Question

why would yeast cells not produce ethanol if they had access to oxygen?

Answer 1

They would produce ethanol, but they wouldn't stop there! If they had access to air, they would keep going and turn the alcohol into vinegar, AKA acetic acid.

To see the difference, here are the structures:



Ok, so apparently with oxygen, yeast add oxygen to the alcohol (and remove 2 hydrogen atoms, hidden by the bond-line notation) but your question asks WHY.

The reason is, in doing this, yeast get energy, it's basically how they EAT to survive, doin this little reaction right here.