Problem 73 - Chemistry and Chemical Reactivity

Question

Decide whether each of the following substances should be classified as a Lewis acid or a Lewis base. (a) \(\mathrm{H}_{2} \mathrm{NOH}\) in the reaction \(\mathrm{H}_{2} \mathrm{NOH}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \longrightarrow\left[\mathrm{H}_{3} \mathrm{NOH}\right] \mathrm{Cl}(\mathrm{aq})\) (b) \(\mathrm{Fe}^{2+}\) (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (Hint: Draw the electron dot structure.)

Step-by-Step Solution

Answer

(a) Lewis base, (b) Lewis acid, (c) Lewis base.

Step 1: Analyze Reaction with HCl

In the given reaction \(\mathrm{H}_{2} \mathrm{NOH}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq})\longrightarrow\left[\mathrm{H}_{3} \mathrm{NOH}\right]\mathrm{Cl}(\mathrm{aq})\), we see that \(\mathrm{H}_{2} \mathrm{NOH}\) accepts a proton (\(\mathrm{H}^+\)) from \(\mathrm{HCl}\) to form \([\mathrm{H}_{3} \mathrm{NOH}]^+\). According to Lewis theory, a species that accepts a pair of electrons is a Lewis acid. Thus, \(\mathrm{H}_{2} \mathrm{NOH}\) behaves as a Lewis base in this reaction because it donates a lone pair to \(\mathrm{H}^+\).

Step 2: Determine Electronic Structure of Fe²⁺

The \(\mathrm{Fe}^{2+}\) ion is a cation with a positive charge, resulting from the loss of two electrons. Lewis acids are species that can accept an electron pair, and \(\mathrm{Fe}^{2+}\) can accept electron pairs because of its vacant orbitals. Hence, \(\mathrm{Fe}^{2+}\) is a Lewis acid.

Step 3: Draw Electron Dot Structure of CH₃NH₂

\(\mathrm{CH}_{3}\mathrm{NH}_{2}\) has a nitrogen atom with a lone pair of electrons, which is capable of donating this pair to form a bond with a Lewis acid. This characteristic defines it as a Lewis base, as Lewis bases are electron pair donors.

Key Concepts

Electron pair donor

An electron pair donor is a chemical species that provides a pair of electrons to another species to form a chemical bond. This concept is central to understanding Lewis acid-base theory. In a chemical reaction, when a species like methylamine (\( \text{CH}_3\text{NH}_2 \)) donates its lone pair of electrons from its nitrogen atom, it acts as an electron pair donor.

This donation process is a defining characteristic of Lewis bases.

• Through donating electrons, it establishes a coordinate covalent bond with an electron-deficient atom or ion.
• Common examples include molecules with lone pairs such as ammonia (\(\text{NH}_3 \)) and water (\(\text{H}_2\text{O} \)).

When you see a chemical formula with a lone pair, think of it as a potential electron pair donor.

Electron pair acceptor

An electron pair acceptor does the opposite of an electron pair donor. It is a species that takes in a pair of electrons during a chemical reaction. In the Lewis acid-base context, this means forming a bond by accepting these electrons from a Lewis base.

For instance, in the reaction involving ferrous ion (\( \text{Fe}^{2+} \)), the positive charge indicates there is room to accept electrons. This cation seeks out electron-rich species to fill its orbital, becoming bonded by accepting electron pairs.

• Electron pair acceptors usually have empty orbitals that can accommodate additional electron pairs.
• They often carry a positive charge or have high electronegativity elements.

Understanding this concept helps clarify why some species easily undergo reactions with electron pair donors.

Lewis base

A Lewis base is a substance capable of donating an electron pair in a chemical reaction. This ability categorizes it distinctively under Lewis's theory of acids and bases.

Methylamine (\( \text{CH}_3\text{NH}_2 \)) is an everyday example of a Lewis base, owing to the lone pair situated on its nitrogen atom.

• Lewis bases always work hand-in-hand with Lewis acids in reactions to donate their electron pairs.
• Recognizing a Lewis base involves identifying molecules or ions with lone pairs of electrons.
• Generally, look for structures that have non-bonded electron pairs available to share.

Lewis bases are indispensable in understanding numerous reaction mechanisms, especially those revolving around coordination chemistry and organic reactions.

Lewis acid

Lewis acids, on the other hand, are species that can accept electron pairs. This concept stands complementary to that of a Lewis base. A classic example of a Lewis acid in the context of this discussion is the \( \text{Fe}^{2+} \) ion which accepts electrons due to its positive charge and vacant orbitals.

• Lewis acids typically possess a positive charge or an electron-deficient structure.
• They interact seamlessly with Lewis bases, forming coordinate covalent bonds.

The role of Lewis acids is pivotal in industrial processes and chemical syntheses. Recognizing Lewis acids involves identifying species with vacant orbitals or the tendency to gain additional electrons during reactions.