[thescepticalchymist]

Hi, I recently did a lab where I carried out a Diels-Alder reaction with anthracene-9-methanol and N-methylmaleimide, using water as the solvent.  Here is the procedure: https://imgur.com/gallery/VimET.  We didn't cover this reaction in lecture yet, so I'm not sure how high a yield one should expect if they did the reaction correctly. 

My guess is that it should be pretty high, since this reaction is green and therefore has high atom economy.  My % yield, however, was low -- only 35.56%.  The main reason I can think of for the low yield is that the mixture may have been heated at too high a temperature (which would cause some starting material to escape from the reaction flask).  And, of course, some product tends to be lost during vacuum filtration. 

I thought of the possibility that the reflux period was not long enough and that the rxn didn't run to completion, but if that were the case, then my melting point would be far from the literature value (which it isn't -- only a few degrees higher than the literature mp). 

Are there any other reasons one could get a high purity but a low yield from a Diels-Alder reaction?

[wildfyr]

I think the list you came up with is pretty darn complete. The only edge case would be that high heat you mentioned cause degradation of the starting materials or product into something that is still removed during the workup. Any professor would gladly accept the answers you gave as reasonable conclusions for yield loss. Good job

[crawlingmcedge]

MP for Anthracene is 160, and maleimide is ~ 100. Not as if it's exactly boiling off unless you've started heating your flask to 200 or something before adding in solvent. Don't see how you're losing product when you have a reflux condenser fitted unless you've really screwed something up.

Seems more of a case of not giving enough time for crystallization or poor handling on your end. 25 ml of water is ludicrous for a 66 mg reaction.

[Enthalpy]

Is Diels-Alder efficient with aromatics?

[Babcock_Hall]

It has been some time since I looked into this, but my recollection is that the calculation of atom economy does not take percent yield into consideration, although there are other metrics that do.  This point may be tangential to the main question at hand.

[wildfyr]

Enthalpy,
Certain electron rich ones certain do diels alder. I would say anthracene derivs are common Diels-Alder substrates.

Babcock,
I agree, my understanding of atom economy is more about byproducts than the raw yield.

[thescepticalchymist]

Is this mechanism accurate? 

https://imgur.com/gallery/muU8V

Again, we didn’t actually go over it in lecture yet so I’m not entirely sure.  Someone said it was incorrect, but it’s possible that they just didn’t understand it.