L1 - Lewis Theory

objectives

Understandings

• A Lewis acid is a lone pair acceptor and a Lewis base is a lone pair donor.

• When a Lewis base reacts with a Lewis acid a coordinate bond is formed.

• A nucleophile is a Lewis base and an electrophile is a Lewis acid.

Applications and skills

• Application of Lewis’ acid–base theory to inorganic and organic chemistry to identify the role of the reacting species.

Guidance

• Both organic and inorganic examples should be studied.

• Relations between Brønsted–Lowry and Lewis acids and bases should be discussed.

Lewis Theory

In Lewis Theory - Acids accept Lone Pairs and Bases Donate Lone Pairs - in these examples coordinate bonds are formed where both electrons are donated by one species.

ACID lone pair acceptor

BF₃ H⁺ AlCl₃

BASE lone pair donor

NH₃ H₂O

A Nucleophile is a species that is attracted to Positive species - it usually has a lone pair or a negative charge

An Electrophile is a species that is attracted to Negative species - it is usually at least partially positively charged

As you can see here - Ammonia is a common example of a Lewis Base as it donates it's lone pair

Lewis theory is more flexible than Brønsted-Lowry theory, because it can accommodate both non-aqueous systems and systems that do not involve hydrogen ions.

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

The advantage of Lewis definition is that it can be applied to systems that do not have hydrogen ions involved at all.

Example: The reaction between ammonia and Boron trifluoride takes place as follows:

:NH₃ + BF₃ → H₃N:BF₃

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.

Examples of Lewis Theory that do not follow Bronsted-Lowry

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.

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.