what is the reason for the use of a solvent?
To be able to speed up the reaction by heating it and to continuously distill off the water formed in the reaction.
Heating the reaction mixture without a solvent present would likely overheat the product.
when toluene is used, must this distilled off before crystallization or is this not necessary?
If the reaction mixture is cooled in the fridge before filtering, most of the product will precipitate even in the presence of the solvent.
(At least as long as you don't use larger amounts than suggested in the literature methods (https://www.thevespiary.org/rhodium/Rhodium/chemistry/phenyl-2-nitropropene.html)
(https://www.thevespiary.org/rhodium/Rhodium/chemistry/phenyl-2-nitropropene.html))