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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?

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    $\begingroup$ This strikes me as a good question. It is a simple concept, but pragmatically I don't think there is, in general, any really good way to just look at the reaction equation and tell. $\endgroup$ – MaxW Feb 12 '17 at 21:36
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    $\begingroup$ Agreed with @MaxW. There is no one-size-fits-all qualitative approach as far as I'm aware. Theoretically, every reaction will reach equilibrium, and is therefore reversible at some point. Whether this equilibrium has to be taken into account is what we mean by adding two-way arrows $$\ce{<=>}.$$The first estimates are whether there is a gas among products (and system is thermodynamically open), then it's "not" reversible. Same when a non-soluble compound or stable complex forms. More generally, think in terms of stability and rel. likelihoods (hence speed). Hunches gets us only so far though $\endgroup$ – Linear Christmas Feb 12 '17 at 21:52
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    $\begingroup$ see also here $\endgroup$ – Linear Christmas Feb 12 '17 at 21:54
  • $\begingroup$ I think the gist is that every reaction will have a forward rate and a backwards rate. If $\ce{K_f >> K_b}$ then we say the reaction is irreversible. $\endgroup$ – MaxW Feb 14 '17 at 16:18
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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


*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?

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