Question

Which of these is a Lewis acid? A Lewis base? (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{BeCl}_{2}\) (c) \(\mathrm{BCl}_{3}\)

Short answer

\(\mathrm{NH}_{3}\) is a Lewis base; \(\mathrm{BeCl}_{2}\) and \(\mathrm{BCl}_{3}\) are Lewis acids.

Step-by-step solution

Identify Lewis Acid and Lewis Base

A Lewis acid is a substance that can accept a pair of electrons, while a Lewis base is a substance that can donate a pair of electrons. We will analyze each compound to determine which category they fall into.

Analyze \\(\mathrm{NH}_{3}\\)

The molecule \(\mathrm{NH}_{3}\) (ammonia) has a lone pair of electrons on the nitrogen atom, which it can donate to an acceptor. Hence, \(\mathrm{NH}_{3}\) is a Lewis base.

Analyze \\(\mathrm{BeCl}_{2}\\)

\(\mathrm{BeCl}_{2}\), or beryllium chloride, has beryllium in a \(sp\) hybridization state and is electron-deficient, which allows it to accept a pair of electrons. Therefore, it acts as a Lewis acid.

Analyze \\(\mathrm{BCl}_{3}\\)

The molecule \(\mathrm{BCl}_{3}\) (boron trichloride) is also electron-deficient due to the boron atom lacking a complete octet, making it capable of accepting an electron pair. Therefore, \(\mathrm{BCl}_{3}\) is a Lewis acid.

Key concepts

Electron Pair Acceptor

In chemistry, an electron pair acceptor is often referred to as a Lewis acid. These substances can accept an electron pair from another substance.
A classic example of a Lewis acid is a molecule that is electron-deficient. This deficiency allows it to attract and accept an electron pair from a donor.

• For instance, beryllium chloride (\( \mathrm{BeCl}_{2} \)) acts as a Lewis acid because the beryllium atom is surrounded by only four electrons, which is less than a full octet.
• Similarly, boron trichloride (\( \mathrm{BCl}_{3} \)) also functions as a Lewis acid. The boron atom in \( \mathrm{BCl}_{3} \) is also electron-deficient, motivating it to accept additional electrons to achieve stability.

When you encounter a molecule that seems lacking in a complete electron shell, this is usually a good sign that you have found an electron pair acceptor or a Lewis acid.

Electron Pair Donor

In contrast to an electron pair acceptor, an electron pair donor is called a Lewis base. It has electrons that it can donate to another molecule, often filling the deficiency present in a Lewis acid.
These electron pairs, typically found as lone pairs on atoms, are key to their identification as donors.

• Take ammonia, \(\mathrm{NH}_{3}\), for example. The nitrogen atom in ammonia has a lone pair of electrons.
• This lone pair is readily available to be donated to a Lewis acid, which makes \(\mathrm{NH}_{3}\) a strong Lewis base.

Whenever you see a molecule with lone pairs, you are likely looking at a potential Lewis base. These lone pairs can interact with Lewis acids to form coordinate covalent bonds, completing the acid's electron requirements.

Molecular Analysis

Molecular analysis is the thorough examination of a compound's structure to determine its potential to act as a Lewis acid or base. By understanding the arrangement of electrons, you can predict how a molecule might behave in different chemical reactions.
This process involves:

• Looking at electron configurations to identify instances of electron deficiency or surplus.
• Checking for lone pairs that signify potential for electron donation.
• Identifying hybridization states that signal a molecule's ability to accept electrons.

With this analysis:

• \(\mathrm{NH}_{3}\), with its lone pairs, is identified as a Lewis base.
• Beryllium chloride, \(\mathrm{BeCl}_{2}\), and boron trichloride, \(\mathrm{BCl}_{3}\), lacking full electron shells, are recognized as Lewis acids.

Paying attention to these factors allows for a deeper understanding of chemical interactions and how different substances will react with one another.