J. 1. Farmer and E. J. Haws
Wolverhampton College of Technology Wolverhampton, England
An Experiment to Illustrate Nucleophilic Aromatic Substitution and Tautomerism
Pyridine and its derivatives undergo nucleophilic substitution more readily than the corresponding benzenoid compounds, the reaction taking a lace preferentially a t the 2 or 4 positions. The most widely quoted example in standard textbooks' is the amination of pyridine with sodamide (the Chichibabin reaction), although the reaction is not normally included in practical courses. An alternative, and we believe a more viable, exam~leof nucleo~hilicsubstitution for inclusion in an undergraduate practical course is the hydrolysis of 2-chloropyridine. This reaction has the added advantage in that the product is obtained as a tautomeric mixture which can be examined by infrared spectroscopy.
2-chloropyridine
2-hydroxypyridine
Zpyridone
2-Chloropyridine is hydrolyzed by the prescribed method and the product analyzed by infrared spectroscopy. The infrared spectrum may be taken as a Nujol mull, KBr disc, or a solution in chloroform or carbon tetrachloride. The student is asked to interpret the spectrum with special emphasis on the 15001800 cm-' and 2000-4000 cm-' regions and to compare his spectrum with that of 3-hydroxypyridine or pyridine. The spectrum of the product shows bands in the region of 1660 and 3150 cm-I which are not present in the spectrum of 3-hydroxypyridine. These bands may be assigned to the C=O and N-H stretching frequencies, re~pectively.~Another hand a t 1620 cm-I,
which is not present in the spectrum of the 3-hydroxy compound may be attributed to the nonaromatic C=C stretching of the amide. From this information the student can deduce that 2-hydroxypyridine exists predominately as the pyridone whereas 3-hydroxypyridine has the normal phenolic structure. Hydrolysis of 2-Chloropyridine
Place 2-chloropyridine (4.8 ml; 5.7 g; 0.05 mole) and potassium hydroxide (5.6 g; 0.1 mole) in a 100-mlflask fitted with an air condenser. Heat the flask carefully, with a bunsen burner, until reaction commences. When the initial reaction subsides continue heating for 15 min, taking care to avoid charring. Cool the mixture and dissolve it in water (75 ml), add decolorizing charcoal (0.5 g) and shake for 5 min. Remove the charcoal by filtration, acidify the filtrate to pH 4 with hydrochloric acid and evaporate to dryness under reduced pressure. Add xylene (75 ml) to the residue and carry out an azeotropic distillation, using a Dean and Stark apparatus, to remove the last traces of water. Filter the hot xylene solution and cool the filtrate when the product crystallizes as colorless needles. Collect the product by filtration under suction and wash with a little petroleum ether (b.p. 40-60'); yield 2.5; m.p. 105-106°C. We wish to thank Mr. P. h a l l , Midland and Yorkshire Tar Distillers Ltd., Wolverhampton for his advice concerning the hydrolysis of 2-chloropyridine. 1 ROBERTS, J. D., AND CASERIO, M., "Basic Principles of Organic Chemistry," W. A. Benjamin, New York, 1964. 2 GIBSON, J. A,, KYNASTON. W., AND LINDSAY, A. S., J . C h . Sac., 4340 (1955).
Volume
47,Number I,January 1970
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41
