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

Lewis acids are chemical species that accept an electron pair during a reaction, while Lewis bases are species that donate an electron pair. To determine if \(\mathrm{BH}_3\) is a Lewis acid or base, we need to consider its electron configuration.

Analyzing \\(\mathrm{BH}_3\\)

\(\mathrm{BH}_3\) is known for having an empty orbital, which means it can accept an electron pair. In the reaction with trimethylamine, \((\mathrm{CH}_3)_3\mathrm{N}\), the nitrogen in the amine donates a lone pair of electrons to the boron atom in \ \mathrm{BH}_3\.

Formation of the Complex

When trimethylamine \((\mathrm{CH}_3)_3\mathrm{N}\) interacts with \mathrm{BH}_3\, it forms a complex where the nitrogen donates its electron pair to boron. This donation confirms that boron is acting as an electron pair acceptor.

Conclusion about \\ \mathrm{BH}_3\\

Since \(\mathrm{BH}_3\) accepts an electron pair from trimethylamine to form the complex, it is acting as a Lewis acid.

Key concepts

Trimethylamine

Trimethylamine, represented as \((\mathrm{CH}_{3})_3 \mathrm{N}\), is an organic compound often used in chemical reactions and processes. It is known to have a distinct fishy odor and appears as a colorless gas at room temperature. In terms of its chemistry, trimethylamine plays a significant role as a Lewis base.This is because the nitrogen atom in trimethylamine has a lone pair of electrons it can donate. Such electron pair donation makes trimethylamine an ideal molecule to interact with other compounds via electron pair sharing. This quality is fundamental in its interaction with other molecules, such as diborane, forming Lewis acid-base complexes.
Being a Lewis base, trimethylamine's ability to donate an electron pair makes it useful in synthesizing various chemical compounds and in acting as a catalyst in different chemical reactions.

Diborane

Diborane, denoted as \(\mathrm{B}_2 \mathrm{H}_6\), is a simple boron hydride and a significant compound in the boron chemistry realm. It is a colorless gas known for its ability to dissociate under specific conditions to form \(\mathrm{BH}_3\) molecules. This dissociation is crucial for its role in Lewis acid-base reactions.One unique characteristic of diborane is that it can act as a "donor" source of \(\mathrm{BH}_3\), which plays an important role in complex formation with amines like trimethylamine. In general, diborane readily accepts electron pairs due to boron's electron-deficient nature, making it an effective Lewis acid. This property allows diborane to react with Lewis bases, helping to form stable complex formulations that are utilized in various chemical industries.

Electron Pair Donation

Electron pair donation is a key concept in Lewis acid-base chemistry that describes how electrons are shared between molecules during chemical reactions. When a molecule donates an electron pair, it typically acts as a Lewis base. In the context of the exercise, trimethylamine acts as the donor. Its nitrogen atom, holding a lone pair, interacts with diborane's \(\mathrm{BH}_3\) fragment by donating its electrons. This donation forms new chemical bonds and ultimately leads to the formation of a Lewis acid-base complex. Electron pair donation is essential for understanding how substances interact and transform in reactions, such as when a base like trimethylamine provides electrons to an acid like \(\mathrm{BH}_3\), showcasing a typical Lewis acid-base reaction mechanism. This capability is not just pivotal in forming complexes but also in altering reactivity and stability among molecules.

Lewis Acid-Base Complex

A Lewis acid-base complex arises when a Lewis acid and a Lewis base form a stable compound through electron pair sharing. This process involves the donation of an electron pair from the base to the acid.For instance, when trimethylamine \((\mathrm{CH}_3)_3 \mathrm{N}\) interacts with diborane's dissociated \(\mathrm{BH}_3\) component, a Lewis acid-base complex is formed. Here, trimethylamine donates a lone pair of electrons to the electron-deficient boron in \(\mathrm{BH}_3\). This interaction effectively bonds the molecules together, creating a stable complex.Such complexes are prominent in coordination chemistry and are pivotal in understanding many biological and industrial processes. The stability and formation of Lewis acid-base complexes are central to catalysis, synthesis of advanced materials, and even pharmaceuticals, highlighting their significant role in chemical transformations.