Question

Deducing a Reaction Mechanism for the Isomerization of Maleic acid to fumaric acid. I'm completing my organic chemistry lab report and answering questions and I have very confused and would appreciate any help and explanation please. I appreciate it.

Test Tube	Solids	Solvents	Precipitate
A	1.0 g maleic acid	3.0 mL 6 M HCl	Yes
B	1.0 g maleic acid	3.0 mL 3 M H2SO4	No
C	1.0 g maleic acid 0.5 g ammonium chloride	3.0 mL distilled water	No
D	1.0 g maleic acid 0.5 g ammonium chloride	3.0 mL 6 M HCl	Yes
E	1.0 g malic acid	3.0 mL 6 M HCl	No
F	1.0 g maleic acid	1.5 mL distilled water 2.0 mL conc. HBr	Yes

Possible Mechanisms for the Isomerization.

1. Maleic acid is activiated by visible light, and then rearranges to form fumaric acid. In this mechanism, the pi-bond is momentarily broken, allowing for the free rotation to give the more stable trans isomer. The double bond then reforms.

2. The rearrangement of maleic acid to fumaric acid is accomplished by the thermal excitation of the molecule. In this mechanism, the pi-bond is broken by the application of heat. As described above, this would allow rotation to occur and form the more stable trans isomer.

3. A nudeophilic addition of a chloride ion to maleic acid again permits free rotation, followed by release of the Cl- ion to produce fumaric acid.

4. An electrophilic addition of a proton to maleic acid, followed by rotation and then deprotonation.

5. Water adds to maleic acid (under acid catalysis) to form laic acid. Malic acid then rotates to a new conformation and elimenates water to form fumaric acid.

6. The addition of a proton to maleic acid leads to the formation of a lactone (cyclic ester) which undergoes a ring opening reaction to yield malic acid. Malic acid is then dehydrated to give fumaric acid.

7. A proton adds to the carbonyl oxygen to give an intermediate with a number of resonance structures. One of these resonance structures can freely rotate and deprotonation to give fumaric acid.

8. A proton adds to the carbonyl oxygen, followed by an addtion of a chloride ion. The product of this 1,4 addition then undergoes a free rotation, and hydrochloric acid is eliminated to produce fumaric acid.

So, I understand that I can go through a process and eliminate the first 7 reaction mechanisms to chose the last one #8 is close to the true state of affairs.

But I am struggling answering these questions, if anyone can help and provide an explanation I'd greatly appreciate it.

Questions #1: 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?

Then these questions:

1. Does sulfuric acid, which ionizes to give a negatively charged sulfate ion, induce the isomerization of maleic acid to fumaric acid? Explain.

Anybody answer this?

Answer 1

1. Is visible light solely responsible for the isomerization of maleic acid to fumaric acid? Explain.

Maleic acid and fumaric acid do not spontaneously interconvert because rotation around a carbon carbon double bond is not energetically favourable. However, conversion of the cis isomer into the trans isomer is possible by photolysis in the presence of a small amount of bromine.Light converts elemental bromine into a bromine radical, which attacks the alkene in a radical addition reaction to a bromo-alkane radical; and now single bond rotation is possible. The bromine radicals recombine and fumaric acid is formed.

2. If the isomerization of maleic acid to fumaric acid is a thermal effect, which test tubes should contain fumaric acid?

Test tube A should contain fumaric acid. Maleic acid is transformed into fumaric acid through the process of heating the maleic acid in hydrochloric acid solution. Reversible addition (of H⁺) leads to free rotation about the central C-C bond and formation of the more stable and less soluble fumaric acid.

3. For mechanism 3 to be correct, which test tubes should contain fumaric acid?

Test tube D should contain fumaric acid. Here chloride ion of ammonium chloride acts nucleophile. A nudeophilic addition of a chloride ion to maleic acid again permits free rotation, followed by release of the Cl- ion to produce fumaric acid.

4. For mechanism 4 to be correct, which test tubes should contain fumaric acid?

Test tube A should contain fumaric acid.

5. For mechanism 5 to be correct, which test tubes must contain fumaric acid?

Test tube F should contain fumaric acid.

6. Which test might rule out mechanism 6 as a possibility?

Test tube B should contain fumaric acid.

7. For mechanism 7 to be correct, which test tubes must contain fumaric acid?

Test tube F should contain fumaric acid.

8. For mechanism 8 to be correct, which test tubes must contain fumaric acid?

Test tubes A and D should contain fumaric acid.

Then these questions:

1. Does sulfuric acid, which ionizes to give a negatively charged sulfate ion, induce the isomerization of maleic acid to fumaric acid? Explain.

No. sulfuric acid is a well known dehydrating agent, thus it causes removel of water from maleic acid to form maleic anhydride.