The Photochemical lsomerization of Maleic to Fumaric Acid An Undergraduate Organic Chemistry Experiment Albert J. Castro. Suzanne R. Ellenberaer. - . a n d J a m e s P. Sluka San Jose State University, San Jose, CA 9519'2 Fumaric acid is found in many plants,' and i t takes part in the respiratory process in man. Maleic acid does not occur naturally, hut it is obtained as its anhydride by the catalytic air oxidation (VzOb, 400-500') of benzene. Historically it was known that these two acids could be obtained by heating malic acid,2 hut the relationship between the two compounds was unknown. HOCHCOIH I CH,C02H ( e - m a l i c acid
acid is of low solubility in water (0.7 gI100 ml, 25") and crystallizes from solution as it is being formed. In the isomerization, the free radical resulting from the attack of a bromine atom on maleic acid can undergo free rotation about its central C to C bond. Upon loss of the bromine atom from this intermediate, starting maleic acid or the more stable fumaric acid is formed.&
CHCO2H 1,
+H20
CHC02H fumaric acid 190%)
maleic acid 14%)
I- n van't Hoffs eooch-makine ".oaoer - of 1874." he deduced the essential feature; of geometrical isomerism besides developing the basic theory of optical isomerism. The recognition of fumaric and maleic acids as geometrical isomers, as proposed by van't Hoff, was due chiefly to the work of Wisliceuus. T h e experiment to he performed was first reported by Wislicenus4 in 1897, and it is a classical example of the isomerization of a less stable to a more stable isomer. From their heats of combustion, maleic acid (327 kcallmole) is 7 kcallmole energy richer than fumaric acid (320 kcallmole). Maleic acid is very soluhlein water (78.8 gI100 ml, 25"). When an aqueous solution of the acid contain& a small drop of bromine isirradiated, i t is rapidly isomerized to fumaric acid. The latter ~
'
Examples are Fumaria officinalis (a fumitory),the lichen Cetraria biandica, and mushrooms (Agaricusand Boletos species). Maiic acid is present in many fruit juices and was isolated by Scheele in 1785 from unrioe malum.. aooiei , aooles . . (Latin. . .. . It has the propeny of anracling pan1 spermalazoas ti e., lney m grate lowaro Ihe po nt of nignest concentralion,. Ma ;c acid s man.fac1ured commercia I) oy !he hyarat on of male c or furnarc ac o It 1s ,sea as an acidulent in foods. 51, 2 (1974). See Sneiders, H. A. M., J. GHEM.EDUC., Wislicenus, J., Chem. Zenfr, 11, 259 (1897);"Beiisteins Handbuch der Organishen Chemie," Vierte Auflage, Julius Springer, Berlin, 1920, 0. 749. The acid-catalyzed isomerization of maleic to fumaricacid is also well known. Furthermore, the isomerization of maleate to fumarate ion can be brought about by isomerase enzymes, which are isoiatabie from strains of Aicaiigenes and Pseudomonas bacteria. Ultraviolet light can cause painful eye injury and serious skin burns. The intensity of the laboratow. lamp's . rays is much qreater than that of the midsummer sun. Dark glasses protect the eyes.~inimizepersonal exoosure to the iamo's ravs. " W s1 c e n s , J , [lot c;r r reponed a near y wan1 tar ve y~elo Lpon heat ng fumar c ac d n a closed lube. a practlca conversion to tho energy-rlcher ma e c ac 0 can oe acnleved ~~
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T h e crvstallization of the latter acid from solution shifts the &ead;predisposed equilibrium further to the right (Principle of Le Chatelier) resulting in a very high yield of product. Experimental Dissolve 3.0 g of maleic acid in 20ml af water contained in a 50 ml Erlenmeyer flask. Add a small drop of hramine and place the container under an ultraviolet lamp (275W GE' Sunlamp) or in direct sunlight for 15-20 min. If a laboratory ultraviolet lamp is used, adjust the lamp6 (CAUTION: See footnote 6.) so that it is 2-3 in. above the flask. Collect the erystralline deposit of fumaric acid an a Biichner funnel (Size No. O), and wash it with afew milliliters of water on the funnel. Dry the crystals on a watch glass overnight in the lahoratory. Weigh the product, calculate the % yield? and determine its melting pant. Fumaric acid melts at 286', but it sublimes when heated above 200" in an open tube.8 Maleic acid melts at 130'. To determine the melting point of the produd, place a finely divided sample (contained between two microscope slide cover glasses) an the heated stage of a Fisher-Johnsmelting point apparatus when the temperature reaches 280". Continue to raise the temperature steadily until the sample melts.
Volume 60 Number 6
June 1983
521
