Defining Lewis acids and bases

Lewis acid-base theory is a general approach to classifying substances based on their ability to accept or donate electron pairs.

• Lewis acid - An electron pair acceptor. Examples include H₃O⁺, CH₃⁺ and Cu²⁺.
• Lewis base - An electron pair donor. Examples include NH₃, H₂O and Cl^-.

When a Lewis acid reacts with a Lewis base, they form a coordination bond - a covalent bond formed by the donation of a lone pair from the Lewis base to the Lewis acid.

Comparing Lewis theory to Brønsted-Lowry theory

Lewis acid-base theory is more general than Brønsted-Lowry theory.

• Brønsted-Lowry theory focuses on proton transfer.
• Lewis theory considers the transfer of electron pairs.

Brønsted-Lowry bases like OH^- and NH₃ are also Lewis bases, as they can donate a lone pair to form a bond with H⁺ (a Lewis acid).

However, Lewis theory can also explain reactions that don't involve proton transfer. For example:

H₃N: + BF₃ ➔ H₃N→BF₃

Here, NH₃ is a Lewis base (electron pair donor) and BF₃ is a Lewis acid (electron pair acceptor), but no proton transfer occurs.

Lewis acids and bases in organic reactions

Lewis acid-base theory is closely related to the concept of nucleophiles and electrophiles in organic chemistry.

• Nucleophile - An electron-rich species with a lone pair, which can act as a Lewis base.
• Electrophile - An electron-deficient species that can accept a pair of electrons, acting as a Lewis acid.

In the reaction between BF₃ and NH₃:

• NH₃ acts as a nucleophile (Lewis base), donating its lone pair.
• BF₃ is an electrophile (Lewis acid), accepting the electron pair into its vacant 2p orbital.

Some compounds can act as both Lewis acids and bases. For example, AlCl₃ can form dimers (Al₂Cl₆) where:

• The electron-deficient Al atom in each AlCl_3_, acting as a Lewis acid, accepts a lone pair from a Cl atom in the adjacent AlCl_3_ molecule
• At the same time, a Cl atom in each AlCl_3_, acting as a Lewis base, donates a lone pair to the Al atom in the adjacent AlCl_3_ molecule.