I'm asked to determine if ethanol is a Lewis acid, Lewis base, or both.

Since $\ce{O}$ has two lone pairs of electrons I would think that it could act as a base, since they could easily be donated. If it was a Lewis acid, it should have an empty orbital, and I can't find such one. All of the carbons are bonded and oxygen's orbitals are full. Still my book tells me it can act as both.

Is the textbook wrong or could one say that the 1s orbital to $\ce{H}$ (bonded to $\ce{O}$) acts as "empty", since $\ce{O}$ is more electro-negative than $\ce{H}$?

  • $\begingroup$ Why is this being down voted, is it a stupid question? Should I delete it? $\endgroup$ Feb 28, 2018 at 19:17
  • $\begingroup$ You're not missing anything. Well, maybe that the textbooks often mistakenly treat Bronsted acid as Lewis ones (donate proton) while proton donors should be treated as adducts, where proton as acid exchanges its ligand for another. $\endgroup$
    – Mithoron
    Feb 28, 2018 at 19:22
  • $\begingroup$ I edited and undownvoted but you should remember that textbooks usually contain some errors. Also you should really change your username, it doesn't do you any good. $\endgroup$
    – Mithoron
    Feb 28, 2018 at 19:28
  • $\begingroup$ @Mithoron My username is a joke about someone trapped inside a website (as you can see him knocking on the screen). Also thank you for the response, but if ethanol cannot act as both, then it should be a base, which doesn't seem right? $\endgroup$ Feb 28, 2018 at 19:46
  • $\begingroup$ Of course it's a base, What's supposed to be not right here? $\endgroup$
    – Mithoron
    Feb 28, 2018 at 20:13

2 Answers 2


Lewis acidity and basicity require some amount of context because they can't function in those roles without at least a putative base or acid, respectively. Certainly the oxygen here has a lone pair so it could be basic, and the $\sigma^{*}$ orbital of the O-H bond (on the hydrogen side) can accept an electron pair. What matters more is the context of what acids and bases are present to interact with these moieties. There is no reason why these pieces can't separately fulfill their roles independent of each other.

  • $\begingroup$ But hasn't the $\sigma$-bond already got an electron pair? How can it accept another one? $\endgroup$ Feb 28, 2018 at 16:59
  • $\begingroup$ Indeed it can't. If it would, H- ion would need to leave. $\endgroup$
    – Mithoron
    Feb 28, 2018 at 19:30
  • $\begingroup$ The other way. It's a proton, ejecting a alkoxide... @Mithoron $\endgroup$
    – Zhe
    Feb 28, 2018 at 19:41
  • $\begingroup$ @cansomeonehelpmeout I'm talking about the antibonding orbital. The bonding orbital, as you've noticed, is already full. $\endgroup$
    – Zhe
    Feb 28, 2018 at 19:42
  • $\begingroup$ He knows this but if you put a lone pair of nucleophile in antibonding orbital it kicks out the one that's already bonding and it would be H- not H+ You'd be oxidating hydrogen here and it still wouldn't be pure acid + base reaction but exchange of ligands of R-O+ cation. $\endgroup$
    – Mithoron
    Feb 28, 2018 at 20:11

Ethanol donate protons to the strong bases forming alkoxide ion. Ethoxide ion is a strong conjugate base which shows that it is a weak acid.

On the other hand, ethanol donate electrons to hydrogen in a reaction with Hydrogen halide and hence it can be considered as strong lewis bases. This is evident from the fact that there are lone pairs on the oxygen atom.

  • $\begingroup$ Is there no way that I can know if it is an Lewis acid/base just by looking at the structure? $\endgroup$ Feb 28, 2018 at 17:22
  • $\begingroup$ As far as I know, electron deficient molecules do act as lewis acids and electron rich molecules act as lewis bases. So from the structure it is possible to identify. But I never mentioned it to be lewis acid, by that reaction I just told it is a weak acid. $\endgroup$
    – MollyCooL
    Mar 1, 2018 at 1:37

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