In the Diels Alder reaction of maleic anhydride and anthracene, why does the initial yellow color of the reaction mixture fade during the reaction?

Question

Ryan Andrews · Accepted Answer

Here's my explanation.

The reaction takes place in two steps.

Step 1

The maleic anhydride and anthracene initially react in a fast step to form a charge transfer complex.

#underbrace("maleic anhydride")_color(red)("colourless") + underbrace("anthracene")_color(red)("colourless") ⇌ underbrace("charge transfer complex")_color(red)("yellow")#

As the adduct forms, it pulls the equilibrium of Step 1 to the left (you do remember Le Châtelier's Principle?) and the yellow colour of the complex fades to be replaced by the pale yellow colour of the adduct.

Ava Andrews · Answer

undefined The initial yellow color fades in the Diels-Alder reaction of maleic anhydride and anthracene due to the formation of a colorless adduct. The newly formed product lacks the conjugated system present in anthracene, leading to a change in absorption properties and the disappearance of the yellow color.