What determines whether a step in a reaction is reversible?

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?




There's no way of figuring that out "from scratch", it is a lot of experience and knowledge. There is no general answer to this but there are some steps which are basically always seen as reversible or irreversible\*.


**protonation/deprotonation:**
unless an extremly strong acid/base (like BuLi) is involved those are always in some kind of equilibrium and are reversible.


**exchanging a good leaving group with a bad one:**
That can be seen as irreversible. 


**gas evolution:**
as @LinearChristmas pointed out: that can be seen as irreversible (if you are working in solution)


**in general: producing a stable structure:**
for example aromatization, that's irreversible




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\*theoretically every reaction is reversible, but if the equilibrium is far on one side we call it irreversible.


For an explanation what makes a reaction reversible see: [Is every chemical reaction in equilibrium?](https://chemistry.stackexchange.com/questions/47262/is-every-chemical-reaction-in-equilibrium/47356#47356)


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