Key Definitions
- Lewis acid: An electron-pair acceptor. It has a vacant orbital, an incomplete octet, or a positive/partial positive charge.
- Lewis base: An electron-pair donor. It has a lone pair of electrons or a π bond available for donation.
Lewis Theory: A Broader Model
The Lewis definition of acids and bases is broader than the Bronsted-Lowry definition. While all Bronsted-Lowry bases are Lewis bases, and all Bronsted-Lowry acids are Lewis acids, the reverse is not always true.
Bronsted-Lowry
Acid: proton (H⁺) donor
Base: proton (H⁺) acceptor
Lewis (broader)
Acid: electron-pair acceptor
Base: electron-pair donor
The Critical Link
Nucleophiles are Lewis bases (they donate electron pairs). Electrophiles are Lewis acids (they accept electron pairs). This connects organic reaction mechanisms directly to Lewis acid-base theory.
Identifying Lewis Acids
A Lewis acid can accept an electron pair because it has:
| Lewis Acid | Why it accepts electron pairs | Type |
|---|---|---|
| BF₃ | B has only 6 electrons (incomplete octet) | Neutral (not a Bronsted acid) |
| AlCl₃ | Al has only 6 electrons (incomplete octet) | Neutral |
| H⁺ | No electrons at all, empty 1s orbital | Also a Bronsted acid |
| Fe³⁺ | Empty d orbitals, positive charge | Transition metal cation |
| Carbocations (R₃C⁺) | Positive charge, incomplete octet on C | Organic electrophile |
Identifying Lewis Bases
| Lewis Base | Electron pair donated from | Type |
|---|---|---|
| NH₃ | Lone pair on N | Neutral nucleophile |
| H₂O | Lone pair on O | Neutral nucleophile |
| OH⁻ | Lone pair on O | Charged nucleophile |
| Cl⁻ | Lone pair on Cl | Charged nucleophile |
| Alkene (C=C) | π bond electrons | Organic nucleophile |
Applying Lewis Theory
Inorganic Example: BF₃ + NH₃
BF₃ (Lewis acid, incomplete octet) reacts with NH₃ (Lewis base, has a lone pair).
BF₃ + :NH₃ → F₃B←NH₃
NH₃ donates its lone pair to B, filling its octet. A coordination bond (coordination bond) forms. The curly arrow goes from the lone pair on N to the empty orbital on B.
Organic Example: Carbocation + Water
(CH₃)₃C⁺ (Lewis acid, carbocation) reacts with H₂O (Lewis base, has lone pairs).
(CH₃)₃C⁺ + :OH₂ → (CH₃)₃C-OH₂⁺
Water donates a lone pair to the carbocation. This is a Lewis acid-base reaction AND a step in the SN1 mechanism (R3.4.9).
Think About It
BF₃ is a Lewis acid but not a Bronsted-Lowry acid. Why?
A Bronsted-Lowry acid must donate a proton (H⁺), but BF₃ contains no hydrogen atoms. It cannot donate H⁺. However, it can accept an electron pair because boron has an incomplete octet (only 6 electrons). This makes it a Lewis acid only.
Common Exam Mistakes
- Confusing Lewis acids with Bronsted-Lowry acids. A Lewis acid accepts electron pairs, not protons.
- Forgetting that nucleophiles = Lewis bases and electrophiles = Lewis acids.
- Drawing the curly arrow the wrong way. It must go from the Lewis base (electron-pair donor) to the Lewis acid (acceptor).
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