Lewis Acid & Base Reactions (HL) (DP IB Chemistry): Revision Note

Lewis acid & base reactions

Formation of coordinate bonds

• A coordinate bond forms when a Lewis base donates a lone pair of electrons to a Lewis acid.

• In the reaction between ammonia (NH₃) and boron trifluoride (BF₃):

  • BF₃ acts as the Lewis acid

  • NH₃ acts as the Lewis base

  • NH₃ donates a lone pair from nitrogen to BF₃

  • This forms a coordinate bond

• Only electron pairs are transferred

  • No protons are involved

• This means neither species acts as a Brønsted–Lowry acid or base in this reaction

• In this reaction, boron forms three sp² hybridised orbitals

  • This leaves a vacant 2p_z orbital

  • This vacant orbital can accept a lone pair of electrons from the nitrogen atom to form a coordinate bond

Boron forms three sp² hybridised orbitals

Electrophiles and nucleophiles

• An electrophile is an electron-deficient species that accepts a lone pair from another reactant to form a new covalent bond

  • Therefore, an electrophile is a Lewis acid

• A nucleophile is an electron-rich species that donates a lone pair to another reactant to form a new covalent bond

  • Therefore, a nucleophile is a Lewis base

Formation of complex ions

• Lewis acid–base behaviour is also seen in the formation of complex ions

• In a complex ion such as hexaaquacopper(II):

  • The metal ion, Cu²⁺, is the Lewis acid

  • The water molecules are Lewis bases

• Copper(II), like other transition metals, can form a complex due to a partially occupied d subshell

Cu²⁺ (aq) + 6H₂O (l) → [Cu(H₂O)₆]²⁺ (aq)

• The Cu²⁺ ion is a Lewis acid and electrophile because it accepts a lone pair of electrons from water

• The water molecules are Lewis bases and nucleophiles because they donate a lone pair of electrons to the Cu²⁺ ion

  • Other ligands such as the cyanide ion, ^–CN,  and ammonia , NH₃ , are examples of Lewis bases and can act as nucleophiles