Reduction of aromatic nitro compounds with hydrazine hydrate in the presence of an iron oxide hydroxide catalyst. I. The reduction of monosubstituted nitrobenzenes with hydrazine hydrate in the presence of ferrihydrite

Abstract

A great variety of monosubstituted nitrobenzene derivatives has been reduced in good yield to the corresponding anilines with hydrazine hydrate in the presence of an iron oxide hydroxide catalyst prepared by precipitation from an aqueous iron(III) solution with sodium hydroxide. The dependence of the rate of reduction on the nature and the position of additional substituents other than the nitro group was determined by measuring the reaction kinetics. The rate is enhanced by electron-attracting substituents and decreased by electron-donating groups, which results in a positive slope of ρ=0.546 for the Hammett plot. Competitive reduction experiments with mixtures of two differently substituted nitrobenzene derivatives revealed that the nitro compound with the more electron-attracting substituent is reduced first.

Introduction

Aromatic amines are important starting materials and intermediates for the manufacture of a great variety of chemicals, such as dyestuffs, pharmaceutical products, agricultural chemicals, photographic chemicals, additives, surfactants, textile auxiliaries, chelating agents and polymers. They are generally synthesized by the reduction of nitroarenes.

Aromatic amines can be prepared by a great variety of reduction methods. Probably the oldest industrially applied method is the reduction of nitrobenzenes with metal (usually iron, but also tin, zinc and aluminium can be employed) and water in the presence of small amounts of acid, first described by Béchamp in 1854 [1]. It would certainly have been replaced much earlier by an alternative reduction method, if it had not been possible to obtain iron oxide pigments as a by-product of the reduction step. The reactions with metals and acid are the most vigorous reduction methods producing merely the amino products. Therefore, if the aromatic moiety contains additional substituents prone to being reduced (as e.g. cyano, azo or further nitro groups), this drastic reduction method will produce a significant amount of by-products. In those cases, the reduction can be carried out selectively by employing sulphides (Zinin-reduction [2]). This selective sulphide reduction is more expensive than the reduction by iron and acid, but is nevertheless widely applied for the partial reduction of one nitro group in dinitroarenes and the selective reduction of nitro functions in azo and anthraquinone dyes. The Zinin-method [2]has a severe drawback, however, because in industry it is accompanied by the production of a large amount of waste products that have to be disposed in an ecologically unfavourable way. Moreover, at low pH values, the evolving of H₂S gas might endanger the operating personnel. These problems with process security and more rigorous environmental legislation in the handling of useless waste products required the development of a safe, economically and ecologically beneficial alternative to these non-catalytic reduction methods still employed in industry.

Nowadays, most large-scale aromatic amines are being produced by catalytic hydrogenation of the corresponding nitroarenes. With a large variety of catalysts (e.g. Ni, Cu, Co, Cr, Fe, Sn, Ag, Pt, Pd, Zn, Ti, Mo, metal oxides and sulphides) and under a wide range of reaction conditions in most cases the corresponding amine is obtained quantitatively without the production of waste products. Because of the exothermic nature of the reaction and the need for a closed high-pressure system, numerous safety precautions have to be taken.

The reduction of aromatic nitro compounds with hydrazine or hydrazine hydrate represents a special variation of the catalytic reduction, where hydrazine is the source of the hydrogen. The hydrogen can be generated by a variety of H-transfer catalysts 3, 4, 5. Especially with the use of noble metal catalysts, such as Pd, Pt or Ru, but also with the application of Ni or Cu, the catalytic hydrazine reduction gives high yields comparable to or better than the catalytic hydrogenation. In the past, the relatively high costs for hydrazine hydrate and for the noble metal catalysts prevented this reduction method from being applied at an industrial scale. However, there are two main reasons which are currently enhancing the attractivity of this catalytic H-transfer reduction: (a) It has been observed repeatedly that several cheap iron(III) compounds, especially a series of iron oxide or hydroxide modifications, exhibit an appreciable activity with regard to catalytic H-transfer 6, 7, 8, the best results being obtained with β-FeO(OH) 7, 9in methanol or ethanol [8]; (b) In cases where the catalytic hydrogenation is not the method of choice, this method offers a safe as well as an ecologically and economically beneficial alternative, above all for small product volumes in fine chemical manufacture, where the reaction can be carried out in multi-use batches under normal pressure.

This prompted us to investigate the H-transfer activity of these cheap and easily synthesizable iron oxide hydroxide catalysts [10]by examining the influence of additional substituents on the rate of the reduction [11]and determining the selectivity [12]of the catalytic reduction for a selection of monosubstituted nitrobenzenes (Fig. 1).

The catalyst was prepared by precipitation of an iron oxide hydroxide from an aqueous iron(III) chloride solution with sodium hydroxide. It showed a much higher activity than the β-FeO(OH) used by Miyata et al. [7]and Ayyangar et al. [8]. This most active iron oxide hydroxide modification was found [10]to be the ferrihydrite Fe₅HO₈·4H₂O [9].