Are Bronsted Lowry acids Lewis acids?

[ratatat]

Are Bronsted Lowry acids Lewis acids? I've heard different opinions on that before. I am confident that not all Lewis acids are Bronsted Lowry though.

Thanks

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[boaz]

Strictly speaking, no. Let's say we have HF and OH-. The Bronsted acid is the HF. Lewis acid is defined as the "electron acceptor"; in this case (as in any Bronsted acid) it is the H of the HF. The HF molecule as a whole cannot be considered an electron acceptor as there is no room for more electrons; HF first dissociates and then the H accepts the electron. But the Bronsted base, OH-, is also the Lewis base. It donates the electron.

 

[bananapudding]

This thread is old but has a high Google search rank, and I'm wondering if the above post is correct. It is my understanding that the definition of Lewis Acid contains all Bronsted Acids.

Lewis acid is defined as the "electron acceptor"; in this case (as in any Bronsted acid) it is the H of the HF

I'm not sure about this. If you look at the HF dissociation from the perspective of Fluorine, it's donating a proton and accepting [half of] its pair of electrons simultaneously. I understand what boaz is getting at---due to the way it's taught we tend to think of Lewis Acids as getting their electrons from another molecule entirely, but it's technically accepting them from the H in this case.

So... all Bronsted Acids are Lewis Acids. No?

 

[Teleologist]

Strictly speaking, no.

Even loosely speaking you're wrong. All Bronsted Lowry acids are Lewis acids. The converse is not true. HF is a B/L acid. It is also a Lewis acid. Yes, HF can accept a pair of electrons. It just accepts them in an antibonding orbital. This leads to HF splitting and forming H+ and F-.

 

[Teleologist]

The HF molecule as a whole cannot be considered an electron acceptor as there is no room for more electrons; HF first dissociates and then the H accepts the electron. But the Bronsted base, OH-, is also the Lewis base. It donates the electron.

1) No, HF doesn't fall apart to make H and F. The bonds aren't homolytically broken like that.

2) "The electron"? What are we talking about here? Radical reactions?

3) Yes, HF has room for electrons - they just go into an antibonding orbital.

 

[Radioactive1112]

All bronsted acids are lowry acids but the reverse is not true.