Nearly every organic reaction is built from one move: a lone pair grabs a proton, or lets one go. Here acetic acid hands its proton to ammonia:
Acetic acid (the acid) donates H+ to ammonia (the base), producing acetate and ammonium. Structures drawn live.
1. A Brønsted–Lowry Acid Donates a Proton; a Base Accepts It
An acid is a proton (H+) donor and a base is a proton acceptor — and a base needs an available lone pair to bond the incoming H.
2. Every Acid–Base Reaction Is a Proton Transfer That Creates a Conjugate Pair
The acid loses H+ to become its conjugate base and the base gains it to become its conjugate acid — the two members of a pair differing by exactly one H+.
3. Curved Arrows Show Where the Electrons Move During Proton Transfer
Two curved arrows draw it: one from the base's lone pair to the acidic H, one from the H–X bond onto X — always following the electrons, never the proton itself.
HCl loses its proton to water: the H–Cl bonding electrons collapse onto chlorine, giving Cl− and H3O+.
4. The Stronger the Acid, the Weaker (More Stable) Its Conjugate Base
Whatever stabilizes an anion (resonance, electronegativity, size, induction) makes its parent acid strong — so phenol is acidic because phenoxide delocalizes into the ring, while ethanol holds its proton tightly and ethoxide is a strong base.
5. pKa Predicts the Direction: the Proton Ends Up on the Weaker Acid
Lower pKa means stronger acid; equilibrium always favors the side with the weaker acid (higher pKa), so the proton comes to rest on it — a gap of a few units drives the reaction to completion.
Acetic acid (pKa 4.8) + hydroxide → acetate + water (pKa 15.7). The weaker acid, water, wins — equilibrium runs right.
6. Brønsted Is a Special Case of the Broader Lewis Definition
A Lewis acid just accepts an electron pair, so every Brønsted base is a Lewis base, but a Lewis acid (BF3, AlCl3) need not supply a proton — proton transfer is the Brønsted-flavored special case.
7. Summary
Acid donates H+, base accepts with a lone pair · proton transfer drawn with two curved arrows · products are a conjugate pair differing by one H+ · strong acid ↔ weak, stable conjugate base · proton settles on the weaker acid (higher pKa) · Brønsted is the proton case of the broader Lewis definition.
Quiz yourself
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Acetic acid is the acid (it loses H+); ammonia is the base (it gains H+). On the right, acetate is the conjugate base of acetic acid, and ammonium is the conjugate acid of ammonia. The two conjugate pairs are acetic acid/acetate and ammonium/ammonia — each differs by exactly one proton.
Ethanol. It is the weaker acid (higher pKa), so by the inverse relationship its conjugate base, ethoxide, is the stronger base. Acetate is a weaker base because it is resonance-stabilized across two oxygens, which is exactly why acetic acid is the stronger acid.
Toward acetic acid + water. The proton moves from hydronium (conjugate acid of water, pKa ≈ −1.7, a strong acid) to acetate, forming acetic acid (pKa ≈ 4.8, the weaker acid). Equilibrium always runs toward the weaker acid / more stable base, so it lies far to that side.
A Brønsted base donates a lone pair to a proton, which is exactly what a Lewis base does (donate an electron pair) — so the two definitions coincide for bases. But a Lewis acid only has to accept an electron pair; it need not supply a proton. Electron-poor species like BF3 or AlCl3 are Lewis acids with no proton to donate, so they fall outside the narrower Brønsted definition.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.