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 2pz 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–
Worked example
Identify the Lewis acid and Lewis base in the following reaction
Answer
- The Lewis acid is water, H2O
- The hydrogen in the water molecule is accepting a pair of electrons leaving an OH- ion
- The Lewis base is the methanoate ion, HCOO–
- The lone pair of electrons in the methanoate ion forms a coordinate bond with one of the hydrogens from the water molecule
- We have seen previously that water can act as a Brønsted-Lowry acid or base, so it should be no surprise that water can act as both a Lewis acid or base depending on how it is interacting with other species
- 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