There are certain concepts that chemists describe as being hybrids of multiple definitions or theories. Bonding, one of the simplest and most fundamental parts of chemistry, is described by either valence shell electron pair repulsion (VSEPR) or molecular orbital (MO) theory (there are other theories, too), depending on what you’re interested in talking about. (Oh, you think that carbon’s sp3 hybridized? Here, have some resonance.) Many of the students in my Gen Chem I class would tell you that they vividly remember our professor turning to us, chalk dust on his sleeves and a wry smirk on his face, and saying, “Everything I’m about to tell you is a lie.” This was the day that we learned what B.S., M.S. and Ph.D. actually stood for. (You know, the whole thing about Ph.D. meaning “piled higher and deeper?”)
Acid/base chemistry is another of those things that professors like to introduce in parts. They show you the simplest aspects of it first, then slowly reveal the underlying sophistication of the whole apparatus and smile as you squirm uncomfortably in your seat. General chemistry was bad enough about this, introducing an element of math that made the very thought of acid and base constants nauseating, but organic chemistry is even more terrible, insisting not only that you relearn the set of definitions of acids and bases that you ignored in Gen Chem, but that you employ it with ease and draw pretty pictures in the meantime.
Thankfully, although acid-base chemistry is more complicated than it was presented to be in high school, it isn’t really all that complicated. After you get past the mess of curved arrows, formal charges and boron trifluorides, it’s really a matter of simply getting the names of your Danish (and English and American) scientists in order.
Don’t believe me? Here, I’ll show you what I mean.
Arrhenius Acids and Bases
The Arrhenius definitions of acids and bases are, by far, the easiest to understand. Perhaps you saw these early in a chemistry course, or maybe you’ve just noticed a trend by looking at typical acid-base reactions. According to these definitions, an acid is something that dissolves in water to produce H+ ions, and a base is something that dissolves in water to produce OH– ions.
This is pretty easy to see if you look at the dissociation of HCl and NaOH, respectively, in water:
HCl (aq) → H+ (aq) + Cl– (aq)
NaOH (aq) → Na+ (aq) + OH– (aq)
It doesn’t take acrobatics of the imagination (the imagination can’t be strained, you say? Hah, try visualizing a chiral molecule after two Redbulls) to see how HCl and NaOH could react to form salt (NaCl) and water (H2O). The sodium and the chlorine get together, and the proton and the hydroxide get together, right?
Turns out, erm, not exactly.
Remember hydronium, that weird little polyatomic ion that was formed in the auto-ionization of water? Well, turns out, when you dissolve an acid in water, you get that little guy.
HCl (aq) + H2O (l) → H3O+ (aq) + Cl– (aq)
That’ll also react with other water molecules to make H5O2+, which is, frankly, a really weird-looking thing…
So, already, our definitions of acids and bases are off. I know what you’re thinking: “Okay, you promised something about Danish chemists… does that have something to do with this?”
Why yes, astute reader! I promised, and I shall not disappoint! Onward we march to our next set of definitions:
Brønsted-Lowry Acids and Bases
This definition is the definition you probably used (or will use) a lot in Gen Chem, especially if you had (or have) a professor who enjoys equilibria a little too much. The Brønsted-Lowry (bet you can’t guess who the Danish chemist is) definitions of acids and bases work pretty well for the part of chemistry where you’re learning about acid strength, Le Chatelier’s Principle and the common-ion effect. Under this set of definitions, an acid is a proton-donor, and a base is a proton-acceptor. This makes a little more sense of some common acid-base reactions that don’t involve hydroxide at all, like those that involve ammonia:
CH3COOH (aq) + NH3 (aq) ⇌ CH3COO– (aq) + NH4+ (aq)
There is no hydroxide to be found, and yet ammonia is a very well-known weak base. These definitions keep us from crying ourselves to sleep at night over inconsistencies in theory (… what? You don’t do that? … I know people who do.). The Brønsted-Lowry definitions of acids and bases also included stuff about conjugate acids and bases, but, as my professor would say, “That’s a whole ‘nother bag of marbles.”
Yet, organic chemists aren’t pleased with this. Why? Well, the reason they give is that there are acid-base reactions that don’t involve the exchange of a proton. I think it’s just because they have a weird obsession with electrons. Either way, we’re brought to our third and final set of definitions, which was written by our favorite American physical chemist.
Lewis Acids and Bases
The definitions of acids and bases written by Lewis actually make a lot more sense than most of us Organic students would like to admit. In a lot of cases, these definitions can be used hand-in-hand with the Brønsted-Lowry definitions, but, just as the group of Brønsted-Lowry acids and bases was broader than the group of Arrhenius bases, so the Lewis acids and bases include but are not limited to Brønsted-Lowry acids and bases. Lewis’s definition is the most general, and it’s a favorite of electron-loving organic chemists for an obvious reason. Here, acids are electron-pair acceptors, and bases are electron-pair donors.
“In what situation could you possibly need those?” you may ask? Well, innocent student, in a lot of reactions that are downright ugly.
One of these is one that my textbook gives, the reaction of bromine trifluoride (BF3) with diethyl ether (C4H10O).
I apologize in advanced for the general shoddiness of this image; my chemical sketcher doesn’t, apparently, put lone pairs on atoms or draw curved arrows (or basically do anything that it should)…
If you look at the above reaction, you’ll notice something: there isn’t an exchange of a proton. Rather, the oxygen in dimethyl ether donates an electron pair (I know you can’t see it here, but it happens) to the electronic-deficient boron in boron trifluoride. That makes diethyl ether the Lewis base in this reaction, while boron trifluoride is the Lewis acid.
This transfer of electrons leads to two new names for acids and bases: an acid is an electrophile (wanting electrons), and a base is a nucleophile (wanting a more positive charge).
Make sense? I hope so, because we aren’t going to dwell here for long. After we talk a bit about organic acid trends, we’re going to move on to something quite a bit different: alkene chemistry.
Questions? Comments? Corrections? Put ’em below. Just go easy on me, okay?