should be observed very carefully. During the condensation of aromatic aldehydes, α-nitroalkenes are usually formed and the reaction is very difficult to stop at the stage of β-hydroxy nitroalkane formation.