Explain why dimethyl maleate is thermodynamically less stable than dimethyl fumarate.
To understand why dimethyl maleate is thermodynamically less stable than dimethyl fumarate, we need to consider the concept of stability in organic compounds.
In organic chemistry, stability refers to the tendency of a molecule to undergo chemical reactions and transform into a more stable state. It is influenced by various factors including bond strength, delocalization of electron density, and steric hindrance.
Dimethyl maleate and dimethyl fumarate are structural isomers, meaning they have the same molecular formula but different connectivity. Both compounds belong to the class of compounds called dienes, which contain two double bonds.
Now, let's analyze the stability difference between dimethyl maleate and dimethyl fumarate:
1. Bond strength:
In dimethyl maleate, the two double bonds are cis to each other, which means the carbon-carbon double bonds are connected on the same side of the molecule. This arrangement leads to steric repulsion between the bulky methyl groups, causing strain on the molecule. As a result, the carbon-carbon double bonds in dimethyl maleate are relatively weaker compared to those in dimethyl fumarate, where the double bonds are trans (connected on opposite sides of the molecule). The stronger bonds in dimethyl fumarate contribute to its higher stability.
2. Electron delocalization:
In dimethyl fumarate, the trans arrangement of the double bonds allows for efficient delocalization of electron density across the conjugated system. This electron delocalization stabilizes the molecule, making it more thermodynamically stable. On the other hand, in dimethyl maleate, the cis arrangement of the double bonds leads to a reduced ability for electron delocalization. Consequently, dimethyl fumarate exhibits greater stability due to the enhanced electron delocalization across the conjugated system.
In summary, dimethyl fumarate is thermodynamically more stable than dimethyl maleate due to the stronger carbon-carbon double bonds resulting from the trans arrangement and the more efficient electron delocalization across the conjugated system. The steric strain caused by the cis arrangement in dimethyl maleate weakens the carbon-carbon double bonds and reduces its overall stability.