Gettlng the Acid out of Your 2,4-DNPH The conventional methods'.2 for the preparation of carbonyl2,4-dinitrophenylhyd1azonesfrequently are not reproducible and often give derivatives with low and ambiguous melting ranges.
OP
,\ c=o
+ ~8-NH 0
aNo2
NO,
'cA =N-NH
0
NO,
+ H,O
In a recent article3 we reported that the main cause for the observed low melting ranges of the common 2,4-DNPH's and their melting variations is the syn-anti isomerizations of these derivatives that occur during melting. These isomerizations are catalvzed hv traces of acids oresent in the crvstals. As described below. we modified the conventional method of 2.4DNPH beparkion simply by r&oval of the aci& from the 2,4-DNPH c&tals by using a base wash. ModifiedMethod. Approximately 1.0g of the carhonyl compound is dissolved in 40 ml of 95%ethanol. To this, 35 ml of a solution of 2.4-dinitrophenylhydrazinein ethanol and sulfuric acidk is added, and the precipitate5 is filtered off on a sintered glass or a Buchner funnel by suction. The filtrate is discarded and 20 ml of a 5%sodium bicarbonate solution is added to the crystals in the funnel. After being stirred with a glass rod or a spatula, the liquid is filtered 0ff.O The solid is washed with 20 ml of distilled water and then recrystallized from the appropriate solvents. Comparison of the melting ranges of 2,4-DNPH derivatives prepared by the conventional and the modified methods revealed that the new method is much superior to the existing literature procedures and gives products with much higher melting range^.^ The modified method can also he applied to the preparation of certain hydroxyketone derivatives previously unattainable by the conventional methods.
' Allen, C. F. H. J. A m . Chrm. Soc. 1930, 52,2955. Brady. 0 . L. J. Chem. Soc. 1931,756. 3Behforour. M.: Bolan. J. L.; Flynt. M. S. J. Org. CChem. 1985, 50. 1186. 'me 2.4dln~mphenylhydr~ine ~oluiionwas prepred accordingto UK1 procedure described in: Shriner, R. L.; Fuson. R. C.: Curtin. D. Y.: Morrill. T. C. ''The Systematic Identificationof Organic Compounds". 6th ad.; Wiley: New York. 1980: p 162. In Some cases water must be added to cause the precipitation. See reference in footnote 3. Vipmus foaming occurs when the base soiuiion is added.
Mohammad Behforouz Ball State University
Volume 63
Number 8
August 1986
723
