Brønsted acids/bases — Acids & Bases

Acid–base chemistry is the most common reaction in this course, and not by a small margin. Protons move on and off oxygen and nitrogen in nearly every mechanism you will draw — activating an electrophile, generating a nucleophile, cleaning up a charge at the end of a sequence. It is also the cleanest place to learn to reason quantitatively about stability, because one number, pKa, captures the whole thing.

The Brønsted–Lowry definition

A Brønsted–Lowry acid is a proton (H⁺) donor. A Brønsted–Lowry base is a proton acceptor. Every acid–base reaction is a proton transfer from the acid to the base, and nothing more mysterious than that.

This definition replaces the older Arrhenius one — acid produces H⁺ in water, base produces OH⁻ in water — which is too narrow to be useful here, because it only works in aqueous solution. Brønsted–Lowry works in any solvent or none, which matters a great deal given that most organic reactions are run in THF, ether, DMSO or dichloromethane rather than water.

Note what a proton actually is: a bare hydrogen nucleus with no electrons. It cannot exist free in solution; it is always attached to something. So "donating a proton" always means handing it to a base's lone pair, and the two halves of an acid–base reaction are inseparable. There is no acid without a base to take the proton.

Worked example — HCl + NH₃

Ammonia's nitrogen has a lone pair; HCl has a polarized H–Cl bond with the hydrogen δ+.

Two arrows, simultaneously: tail on nitrogen's lone pair, head on the hydrogen, forming a new N–H bond; and tail on the H–Cl bond, head onto chlorine, which leaves as chloride.

Products: NH₄⁺ and Cl⁻. HCl is the acid, NH₃ the base. Check the charges: 0 + 0 on the left, +1 − 1 on the right. ✓

A proton transfer, drawn as what it actually is: a two-arrow nucleophilic substitution at hydrogen. Ammonia's lone pair is the nucleophile, the δ+ hydrogen is the electrophile, and chloride is the leaving group. Nothing here is special to acid–base chemistry — the mechanisms in the modules ahead are longer, but they are not made of anything else.The base’s lone pair is the nucleophile. The acidic proton is the electrophile. The conjugate base is the leaving group. Same three roles as every mechanism ahead.

The second arrow is not optional. Without it, the hydrogen would end up with two bonds.

Brønsted acid–base chemistry is a special case of the nucleophile/electrophile framework from Module 2. The base's lone pair is the nucleophile; the acidic proton is the electrophile; the conjugate base is the leaving group. The arrows are the same arrows, and the rules for drawing them are unchanged. Proton transfer is the simplest case of the general pattern, and it is worth practising for that reason alone: the mechanisms later in this book are longer, but they are not made of anything else.

Which side does the equilibrium favor?

Every proton transfer is an equilibrium, drawn with ⇌ even when it appears to go to completion. The reaction favors the side with the weaker acid and weaker base — which is to say, the more stable, lower-energy side.

A useful way to hold this: the proton ends up where it is held most tightly. If the base on the left holds protons more strongly than the base on the right, the proton moves left to right and stays there. The next section quantifies this exactly, and the rule becomes a one-line pKa comparison.

A rough guide worth internalizing now: a difference of about 4 pKa units means the equilibrium lies roughly 10⁴ to one — effectively complete for practical purposes. Chemists say a reaction "goes" when the pKa gap is comfortably positive in the right direction, and organic synthesis is full of steps designed to make that gap large.

Proton transfers are fast

One practical fact deserves stating early. Proton transfers between electronegative atoms — oxygen to oxygen, oxygen to nitrogen — are among the fastest reactions in chemistry, often diffusion-limited. They are effectively never the slow step in a mechanism.

This has a consequence you will lean on constantly: in a multi-step mechanism you can move protons around freely between heteroatoms without worrying about the cost, and a mechanism is not wrong for including a proton-shuffling step. What you cannot do for free is remove a proton from carbon, which is slow and which is why enolate chemistry in Module 11 needs its own treatment.

Not every hydrogen is acidic. Hydrogens on carbon are, with rare exceptions, not removable by anything you will meet in the first half of the course — an alkane C–H has a pKa around 50, which is beyond any practical base. When asked to find the acidic proton in a molecule, look on oxygen and nitrogen first, then at carbons adjacent to a carbonyl or in a terminal alkyne, and only then consider anything else.

Recognizing the acid and the base

In an organic mechanism, the acid is often not a bottle of HCl. It might be the solvent, a protonated intermediate generated two steps earlier, or the substrate itself. Likewise the base might be a carboxylate, an amine, a halide, or just water.

The practical test is structural: an acid needs a hydrogen attached to something that can survive on its own after the hydrogen leaves. A base needs an available lone pair or negative charge. Many species satisfy both tests — water, alcohols, and amines can each act as either — and which role they play depends entirely on what else is in the flask. Species like this are amphoteric, and the next section but one takes that idea up properly.

What carries forward

The Brønsted framework is the basis of the pKa scale, which is the most quantitative tool in the course and which sets leaving-group ability, nucleophile strength, and whether a given base can deprotonate a given substrate. The Lewis definition in the next section generalizes it to reactions with no proton at all. And in Modules 9 through 11, acid and base catalysis is what makes half the reactions in the module run at a useful rate.