Cl in HCl has three lone pairs of electrons, so it can donate a lone pair and thus becomes a Lewis base. Just like water is called a Lewis base because oxygen has two lone pairs and can donate a lone pair. Am I right?

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    $\begingroup$ It can, but most commonly considered Lewis acids do not interact well with HCl. Still, PtCl4 dissolves in HCl(aq) forming H2[PtCl6]. $\endgroup$
    – permeakra
    Oct 9, 2020 at 12:44
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    $\begingroup$ Please don't believe that all molecules having atoms with lone pairs of electrons are Lewis base. It is a necessary condition but it is not sufficient. $\ce{O2, CO2, N2}$ are not Lewis bases. $\endgroup$
    – Maurice
    Oct 9, 2020 at 13:00
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    $\begingroup$ @Maurice Hmm, a naked proton is quite a very strong Lewis acid, able to react with many very weak Lewis bases. "Dioxidanylium ($\ce{HO2+}$) is the conjugate acid of dioxygen. The proton affinity of dioxygen (O2) is 4.4 eV." $\endgroup$
    – Poutnik
    Oct 9, 2020 at 13:14
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    $\begingroup$ @Maurice They are very weak bases, but all of them can coordinate to form complexes, or even be protonated with carborane superacid. $\endgroup$
    – Mithoron
    Oct 9, 2020 at 13:38

1 Answer 1


The strength of Lewis acids relative to your Lewis base has been commented on. A fuller answer to your question needs to include the Lewis acid you are considering.

The original Bronsted-Lowry theory of comparing acidity and basicity, or acids and bases, involved $\ce{H^+}$ and $\ce{OH^-}$ and the transfer of a proton.

The broader Lewis theory allowed consideration of other cations besides the proton as acids, and other nucleophiles than hydroxide ion to be bases. So water can be considered a Lewis acid relative to chloride ion because $\ce{H2O}$ hydrates chloride ions by hydrogen bonding. And water is a Lewis base relative to metal cations because water hydrates the metal ions through electron donation from oxygen.

So the answer to the question is yes and no. Yes, $\ce{HCl}$ can be considered a Lewis base - relative to a very strong acid, as mentioned in the comments - but in the ordinary world, I would not first think of $\ce{HCl}$ as an alkali. Stretching the word "base" by adding "Lewis" in front of it needs more, like adding the reference acid, to make the comment clear rather than confusing.


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