Question

Indicate whether each of the following is a Lewis acid or base. (a) \(\mathrm{OH}^{-} ;\) (b) \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{B} ;\) (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\)

Short answer

(a) \(\mathrm{OH}^{-}\) is a Lewis base. (b) \((\mathrm{C}_{2} \mathrm{H}_{5})_{3} \mathrm{B}\) is a Lewis acid. (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) is a Lewis base.

Step-by-step solution

Identify the Lewis acid or base - \(\mathrm{OH}^{-}\)

The \(\mathrm{OH}^{-}\) ion has an extra lone pair of electrons which can be donated. Therefore, \(\mathrm{OH}^{-}\) is a Lewis base.

Identify the Lewis acid or base - \((\mathrm{C}_{2} \mathrm{H}_{5})_{3} \mathrm{B}\)

\((\mathrm{C}_{2} \mathrm{H}_{5})_{3} \mathrm{B}\) or triethylborane is a species that is capable of accepting a lone pair of electrons due to the presence of an empty p-orbital on boron. So, it is a Lewis acid.

Identify the Lewis acid or base - \(\mathrm{CH}_{3} \mathrm{NH}_{2}\)

\(\mathrm{CH}_{3} \mathrm{NH}_{2}\) or methylamine has a lone pair of electrons on the nitrogen atom which can be donated. So, \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) is a Lewis base.

Key concepts

Chemical Bonding

Chemical bonding is the process that holds atoms together in molecules and compounds. It is one of the key concepts in chemistry, as it explains how elements combine and interact to form the substances we encounter in the world around us. There are several types of chemical bonds, including ionic, covalent, and metallic bonds. Each type of bond involves the distribution and sharing of electrons between atoms.

In the context of Lewis acids and bases, chemical bonding particularly pertains to the sharing of electron pairs. A Lewis base donates an electron pair to form a covalent bond, while a Lewis acid accepts an electron pair. Understanding this electron movement is crucial for identifying whether a substance acts as an acid or a base. For instance, in the given exercise, \
\(\mathrm{OH}^{-}\) donates an electron pair, making it a Lewis base, whereas triethylborane \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3}\mathrm{B}\) accepts an electron pair due to its empty p orbital, acting as a Lewis acid.

Lone Pair of Electrons

A lone pair of electrons refers to a pair of valence electrons that are not shared with another atom in a chemical bond. These pairs are important in chemical reactions because they often participate in chemical bonding by being donated or shared with other atoms.

Lone pairs can have a significant effect on molecule shape and reactivity. In the textbook exercise, \
\(\mathrm{OH}^{-}\) and \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) both have lone pairs of electrons, which can be used to form bonds with other atoms or ions that have vacant orbitals, such as Lewis acids. Understanding the behavior of lone pairs is essential for predicting the outcome of chemical reactions and for recognizing the role of a substance as a Lewis base, as seen with the hydroxide ion and methylamine.

Electronic Structure

The electronic structure of an atom or molecule describes the arrangement of electrons in its atomic or molecular orbitals. This concept is fundamental to chemistry since it dictates an element's chemical properties and reactivity, based on its ability to lose, gain, or share electrons.

When discussing Lewis acids and bases, we focus on the valence shell electrons or the electrons in the outermost shell. A complete valence shell typically leads to chemical stability, while an incomplete shell makes an atom more reactive. For example, boron in \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3}\mathrm{B}\) seeks to complete its valence shell by accepting a lone pair of electrons - thus, its electronic structure explains its role as a Lewis acid. Observing the electronic structure can predict how an atom or molecule will behave in a chemical reaction and whether it will act as a Lewis acid or base.