Deducing A Reaction Mechanism for the Isomerization of Maleic Acid to Fumaric Ac

Question

Deducing A Reaction Mechanism for the Isomerization of Maleic Acid to Fumaric Acid. Can you please help me answer these questions? I am having a hard time understand why which mechanisms do and dont work for the following questions. THANK YOU!

ECHANISM 1. Is visible light solely responsible for the isomerization of maleic acid to fumaric acid? Explain. 2. If the isomerization of maleic acid to fumaric acid is a thermal effect, which test tubes should contain fumaric acid? 3. For mechanism 3 to be correct, which test tubes should contain fumaric acid? 4. For mechanism 4 to be correct, which test tubes should contain fumaric acid? 5. For mechanism 5 to be correct, which test tubes must contain fumaric acid? 6. Which test might rule out mechanism 6 as a possibility? 7. For mechanism 7 to be correct, which test tubes must contain fumaric acid? 8. For mechanism 8 to be correct, which test tubes must contain fumaric acid? ESTI ONS .1. Does sulfuric acid, which ionizes to give a negatively charged sulfate ion, induce the isomerization of maleic acid to fumaric acid? Explain n.2. Based upon your observations, would you expect fumaric acid to form in a test tube containing 1 g of maleic acid, 0.5 g NH4Cl, and 3 mL of 3M H2SO4, after it was heated? Explain.

Explanation / Answer

1. Visible light is not solely responsible for isomerization of maleic acid to fumaric acid. The isomerization can be brought about thermally, i.e., by heating maleic acid in a water bath as denoted by mechanism 2. When heated, maleic acid is excited and the pi bond is broken facilitating rearrangement about C2-C3 bond and a new pi bond formation. The isomerization may also be brought about by nucleophilic addition as denoted in mechanism 3. Hence, irradiation of maleic acid with visible light is not the only method to obtain fumaric acid.

2. Test tubes marked A, B and E should contain fumaric acid. The reason is dissolution of a solid into acid is usually accompanied by evolution of heat. Sulfuric acid produces the more exothermic dissolution, i.e, this dissolution produces more heat. Since heat is produced, the temperature rises and hence the isomerization of maleic acid to fumaric acid takes place. Infact, it may be possible that fumaric acid is only obtained in test tube B as the other two test tubes may fail to produce significant thermal energy to bring about the isomerization.

3. Ammonium chloride, NH4Cl is an electrolyte that decomposes in aqueous or acid solution into ammonium ions, NH4+ and chloride ions, Cl-. The Cl- adds to the pi bond of maleic acid and leads to rotation and ultimately release of the Cl- to form fumaric acid. Hence test tubes C and D must contain fumaric acid.

4. Test tubes A, B and E must contain fumaric acid for mechanism 4 to be correct. All the three acids are strong electrolytes and dissociate into protons in aqueous solution. The proton then adds to the pi bond in maleic acid and causes rotation of the carbocation (as per mechanism 4) to form fumaric acid. The carbocation intermediate 4 (as shown) is stabilized by the negative ions in acidic solutions.

5. Test tube F must contain fumaric acid if mechanism 5 is to be correct. Pure water is a bad conductor of electricity. However, in presence of a trace of acid like HBr, water becomes a good conductor and can act as a source of protons. Therefore, mechanism 5 is likely and we will get fumaric acid in test tube F.

You haven’t provided the remaining mechanisms.

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