Question

I have a couple of pre lab questions regarding N-phenylmaleimide.

1) why it is necessary to prevent N-phenylmaleimide from overheating when recrystallizing???

2) what would be the precautions, in terms of reagents, to be taken in consideration for the preparation of N-phenylmaleimide???

3) what is the reaction solvent for preparing N-phenylmaleimide from maleanilic acid?? what is the recrystallization solvent of this??

*there is no data these are pre-lab worksheet 

Answer 1

Please note that I am assuming you are using above scheme for your synthesis. If it is different please mention it in the comment box and accordingly I will update my answer.

1) N-phenylmaleimide is an active dinophile and hence over heating would result into Diels-Alder adduct formation. Also the strained five membered amide ring may get hydrolysed to give intermediate Maleanilic acid. Also the melting point of the product is 88-89^oC.
Hence excess of heating is avoided while recrystalization.

2) i) In the first step of synthesis the solvent must anhydrous and condition while preparation should be maintained anhydrous. Small amount of moisture may hydrolyse the maleic anhydride and results into lower yield of intermediate Maleanilic acid.
ii) Aniline should be dried over molecular seves to have better yield.
iii) Solvent ethyl ether should be freshly distilled and anhydrous.
iv) Reaction mixture should not be allowed to rise above room temprature and after an hour it shuld be cooled below 15oC to get maximum crystalysed intermediate.
v) In the second step, anhydrous acetic anhydride and sodium acetate is used as dehydrating agent to give cyclization product as N-phenylmaleimide.
vi) the final product melting point is 88-89^oC hence reaction mixture or recrystalization temprature should be below that to avoide melted product formation.

3) For the first step diethyl ether is the solvent and for the second step aceticanhydride itself act as a solvent.
Cyclohexane is used as a recrystalization solvent.

Instead of acetic anhydride you may use P2O5, PCl5 may be used at the cyclization step.