18.1.1 Lewis Theory

• A more general definition of acids and bases was given by G.N. Lewis who defined them as:
  • A Lewis acid is an electron pair acceptor
  • A Lewis base is an electron pair donor

General mechanism for Lewis acids and bases

• This enabled a wider range of substances to be classed as acids or bases
• This can be shown in the following examples in which a hydroxide ion, OH^-, and ammonia, NH₃, donate a pair of electrons to a hydrogen ion

The OH^- ion and ammonia act as Lewis bases in both examples by donating an electron pair

Brønsted-Lowry Acids and Bases

• A Brønsted-Lowry acid is a species that can donate a proton
  • For example, hydrogen chloride (HCl) is a Brønsted-Lowry acid as it can lose a proton to form a hydrogen (H⁺) and chloride (Cl^–) ion

HCl (aq) → H⁺ (aq) + Cl^– (aq)

• A Brønsted-Lowry base is a species that can accept a proton
  • For example, a hydroxide (OH^–) ion is a Brønsted-Lowry base as it can accept a proton to form water

OH^– (aq) + H⁺ (aq) → H₂O (l)

• In an equilibrium reaction, the products are formed at the same rate as the reactants are used
• This means that at equilibrium, both reactants and products are present in the solution
• For example, ethanoic acid (CH₃COOH) is a weak acid that partially dissociates in solution
• When equilibrium is established there are CH₃COOH, H₂O, CH₃COO^– and H₃O⁺ ions present in the solution

Lewis Acid, Lewis Base, Brønsted-Lowry acid or Brønsted-Lowry base

• A point to consider when thinking about Lewis acids and bases as well as Brønsted-Lowry acids and bases is the donating and accepting of protons
  • Brønsted-Lowry acid and base theory restricts the acids to proton donors only

• A Lewis acid can accept a pair of electrons and it can also accept a proton
  • As a lone pair of electrons present a proton can be accepted, the OH^- ion is a good example of this

• To be defined as a Brønsted-Lowry acid or base, a proton must be donated or accepted
• This does not of course occur in every reaction
• For example:
  • The lone pair on the nitrogen atom in ammonia, NH₃ , can be donated to the boron atom in boron trifluoride, BF₃, creating a molecule of NH₃BF₃
  • 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
  • Only electron pairs are being donated and accepted

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

Hybridisation of the boron atom 

• The following molecules can behave as either Lewis bases and Brønsted-Lowry acids
  • Lewis bases as they can donate an electron pair
  • Brønsted-Lowry base as they can accept a proton

Examples of molecules that can behave both as Lewis bases and Brønsted-Lowry base

• 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)

Hexaaquacopper(II) complex 

• Given that Lewis acids can donate a pair of electrons, they can be classed as electrophiles
  • An electrophile is a electron-deficient species that can accept a lone pair from a nucleophile, in the same way the Cu²⁺ ion accepts electron pairs from water
• The cyanide ion, ^-CN , water, H₂O , ammonia , NH₃ , are examples of Lewis bases and they can also act as nucleophiles
  • Nucleophiles are electron rich species with at least on pair of lone electrons

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