Question

Classify each of the following species as a Lewis acid or a Lewis base: (a) \(\mathrm{CO}_{2},(\mathrm{~b}) \mathrm{H}_{2} \mathrm{O},(\mathrm{c}) \mathrm{I}^{-},(\mathrm{d}) \mathrm{SO}_{2},(\mathrm{e}) \mathrm{NH}_{3},\) (f) \(\mathrm{OH}^{-},(\mathrm{g}) \mathrm{H}^{+},(\mathrm{h}) \mathrm{BCl}_{3}\)

Short answer

CO2, SO2, H+, BCl3 are Lewis acids; H2O, I-, NH3, OH- are Lewis bases.

Step-by-step solution

Defining Lewis Acids and Bases

A Lewis acid is a chemical species that can accept an electron pair, while a Lewis base is a species that can donate an electron pair.

Analyzing CO2

\( \mathrm{CO}_2 \) can accept an electron pair into its carbon atom, making it a Lewis acid.

Analyzing H2O

\( \mathrm{H}_2\mathrm{O} \) has lone pairs on oxygen that can be donated, thus acting as a Lewis base.

Analyzing I-

\( \mathrm{I}^- \) has extra electrons and can donate an electron pair, classifying it as a Lewis base.

Analyzing SO2

\( \mathrm{SO}_2 \) can accept an electron pair through sulfur, making it function as a Lewis acid.

Analyzing NH3

\( \mathrm{NH}_3 \) has a lone pair of electrons on nitrogen, allowing it to act as a Lewis base.

Analyzing OH-

\( \mathrm{OH}^- \) can donate an electron pair due to the lone pairs on oxygen, so it is a Lewis base.

Analyzing H+

\( \mathrm{H}^+ \) is a proton with no electrons, needing to accept an electron pair, therefore it is a Lewis acid.

Analyzing BCl3

\( \mathrm{BCl}_3 \) can accept an electron pair into the empty orbital of boron, making it a Lewis acid.

Key concepts

Electron Pair Donation

In chemistry, electron pair donation refers to the capability of a chemical species to donate, or give up, a pair of electrons to another species. This is a key trait of Lewis bases.

Lewis bases possess lone pairs of electrons that can be shared with another chemical entity. The ability to donate these electrons allows them to form new chemical bonds with Lewis acids.

For example:

• Water (\(\mathrm{H}_2\mathrm{O}\)) can donate an electron pair through its oxygen atom.
• Ammonia (\(\mathrm{NH}_3\)) has a lone pair on nitrogen, making it a donor.

This electron pair donation is crucial in various chemical reactions and plays an essential role in molecular interactions.

Electron Pair Acceptance

Electron pair acceptance is the ability of a species to accept or take in a pair of electrons from a donor. This is the defining property of Lewis acids.

Lewis acids have empty orbitals that can accommodate these electron pairs. By accepting an electron pair, they form a new bond with the Lewis base.

Consider these examples:

• Carbon dioxide (\(\mathrm{CO}_2\)) can accept electron pairs into its carbon atom.
• Boron trichloride (\(\mathrm{BCl}_3\)) accepts electron pairs into the empty orbital of boron.

This interaction is fundamental to understanding how molecules engage and react with each other.

Chemical Species Classification

Classifying chemical species as either Lewis acids or Lewis bases helps us understand their behavior in reactions.

This classification is based on their ability to either donate or accept electron pairs.

• Lewis bases are often negatively charged or neutral molecules with lone electron pairs.
• Lewis acids are typically positively charged or have vacant orbitals ready to accept electron pairs.

Examples of classification include:

• Sulfur dioxide (\(\mathrm{SO}_2\)) acts as a Lewis acid.
• The hydroxide ion (\(\mathrm{OH}^-\)) functions as a Lewis base.

By classifying these species, we predict and understand their participation in chemical reactions.

Lewis Acid

A Lewis acid is a chemical species with the ability to accept an electron pair from a Lewis base. This makes them electron pair acceptors.

Lewis acids often have incomplete octets or positively charged ions that attract additional electrons.

Examples include:

• Hydrogen ion (\(\mathrm{H}^+\)) lacks electrons and readily accepts a pair.
• Boron trichloride (\(\mathrm{BCl}_3\)) with an empty orbital on boron suitable for electron pair acceptance.

Understanding Lewis acids is essential for exploring many reactions, including catalysis and complexation.

Lewis Base

Lewis bases are compounds that can donate an electron pair to a Lewis acid, forming a coordinate covalent bond.

These species usually have one or more lone pairs of electrons.

Examples of Lewis bases include:

• The iodide ion (\(\mathrm{I}^-\)) that carries extra electrons, making it a donor.
• The ammonia molecule (\(\mathrm{NH}_3\)), with lone pairs on the nitrogen atom.

Knowing how these bases interact with acids helps in understanding formation of adducts and the mechanisms of many reactions.