Lewis Acid-Base Definitions
- Lewis Acid: An electron-pair acceptor.
- Lewis Base: An electron-pair donor.
- Forms a coordination (dative covalent) bond where both bonding electrons originate from the Lewis base.
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.
Brønsted-Lowry vs Lewis Theory
Brønsted-Lowry Theory (R3.1)
Acid: Proton (\(\text{H}^+\)) donor
Base: Proton (\(\text{H}^+\)) acceptor
Lewis Theory (Broader)
Acid: Electron-pair acceptor
Base: Electron-pair donor
Unifying Organic and Inorganic Acid-Base Theory
Fundamental IB Synthesis:
- Nucleophiles = Lewis bases (they donate electron pairs to form bonds).
- Electrophiles = Lewis acids (they accept electron pairs to form bonds).
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).
Lewis Acids without Protons (BF₃)
\(\text{BF}_3\) is a Lewis acid because boron has an incomplete octet (6 valence electrons) with an empty \(2p\) orbital that can accept an electron pair. However, it cannot donate a proton (\(\text{H}^+\)), so it is not a Brønsted-Lowry acid.
Hierarchy of Acid-Base Theories
- All Brønsted-Lowry bases are Lewis bases (they use a lone pair to bind \(\text{H}^+\)).
- Not all Lewis acids are Brønsted-Lowry acids (e.g. \(\text{AlCl}_3, \text{FeBr}_3, \text{BF}_3\)).
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