Question

How do you predict lewis acid / base reactions?

Answer 1

You identify the Lewis acid and the Lewis base and join them to form an adduct with a coordinate covalent bond.

LEWIS ACIDS

A Lewis acid will have a vacant orbital. Various species can act as Lewis acids.

1. All cations (e.g., Cu²⁺, Fe²⁺, Fe³⁺)
   Cu²⁺ + 4NH₃ → Cu(NH₃)₄²⁺

2. An atom, ion, or molecule with an incomplete octet of electrons (e.g., BF₃, AlF₃).
   BH₃ + NH₃ → H₃B⁻-N⁺H₃

3. Molecules in which the central atom can expand its octet (e.g., SiBr₄, SiF₄).
   SiF₄ + 2F⁻ → SiF₆²⁻

4. Molecules with double or triple bonds between two atoms of different electronegativities. Examples are CO₂ and SO₂.
   O=C=O + OH⁻ → HO-C(=O)O⁻

5. Brønsted-Lowry acids (e.g. HBr, HNO₃).
   H-Br + H₂O → H₃O⁺ + Br⁻

LEWIS BASES

A Lewis base will have lone-pair electrons. Various species can act as Lewis bases.

1. Anions (e.g. OH⁻, CN⁻, CH₃COO⁻)
   HO⁻ + H-CN → HO-H + CN⁻

2. Atoms with lone pairs (e.g. NH₃, H₂O:, CO
   O=C=O + OH⁻ → HO-C(=O)O⁻

3. Molecules with double or triple bonds (e.g, (CH₃)₂C=CH₂)
   (CH₃)₂C=CH₂ + H-Br → (CH₃)₃C⁺ + Br⁻