Lab report - Experiment 6 PDF

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lab report 6 organic chemistry...

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Lab report 6 Mechanism of isomerization of Maleic Acid 11/1/2020 Introduction In this lab, observations from a number of simple experiments will be used to deduce certain facts about the isomerization of cis-butenedioic (maleic) acid to trans-butenedioic (fumaric) acid. The possible reactions include thermal, photochemical, initial electrophilic attack on the carbon carbon double bond, initial nucleophilic attack on the carbon-carbon double bond and involvement of a heavy metal. After dissolving maleic acid/malic acid/chlorosuccinic acid into specific solutions, evaluation and observation of the precipitate will be used to determine whether fumaric acid has formed. Results Test Tube Solids

Solution

Result (after cooling)

1

0.25 g maleic acid

5 ml HCl solution

Precipitate formed

2

0.25 g maleic acid

5 ml H2SO4 solution

clear

3

0.25 g maleic acid

5 mL conc. HBr

Precipitate formed

4

0.25 g maleic acid and 0.5 g NH4Cl

5 mL H2O

Clear

5

0.25 g maleic acid and 0.5 g NH4Cl

5 ml H2SO4 solution

Precipitate formed

6

0.25 g malic acid

5 ml HCl solution

Clear

7

0.25 g chlorosuccinic acid

5 ml HCl solution

Clear

Discussion: 1) For each mechanism listed, explain which reaction mixtures, identified by test tube number, should have produced fumaric acid if that mechanism were correct. (See below for example)

Mechanism

Produces fumaric acid in tubes

Reasoning

Thermal

1, 2, 3, 4, 5

All these tubes have heat and maleic acid. If heat is sufficient to isomerize maleic acid then fumaric should form in these tubes.

Photochemical

1, 2, 3, 4, 5

All of these tubes have maleic acid and been exposed and may have undergone absorption of light, which supplies the energy for disruption of pibond that give furmaric acid from rotation and relaxation.

Initial electrophilic attack on the carboncarbon double bond

1, 2, 3, 5

All of these tubes have an acidic solution and maleic acid. Free rotation of the carbocation intermediate allow for loss of the proton to yield either maleic or fumaric acid.

Initial electrophilic attack on the carboncarbon double bond (chloride ion)

1

This tube has the addition of a chloride ion (in HCl) which may be eliminated to yield chlorosuccinic acid.

Initial electrophilic attack on the carboncarbon double bond (water)

4

This tube has the addition of water that will yield malic acid.

Initial nucleophilic attack on the carboncarbon double bond

Not observed

Involvement of a heavy metal

Not observed

2) For each mechanism you have eliminated, state which results disproved the mechanism. Mechanism

Results which disprove mechanisms

Reasoning

Thermal

2, 4

After the solutions were cooled, tube 2 and 4 had no precipitate which means that fumaric acid was not formed.

Photochemical

2, 4

No precipitate was formed in these tubes despite them having light absorption.

Initial electrophilic attack on the carbon-carbon double bond

2

Initial electrophilic attack on the carbon-carbon double bond (chloride ion)

n/a

Initial electrophilic attack on the carbon-carbon double bond (water)

4

Initial nucleophilic attack on the carbon-carbon double bond

n/a

Tube 2 had no precipitate despite it is an acidic solution.

Tube 4 had no precipitate despite its potential for the addition of water that yield malic acid.

3) Present your support for the best mechanism(s) given or, if you have eliminated all of them, discuss some characteristics of a viable mechanism for the isomerization (dependence on H+ or X-, etc.). (Consider the significance of Tube # 3.) Most of the mechanisms are eliminated. In tube 1, there was a reversible addition of H+ that lead to free rotation on the carbon-carbon double bond. The conversion of maleic acid to fumaric acid (the rotation around the carbon-carbon double bond) is not energetically favorable. In tube 3, I think that the bromine in the solution facilitated the rotation. Absorption of light converts the bromine into a bromine radical that will attack the alkene, enabling bond-rotation as bromine radicals recombine and fumaric acid can be formed. 4) Given that the isomerization is an equilibrium reaction, why does the fumaric acid continue to accumulate over the course of the reaction time? Is the reverse isomerization occurring as well? Why or why not? Isomerization can be induced by heating so the accumulation could attribute to the increase of temperature over time (during the heating process). Reverse isomerization could have happened because for tube 2 it had precipitate after heating but after the cooling process the precipitates disappeared. Conclusion: Although multiple mechanisms are proposed to be possible, after concluding substantiated, inconsistent data, most of them are disproved. The involvement of H+ seemed to be a necessary factor, along with the addition of bromine as a catalyst to photolysis. There were also cases where the reaction seemed to be proceeded initially, but the precipitate disappeared. This could be attributed to the change in temperature or perhaps reverse isomerization....