Identification of Unknowns by Melting Point and Thin-Layer Chromatography in Combination Samuel G. Levine North Carolina State University, Raleigh, NC 27695 Melting-point determination and thin-layer chromatography (TLC) are among the fundamental techniques that are taught early in the laboratory part of most organic chemi s t courses. ~ We have develo~edan exoeriment in which the stuient gains experience witi these techniques while applyine them in a coordinated wav to the identification of unknown substances. The 12 compounds that have been used as unknowns are listed in the table. They fall into four groups, I-IV, each consistine of substances of similar melting point. A gap of at least 10 "%eparates the melting ranges o f ' s ~ c c e s s i v ~ ~ o u p s , The student receives a copy of columns A and B of the table, representing the melting behavior of all possihle unknowns. Helsbe also receives a 50-mg sample of one of these compounds in powdered form and is expected to identify the substance by melting point and TLC in conjunction with available authentic samples. The student first determines the melting point of the unknown, thereby narrowing the possibilities to the three membersof one melting point group. A few milligrams of the sample are then set aside for later use in a mixture meltingpoint determination and the remainder is dissolved1 in 1ml of CHzCI2.A choice among the three members of that group is then accomplished by TLC employing 5% absolute ethanol in hexane. That eluting.system was found to give . good separation within each melting point group as indicated in column C of the tahle. If, for example, the unknown has a melting point which falls in group 111, the student will compare its Rr with those of authentic samples of acenaphthene. m-toluamide. and methvl 4-nitrobenzoate. This should lead t o a tentstive identification of the unknown as, for examole. m-toluamide. This conclusion mav then be confirmed b;ddtermining that (1) asolution containing both mtoluamide and the unknown eives a single spot bv TLC and (2) a mixture of the two solid; melts without dep;ession. Since most students can complete the above procedure in about one hour, two or more unknowns may be given, depending on the length of the laboratory period. I t is also
'Two of the 12 compounds (mtoluamlde and succlnlmlde) are sparingly soluble and these solutlons should be flltered before use. 2Lange's Handbook of Chemistry. 12th Ed.; Dean. J. A,, Ed.: McGraw-HIII: New York. 1979 p 7-48. This table Is not Included In the latest (13th)edltlon.
972
Journal of Chemical Education
possible to expand the scope of the experiment by adding compounds to the melting point groups or by adding additional melting point groups. For this purpose it is helpful to consult the "Melting Point Index of Organic Compounds" in Lange's Handbook of Chemistry.2
Materials and Methods Manv of the com~oundslisted in the table are commonlv availabie in most ciemistry departments but the entire s& mav be ourchased in minimum amounts (25 -e-1 ke) . from ~ l d r i c hior approxmately SlSO. The melting points recorded in the tableare thoseawen on the label and were found to be reliable. All appearid to be pure by TLC. Authentic samples were labeled by name and were made available both as powdered solids and as Y o (wlv) solutions1 in CH2CI,, whirh were dispensed from small dropping hottles. Thin-layer chromatography was performed with Bakerflex plastic-barked silica gel plates IH2-t'. Solutions in CHqCI, were sootted alone a lieht oencil line 6 mm from one endusing 2 p ~ ~5fiL o r disposagle micropipets (Drummond). Development with 5% abs. ethanol in hexane was allowed to proceed to a mark 10 mm from the other end (6-7 min). Visualization was achieved with low wavelength UV light. Compounds Used as Unknowns A
Compound
B mp 'C
C Rd5% abs.EtOH/hexane)
69-72 68-71 70-73
0.82 0.24
Group I
biphenyl 2.5dimethylphenal 44tr0benzyl~hl0ride Group Il
I-~hior~4-nitrobenz~n~ naphthalene
83-84 80-62 Mydroxy-3-m~hoxyben2aIdehyde 81-83
wmp m
acenaphthene mtol~amlde methyl 44trobenwate
93-95 94-96 94-96
Group IV
benzolo add hans-stllbene succlnlmlde
122-123 122-123
122-124
0.40
