Lewis Acid & Base Reactions (HL) (DP IB Chemistry)

Lewis Acid & Base Reactions

Formation of coordinate bonds

• When ammonia, NH₃, and boron trifluoride, BF₃, react a coordinate bond is formed 

• This occur as the lone pair on the nitrogen atom in NH₃ can be donated to the boron atom in BF₃, creating a molecule of NH₃BF₃

• NH₃ reacts as a Lewis base as is donates a lone pair of electrons and BF₃ acts as a Lewis acid

• Only electron pairs are being donated and accepted

• In this case, neither compound reacts as an Brønsted-Lowry acid or Brønsted-Lowry base as no protons (H⁺ ions) are being donated or accepted

Diagram showing boron trifluoride acting as a Lewis acid and ammonia acting as a Lewis base

Ammonia donates a lone pair of electrons to form a coordinate bond 

• Here boron forms three sp² hybridised orbitals leaving a vacant 2p_z orbital which allows the lone pair on the nitrogen atom to form a dative covalent bond

Boron forms three sp² hybridised orbitals

Hybridisation of the boron atom 

Electrophiles and Nucleophiles

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

  • An electrophile is therefore a Lewis acid

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

  • A nucleophile is therefore a Lewis base

Lewis acids behave as electrophiles and Lewis bases act as nucleophiles

The carbocation, C⁺(CH₃)₃ is accepting a pair of electrons from the bromide ion, Br^–

• Lewis acids and bases can also be seen in complex ions

• In the case of a complex ion, such as hexaaquacopper(II), the water molecule is acting as a Lewis base and the metal ion is acting as a Lewis acid

  • 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)

Diagram showing the formation of the hexaaquacopper(II) complex

Hexaaquacopper(II) complex 

• The Cu²⁺ ion acts as a Lewis acid and electrophile as it is accepting a lone pair of electrons from water

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

  • The cyanide ion, ^–CN  and ammonia , NH₃ , are examples of Lewis bases and they can also act as nucleophiles