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 π 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.
Procedures:
Part A. Conversion of maleic acid to fumaric acid
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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.
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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.
- The solution was cooled to room temperature by placing the small beaker with its contents in an ice bath.
- The reaction mixture was filtered by suction using the setup below.
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The crystals were washed with about 1 cm3 of cold water and dried the crystals by drawing air through them for a few minutes.
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The crystals was transferred into a weighed water glass and dried in an over at about 120for 10 minutes.
- The dried crystals of fumaric acid were dried and weighed.
- The melting points of the two isomers were measured and recorded using the electrical melting point apparatus.
B. Addition of Bromine to Fumaric Acid
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About 1 g of fumaric acid was weighed in a clean and dry 50 cm3 beaker. 10 cm3 of stock solution of bromine and swirled slightly to mix.
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The beaker was covered with a water glass. The beaker was placed inside a 100 cm3 beaker which is about one third full of water. The water bath was heated to boiling for about 10 minutes.
- White precipitates would be formed after 10 minutes and the colour of the mixture would be turned to light yellow. If the solution became colourless as it was being heated, a few milliliters of the bromine solution was added.
- The solution was removed from the heat and cooled in an ice bath.
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The product was collected by suction filtration as the setup in part A. The crystals was washed about 1 cm3 of cold water and dried the crystals by drawing air through them for a few minutes.
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The crystals was transferred into a weighed watch glass and dried in an oven at about 120 for 10 minutes.
- The dried crystals of 2,3-dibromosuccinic acid was weighed.
- The melting points of two isomers were measured and recorded using the electrical melting point apparatus.
Results:
- Conversion of maleic acid to fumaric acid
Mass of maleic acid used = 2.004 g
Mass of fumaric acid used = 1.651 g
Percentage yield
Melting point of maleic acid: 138
Melting point of fumaric acid: 254
B. Addition of Bromine to Fumaric Acid
Mass of fumaric acid used = 1.001 g
Mass of 2,3-dibromosuccinic acid(dried) = 1.3858 g
Percentage yield
= no. of mole of C4H4O4Br2 / no of mole of C4H4O4
Melting point of 2,3-drbromosuccinic acid: 255
Discussion:
Geometrical isomer is a stereomer which is superposable with its own mirror image. However, a pair of diastereomers is not mirror images of each other. The cis-trans isomerism is a kind of diastereomerism, where its existence is due to hindered and restricted rotation of double bond. Maleic and fumaric acid are two of the examples shown below.
Generally, they have similar but not identical chemical properties and very different physical properties, e.g. melting point, solubility, density and stability.
Maleic acid is highly toxic; it reacts with thiol groups on proteins in human body and thereby renders the proteins unable to react in their normal way. Fumaric acid is not toxic and it is a common and important compound formed when human body metabolizes food.
For cis-isomer, the two identical groups (hydrogen atoms) are on the same side. (Latin: cis = on this side), and for trans-isomer, the two identical groups are on the opposite side. (Latin: trans = across)
(Q2) Maleic acid has a lower melting point than fumaric. Although maleic acid has a stronger dipole moment, the formation of intramolecular hydrogen bond reduces the formation of intermolecular hydrogen bond. This reduces the extent of the hydrogen formed between adjacent molecules in the crystal and causes a lower melting point than that of fumaric acid. And, the trans-isomer is more symmetrical in shape and can be packed more regularly in the solid crystal.
(Q1) Fumaric acid is more stable than maleic acid (7 kcal lower heat of combustion per mole), the equilibrium lied much towards the fumaric acid side. This is because in fumaric acid, the two bulky carboxyl groups are further apart and experience less steric repulsion. As we started with a solution of maleic acid, then as fumaric acid was formed it precipitates from solution.
(Q3) cis-butenedioic acid could loss a molecule a molecule of water from each molecule of acid when two carboxyl groups (-COOH) react to form the butenedioic anhydride (maleic anhydride) at about 140. Although trans-butenedioic acid has two carboxyl groups, there was insufficient energy to overcome the barrier of rotation about the C=C double bond in the same temperature.
(Q4, 5) As the melting point of the product in part B is 255, thereochemistry of the acid is meso-2,3-dibromosuccinic acid. Meso-compound does have an axis of symmetry so they are not chiral. Nor are they optically active. The chemical reaction mechanism for the reactions of fumaric acid with molecular Br2 is shown below.
Equation 1
Equation 2
The product from both equation is also meso-2,3-dibromosuccinic acid.
(Q6) The structure of the product from the addition of molecular bromine to maleic acid are 2R,3R-dibromosuccinic acid and 2S,3S-dibromosuccinic acid
Mechanism 1
Mechanism 2
Mechanism of isomerization of maleic acid to fumaric acid
In the mechanism of the experiment part A, the first step was a simple acid base reaction with the carbonyl oxygen and a proton. The proton was shown as a bare proton, but in reality the proton would be solvated with water. After the positive charge was formed on the oxygen atom, then it could be delocalized throughout the conjugated system. The carboxylic acid groups on maleic acid are close together and cause internal stress in the molecule because of the repulsion between these two groups. When the positive charge is delocalized the carbon – carbon double bond becomes a single bond and the sigma bond is now free to rotate. The steric interaction between the two hydroxyl groups causes the hydroxyl groups to move away from each other and form the more stable trans isomer. The rest of the mechanism involves the delocalization of the cation to reform the C=C bond and loss of the catalyst.
When the small beaker in the water bath was removed, we MUST not place it in the ice water immediately because it is very dangerous as the beaker would crack and the solution will bounce out. We should cool the solution to room temperature and then place it to ice water to make the solution much saturated. The solubility of fumaric acid is much insoluble than maleic acid since there is no net dipole moment in fumaric acid. When placing the beaker in the lower temperature environment, more fumaric crystal would be formed by saturation.
Conclusion:
In the experiment, we had to make sure that all water was evolved from the sample and all remaining substance was pure, otherwise, the percentage yield in the result would become meaningless.
Hazards
The whole experiment part B had to process in the fume cupboard as bromine gas is toxic and has strong disagreeable odor, and having a very irritating effect on the eyes and throat.
Maleic acid is a strong toxic; we have to use gloves while handling this compound and concentration hydrochloric acid is corrsive. Avoid contact them with face and hands.
Solid compounds from the experiment should be placed into the appropriate labeled waste jars in the side hood.
The stock solution of bromine water was prepared by dissolving 232.5 g of Br2 and 187.5 g KBr in water and diluting to about 750 ml.
