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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R3.4 - Electron-pair sharing reactions

3.4.1 Nucleophilic 3.4.2 Nucleophilic Substitution Reaction 3.4.3 Electrolytic Fission and Ionic Formation 3.4.4 Electrophilic 3.4.5 Electrophilic Addition to Alkenes 3.4.6 Lewis Acids and Bases (AHL) 3.4.7 Lewis Acid-Base Reaction and Co-ordinate Bonds (AHL) 3.4.8 Complex Ions and Ligand Co-coordination (AHL) 3.4.9 SN1 and SN2 Reaction (AHL) 3.4.10 Leaving Group and Substitution (AHL) 3.4.11 Electrophilic Addition of Alkenes (AHL) 3.4.12 Major Product of Addition Reaction (AHL) 3.4.13 Electrophilic Substitution of Benzene (AHL)

Lewis Acids and Bases HL Only

Specification Reference R3.4.6

Quick Notes:

  • A Lewis acid is an electron-pair acceptor.
  • A Lewis base is an electron-pair donor.
  • This theory applies broadly to both organic and inorganic reactions.
  • All nucleophiles are Lewis bases.
  • All electrophiles are Lewis acids.
  • Common Lewis acids: H⁺, BF₃, AlCl₃, transition metal ions.
  • Common Lewis bases: OH⁻, NH₃, H₂O, CN⁻, Cl⁻.

Full Notes:

Definition of Terms

Lewis acid: A species that accepts an electron pair.

Lewis base: A species that donates an electron pair.

This theory is more general than the Brønsted–Lowry definition, which is limited to proton transfer (see R3.1.1) .

Lewis Acid–Base Reaction

A Lewis acid–base reaction follows the general reaction:

Lewis base + Lewis acid → Lewis acid–base complex

Example Lewis Acid–Base Complex

BF₃ (acid) + NH₃ (base) → F₃B←NH₃ (complex)

IB Chemistry diagram showing BF₃ as a Lewis acid accepting a lone pair from NH₃, the Lewis base, to form a coordinate bond.

Here, NH₃ donates its lone pair to the empty p orbital on BF₃.

The NH₃ acts as the Lewis base (electron pair donor) and the BF₃ acts as the Lewis acid (electron pair acceptor).

The arrow (←) shows the donation of an electron pair.

Reactivity 3.1 — Linked Course Question

Reactivity 3.1 — Linked Course Question

What is the relationship between Brønsted–Lowry acids and bases and Lewis acids and bases?

Both the Brønsted–Lowry and Lewis definitions describe acids and bases, but from different perspectives:

  • Brønsted–Lowry:
    • Acid = proton (H⁺) donor
    • Base = proton (H⁺) acceptor
    • Focus: proton transfer
  • Lewis:
    • Acid = electron pair acceptor
    • Base = electron pair donor
    • Focus: electron pair interactions

Relationship Between the Two:

  • All Brønsted–Lowry acids and bases fit the Lewis model. For example, when NH₃ accepts a proton, it uses its lone pair—so it is both a Brønsted–Lowry base and a Lewis base.
  • Not all Lewis acids/bases are Brønsted–Lowry. Some Lewis acids (like BF₃ or AlCl₃) don’t involve proton transfer—they just accept electron pairs. These have no Brønsted–Lowry counterpart.

Summary