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Lewis theory extends the idea of acids still further by looking at reactions from the perspective of the electron pairs involved. |
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Lewis theory
Lewis theory is more flexible than Brønsted-Lowry theory, because it can accomodate both non-aqueous systems and systems that do not involve hydrogen ions.
Lewis extended the theory of acids to cover both non-aqueous systems and systems that do not involve proton transfers. He defined a Lewis acid from the point of view of the electrons rather than from the point of view of hydrogen ions (protons). An electron pair donor becomes a Lewis base and an electron pair acceptor is a Lewis acid.
To see how this affects Arrhenius acid - base behaviour, consider the reaction between a hydrogen ion and a hydroxide ion.
H+ + OH- 
H2O |
In this reaction the H+ ion is accepting a lone pair of electrons from the hydroxide (OH-) ion. According to Lewis' definition the H+ is an acid (as we already know from Arrhenius).
The hydroxide ion is donating a lone pair of electrons and is defined as a Lewis base
Summary
- Lone pair acceptor - Lewis acid
- Lone pair donor - Lewis base
The advantage of Lewis definition is that it can be applied to systems that do not have hydrogen ions involved at all.
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Example: The reaction between ammonia and Boron trifluoride takes place as follows:
In this reaction the lone pair on the ammonia is coordinating into the empty orbital of the electron deficient boron trifluoride. The Ammonia molecule is effectively donating a pair of electrons (:) to the boron trifluoride molecule to form the covalent bond and is therefore a base (by Lewis' definition) The Boron trifluoride is accepting the lone pair of electrons and is therefore a Lewis acid. |
Scope of Lewis' theory
Lewis theory gives an alternative slant on acid base reactions and extends the ideas of Arrhenius and Brønsted Lowry to sytems that do not involve hydrogen ions.
- All Arrhenius acids are also Brønsted Lowry acids and Lewis acids
- All Brønsted Lowry acids are also Lewis acids
- Lewis acids do NOT have to be either Brønsted Lowry or Arrhenius acids
The advantage of Lewis theory is that it extends to all dative coordinate bond systems. These are particularly common in transition metal chemistry and solvation (hydration when water is the solvent)
Transition metal complexes
All transition metals form coordinate bonds with ligands. This means that they accept electron pairs from the ligands. They behave as Lewis acids. The reacting ligands are Lewis bases.
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Hydration
The dissolution of an ionic substance in water involves the water molecules forming dative coordinate bonds to the positive hydrogen ions by donation of a lone pair from the oxygen atom of water. The positive hydrogen ions could therefore be considered to be a Lewis acid. The coordinating water molecules are Lewis bases.
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Summary
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