Nitration of Aromatic Compounds on Silica Sulfuric Acid

Abstract

Nitration of organic compounds has long been a very active and rewarding area of research and is the subject of a large body of literature. Also nitro-aromatics compounds are extensively utilised and act as chemical feedstocks for a wide range of useful materials such as dyes, pharmaceuticals, perfumes, and plastics. Therefore, the nitration of aromatic rings has received considerable attention of late, due to unsolved problems pertaining to regioselectivity, overnitration and competitive oxidation of substrates. Nitration of phenol taken as a special case has been studied by various nitrating agents under different conditions. The nitration of benzene and toluene is one of the most important routs to substituted aromatics in the production of chemical intermediates. The industrial nitration of toluene is performed by means of mixed acid, a mixture of nitric acid, sulfuric acid, and water which leading to excessive acid waste streams and added expense. Also, the separation of the products from the acid is often a difficult and energy consuming process that habitually implies a basic aqueous work-up. Moreover, sulfuric acid is corrosive and is dangerous to transport and handle. The above mentioned disadvantages of the commercial manufacturing process currently used has led to a substantial effort to develop viable alternatives, inter idea using solid acid catalyst, other sources of NO2, organic nitrating agents, metal nitrates, other acids replacing sulfuric acid such as inorganic acidic salts (NaHSO4·H2O, Mg(HSO4), Oxone, ...) and silica sulfuric acid, etc. Our goal, in undertaking this line of work, was three-fold: a) to overcome the limitations and drawbacks of the reported methods such as tedious work-up, strongly acidic media, oxidation ability of the reagents and safety problems (storage, handling, using and also presence of toxic transition metal cations such as Cr, Hg, Cu,... within molecular structure of the reagents), (b) to perform solvent-free organic synthesis which seems to be a highly useful technique, especially for industry possessing many advantages like: reduced pollution, low costs, as well as simplicity in process and handling (these factors are especially important in industry), (c) moreover, to develop an high-yielding onepot synthesis of nitro-aromatics using a novel combination of reagents. In addition, any reduction in the amount of liquid acids needed and/or any simplification in handling procedures would be highly convenient in terms of risk control, economic advantage and environment protection. Recently, we have reported the preparation of silica sulfuric acid I as a stable acidic reagent (solid acid) and showed its catalytic activity in synthetic methodology. In continuation of our studies in this regard, we have found that aromatic rings can be nitrated by using a combination of silica sulfuric acid I, NaNO3 and wet SiO2 under solvent free conditions. Therefore, we wish to report here a one-pot solid phase procedure for the nitration of different kind of aromatic compounds.