Why is toluene used in Diels Alder reaction?
Toluene is used because it is a relatively high-boiling inert solvent.
Toluene is able to dissolve unsaturated hydrocarbons better than most polar solvents.
The normal boiling point of toluene is 111 °C, which is higher than the boiling point of most solvents used in undergraduate organic chemistry labs.
The high boiling point means that the Diels-Alder reaction will have a faster reaction rate.
In some cases, xylene (b.p. 140°C) is the solvent of choice because its boiling point is even higher than that of toluene.