Isomerization of maleic acid (cis) to fumaric acid (trans)


Perform an isomerization reaction of C=C double bonds in acidic medium.


Double bonds are rigid structures where interconversion between Z- and E-isomers (cis and trans) does not occur. However, in acidic media protonation of the double bond can occur, with a consequent change in hybridization.

Isomerization of maleic acid (cis) to fumaric acid (trans)

Then, (unlike the π double bond) the σ bond allows rotation of the substituents around the bond, which through an additional loss of protons regenerates the double bond again. Under equilibrium conditions, the thermodynamically most stable double bond is formed. Maleic acid is isomerized to fumaric acid by heating with acid catalysis.

Experimental procedure

In a conical vial fitted with a rotating paddle, place 2 g of maleic acid, 3 mL of deionized water and 2.5 mL of concentrated HCl. Attach the vial to a condenser and reflux with stirring for 20 min. A precipitate will appear and become more visible when the reaction liquid cools after the reflux time is completed.

After reaching room temperature, separate the precipitate completely by vacuum filtration. Recrystallize the solid in water; dry, weigh and determine the yield (melting point = 130 °C) and calculate the reaction yield (estimated 85 %). Verify the purity of the product by CCF using ethyl acetate / EtOH (absolute) (1: 1) as eluent.

International Chemical Identifier

The IUPAC InChI key identifiers for the main compounds used in this experiment are provided to facilitate the nomenclature and formulation of chemical compounds and the search for information on the Internet for these compounds.


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