Lewis AcidsThe worst acids

What makes something an acid?

Lewis: An acid is an electron pair acceptor

What on Earth Does that Mean?While technically the Lewis definition applies to all acids, it’s only used for some weird cases.

For example, stick some AlCl3 in water, and measure the pH, and it comes out acidic.

How that HappensPut some Al3+ in water, and you have this:

Al3+ H—O—H

Now, 3+ is an okay charge for aluminum to have, and it’s pretty okay with that, but it wouldn’t really mind having some extra electrons around.

And hey look—oxygen has some.

How that HappensSo if they share those electrons:

Al2+

H—O+—H

The aluminum is now a bit happier, but now oxygen is none too pleased.

It could take its electrons back from aluminum, but it also has another option: take its electrons back from hydrogen, and in the process lose an H+…

How that HappensNow we have

Al2+

H—O H+

We just made H+

That makes my thing an acid, even though it doesn’t have a hydrogen of its own.

What makes a good Lewis acid?1. Unfilled octets. Boron compounds are frequently quite

good Lewis acids. They seriously want some electrons.

1. High charge. The bigger the charge, the more the element would like some electrons

When do you use this?1. When you can’t explain acidity any other way.

1. Interactions that happen outside of water, where H+ is never involved.

1. Basically…almost never for general chemistry.

What about the base?1. In the example before, water was the Lewis base—the

thing that gave the electrons.

1. A Lewis base must have available electrons—essentially anything with a lone pair (or even better, a negative charge) will do.

1. There are some features that can make those electrons more available. We may get to them later.