Trends in Lewis Acid Strength

In my previous chemistry post on various ways of looking at acid-base chemistry, I talked a little bit about Lewis acids and bases. Of course, if you remember your Gen Chem (or read my post on acid-base strength), you know that acids and bases come in a colorful variety of strengths. Before we completely turn our discussion away from acids and onto more interesting things, there are a few important things to note about trends in acidity.

Remember all that (it’s a WaniKani joke) about pKa and equilibrium? Well, not that it’s not important, but we’re leaving it behind. That’s right, folks, this is chemistry without numbers, the best kind of chemistry that exists, probably.

Are you as excited by that as I am? Great! Let’s get started.

The acidity of a compound is directly related to how stable its conjugate base is. For an acid, HA, the more stable A is, the stronger HA is (and the weaker A is). This is because nature doesn’t like forming less stable things from more stable things, so once your acid makes its conjugate base, your base isn’t going to want to go back to being the acid. This tips equilibrium in favor of the weak base, therefore making the acid stronger. tl;dr: if A is happy, HA is strong.

What contributes to the conjugate base (A) being stable? Well, a number of things. Five things, to be exact.

Negative Charge on an Electronegative Atom

It’s no surprise that electronegative atoms like negative charges. We’ve been taught since Gen Chem that fluorine will practically get in fistfights with other atoms for electrons, often blowing things up or setting things on fire in the process, and oxygen is still selfish enough to take more than its fair share of electrons from bonds like the sibling who’s supposed to be sharing the iPad with you. Therefore, it’s not difficult to imagine that electronegative atoms with negative charges on them are happier (read: more stable) than less electronegative atoms. Because electronegative atoms handle charge better, an acid that forms a conjugate base with a charge on a more electronegative atom will be stronger than an acid that forms one with a charge on a less electronegative atom.

Note: this rule only works within rows of the periodic table. When you start comparing acids from different rows, another thing comes into play…

Size of the Charged Atom

When it comes to stability, spreading charge out is key. The less charged an atom feels, the happier it will be. This is where we get this rule; charges on bigger atoms will be more stable than charges on smaller atoms. Think about it; when you’re a little bitty fluorine atom, you have nowhere to put that negative charge, but if you’re a gigantic iodine, it’ll be far away from the nucleus and spread out all over. Therefore, acids that form conjugate bases with charges on larger atoms will be more acidic. For example, out of the binary acids HF, HCl, HBr and HI, which do you think will be the most acidic? That’s right, hydriotic acid—which, if you were paying attention in Gen Chem, was exactly what your professor said.

This is a trend that carries cross rows of the periodic table, in case you didn’t catch that; this is the one you should refer to if your charged atoms come from different periods.

The next rule plays off of the same principle as this one, although it’s a lot more of a pain in organic chemistry students’ backsides…

Delocalization of Charge (Resonance)

One of the first things you learn about in organic chemistry is resonance. I meant to write a post about resonance eventually, but for now, just suffice it to say that resonance describes a phenomenon where electrons are delocalized, or where they aren’t strictly shared between only two atoms. Delocalized electrons move around through unhybridized p orbitals, distributing charge across parts or wholes of molecules instead of keeping it on a single atom.

As we saw in the rule about charged atom size, molecules are much more stable if they can move their charges around (and feel less charged). Therefore, one acid will be stronger than another if it forms a conjugate base with resonance.

A good example of resonance contributing to acid strength can be found in acetic acid, CH3COOH.  If you’re having unpleasant flashbacks, you’re probably getting ready to stop me: “Wait a second, but acetic acid is a weak acid!” Yessir, you’re absolutely right, but it’s still quite more acidic than ethanol (CH3CH2OH). The reason behind this is simple: when acetic acid undergoes an acid-base reaction, its conjugate base, the acetate anion, has resonance. Ethanol, on the other hand, just ends up making the ethoxide anion, CH3CH2O, with a negative charge stuck on its oxygen.

If you need a visual illustration of what happens in the acetate ion, here it is; keep in mind that the true ion is a hybrid of both of these structure (and thus, the charge on each oxygen is really ~1/2).

800px-Acetate-resonance-2D-skeletal

You follow? All right, cool. Three down, one to go!

The Inductive Effect

When this concept was first introduced to us, our professor spent several minutes desperately trying to make it clear that this was not resonance. This is a point I will try to make clear to you, as well; the inductive effect is concerned with electrons in sigma bonds, not pi bonds. Okay?

So, what is the inductive effect, you ask? Well, allow me to draw you a mental picture. Imagine you have a molecule (I’ll use ethanol, because it’s what my book uses) that, when you pull a proton off, has a negative charge on a single electronegative atom (in this case, oxygen). That atom has a full negative charge, which it’s not particularly happy about. But what if you were to replace the hyrdogens on the other carbon atom with chlorine atoms? Well, as we know, chlorine is reasonably electronegative, so those chlorine atoms will pull stronger on the electrons in their bonds, pulling some of the charge in the molecule toward themselves (there’s a more in-depth explanation of this that I think I understand, but I can’t find a reliable source in my textbook or on the internet to back my ideas up, so I’ll leave it at what they think is relevant to know). This induction of electrons through sigma bonds results in charge that is spread out, again giving us ion stability. Therefore, an acid will be stronger if its conjugate base will have inductive effects at play within it.

Finally, we’re down to the last one. Don’t get too comfy, though, ’cause this can get a bit tricky…

Percent S-character of Hybridized Orbitals on Charged Atom

This is where my professor rounded on us and explained that atoms can act more electronegative than they really are. This promptly blew the minds of several students, and we spent the next half hour cleaning brain matter off of the floor.

Just kidding. Sapphire and I thought it was kind of cool, though.

If you’re familiar with the concept of hybridization, then the phrase “percent s-character” should be vaguely familiar to you. If you’re not familiar, go watch this excellent Crash Course on orbitals, then come on back.

Lovely, right? Gotta love the VlogBrothers. Right, back to acids.

So, the percent s-character of a hybrid orbital is, essentially, how much it resembles an s orbital. As the number of hybrid orbitals goes up in a molecule, percent s character goes down, and vice versa. You can also think of s-character as increasing with the presence of multiple bonds. If that’s hard for you to visualize, I’ll just give you the numbers:

sp3 hybridization—3 p orbitals + 1 s orbital—s-character = 25%

sp2 hybridization—2 p orbitals + 1 s orbital—s-character = 33%

sp hybridization—1 p orbital + 1 s orbital—s-character = 50%

Make sense? Okay, nice. But what does that have to do with acidity?

Well, it turns out that the more s-character the orbitals of an atom have, the closer its electrons are held to its nucleus. The closer its electrons are held to its nucleus, the lower in energy its electrons are. The lower in energy its electrons are, the less it minds having charge (or the more electronegative it’s going to act). Hm, ring any bells? That’s right, we’re going back to “Negative Charge on an Electronegative Atom.” If the acid produces a conjugate base that has a negative charge on an atom with orbitals that have high s-character, that conjugate base is happy, and it won’t be eager to form into the acid again. Thus, the more s-character the orbitals of the charged atom in the conjugate base have, the stronger the acid.

All right! Good job, team! We’ve made it through the drudgery that is acid-base chemistry! What new, exciting journeys shall we take now? Well, we’re going to wander off the beaten path of Gen Chem and to the magical land of alkene chemistry.


Questions? Comments? Corrections? Put them below. Want to flame or troll? Allow me to direct you to a few better outlets.

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