Where Brønsted–Lowry tracks a proton, Lewis theory tracks the electron pair — the deeper description that also names the nucleophile vs. electrophile.
The whole topic in one line: acid + base → adduct. All structures rendered live.
1. A Lewis Acid Accepts an Electron Pair; a Lewis Base Donates One
An acid is an electron-pair acceptor and a base is an electron-pair donor — complementary roles, like plug and socket, always defined relative to a partner.
2. A Lewis Acid Has an Empty Orbital or a δ+ Atom
An acceptor needs a low-energy place for electrons — spot it by one of three features:
- Incomplete octets — BF3, AlCl3 (six valence electrons, empty p orbital).
- Positive charge — H+, carbocations, metal cations (Mg2+, Zn2+, Fe3+).
- δ+ atoms in polar bonds — a carbonyl carbon or alkyl-halide carbon.
3. A Lewis Base Has a Lone Pair or a π Bond
Two signatures mark a donor — a lone pair or the loosely held electrons of a π bond:
- Lone-pair donors: NH3 and amines, H2O, ethers, OH−, halides (Cl−, Br−).
- π donors: an alkene C=C or an aromatic ring — a good base with no lone pair and no proton, the tell that Lewis reaches past Brønsted.
4. The Base Donates Into the Acid to Form a Dative (Lewis Adduct) Bond
The curved arrow runs base → acid: both electrons come from the base, giving a dative bond and a Lewis adduct — in NH3 + BF3, N attacks B (N becomes +1, B becomes −1) with no proton moving.
5. The Lewis Definition Subsumes Brønsted and Equals Nucleophile/Electrophile
The theories nest — a proton is just one Lewis acid, so every Brønsted reaction is Lewis but not the reverse — and in mechanisms a Lewis base is a nucleophile, a Lewis acid an electrophile, the same donor → acceptor flow.
6. Lewis Acid Catalysts Work by Activating Electrophiles
In EAS, AlCl3 or FeBr3 coordinates to a halogen lone pair, weakening the bond to generate a stronger electrophile (Br+ or R+) that the ring's π system attacks — the acid is not consumed.
7. Summary
Acid = electron-pair acceptor (empty orbital / δ+) · base = electron-pair donor (lone pair / π bond) · arrow base → acid forms a dative Lewis adduct · subsumes Brønsted (a proton is one Lewis acid) · base = nucleophile, acid = electrophile · Lewis acids catalyze by activating electrophiles.
Quiz yourself
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BF3 is the Lewis acid — boron has only six valence electrons and an empty p orbital, so it accepts the electron pair. NH3 is the Lewis base — nitrogen has a lone pair it donates. The curved arrow runs from N to B, forming a dative N–B bond.
A Brønsted acid donates H+, and a proton is simply the smallest possible electron-pair acceptor (empty 1s orbital) — so accepting a base's lone pair makes it a Lewis acid too. But species like BF3, AlCl3, and carbocations accept electron pairs without having any proton to donate, so they are Lewis acids yet not Brønsted acids.
Its π bond is a region of accessible electron density. Toward a strong electrophile the C=C π electrons are donated to form a new bond, so the alkene behaves as a Lewis base (nucleophile) even with no lone pair — something the proton-based definitions cannot describe.
FeBr3 is a Lewis acid. It coordinates to a lone pair on Br2, weakening the Br–Br bond and generating a much stronger electrophile (effectively Br+). The benzene π system — a Lewis base — then attacks that activated electrophile. FeBr3 is regenerated, so it acts catalytically.
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.