Question

Trimethylamine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N},\) is a common reagent. It interacts readily with diborane gas, \(\mathrm{B}_{2} \mathrm{H}_{6}\). The latter dissociates to \(\mathrm{BH}_{3}\), and this forms a complex with the amine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N} \rightarrow \mathrm{BH}_{3} .\) Is the \(\mathrm{BH}_{3}\) fragment a Lewis acid or a Lewis base?

Short answer

The \(\mathrm{BH}_{3}\) fragment is a Lewis acid.

Step-by-step solution

Understanding Lewis Acids and Bases

In the context of Lewis theory, Lewis acids are chemical species that can accept an electron pair, while Lewis bases are species that can donate an electron pair. The direction of donation or acceptance determines the nature of the molecule in terms of Lewis acidity or basicity.

Analyzing the Reaction

In the given reaction, diborane ( \(\mathrm{B}_{2} \mathrm{H}_{6}\)) dissociates into \(\mathrm{BH}_{3}\). This \(\mathrm{BH}_{3}\) fragment then forms a complex with trimethylamine. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}\) is a known Lewis base because it has a lone pair of electrons on the nitrogen atom that can be donated to form bonds.

Identifying the Role of BH3

Since the nitrogen in trimethylamine donates an electron pair to \(\mathrm{BH}_{3}\), it indicates that the \(\mathrm{BH}_{3}\) fragment is accepting this electron pair. According to Lewis acid-base theory, an electron pair acceptor is classified as a Lewis acid.

Conclusion

Based on the interaction where \(\mathrm{BH}_{3}\) accepts an electron pair from trimethylamine, \(\mathrm{BH}_{3}\) serves as the Lewis acid in this reaction.

Key concepts

Electron Pair Acceptor

In the world of chemistry, certain molecules have the ability to accept electron pairs, and these are known as electron pair acceptors. One common example is the molecule \(\mathrm{BH}_{3}\) which plays a crucial role in many reactions. When we say that a molecule is an electron pair acceptor, it simply means it has empty orbitals that are ready to receive an electron pair from another molecule. This is a defining characteristic of a Lewis acid.
\[ \mathrm{BH}_{3} + \mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N} \rightarrow \mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N:BH}_{3}\]
In this example, \(\mathrm{BH}_{3}\) forms a complex with trimethylamine, \(\mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N}\), by accepting its electron pair. This interaction highlights the central role of \(\mathrm{BH}_{3}\) as an electron pair acceptor, thereby classifying it as a Lewis acid. It does not just happen in isolation; understanding this concept is key to predicting how different molecules might interact in a chemical reaction.

Electron Pair Donor

In contrast to electron pair acceptors, electron pair donors are molecules that offer up their electron pairs to other molecules. A classic example is trimethylamine, \(\mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N}\), which acts as a Lewis base. This means it has a pair of electrons on the nitrogen atom that it can donate.

• This donation ability is what makes it a Lewis base and allows it to interact with electron pair acceptors.
• The key feature is the presence of lone pair electrons, which are not involved in bonding and are hence available for donation.

In the reaction involving \(\mathrm{BH}_{3}\) and trimethylamine, the nitrogen atom in trimethylamine uses its lone pair to bond with \(\mathrm{BH}_{3}\). This illustrates its role as a donor, initiating a stable bond formation. Recognizing these donating abilities helps in understanding various reaction mechanisms, particularly those involving complex formations with transition metals or other Lewis acids.

Chemical Reactions

Chemical reactions are processes where molecules interact, break, and form new bonds. In the reaction between \(\mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N}\) and \(\mathrm{BH}_{3}\), a new complex is formed when the nitrogen in trimethylamine donates an electron pair to \(\mathrm{BH}_{3}\).

• This interaction is fundamental in many areas of chemistry, especially in synthesis and catalysis.
• Understanding these reactions requires knowledge of how electron pairs are exchanged, transferred, or shared among different molecules.

In this specific reaction, recognizing \(\mathrm{BH}_{3}\) as a Lewis acid allows chemists to predict its interaction behaviors, involving acceptance of electron pairs. Similarly, identifying \(\mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N}\) as a Lewis base leads to insights into its ability to donate electron pairs, thus facilitating a comprehensive understanding of the reaction dynamics.

Complex Formation

Complex formation is a pivotal concept in chemistry, involving interactions where a new compound, or complex, is formed. This happens when one molecule donates an electron pair to another, establishing a coordinate covalent bond.

• The formed complex often has different properties than the individual reactants.
• In many cases, these complexes are crucial for biological systems and industrial processes.

In the case of \(\mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N}\) interacting with \(\mathrm{BH}_{3}\), the formation of the \(\mathrm{(CH}_{3}\mathrm{)}_{3}\mathrm{N:BH}_{3}\) complex illustrates this concept perfectly. The nitrogen from trimethylamine provides an electron pair that is accepted by \(\mathrm{BH}_{3}\), resulting in a stable structure. Such an understanding of complex formation is fundamental for chemistry students, allowing them to predict and explain how molecules will behave in new reactions.