Lewis Acids and Bases

The Lewis Concept of Acids and Bases

In 1923, G.N. Lewis proposed a broader definition that shifted the focus from the proton to the electron pair. This concept remains central to understanding a vast range of chemical reactions, especially those that do not involve hydrogen ions.

A Lewis acid is any species (atom, ion, or molecule) that can accept an electron pair. A Lewis base is any species that can donate an electron pair.

The key difference from the Brønsted-Lowry theory is that a Lewis acid does not need to have a proton to donate. The reaction is simply the formation of a coordinate covalent bond between the electron-pair donor (base) and the electron-pair acceptor (acid).

The Classic Example: Boron Trifluoride and Ammonia

Boron trifluoride ($BF_3$) is the textbook example of a Lewis acid. The boron atom has only six electrons in its valence shell (three from its own bonds to fluorine), making it electron-deficient and highly eager to accept a lone pair. Ammonia ($NH_3$) has a lone pair on nitrogen, making it a perfect Lewis base.

The reaction is a direct combination:

The arrow from the nitrogen lone pair to the boron shows the donation. The product, $BF_3:NH_3$, is a stable adduct (a compound formed by direct combination) where the boron now has a complete octet.

Identifying Lewis Acids and Bases

The rule is straightforward: look for species that can accept a lone pair (acids) and species that have a lone pair to donate (bases).

Common Lewis Acids (Electron-Pair Acceptors):

• Electron-deficient molecules: $BF_3$, $AlCl_3$, $BCl_3$. These have an incomplete octet.
• Cations: $H^+$, $Co^{3+}$, $Mg^{2+}$, $Fe^{3+}$. Positive charge creates a strong pull on electron density.
• Molecules with a polar double bond: $CO_2$, $SO_2$ (the central atom can accept a lone pair).

Common Lewis Bases (Electron-Pair Donors):

• Molecules with a lone pair: $H_2O$, $NH_3$, amines ($R_3N$), alcohols ($ROH$).
• Anions: $OH^-$, $F^-$, $Cl^-$, $CN^-$. The negative charge makes the lone pairs more available. …