I've been running acetylation reactions with acetic anhydride using pyridine as the base with with either DMAP or n-methylimidazole as catalysts (or acetic anhydride and TEA with no catalyst). The unsaturated alcohol standard I'm acetylating is in toluene. Upon heating (or with enough time), the solutions are turning dark yellow or red. This colored component sticks around as a large amount of an organic-soluble oily residue.
I know TEA can turn yellow due to the presence of "N-oxides". Is that what's happening here? Am I forming some sort of oxidized byproduct? I've also been having loss of my alcohol during this reaction, might I be somehow oxidizing my double bond? I've seen papers researching acetylation prep their reagents to remove olefins - is this why?
Hi Matthew, same thing is happening to me during my methacrylation reaction. When I mix TEA, DMAP and methacrylic anhydride in minutes, mixture turns into yellow even without adding them into my polymer solution. Still I have no idea why.
Combination of Ac2O/Et3N/DMAP makes an extremely active acetylating agent. It can, for example, acetylate the enol form of acetic anhydride itself, producing colored material (eventually making an oligomeric mixture).
This is the reason why during DAMP-catalyzed acylations, it is recommended to add DMAP last, when the substrate for acylation is already in the reaction mixture, and use only catalytic amount of DMAP.
Also, quite often DMAP is used without real need for it - which only results in generating unnecessary by-products.
Ah! I havent used that exact combination but I can imagine this happening to varying degrees given my conditions (hard to NOT add too much catalyst given my scale, and I need to heat to get around some steric hindrance ). Thanks!
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