# Is HCl a lewis base?

Cl in HCl has three lone pairs of electrons. So it can donate a lone pair and thus become a lewis base. Right? Just like water is called a lewis base because oxygen has 2 lone pairs and can donate a lone pair.

• It can, but most commonly considered Lewis acids do not interact well with HCl. Still, PtCl4 dissolves in HCl(aq) forming H2[PtCl6]. – permeakra Oct 9 '20 at 12:44
• 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. – Maurice Oct 9 '20 at 13:00
• @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." – Poutnik Oct 9 '20 at 13:14
• @Maurice They are very weak bases, but all of them can coordinate to form complexes, or even be protonated with carborane superacid. – Mithoron Oct 9 '20 at 13:38

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.