A DEMONST~ATION OF CIS-TRANS ISOMERIZATION OLIVER GRUMMITT
THE catalyzed isomerization'a2,a of maleate esters by halogens, hydrogen halides, oxides of nitrogen, sulfur, and various metals suggested the possibility of demonstrating this interesting reaction to a class or laboratory in organic chemistry. Dimethyl maleate is a convenient ester for this purpose because its geometrical isomer, dimethyl fumarate, is a solid which is virtually insoluble in the liquid dimethyl maleate. Trial experiments with various catalysts showed that bromine is one of the most active, especially if the reaction is carried out in a strong light, as indicated by: H.C-C0.0CH8
II
H.C-CO.OCHa Dimethyl maleate
Bromine Light
H,c-co.ocHa
II
CH80.0C-C.H Dimethyl fumarate
Procedure.-Into three 20- by 150-mm. test tubes measure 10-ml. portions of dimethyl maleate. Into two of the test tubes place two drops of a five per cent solution of bromine in carbon tetrachloride. Wrap one of these tubes with dark opaque paper. Place the three tubes in a beaker of boiling water so that each receives about the same light from a 75100-watt bulb which is placed a foot away. At the end of five minutes remove the tubes and cool with shaking under the cold-water tap. The irradiated sample containing the catalyst rapidly solidifies as colorless crystals of dimethyl fumarate. The wrapped tube containing bromine appears unchanged. The third tube which contained no catalyst but which was irradiated also appears unchanged.
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FREUDENBERG, "Stereochemie," F. Deuticke, Leipzig and Vienna, 1933; Article by K ~ NP.. 917. ~GRIONARD AND BAUD, "Trait6 de chemie organique I," Masson e t Cie., Paris, 1935, p. 1088. 'GEBERT, T-EVITCX, AND WALLIS,J. OOI Ckm., 3, 361 (1939).
Western R e s e w University, Clevehnd, Ohio
These experiments not only show the reaction of cis-trans isomerization but they also illustrate the remarkable effect of a catalyst and of light. Attention should also be called to the fact that the bromine color does not disappear a t room temperature. This is in keeping with the general concept that "negativelysubstituted" ethylenic bonds are relatively inert in addition reactions. I n the sample which undergoes isomerization the bromine color gradually disappears in the course of heating. According to the theory3 for this process there is 6rst addition of bromine to the ethylenic bond with subsequent elimination. The dibromo intermediate no longer contains a double bond and is therefore incapable of geometric isomerism. When the double bond is re-established. the form ~ r o duced is the more stable one-in this ca$e the tra' or fumarate compoun~~ If it is desired to prepare the maleate and to recover the dimethyl fumarate, the following d&ctions may be used: 58 g. (0.5 mal) of maleic anhydride, 160 g. (200 ml., 5.0 molr) of methyl alcohol, and 4 to 5 ml. of concentrated sulfuric acid are reamed on the steam bath for 4 t o 5 hours. The condenser is then set for distillation and the excess water and alcohol are distilled off. The residue is washed with two 20-ml. portions of water, two 20-ml. portions of five per cent sodium bicarbonate solution, and then once more with water. I t is dried over Drierite or anhydrous magnesium sulfate and distilled, the fraction boiling a t 200' to 207°C. being collected. Yield, 50 g.. 70 per cent of the theoretical.
The dimethyl fummate p=pared above may be purified cr~stauization from m' p. 101°-20C.
