Problem 82 - Chemistry and Chemical Reactivity

Question

Decide whether each of the following substances should be classified as a Lewis acid or a Lewis base. (a) \(\mathrm{BCl}_{3}\) (Hint: Draw the electron dot structure.) (b) \(\mathrm{H}_{2} \mathrm{NNH}_{2},\) hydrazine (Hint: Draw the electron dot structure. (c) the reactants in the reaction \(\mathrm{Ag}^{+}(\mathrm{aq})+2 \mathrm{NH}_{3}(\mathrm{aq}) \rightleftharpoons\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}(\mathrm{aq})\)

Step-by-Step Solution

Answer

(a) Lewis acid, (b) Lewis base, (c) \( \mathrm{Ag}^{+} \) is a Lewis acid; \( \mathrm{NH}_{3} \) is a Lewis base.

Step 1: Understanding Lewis Acids and Bases

A Lewis acid is a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair. Typically, Lewis acids have incomplete octets, while Lewis bases have lone pairs of electrons that can be shared.

Step 2: Analyze \( \mathrm{BCl}_{3} \)

\( \mathrm{BCl}_{3} \) is made up of one boron and three chlorine atoms. The boron atom in \( \mathrm{BCl}_{3} \) has only six electrons in its outer shell, making it electron-deficient. Therefore, it can accept a pair of electrons to complete the octet around boron.

Step 3: Conclusion for \( \mathrm{BCl}_{3} \)

Since \( \mathrm{BCl}_{3} \) can accept a pair of electrons, it is classified as a Lewis acid.

Step 4: Analyze \( \mathrm{H}_{2} \mathrm{NNH}_{2} \)

Hydrazine (\( \mathrm{H}_{2} \mathrm{NNH}_{2} \)) consists of two nitrogen atoms, each bonded to two hydrogen atoms. Each nitrogen atom has a lone pair of electrons. Hence, \( \mathrm{H}_{2} \mathrm{NNH}_{2} \) can donate its lone pairs to act as a Lewis base.

Step 5: Conclusion for \( \mathrm{H}_{2} \mathrm{NNH}_{2} \)

\( \mathrm{H}_{2} \mathrm{NNH}_{2} \) has lone pairs that can be donated, so it should be classified as a Lewis base.

Step 6: Analyze the Reaction \( \mathrm{Ag}^{+} + 2 \mathrm{NH}_{3} \)

In the reaction \( \mathrm{Ag}^{+} + 2 \mathrm{NH}_{3} \rightarrow \left[ \mathrm{Ag}\left( \mathrm{NH}_{3} \right)_{2} \right]^{+} \), \( \mathrm{Ag}^{+} \) ion is electron-deficient and can accept electron pairs, making it a Lewis acid. \( \mathrm{NH}_{3} \), having a lone pair on nitrogen, can donate electrons, making it a Lewis base.

Step 7: Conclusion for the Reaction

In the given reaction, \( \mathrm{Ag}^{+} \) acts as the Lewis acid and \( \mathrm{NH}_{3} \) acts as the Lewis base.

Key Concepts

Electron Pair Acceptor

In the context of Lewis Acid-Base Theory, a substance that acts as an electron pair acceptor is classified as a Lewis acid. Lewis acids have the ability to accept a lone pair of electrons from another molecule. This is often due to their electron-deficient nature, i.e., they have fewer than eight electrons in their valence shell.
To understand this better, let’s look at the example of boron trichloride (\(\mathrm{BCl}_3\)).

• The boron atom in \(\mathrm{BCl}_3\) only has six electrons in its valence shell, which makes it electron-deficient or an electron pair acceptor.
• Boron seeks to achieve a stable electronic configuration by accepting a pair of electrons to complete its octet.

This electron-accepting property is the hallmark of Lewis acids, which include metal ions and molecules like \(\mathrm{BCl}_3\) that have incomplete valence shells.

Electron Pair Donor

A Lewis base is known as an electron pair donor. These substances have lone pairs of electrons that they can share with a Lewis acid. For instance, when examining hydrazine (\(\mathrm{H}_2\mathrm{NNH}_2\)), we notice that each nitrogen atom in hydrazine has a lone pair of electrons.
This characteristic allows hydrazine to donate these electron pairs, which is a key behavior of Lewis bases.

• The ability to donate lone pairs makes compounds like \(\mathrm{H}_2\mathrm{NNH}_2\) ideal candidates for forming coordinate covalent bonds with Lewis acids.
• Common molecules acting as Lewis bases include ammonia (\(\mathrm{NH}_3\)) and anions like \(\mathrm{OH}^-\).

Therefore, the presence of lone electron pairs is crucial for a substance to function as an electron pair donor.

Lewis Structure

The Lewis structure is an essential tool for representing molecules and ions by showing their valence electrons as dots. This helps to visualize how atoms share electrons in a stable configuration.
Creating a Lewis structure involves:

• Identifying the total number of valence electrons involved in a molecule.
• Representing bonds as pairs of shared electrons between atoms.
• Ensuring that atoms (mainly in the main group elements) strive to have a complete octet, except for some exceptions.

In Lewis structures, central atoms like boron in \(\mathrm{BCl}_3\) may not achieve an octet immediately, highlighting its opportunity to act as an electron pair acceptor. Conversely, Lewis bases, shown as having lone pairs (like the nitrogen in ammonia or hydrazine), demonstrate their electron-donating capabilities within these structures.

Incomplete Octet

The incomplete octet is a term often encountered when discussing the electron configurations of certain Lewis acids. Unlike most elements that obey the octet rule, some elements such as boron are stable with fewer than eight electrons in their outer shell.
An incomplete octet occurs naturally in molecules such as \(\mathrm{BCl}_3\):

• In \(\mathrm{BCl}_3\), boron achieves a stable state with only six electrons.
• This makes boron eager to accept electron pairs to fulfill its octet, hence functioning as a Lewis acid.

The concept of an incomplete octet highlights the unique behavior of some molecules in chemical reactions, especially in forming complexes with electron pair donors to achieve stability.