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