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H. Dupont Durst State University of New York at Buffalo Buffalo, 14214 and G e o r g e W. G o k e l The Pennsylvania State University University Park, 16802
A Classification Test for Aldehydes hv0hing Phase Transfer Catalysis
Aldehydes a r e normally distinguished from ketones h y examining t h e infrared stretching frequencies for the carhonyl functionality. I n addition, t h e presence of a n aldehyde CH stretch in t h e infrared or t h e downfield aldehyde proton signal in t h e nuclear magnetic resonance spectrum is often indicative. Although common spectroscopic methods have largely supplanted t h e more classical methods for distinguishing aldehydes from ketones in m a n y applications, aldehyde classification tests remain verv useful in actual lahoratorv. ~ .r a c t i c e a s well as a n important pedagogical device in qualitative oreanic chemistrv. I 4 v r traditional rest+ have been utilized in a n effort t o d i s tineuish uldehvdes from ketones. Four uf these tests, t h e Heneiict's, t h e Fkhling's, t h e Tollens', a n d t h e Bayer's involve t h e oxidizahilitv of t h e aldehvde function. I n t h e Fehling and Benedict tests,-oxidation of t h e aldehyde is accompanied b y reduction of copper(I1) t o copper(1) which precipitates a s its brick r e d oxide. I n t h e Tollens' test, t h i s oxidation results in production of a metallicsilver mirror from silver(1) ion. I n t h e Bayer permanganate test, M n 0 2 precipitates in a positive test. T h e only non-oxidative t e s t in t h i s group is t h e Fuchsin aldehyde test which involves the dyep-rosaniline hydrochloride: a positive t e s t is indicated by a d e e p purple solution. E a c h of t h e above procedures suffers from t h e s a m e basic deficiency: i n each case t h e positive reaction will be indicated onlv for a n aldehvde which is soluble in a n aqueous, o r essentially aqueousw, medium. T h e Benedict's a n d Fehling's solutions a r e of histdrical interest h u t t e n d to he unreliable. T h e Tollens, Bayer, a n d Fuchsin tests a r e more reliable but work well onlv with those aldehydes which a r e soluble in t h e m e d i u m c o n t k n g t h e aldehyde classification reagent. ~
A new reagent has recently been introduced which reacts with aldehydes and, after air oxidation, forms acolored complex. Use of this (I),' compound, 4-amino-3-hydrazino-5-mereapto-1,2,4-triazole causes both the hydrazine and the amino function of the heterocycle to read with the aldehyde to form an a m i d (eqn. (1)). Air oxidation of the aminal ((II),R 1or R2 = H) leads to the formation of a bicyclic heterocycle containing six nitrogens (eqn. (2)). As this is a highly conjugated system, the reaction of an aldehyde with the reagent gives a ornduct which. suhseouent to air oxidation. undereoes a color chanee
ethylene glycol is readily oxidized, spurious results are frequently observed and. therefore. careful blanks must be run with eaeh unknown dcterminntim: (1, hrisusr cthvlcne glgcul is n h~ghlyhyr hr u a l w ur alcohd x h h l ~ dmxylatea medium, the ~ l d r h y d muit fur g w d results
