Question

Identify the Lewis acid and Lewis base from among the reactants in each equation. a. \(\mathrm{Fe}^{3+}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}(a q)\) b. \(\mathrm{Zn}^{2+}(a q)+4 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}(a q)\) c. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}(g)+\mathrm{BF}_{3}(g) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NBF}_{3}(s)\)

Short answer

a. Lewis acid: \(\mathrm{Fe}^{3+}\), Lewis base: \(\mathrm{H}_{2} \mathrm{O}\); b. Lewis acid: \(\mathrm{Zn}^{2+}\), Lewis base: \(\mathrm{NH}_{3}\); c. Lewis acid: \(\mathrm{BF}_{3}\), Lewis base: \(\mathrm{(CH}_{3})_{3} \mathrm{N}\)

Step-by-step solution

- Identify Lewis acid and Lewis base in reaction (a)

According to Lewis theory, a Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. In the given reaction (a), \(\mathrm{Fe}^{3+}\) is the electron pair acceptor, as it coordinates with water molecules. Thus, \(\mathrm{Fe}^{3+}\) is the Lewis acid. Water \(\mathrm{H}_{2} \mathrm{O}\) donates an electron pair to \(\mathrm{Fe}^{3+}\) and is therefore the Lewis base.

- Identify Lewis acid and Lewis base in reaction (b)

In reaction (b), \(\mathrm{Zn}^{2+}\) is the species that accepts electron pairs from the \(\mathrm{NH}_{3}\) (ammonia) molecules and, as such, is the Lewis acid. The ammonia molecules \(\mathrm{NH}_{3}\) each donate an electron pair to \(\mathrm{Zn}^{2+}\), making them the Lewis bases.

- Identify Lewis acid and Lewis base in reaction (c)

For reaction (c),\ \((\mathrm{CH}_{3})_{3} \mathrm{~N}\) acts as an electron pair donor when it interacts with \(\mathrm{BF}_{3}\), so it is the Lewis base. \(\mathrm{BF}_{3}\) accepts the electron pair, making it the Lewis acid.

Key concepts

Lewis theory of acids and bases

The Lewis theory of acids and bases is an alternative to the more commonly known Brønsted-Lowry theory. Instead of focusing on protons, Gilbert N. Lewis proposed a broader approach, defining acids and bases in terms of electron pairs. A key principle of this theory is that an acid is any substance that can accept a pair of electrons, whereas a base is any substance that can donate a pair of electrons. Unlike the Brønsted-Lowry definition which is limited to aqueous solutions, the Lewis theory applies to a wider range of reactions, including those in gaseous phase or involving nonprotons.

This interpretation of acids and bases is especially useful in coordination chemistry, where complex formation involves the sharing of electron pairs. Identifying Lewis acids and bases in reactions is crucial for understanding the molecular interactions that occur during chemical processes.

Electron pair acceptor

In the context of Lewis theory, an electron pair acceptor is considered a Lewis acid. This role is not limited to hydrogen ions; any positively charged or electron-deficient species can act as a Lewis acid. For example, in the provided exercises, \(\mathrm{Fe}^{3+}\) and \(\mathrm{Zn}^{2+}\) ions are electron-deficient metal cations and can accept electron pairs, thus functioning as Lewis acids. Even neutral molecules with an empty orbital, like \(\mathrm{BF}_{3}\), can accept electron pairs and are therefore classified as Lewis acids.

The ability to accept electron pairs is essential for the formation of coordinate covalent bonds. This characteristic hugely impacts the formation of complexes, which are central in coordination chemistry and have numerous applications in catalysis, materials science, and biochemistry.

Electron pair donor

A Lewis base is an electron pair donor. In other words, any species that has a lone pair of electrons and is capable of sharing it can be classified as a Lewis base. In our exercise examples, water \(\mathrm{H}_{2} \mathrm{O}\) and ammonia \(\mathrm{NH}_{3}\) are classic Lewis bases. These molecules have lone pairs of electrons that can be shared with electron-poor species or metal ions to form coordinate covalent bonds.

Beyond simple molecules, larger organic compounds and even anions can be electron pair donors. Appreciating the role of electron pair donors is key to understanding a wide array of chemical reactions, from the fundamental to the complex, such as those involving organic synthesis or biological enzyme activity.

Coordination Chemistry

Coordination chemistry explores the chemical compounds featuring coordination compounds, where central metal atoms or ions are surrounded by molecules or anions, known collectively as ligands. Ligands are typically Lewis bases as they offer electron pairs that form coordinate bonds with the metal center, which acts as a Lewis acid. These interactions lead to the creation of complex species with varied geometries and coordination numbers.

In our textbook examples, the reactions showcase the formation of coordination compounds in aqueous solution. The water in reaction (a) and the ammonia in reaction (b) are coordinating to metal ions, creating octahedral \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) and tetrahedral \(\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\) complexes respectively. Understanding coordination chemistry is pivotal for fields like bioinorganic chemistry, catalyst design, and materials science.