John P. ldoux and Wendell Plain
Florida Technological University Orlando, Florida 32816
I I
Selective Reduction of Dinitrobenzenes An organic /aborafory experimenf
Selective transformations of various types constitute a very important segment of organic chemistry. One such reaction which seems to intrigue undergraduate organic students is the selectivereduction of nitroaromatic compounds. The quest,ion concerning which of two nitro groups present on some aromatic nucleus will be reduced is one which generally receives most of their attention. For thesodium hydrosulfideinduced reduction of z,yf-dinitrobiphenyls (1) and for the extent of 4-nitro versus 2-nitro group reduction in the Raney copper catalyzed reduction of 2,4-dinitroalkylbenzenes (B), there seems to be no question that the selectivity site is governed by steric factors. However, for a series of l-substituted-2,4-dinitrobenzenes whwr the 1-subsrituents h > ~ vcomplmble c stcric consiclcrstions bur diffrrwr vol;w dlaracrrrs the ~ r l c c t i ~ i t v appears to be controlleh by electronic effects. For example, resonance theory predicts the same degree of electron-withdrawal or -donation at the 2-position rn a t the bposition by a l-substituent, but different degrees by an inductive effect. Thus the position from which electron-withdrawal is the greatest, or conversely the position $0 which electron-donation is the least, will be the site at which reduction occurs. Thus the more electrophilic nitro group is the one which is reduced. For example, an allcyl group has an electrondonating inductive effect, and this effect is felt least at the Cnitro group in a I-alkyl-2,4-dinitrobenzene. As a result, the 4-nitro group is more electrophilic than the 2-nitro group, and it is the 4-nitro group then that is predominantly selectively reduced. This simple explanation and the data discussed in references (1) and (2) are presented to the students for their consideration, and then different sets of students are instructed to carry out the reduction of m-dinitrobenzene, 2,4dinitropheuol, or 2,4-dinitrobenzoic acid. Some students are instructed to prepare and use an ammonium sulfide reagent (5) for the reduction and others the sodium hydrosulfide reagent (4). We have found the hydrosulfide reagent to be, in general, asuperior reagent for the selective reduction. That is, product purification is simpler because of the absence of elemental sulfur which is a by-product from ammonium sulfide reductions. As a result, a purer product and a higher yield of the product are generally obtained with the hydrosulfide reagent. In their summary report, the students are asked to
explain why their particular selective reduction occurs as their results indicate.' They are also asked to compare their results with those students who carried out the same reaction but with a different reducing agent and explain any differences. We have found that the students respond quite favorably to an experiment such as the one outlined here. It ~rovidesthem the opportunity to apply what they have been taught concerning substituent effects and to examine the preparation of the same compound by the same basic reaction but under slightly different conditions. Experimental Reducing agenta. The ammonium sulfide reagent is prepsred according to Murray and Waters (8)and the sodium hydrosulfide reagent according to Hodgson and Ward (4). General Pwcedure for Reduction. The l-suhstituted-2,P dinitrobenzene (10 g) was dissolved in the minimum amount of hot methanol (-150 ml) in a 1-neck, 500-ml round-bottomed flask. A reflux condenser was then attached and the alcoholic solution (-200 ml) of the reducing agent added slowly (-5 min) with swirling. The reaction mixture was then brought to gentle reflux for 20 min. At the end of that time, the reaction mixture was filtered hot, the solvent pulled off under vacuum and worked up as indicated below. Preparatzon of m-Nitroaniline. The crude material was simply recrystallized twice from water. Lit. mp 114'C (Ga). Yields were typically in the 65807' range when using the sodium hydrosulfide reagent and were always less when using the ammoniumsulfide reagent. Preparation of %Amino-4-Nitrophenol. The crude material was dissolved in hot water, acidified with glacial acetic acid (-5 ml), and cooled. This material was filtered and then recrystallized from water. Lit. mp 142143'C (anhydrous), 80-90°C (wet) and decomposes 195-198°C (Gb). Yields from sodium hydrosulfide reduotion were 85-92% and from ammonium sulfide reduction 50-60%. P~eparationof 4-Amino-.%Nitrobazoie Acid. The crude material was treated in the same manner in the case of the phenol and water was also used as the recrystallization solvent. Lit. mp 2 3 9 T (Gc). Yields from sodium hydrosulfide reduction were 80-93% and from ammonium sulfide reduction 50-60%.
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Literature Cited (1) Ioonx. J.P.. 3. CAen. Soc. (C), 435 (1870). (2) JONBB,W. H.. BTAL.Ann.N. Y. Acod.Soi.. 158,471(1969). (3) Mmnbr, M. 3.. Ann WATena, D. E.. J . Am=. Chsnr. Soc.. 60,2818
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(4) Hooasow. H. H.. A N D WARD.E.R., 3. CAem 5 0 0 . . 242 (1947). (5) "Dictionary of Organic Campounda' (4th ad.), Oxford University Press, New York, 1963: (a) p. 2428: (b) p. 184; (a) p. 176. 1 It is pointed out to those students working with the benzoic acid that under the conditions of the reaction it exists redom minantly as the carhoxylate ion
Volume 49, Number 2, February 1972
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