So I presented the results of my independent project yesterday in organic lab, and all in all things went well. I did learn something interesting about eliminations involving nitrogen, however.
Everyone knows that hydroxide is a pretty bad leaving group. But protonate it to make cationic water, and that sucker’ll split like Louie Anderson after a pot pie. An adjacent nitrogen atom, on the other hand, actually makes hydroxide into a superb leaving group under elimination conditions! Why? Consider a solution of ammonia and water. Basic ammonia immediately rips a proton from water to form ammonium hydroxide, right? Believe it or not, E2 elimination of hydroxide next to a nitrogen atom is an analogous process. In fact, one can argue that nitrogen-adjacent E2 is even more favorable than ammonium formation, owing to the formation of a carbon-nitrogen pi bond. In both cases, nitrogen forms a bond to something (either hydrogen or carbon) and boots out hydroxide. Nitrogen’s just that good of a Lewis base!
This is relevant to my project because my first reaction is between some nitrogen functionality and glyoxal to form RNCHCHNR (ethylene diimine?). The nitrogens attack glyoxal’s carbonyl carbons, rendering its oxygens negatively charged. Glyoxal’s oxygens have to leave somehow, and I figured they were just protonated twice then eliminated. In fact, the second protonation is unnecessary! Glyoxal’s O’s pick up protons from somewhere, then elimination of hydroxide occurs by the mechanism above.