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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2$\begingroup$ It can, but most commonly considered Lewis acids do not interact well with HCl. Still, PtCl4 dissolves in HCl(aq) forming H2[PtCl6]. $\endgroup$– permeakraCommented Oct 9, 2020 at 12:44
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2$\begingroup$ 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. $\endgroup$– MauriceCommented Oct 9, 2020 at 13:00
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2$\begingroup$ @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." $\endgroup$– PoutnikCommented Oct 9, 2020 at 13:14
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1$\begingroup$ @Maurice They are very weak bases, but all of them can coordinate to form complexes, or even be protonated with carborane superacid. $\endgroup$– MithoronCommented Oct 9, 2020 at 13:38
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1 Answer
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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.
answered Oct 9, 2020 at 14:34