Lap report 6 - Using Various Mechanisms for the Isomerization of Maleic Acid to Fumaric Acid PDF

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Using Various Mechanisms for the Isomerization of Maleic Acid to Fumaric Acid...

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Natalie Loveridge CHEM 2221L 11/2/20 20 Using Various Mechanisms for the Isomerization of Maleic Acid to Fumaric Acid Introduction: The purpose of this experiment is to form fumaric acid from maleic acid using a number of reaction conditions. Maleic acid, like all substituted alkenes, is a geometric isomer of the cis nature, meaning that it has its two substituted groups (which are both CO2H groups) on the same side of the double bond. Under proper conditions and through certain processes, one cis isomer can be transformed into a trans (the substituted groups are on opposite sides of the double bond) isomer, and vice versa. In this experiment, our cis isomer, maleic acid, will be transformed into its trans isomer, fumaric acid. In order to transform substituted alkenes into their isomers, however, an intermediate with free rotation between the carbon – carbon bond must be created. There were several mechanisms analyzed in this experiment that explored the formation of this intermediate and the formation of fumaric acid from malic acid. There are multiple possible mechanisms that can be used for the transformation of maleic acid to fumaric acid; those mechanisms are thermal, photochemical, initial electrophilic attack on the carbon-carbon double bond, initial nucleophilic attack on the carbon-carbon double bond, and the involvement of a heavy metal. With the thermal mechanism, heat breaks the pi bond and the structure is allowed to rotate into the trans isomer, and when the heat is removed, the pi bond reforms and fumaric acid is the result. In the photochemical mechanism, light absorption disrupts the pi bond enough for rotation, and fumaric acid is formed. Initial electrophilic attack on the carbon-carbon double bond is slightly more complicated; an electrophilic attack of a proton on the double bond causes a carbocation intermediate to form that allows for rotation and the yielding of fumaric acid (shown below).

This carbocation can either lose a hydrogen and become fumaric acid or it can become chlorosuccinic acid (through the attack of a chloride ion) or malic acid (though the addition of water). Then, the loss of HCl in chlorosuccinic acid or the loss of H2O from malic acid yields fumaric acid (shown below).

In the fourth proposed mechanism, nucleophilic attack of the double bond, a nucleophile attacks the double bond (which allows it to rotate) and then leaves, forming fumaric acid (shown below).

The final probable mechanism, involvement of a heavy metal, involves a metal catalyzing the reaction to form fumaric acid though an inorganic mechanism. In the table below, the mechanisms in certain test tubes are hypothesized. Aside from the contents in the test tubes, they were also put in a hot water bath for 40 minutes. In test tube 1 there was 0.25 g maleic acid with 5mL of HCl solution. In test tube 2 there was 0.25 g maleic acid with 5 mL of H2SO4 solution. In test tube 3 there was 0.25 g maleic acid with 5 mL of concentrated HBr solution. In test tube 4 there was 0.25 g maleic acid and 0.50 g of NH4Cl with 5 mL of H2O. In test tube 5 there was 0.25 g maleic acid and 0.50 g of NH4Cl with 5 mL of H 2SO4 solution. In test tube 6, there was 0.25 g malic acid with 5 mL of HCl solution. Finally, in test tube 7, there was 0.25 g chlorosuccinic acid with 5 mL of HCl solution.

Mechanism

1) Thermal

2) Photochemical

Produces fumaric acid in Reasoning tubes

1, 2, 3, 4, 5

These tubes all contained maleic acid that should be converted successfully to the solid precipitate because they were heated. If this was the correct mechanism, all of these tubes should have formed precipitates.

1, 2, 3, 4, 5

All of these tubes had maleic acid that was exposed to regular room light. If the photochemical mechanism was what broke the pi bond of malic acid, these test tubes MAY form precipitates, even though the light that they were exposed to was not very strong.

3a) Electrophilic 1, 2, 3, 4, 5 attack (Hydrogen)

These tubes have maleic acid and a source for the proton.

3b) Electrophilic attack (Hydrogen- 1, 4, 5, 7 Chloride)

Tubes 1, 4, and 5 contain Maleic Acid, a proton source, and a chloride ion source. Tube 7 contains the chlorosuccinic acid intermediate and HCl needed to form fumaric acid.

3c) Electrophilic attack (Hydrogen- 4, 6 Water)

Tube 4 has the proton source, water, and maleic acid for this mechanism. Tube 6 contains the intermediate malic acid and the HCl to from fumaric acid.

4) Nucleophilic

These two tubes both have maleic acid and a nucleophile

4, 5

attack

for nucleophilic attack.

5) Involvement of a Heavy metal

1, 6, 7

These three tubes use HCl solutions that may involve metal impurities.

Procedure: This experiment was carried out as described in the given manual procedure. No changes occurred, aside from the re-performance of test tube 2 because the first experiment did not yield the proper result. Results: The final solution and solid mixtures were analyzed for the creation of a precipitate. Test tubes 1, 3, and 5 all formed precipitates – the rest did not. Test tube 1 contained the mixture of 0.25g of maleic acid and 5 mL of HCl solution. Test tube 3 contained the mixture of 0.25g of maleic acid and 5ml of concentrated HBr, and this was the quickest precipitate to form. Test tube 5 contained 0.25g of maleic acid, 0.50g of NH4Cl, and 5ml of H2SO4 solution. Test tube

Solids

Solutions

Precipitate formed?

1

0.25 g maleic acid

5mL HCl solution

Yes

2

0.25 g maleic acid

5 mL H2SO4 solution

No

3

0.25 g maleic acid

5 mL conc. HBr

Yes, quickest to form

4

0.25 g maleic acid and 0.50 g of NH4Cl

5 mL H2O

No

5

0.25 g maleic acid and 0.50 g of NH4Cl

5 mL H2SO4 solution

Yes

6

0.25 g malic acid

5 mL HCl solution

No

7

0.25 g chlorosuccinic acid

5 mL HCl solution

No, yellow color change

Discussion/ Conclusion:

After careful analysis, it was realized that pretty much all of the reaction mechanisms for the isomerization of fumaric acid can be eliminated as probable options for the mechanism. Specifically, the thermal, photochemical, hydrogen-chloride electrophilic attack, hydrogen-water electrophilic attack, initial nucleophilic attack, and the involvement of a heavy metal mechanisms can be eliminated as possible reaction mechanisms for the isomerization of fumaric acid. The reasoning for this is because, even though all test tubes were in a hot water bath, test tubes 2 and 4 did not form precipitates. Therefore, this suggests that heat alone is not good enough to convert maleic acid to fumaric acid and does not support the thermal mechanism. Similarly, test tubes 2 and 4 did not form precipitates even though all of the tubes with maleic acid were exposed to light, suggesting that light absorption is not the proper reaction mechanism. In the hydrogenchloride electrophilic attack, half of the hypothesized possible reactions did not form precipitates. Test tubes 4 and 7 did not form precipitates even though they both had the means to go through the hydrogen-chloride electrophilic attack. The hydrogen-water electrophilic attack mechanism was eliminated due to no precipitates forming in test tubes 4 or 6, which were the only test tubes with the means to go through this mechanism. The nucleophilic attack mechanism can be eliminated because no precipitated of fumaric acid formed in test tube 4. Lastly, the involvement of a heavy metal mechanism can be completely eliminated because test tubes 6 and 7 did not give fumaric acid even though they both contained HCl. As one can see, the majority of these mechanisms do not work. There is not a single proposed reaction mechanism that is 100% correct, as only test tubes 1, 3, and 5 reacted to produce fumaric acid. The hydrogen-chloride electrophilic attack is the closest possible mechanism to the actual mechanism for the isomerization of fumaric acid. However, the actual reaction mechanism is a lot simpler; all one needs is a proton donor and a halogen donor. We know this because the 3rd test tube reacted with a bromide ion instead of the typical chloride ion; this mechanism is not limited to chloride and can instead be used with any halogen. Additionally, since test tube 7 did not react, it can be assumed that there is no chlorosuccinic acid intermediate. Isomerization is an equilibrium equation, yet the fumaric acid precipitate continues to accumulate over time. This is because the isomerization of fumaric acid to maleic acid is still happening, but it is at a significantly lower rate than the isomerization of maleic acid to fumaric acid only because converting maleic acid to fumaric acid is a cis-to-trans compound conversion which is more thermodynamically favorable than a trans-to-cis compound conversion....