W. B. Burton Shell Development Company Biological Sciences Research Center Modesto. CA 95352
Solid State Photochemical lsomerization A convenient laboratory experiment
T h i s experiment demonstrates the novel use of henzophenone in t h e d u a l role a s a solid s t a t e sensitizer in a photochemical isomerization a n d a s a n indicator in t h e preparative layer chromatography (plc) used for purification of t h e isomeride. T h e photoisomerization of dieldrin (I) is straightforward with only o n e major photoproduct (11) (Fig. 1).
Procedure The isomerization was &rried out with crystalline dieldrin, 99% mlm, 500 mg admixed with henzophenone,' 25 mg, in a 50 mm diameter glass Petri dish. This mixture was exposed to two General Electric G8T5 germicidal lamps a t a distance of 5 cm. The conversion was monitored by tle analysis. Fifty percent conversion was observed in 4 hr. After 10 hr the product was collected in acetonk andanalyzed hy tlc. Photodieldrin and its impurities or precursor were separated by thin-layer chromatography (tle) using E. Merck-Dannstadt Silica Gel 60 F254 precoated 0.25 mm plates. The sensitizer was also used as an indicator toshow the separation of the dieldrin from its photoproduct, as it migrated between the other two compounds with a hexane-aeetone (32) developing solvent. The fallowing Rfvalues were obtained Compound Dieldrin Benzophenone Photodieldrin
the Ri value of the henzophenone and determine the Ri value of the photoisomer. The section 2 cm below the indicator hand was collected and extracted with methylene chloride and acetone. The solvent was removed on a rotary flash evaporator and the residue recrystallized from a mixture of methylene chloride-hexane, 1:fi vlv (ethanol may also be used). The yield was 285 mg, 57%of theory. The photodieldrin, m.p. 196-8% was characterized by ir2 and tlc (Fig. 2) techniques as photodieldrin with no detectable impurities. Other smaller batches (10-50 mg) have been made in yields to 85% by this procedure.
,
O,.,d,," 9b,o,.ome, Figure 1. Photochemical isomerization of dieldrin.
0,
o,e,6.,"
-n
RrValues 0.60 0.54 0.49
The preparative layers used were E. Merck-Darmstadt Silica Gel 60 F254 precoated 2 mm plates. These plates have a relatively high preparation efficiency and allowed for a fairly rapid development time. The photochemical mixture in acetone solution was streaked as a fine line on the Silica Gel layers 20mm from the bottom edge a t the rate of 150 mglplate and developed with herane-acetone in a volume ratio of 3 to 2. The atmosphere in the developing chamber was saturated with respect t o the aeveloping solvent mixture. The ple plate was removed when the moving solvent front had reached the designated migration mark. The migrated solvent mixture was removed by evaporation and the separated components of the mixture remained absorbed in different places on the layered material. The plate was exposed to short wavelength ultraviolet light t o visualize
'Ivie, C. W., and Casida, J. E., J. Agrie. and Food Chem. 19,405-416 (1971). 2Pollard, G. E., Shell Development Company unpuhlished data.
Figure 2. Thin-layer chromatography of photodieldrin.
Volume 56. Number 7, July 1979 / 483
