Question

Identify the Lewis acid and the Lewis base in the following reactions: (a) \(\mathrm{AlBr}_{3}(s)+\mathrm{Br}^{-}(a q) \longrightarrow \mathrm{AlBr}_{4}^{-}(a q)\) (b) \(6 \mathrm{CO}(g)+\mathrm{Cr}(s) \longrightarrow \operatorname{Cr}(\mathrm{CO})_{6}(s)\) (c) \(\mathrm{Cu}^{2+}(a q)+4 \mathrm{CN}^{-}(a q) \longrightarrow \mathrm{Cu}(\mathrm{CN})_{4}^{2-}(a q)\)

Short answer

(a) Lewis acid: \( \mathrm{AlBr}_3 \), Lewis base: \( \mathrm{Br}^- \). (b) Lewis acid: \( \mathrm{Cr} \), Lewis base: \( \mathrm{CO} \). (c) Lewis acid: \( \mathrm{Cu}^{2+} \), Lewis base: \( \mathrm{CN}^- \).

Step-by-step solution

Understand Lewis Acid and Base

A Lewis acid is a substance that can accept a pair of electrons to form a new bond. A Lewis base, on the other hand, is a substance that can donate a pair of electrons to form a new bond.

Analyze Reaction (a)

In the reaction \( \mathrm{AlBr}_3(s) + \mathrm{Br}^-(aq) \rightarrow \mathrm{AlBr}_4^-(aq) \), \( \mathrm{AlBr}_3 \) accepts an electron pair from \( \mathrm{Br}^- \). Therefore, \( \mathrm{AlBr}_3 \) is the Lewis acid and \( \mathrm{Br}^- \) is the Lewis base.

Analyze Reaction (b)

In the reaction \( 6 \mathrm{CO}(g) + \mathrm{Cr}(s) \rightarrow \operatorname{Cr}(\mathrm{CO})_6(s) \), each \( \mathrm{CO} \) molecule donates an electron pair to the \( \mathrm{Cr} \) metal. Therefore, \( \mathrm{CO} \) is the Lewis base, and \( \mathrm{Cr} \) is the Lewis acid.

Analyze Reaction (c)

In the reaction \( \mathrm{Cu}^{2+}(aq) + 4 \mathrm{CN}^-(aq) \rightarrow \mathrm{Cu}(\mathrm{CN})_4^{2-}(aq) \), \( \mathrm{CN}^- \) donates an electron pair to the \( \mathrm{Cu}^{2+} \). Thus, \( \mathrm{Cu}^{2+} \) is the Lewis acid, and \( \mathrm{CN}^- \) is the Lewis base.

Conclusion

For each reaction, identify the electron donor as the Lewis base and the electron acceptor as the Lewis acid. This ensures clarity in distinguishing between the two.

Key concepts

Electron Pair Donation

Electron pair donation forms the cornerstone of many chemical reactions, especially Lewis acid-base reactions. In these interactions, a Lewis base donates a pair of electrons. This action occurs because the Lewis base has extra electrons it can share. Think of it as a partner in a dance, sharing its space and energy with another molecule.
Common substances that act as electron pair donors include molecules or ions with lone pairs of electrons.

• For example, in reaction (a) -- \( \mathrm{Br}^{-} \) is the donor as it gives an electron pair to \( \mathrm{AlBr}_{3} \).
• In reaction (b), each \( \mathrm{CO} \) molecule donates its lone pairs to \( \mathrm{Cr} \).
• In reaction (c), \( \mathrm{CN}^{-} \) donates its electron pairs to the metal ion \( \mathrm{Cu}^{2+} \).

These electron donations are critical for forming new bonds and stabilizing the resulting chemical complexes.

Electron Pair Acceptance

While one party donates in a Lewis acid-base reaction, the other accepts. This electron pair acceptance is a defining characteristic of a Lewis acid. A Lewis acid is a chemical species that can accept an electron pair, often because it has an empty orbital ready to accommodate additional electrons.
As the receptor of electrons, the Lewis acid helps to stabilize the complex by creating new bonds.
In reaction (a), \( \mathrm{AlBr}_{3} \) functions as the Lewis acid by accepting an electron pair from \( \mathrm{Br}^{-} \). When you consider reaction (b), the \( \mathrm{Cr} \) atom is the electron pair acceptor, taking in pairs from multiple \( \mathrm{CO} \) molecules. Lastly, for reaction (c), \( \mathrm{Cu}^{2+} \) readily accepts electron pairs from \( \mathrm{CN}^{-} \) ions. The concept of acceptance illustrates the dual nature of bonding interactions in these kinds of reactions.

Lewis Base

The Lewis base is crucial in forming chemical bridges in reactions. It is defined by its ability to donate electrons to another species. These electron donors often have extra electrons, either in lone pairs or within the structure of the molecule.
Such bases are highly reactive and actively seek out Lewis acids to donate their extra electrons and form stable structures.

• In reaction (a), \( \mathrm{Br}^{-} \) plays the role of the Lewis base, providing its electron pair to \( \mathrm{AlBr}_{3} \).
• Reaction (b) sees \( \mathrm{CO} \) as the Lewis base, with its carbon atom contributing electrons to \( \mathrm{Cr} \).
• For reaction (c), \( \mathrm{CN}^{-} \) acts as the Lewis base, forming a complex with \( \mathrm{Cu}^{2+} \).

Recognizing a substance's role as a Lewis base makes it easier to predict and understand chemical behavior and bonding patterns.

Lewis Acid

A Lewis acid's defining feature is its ability to accept an electron pair during interaction with a Lewis base. This property is fundamental to creating new, stable compounds.
Many substances can act as Lewis acids, especially those with an empty orbital or an ability to expand their valence shell.
In the described reactions, identifying the Lewis acid is simple once you've noted what is accepting the electron pair. Reaction (a) shows \( \mathrm{AlBr}_{3} \) as the acceptor, while in reaction (b), \( \mathrm{Cr} \) takes on that role. For reaction (c), \( \mathrm{Cu}^{2+} \) is the Lewis acid, forming a bound structure with the electron-donating \( \mathrm{CN}^{-} \) ions. By understanding a Lewis acid's role, you gain insight into molecular interactions and compound formation.

Reaction Analysis

Analyzing Lewis acid-base reactions requires identifying both the electron donor and acceptor. Such analysis allows you to categorize each component in a reaction accurately.
Take each chemical structure and determine:

• Which molecule donates electrons? This is the Lewis base.
• Which molecule accepts these electrons? This becomes the Lewis acid.

Examining the reactions provided, you can see:
In reaction (a), \( \mathrm{Br}^{-} \) donates to \( \mathrm{AlBr}_{3} \).
In reaction (b), \( \mathrm{CO} \) molecules donate to \( \mathrm{Cr} \).
In the final example, reaction (c), \( \mathrm{CN}^{-} \) ions donate electrons to \( \mathrm{Cu}^{2+}. \)
Thorough reaction analysis is a valuable skill for understanding and predicting the outcome of chemical processes.