Objective: To study the interconversion of two geometric isomers, maleic acid (cis isomer) to fumaric acid (trans isomers), the differences in physical properties between this pair of cis-trans isomers and determine the stereochemistry of addition of bromine to butenedioic acid.
Chemicals and Apparatus:
2 grams of maleic acid, 10 cm3 of concentrated hydrochloric acid, 10 cm3 of bromine water, one 50 cm3 beaker, one 100 cm3 beaker, one 250 cm3 beaker, one 10 cm3 measuring cylinder, one piece of spatula, one piece of water glass, one glass rod, a few pieces of filter paper, a set of Buchner funnel, a filter flask with side tube, three glass capillary tubes, a set of melting point apparatus, one long hollow glass tubing, one electronic balance, one heater and the oven
Stereochemistry is always encountered throughout organic chemistry. And stereochemistry isomerism can be divided into two classes, geometrical isomerism and optical isomerism. Although these categories are not mutually exclusive, it is not uncommon to find compounds that exhibit only one or the other of the two types. Most geometrical isomers result from cyclic systems or restricted rotation about double bonds. The preparation of fumaric acid by isomerization of maleic acid and addition of molecular bromine to fumaric acid was illustrated in the experiment.
Geometric isomers can be interconverted if the double bond is temporarily converted to a single bond, about which rotation is relatively free. For example, an electrophile adds to the double bond, rotation becomes possible. Loss of the electrophile then regenerates the double bond. If rotation occurred in the intermediate, the result is the other isomer.
The result of this kind of transformation is an equilibrium mixture of the cis and trans isomers. Frequently, the trans isomer is more stable, so the equilibrium mixture would contain more of the trans isomer. The greater the difference is in the stability of the isomers, the greater the concentration of the trans isomer will be at equilibrium.
Maleic acid and fumaric acid are geometric isomers of butenedioic acid. Each of these isomers has its own distinctive properties such as melting point, solubility, density and stability.
In part A of the experiment, Maleic acid could be converted to the more stable fumaric acid when heated with hydrochloric acid as Fumaric acid is less soluble in water than maleic acid. The hydrochloric acid served as an acid-catalyst of the reation. Thus, the Ï€ bond was broken. Rotation about the sigma bond occurred readily. Loss of a proton could be lead to the formation of either the cis or the trans isomer. Under equilibrium conditions, the thermodynamically favored product predominated.
In part B of the experiment, we would react the resulted isomer in part A, i.e. fumaric acid, with molecular bromine to form
2,3-dibromosuccinic acid. The first step in the mechanism of the electrophilic addition of bromine to an alkene was the formation of a cyclic bromonium ion. Nucleophilic attacked by bromide anion caused the cyclic bromonium ion ring to open. The net result was the addition of two bromines to the double bond.
Part A. Conversion of maleic acid to fumaric acid
1. About 2 grams of maleic acid was weighed out in a clean dry 100 cm3 beaker. 10 cm3 of deionized water was added and warmed slightly to dissolve the acid.
2. 10 cm3 of concentrated hydrochloric acid was added to the beaker and it was covered with a watch glass. Then the beaker was placed inside a 250 cm3 beaker which is about one third full of water. The water bath was heated to boiling for about 5 minutes or until a solid material formed in the smaller beaker.
3. The solution was cooled to room temperature by placing the small beaker with its contents in an ice bath.
4. The reaction mixture was filtered by suction using the setup below.