Paul F. Schatz University of Wisconsin Mad~son,WI 53706
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Friedel-Crafts Acvlation .
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An experiment incorporating spectroscopic structure determination
In our introductory undergraduate organic laboratory course, one of the experiments demonstrates Friedel-Crafts acylation and the effect of suhstituents on the orientation of electrophilic aromatic substitution. T h e experimental procedure used is applicahle to a wide variety of aromatic compounds, demonstrates several techniques (manipulation of highly reactive chemicals, use of agas trap, simple and reduced pressure distillation), and affords only one product in good yield.' Moreover, by requiring the student t o characterize the starting material and product with nuclear magnetic resonance and infrared spectroscopy, the experiment serves a s a demonstration of the usefulness and complementary nature of these two techniques. As an illustration of this last point, consider the results with the three isomers of xylene. T h e nuclear magnetic resonance spectra of ortho, meta, and para xylene are virtually identical. However, analysis of the absorptions due to out-of-plane CH deformation vibrations in the 1000 t o 650 cm-' region of the infrared spectra (see Fig. 1)allows one to distinguish between the isomers (1,Z). After acylation, the converse is true. Since the products are all 1,2,4-trisuhstituted benzenes, the infrared spectra are not useful for determining the relative positions of the substituents. On the other hand, analysis of the nuclear magnetic resonance spectra in the region between 6 and 8 ppm (see Fig. 2) allows one to unambiguously assign a structure to each oroduct. A list of aromatic compounds used along with the boiling points and yields of the ketonic products is presented in the, table. Experimental [Safety Note: Caution: Anhydrous a l u m i n u m chloride r e a c t s vigorously w i t h w a t e r f o r m i n g corrosive a n d noxious hydrogen chloride. T h e w o r k u p of this reaction m u s t h e CARRIED OUT I N A FUMEHOOD.] In a 250-ml round-bottomed flask equipped with adroppinpfunnel, glass stopper, and a reflux condenser fitted with an adapter which is connected to a gas trap2 are placed 27 g (0.20 mole) of anhydrous aluminum chloride, 50 ml of diehloromethane, and a magnetic stir bar. The stirred suspension is cooled with an icewater bath. A mixture of 16 g (0.20 male) of acetyl chloride and 25 ml of diehloromethane
1 For example, the product obtained by acylation of toluene is greater than 99% 4-methylacetophenone by gas chromatography analysis. %Anyone of several types of gas traps may he used ( 3 , 4 , 5 , 6 )
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