Microscale Group Test for Carbonyl Compounds V. Horak and R. F. X. Klein Georgetown University, Washington, DC 20057 Modern spectroscopic methods have drastically reduced derivatization of unknown compounds for structural analvsis and identification, even in undergraduate laboratori& However, rapid and simple qualitative tests requiring minimal quantities are still useful and instructive. The oualitn~~~~~~tivc gruup test described in this note for rharacwrization of carbonyl rompmmds is desicned for soohomore oreanir lab and serves three purposes: first, i t demonstrates principles of derivatization with one of the most frequently used reagents, 2,4-dinitrophenylhydrazine (2,4-DNPH); second, it is a TLC experiment which visuallv demonstrates seoaration of cdored compounds of different polarities'; and third, to mirroscal~exoerimmtation it is on excellent inrrod~~ction which can be performed in any standard student laboratory. The advantages of microscale experimentation, recently discussed by Rawls2, has helped renew interest in this technique. Microscale experiments requiring specialized equipment are not usuallv performed in undereraduate laboratories. However, our procedure utilizes onlyihe simplest glassware and does not require exact handline of either reaeents or samples. Derivatization followed by characterization of products with TLC demonstrates the advantage of this combination for rapid qualitative analyses3. The TLC system in this experiment gives distinct and visual separation of the reagent, 2,4-DNPH, from the hydrazones of the tested carbony1 compounds; no additional chemical or light visualization technique is required, and students can observe the chromatogram develop.
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Procedure A small amount of the carbonyl compound (i.e., one drop of a liquid or a spatula-tip of a solid) is added to a two-phased reaction system consisting of 1mL of asat'd soln. of 2,4-DNPH in CH2,CI2, 1mL of conc. HCI, and 1mL of H2O in a test tube. After oceaslonal shaking for 15-20 min4 the organic layer is transferred with a disp. pipet into a second test tube and about 1 rrL spotted on a TLC strip alongside a spot of the original 2,4-DNPH soln. (BakerFlex Silica Gel IB and Alumina IB plates are cut into 10- X 1.5-cm strips and samples spotted about 1 cm from the bottom.) After brief air drying the plate is developed with either CHzClzor toluenemHF (41) in a small jar furnished with filter paper to maintain a sat'd solvent atmosphere. The procedure was tested with a variety of aldehydes and ketones; R p data are reported in the table. In all systems, each hydrazone gave a yellow or orange colored spot with an RF value distinctly higher than that of 2,4-DNPH; even compounds containing highly polar groups (e.g., -NO2 or -OH) showed higher R p values. Wlth Alumina, however, in addition to the hydrazone and 2,4-DNPH spots, unidentified darker colored spots with lower Rp values were noted.
806
Journal of Chemical Education
RFValues
Compound Solvent System:.
Silica Gel IB
Aiumlna 18
I
I1
I
0.64
-
I1
2.4-3NPH
Acetophenone Anisaldehyde Benzaidehyde Benzoin Citmi Cycloheptanone Cyclohexanane Cydopentanone 3,4-Oimethoxybenraidehyde
Fenchone Formaldehyde Heptanal +Hydroxyacetophenone lsophorone Mesityi Oxide Methyl Ethyl Ketone 1-Napthylaldehyde mNitrobenzaldehyde pNitrobenzaldehyde Vanilline
DWC.~
-
" SolveM Systems I: mevlylene chloride; 11: Wlveneltetahydrofuran (41) bconsiderablestreaking
The procedure reported here is typically for detection of ndat carbonyl compounds in the 5-10 mg range, as described; with dilute solutions, detection down to 10 pg/drop is possible.
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There is a surprising lack of laboratory TLC experiments in the literature. The latest Chern. Abstracts Collective Index reports only six examples, and five of these are in foreign language journals. For the lone exception, which utilizes TLC to separate several FDRC dyes, see: McKone, H. T.. et al., J C ~ MEouc., . 53, 722 (1976). Rawls, R., Chern. Eng. News, 62, 20-27 (1984). Derivatizationof carbonyl compounds with 2.4-DNPH followed by TLC analysis has been utilized by several groups for specific research applications (but never before as a generalized qualitative group test.) For example, see (1)Kolbe. M., et al., FresenIus'Z. Anal. Chern., 281, 365-69 (1976). (2) Grodzka-Zapytowska, S., et ai.. Chern. Anal. (Warsaw),22, 549-52 (1976).and (3)Black, Jr., A. C., Proc. Iowa Acad. Sci.. 85,99-102 (1978). 'For most carbonyl compounds, the reaction is essentially complete within 2-3 min; the 20-min time scale is to cover possible inefficientmixing by the students andlor unusually slowly reacting compounds.
