In the Diels Alder reaction of maleic anhydride and anthracene, why does the initial yellow color of the reaction mixture fade during the reaction?
Here's my explanation.
The reaction takes place in two steps.
The maleic anhydride and anthracene initially react in a fast step to form a charge transfer complex.
The maleic anhydride and anthracene react to form the product.
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.
(From Organic Synthesis International)