What Is a Lewis Acid and a Lewis Base? (HL)

A Lewis acid is an electron-pair acceptor, while a Lewis base is an electron-pair donor. When they react, the Lewis base donates both electrons that form a new coordination bond.

This is Additional Higher Level content in Reactivity 3.4, Electron-pair sharing reactions.

How the reaction works

A Lewis base must have an available electron pair, usually a lone pair. A Lewis acid must have somewhere to accept that pair, such as an incomplete valence shell, a vacant orbital, or a positive charge.

For example, ammonia reacts with boron trifluoride:

$\text{NH}_3(g) + \text{BF}_3(g) \rightarrow \text{H}_3\text{NBF}_3(g)$

Nitrogen in $\text{NH}_3$ donates its lone pair, so $\text{NH}_3$ is the Lewis base. Boron in $\text{BF}_3$ has only six electrons around it and accepts the pair, so is the Lewis acid.

The resulting species is a Lewis acid-base adduct. Although the new bond is described as a coordination bond because both bonding electrons originated from nitrogen, after formation it behaves like an ordinary covalent bond. The distinction describes the bond’s origin, not a permanently different bond type.

A common misconception is that all acid-base reactions require proton transfer. That describes BrønstedtoLowry reactions, but Lewis acid-base theory is broader: no proton needs to be transferred. For example, $\text{BF}_3$ is a Lewis acid even though it cannot donate a proton.

Exam technique

If asked to identify a Lewis acid or base, state both the role and the evidence: “$\text{NH}_3$ is a Lewis base because it donates a lone pair.” In a mechanism, draw the curly arrow from the lone pair on the Lewis base toward the atom accepting the pair; reversing the arrow loses chemical accuracy.