Question

1. Are dimethyl maleate and dimethyl fumarate constitutional isomers or geometric isomers? Explain.

2. Why are the melting points and boiling points different for dimethyl maleate and dimethyl fumarate?

3. What two methods could be used to purify the dimethyl fumarate?

Answer 1

Ans:

Constitutional isomers (Structural isomers): are compounds with the same molecular formula and different structures

Geometrical isomers: are compounds with the same molecular formula, same structure but different spatial arrangements due to the restricted rotation; may be across a double bond.

Ans 1:

Dimethyl maleate and dimethyl fumarate are geometrical isomers as can be seen clearly from their structures below.

Dimethyl maleate is a cis isomer and dimethyl fumarate is a trans isomer.

Ans 2:

Because of their different spatial arrangements, dipole moment of these two molecules differ. As a result, they possess different boiling and melting point irrespective of the same connectivity.

In cis- trans geometrical isomers, usually cis will have larger dipole moment than trans isomer.

Dipole moment of dimethyl maleate > Dipole moment of dimethyl fumarate.

BP of dimethyl maleate (204 ^oC) > BP of dimethyl fumarate (193 ^oC)

Ans 3:

Since BP of dimethyl fumarate is 193 ^oC, distillation under reduced pressure (vacuum distillation) is one of the methods to purify.

Column chromatography is another method to purify dimethyl fumarate.