Is it possible for a compound to act as a Lewis acid but NOT act as a Brønsted acid? When would this happen? Could you please give a few examples where/why this happens ?


Certainly. There are many compounds that can accept a pair of electrons from a Lewis base, yet don't have any Brønsted-Lowry acid (proton) moiety. The two are really rather different things, although there are of course many compounds that are both Lewis and Brønsted-Lowry acids ($\ce{Al(OH)3}$ for example).

Some examples of Lewis acids that are not also Brønsted-Lowry include:

Metal cations like $\ce{Li+}$, $\ce{Na+}$, $\ce{Mg^2+}$, etc.

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  • $\begingroup$ Water isn't Lewis acid - it would need to make an adduct where it would accept electron pair. Normal action as proton donor is an exchange of base in adduct. It seems it's sometimes considered as you are doing it but I think it shouldn't be. $\endgroup$ – Mithoron May 1 '17 at 19:26
  • $\begingroup$ Although I've always seen water to be considered as a Lewis acid and considered it as such, technically, I think you're probably right. Even though water is in equilibrium with the strong lewis acid $\ce{H+}$, that still isn't the same as $\ce{H2O}$. I figured my answer would be better without a controversial (or possibly wrong) example and changed it to a more certain example. Thx for the input. $\endgroup$ – airhuff May 1 '17 at 19:47
  • $\begingroup$ Yeah, if there's only one other guy connected with post it pings him. If someone isn't connected even @ doesn't work. $\endgroup$ – Mithoron May 1 '17 at 19:51
  • $\begingroup$ @ airhuff What about HCl? $\endgroup$ – devb May 2 '17 at 1:35
  • $\begingroup$ @Nightshade , HCl is a Brønsted–Lowry acid, as it readily donates it's proton. $\endgroup$ – airhuff May 2 '17 at 2:02

In addition to above nice answers, there is a trick that can be used to confuse students: A compound for example $\ce{Al2O3}$;

It can acts as a Lewis acid and Brønsted base but not as Brønsted acid

Consider these reactions:

Recall the Lewis Theory of acids and bases which states that an acid is an electron pair acceptor and a base is an electron pair donor.

Applying the theory to $\ce{Al2O3}$:

$$\ce{Al3+(aq) + 6 H2O(l) <=> Al(H2O)6^3+(aq)~~~~~~~(1)}$$

Thus, in (1) the $\ce{Al(H2O)6^3+}$ ion is formed when an $\ce{Al^3+}$ ion acting as a Lewis acid picks up six pairs of electrons from neighbouring water molecules acting as Lewis bases to give an acid-base complex, or complex ion

the Brønsted-Lowry theory is also observed:

$$\ce{Al2O3 + 6H+ -> 2Al^3+ + H2O~~~~~~~(2) a typical Brønsted-Lowry base }$$

The compound accepts the proton

This compound somewhat behaves as both a Lewis acid and Bonsted-Lowry base but not a Brønsted-Lowry acid.

Clearly, the compound cannot be considered to be Brønsted-Lowry acid since the compound in question has no protons to donate.

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There is also boric acid which is electron deficient but instead of donating a proton it accepts hydroxyl ions.

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