When I place a chunk of 99,99% pure sodium metal in a beaker of (100% chemically pure) acetone, the sodium metal starts reacting and dissolving into the acetone and it forms a yellow/brown liquid. When the acetone evaporates it leaves a dark-brown goo that is like a gel. And it smells like tar or something similar. What is this substance?
I can suggest only two possibilities:
Your acetone or sodium are not pure. Acetone contains water, or something.
Sodium might react like magnesium does, though it required different conditions. That's possible, but maybe you just discovered a new reaction or had some weird impurities that served as a catalyst.