Selective Reductions in the Teaching Laboratory Alan G. Jones Department Of Education, College of Further Education, Homefield Rd.. Douglas, Isle of Man, British Isles This article is an extension of a n experimental exercise reported in this Journal in 1975.' Aromatic amines (arylamines) are generally prepared by the reduction of the corres~ondinaaromatic nitro compounds. In the laboratory, tin and concentrated hydrochloric acid are generally employed as the reducing medium. Practical demonstration of this type of reaction is usually confined t o the reduction of nitrobenzene to aminobenzene (aniline).2 A more interesting reduction involving the substituted aromatic nitro compound 3-nitrophenylethanone, known commercially as meta-nitroacetophenone (1). yields the corresponding primary aromatic m i n e (2)
Students appear to appreciate the theoretical concepts involved in these practical exercises more easily because of the interrelationships. This becomes more manifest when attention is drawn to the f a d that the products 2 and 3 can both be further reduced t o yield the same product, namely 1-(3minophenyl)ethanol(4).
This product, 3-aminophenylethanone, is a crystalline solid and hence easier for purification hy students. An additional benefit ofthischoiceof nitrocompound is that it can beused to demonstrate the selectivity of certain reducing media. An examination of the structure of this compoundindicates a second reducible group (the carhonyl group). Thus the reduction using tin i n d hydrochloric k i d i s selective in reducing only the -NO2 (nitro) group. Students mav consider whether i t is oossible to reduce the carhonyl group"in 3-nitrophenylethanbne without affecting the nitro eroun. Carbonvl comnounds are known to be caoable of red;ctibn, aldehides bLing reduced to primary aicohols while ketones are reduced to secondary alcohols. Lithium tetrahydridoaluminate (lithium aluminum hydride, LiA1H4) and sodium tetrahydridohorate (sodium horohydride, NaBH4) are known to be selective reducing agents for carhonyl compounds. The former is accepted as too hazardous for use by students, but NaBH4is perfectly safe and easy to use. Considering the ease and simplicity of NaBH4 reductions. i t is s u r ~ r i s i n how e seldom these are oerformed in the teaching laboratory. The reduction of 3-nitrophenylethanone (1) to the corresponding secondary alcohol, l-(3-nitrophenyl)etbanol (3) proceeds smoothly.
Students thus become aware that a n organic compound may he synthesized by different routes. Melting point, mixed melting points, and infrared analysis confirms the nature of the final product.
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Reduction of the Nitro Group in 3-Nitrophenylethanone The experimental details are reported in the earlier paper. However, since tin salts produce gelatinous solutions, some difficultyin filteringthe reaction mixture may he experienced. To overcome this problem an alternative procedure may he adopted: To the cooled reaction mixture 40%sodium hydroxide (24 mL) can be added with stirring and cooling to liberate the free amine. The resultant yellow suspension should be extracted twice with ether (30 mL), the two ether extracts combined and dried over anhydrous sodium sulfate in a stoppered flask. Unreacted tin and the tin salts are insoluble in ether and are thus left suspended in the aqueous alkaline layer. Removal of the drying agent by filtration and evaporation of the ether, carefully in a fume cupboard using an electric water-hath, yields a yellow solid that, when recrystallized from water, produces the brilliant yellow product, 3-aminophenylethanone (2). Reduction of the Carbonyl Group in 3-Nitrophenylethanone The experimental details are reported in the earlier paper.
'Jones, A. G. J. Chem. Educ. 1975,52,668-669. Smith. 6. V.; Waldron, N. M. Vogel'sElementary Practicalhgm ic Chemistry. Vol. 1, Preparations, 3rd ed.: Longman: New York, 1980; pp 269-270.
Volume 88 Number 7 July 1989
611
