Josef E. Herz
Centro de lnvestigaci6n y de Estudios Avanzados del I.P.N. Mexico City
Optimizing Experimental Conditions The use of TLC to follow the course of a reaction
In a recent issue of THIS JOURNAL,^ Bozak describes the acetylation of ferrocene applied as a student practice of column chromatography. We would l i e to describe a student research project, which is part of a laboratory course in organic chemistry a t the CIEA-IPN, and utilizes this reaction to illustrate the use of thin-layer chromatography to follow the course of a reaction. The acetylation of ferrocene with acetic anhydride, in the presence of 85% metaphosphoric acid under the conditions described by Graham,%yields not only acetyl ferrocene, but also various isomeric diacetyl ferrocenes, due to the great reactivity of the mono acetyl derivative. The problem for the student consists in finding the best experimental conditions (reaction temperature and time) at which the reaction uses up most of the starting material without going further than acetyl ferrocene. The course of the reaction can be followed nicely by TLC. No developing reagent is necessary, since the spots can be seen easily and the various ferrocenes distinguished by their color (ferrocene:yellow, acetyl ferrocene:orange, diacetyl ferrocenes:dark red). Ferrocene (4.6 g) is dissolved with agitation in 125 rnl acetic anhydride. The flask is then cooled in a water bath, keeping the inside temperature at 25OC while I0 ml85% phosphoric acid is added. After completion
BOSAK, R. E., J. CEEM.EDUC., 43.73 (1966). GRAHAM,P.T.,ET AL., J. Am. Chem. Soc. 79, 3416 (1957). a SCHL~GL, K. H., ET AL., Monatsch. Chem. 9 2 , 533 (1961).
of the addition, the water bath is exchanged for a bath at 30°, 45" or 60°C and a 2-5-ml sample is removed from the reaction mixture, poured on ice, neutralized with NaHCOa, and the aqueous solution extracted with methylene chloride or ether. After drying and evaporation of the solvent, the ferrocenes are dissolved in a small amount of benzene or methylenechloride and spotted on a 20 X 20-cm TLC plate covered with Silicagel G. The procedure is r e peated at regular intervals (30 min to 1 hr a t lower temperatures, every 5 min at 60°C). After all the samples have been spotted, the plate is developed in a jar equilibrated with benzene ethanol (30:l). In this system ferrocene has a Rf = 0.9, acetyl ferrocene Rf = 0.53, and diacetyl ferrocene Rf = 0.25.s Our best experimental conditions were 20°C for 16 hr.
1
FERROCENE
ICETILFFRROCENE
1.1'-DIICETYLFERROCENE I N 0 ISOMERS
Volume 43, Number 1 1 , November 1966 / 599
