Solvent Free Facile Room Temperature Reduction of Aromatic Carbonyl and Nitro Compounds by Zn/Conc. HCl System-An Experimental and DFT Study

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This experimental and DFT studies involve a novel technique for the reduction of aromatic carbonyl/nitro compounds to the corresponding alcohols/amines, in high yield under laboratory conditions.The reducing species is the nascent hydrogen generated by the Zn/HCl system.The novelty of this work is that the comparison of yield with and without solvent.The yield is increased by many folds in the solvent free method compared to the solvent method reported earlier.The technique followed is to make a 'slurry' of the substrate with zinc dust (zinc slurry) and to add (in small portion of the dry slurry) to the optimized amount of conc.HCl, over a period of 3 to 4 h at room temperature.In this technique the substrate, adsorbed on zinc dust being very proximal to the site of generation nascent hydrogen, the reduction is very effective and the yield is high.The novelty is that zinc dust acts as catalyst (adsorbent role) and reactant (hydrogen generation role).The DFT study with B3LYP/6.311g++ (d,p) basis set revealed that the stability of first formed free radical (energy factor) and the homo nuclear nature of carbonyl and nitro group (charge factor) decide the yield.The electrostatic potential calculated by DFT studies correlates well with Mullikan charges in deciding the charge factor.The free radical mechanism was confirmed by the formation of pinacol coupled product in one instance.

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