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edited by N. H. ETTINGER George Washington High School B~stletonAve, and Verree Road Philadelphia, PA 19116
QuantitativeApplication of Thin-Layer Chromatography in the Analysis of Organic Compounds Determining the Molecular Weights of 2,4-Dinitrophenylhydrazones and Dicarboxylic Acids Vincenzo Giuliano La Salle University, Philadelphia. PA 19141 John Paul Rieck' Vineland High School, Vineland, NJ 06360 Thin-layer chromatography (TLC) is a useful solid-liquid partitioning technique used primarily for the separation and qualitative analysis of microgram quantities of material. I n addition to being an excellent qualitative technique, TLC can also prove to b e a useful aid in thechemical laboratory a s a quantitative method for determining t h e molecular weights of organic compounds. This simple, easy, and inexnensive method mav orove interesthe. for adantation hv students and instructors desiring a gotd illustra'tion of thk importance of oolaritv on solubilitv and a uniaue demonstration of the eLffectiv&ess of TLC as a quantitakve tool. In addition to aldehyde derivatives and carboxylic acids, this technique works well for identifying unknowns in other homologous series of organic compounds (such as amino acids, monocarboxylic acids, esters, and derivatives of ketones and amines, for examples) provided that the unknowns are relatively pure and are known to belong to the same series as the standards. The theoretical basis for molecular weight prediction in organic compounds is based on the assumption that increasing the number of carbons on a straight chain decreases the polarity of the molecule in a proportional fashion. In a chromatographic system consisting of a solid stationary phase and a liquid mobile phase, therefore, organic solutes of varying chain lengths should ascend the stationary phase a t varying rates, which are proportional to the size of the molecule. The maximum separation distance between solutes is of great importance in such a method since this increases the accuracy with which a molecular weight prediction can he made. The maximum separation between solutes of varying chain lengths can be achieved only if certain "ideal" parameters are met; namely, the adsorptivity of the stationary phase, the solubility in the mobile phase, and saturation of the developing system-factors that directly affect the rate a t which a solute ascends the stationary phase. The organic
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All chemcal~ and suop les may be obtamed from Aldrlch Chemlcal Company, Meruchen, N, Propanol was not available for study. 'Reference 2gives a good method for preparation. The hydrated variety (20% water by mass) was used.
compounds used in this study, 2,4-dinitrophenylhydrazones and dicarhoxylic acids, were tested in a variety of solvent mixtures of varying polarities, and the "ideal" solvent mixtures were considered to be those that effected the optimum separation distance between solutes.
2,4-Dinitrophenylhydrazone Preparation from Aldehydes3 The 2,4-dinitrophenylhydrazones (aldehyde derivatives) were
prepared by reacting a series of straight-chain aldehydes with 2,4dinitrophenylhydrazinereagent4in an acid medium.
0 R-C-H
11
+
H2NNH
NO^ Hi
tthanoi
0,N
(Caution: Preparation of the reagent was done with extreme caution since conc. sulfuricacid.i hiehlvcausticand 2.4-dinitroohenvlhydrazines as the dry solid is poikn&dly explosive and shock sensitive.) The derivative preparation procedure that was employed is a modification of a standard procedure found in most organic laboratory texts (1,2). A measured volume of 2.4-dinitrophenylhydrazine reagent (5.0 mL) was placed in five separate prelabeled Erlenmeyer flasks(20 mL). Five drops ofaseriesof straight-chainaldehydeswas placed in each respective flask.The aldehydes that were used gave yellow-to-orange precipitates immediately. (Caution: Since many 2,4-dinitrophenylhydrazonesare suspected carcinogens, physical contact was avoided where aossible.)The derivativeswere dried in a desacator fur 2 1 h prmr tc, uic in'I'I.C. hlelr~ngp
