What determines whether a step in a reaction is reversible?

Question

What makes a step in a reaction not reversible? I'm learning all these mechanisms and having to memorize which steps are reversible. Is there a way to figure out whether it's reversible or not?

Answer 1

Mostly it has to do with acid/base chemistry. A stronger base reactant on the left forming a weaker base on the right is indicated with an irreversible arrow. For example most (but not all) Sn2 are irreversible because the leaving group is much less basic than the nucleophile.

I think if the answer is not derived from acid/base chem, then you will need to take it on a case-by-case basis. Many chemical reactions are known where every single step is an equilibrium (Fischer esterification, acetal, imine and enamine formation, for example).