Question

Use Lewis structures to show how the following reaction can be viewed as the displacement of one Lewis base by another Lewis base from a Lewis acid. Identify the two Lewis bases and the Lewis acid. $$ \mathrm{NH}_{2}^{-}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{NH}_{3}+\mathrm{OH}^{-} $$

Short answer

In the reaction, \( \mathrm{NH}_{2}^{-} \) acts as the Lewis base that displaces the \( \mathrm{OH}^{-} \) base from the water (\( \mathrm{H}_{2}O \) ), which is acting as a Lewis acid. \( \mathrm{NH}_{3} \) and \( \mathrm{OH}^{-} \) are the products formed.

Step-by-step solution

Identify Lewis Acids and Bases in Reactants

A Lewis acid is a species that accepts an electron pair, while a Lewis base donates an electron pair. In the reaction, \( \mathrm{NH}_{2}^{-} \) has a lone pair of electrons and is negatively charged, indicating it is a Lewis base. \( \mathrm{H}_{2}O \) can act as both a Lewis acid and a Lewis base, but in this reaction, it serves as a Lewis acid because it will accept an electron pair.

Show Displacement Using Lewis Structures

Draw the Lewis structures for the reactants. \( \mathrm{NH}_{2}^{-} \) donates its lone pair to \( \mathrm{H}_{2}O \), forming \( \mathrm{NH}_{3} \) and displacing the \( \mathrm{OH}^{-} \) group from \( \mathrm{H}_{2}O \) as the product.

Draw the Lewis Structures of the Products

After the electron pair is donated from \( \mathrm{NH}_{2}^{-} \) to \( \mathrm{H}_{2}O \) the Lewis structures of the products show that \( \mathrm{NH}_{3} \) is formed without a charge, and \( \mathrm{OH}^{-} \) is formed with a negative charge. This indicates the completion of the displacement reaction.

Identify the Displaced Lewis Base

From the products, we can see that \( \mathrm{OH}^{-} \) has been displaced as a Lewis base. It originally was part of the \( \mathrm{H}_{2}O \) molecule, which served as the Lewis acid.

Key concepts

Lewis Acid and Base Reaction

Understanding the role of acids and bases is pivotal in chemistry, especially in a Lewis acid and base reaction. A Lewis acid is not restricted by the presence of hydrogen. Instead, it is characterized as a species that can accept an electron pair. A Lewis base, on the other hand, is a species that donates an electron pair.

When these two types of chemicals react, the base donates a pair of electrons to the acid, facilitating a reaction that results in the formation of new products. In the given exercise, the Lewis base is identified as \( \mathrm{NH}_{2}^{-} \) because of its lone electron pair and negative charge, indicating its ability to donate electrons. The water molecule \( \mathrm{H}_{2}O \) acts as a Lewis acid since it accepts the electron pair, despite water often being a Lewis base in other contexts.

Electron Pair Displacement

Electron pair displacement is the movement of an electron pair from one atom to another during a chemical reaction. This is a central feature of Lewis acid-base chemistry. In this scenario, the displacement of an electron pair leads to the Lewis base (the donor) forming a bond with the Lewis acid (the acceptor).

For instance, in the exercise, \( \mathrm{NH}_{2}^{-} \) donates an electron pair to \( \mathrm{H}_{2}O \)—the electron pair is effectively 'moving' from the nitrogen to the oxygen. This displacement is visualized through the change in the structure of the molecules in the reaction, leading to the formation of ammonia (\(\mathrm{NH}_{3}\)) and a hydroxide ion (\(\mathrm{OH}^{-}\)), showing how one Lewis base can be displaced by another.

Drawing Lewis Structures

Drawing Lewis structures is a way to represent molecules using dots to signify valence electrons. It helps in visualizing the bonding between atoms and the lone pairs of electrons that may exist. These structures are incredibly useful in predicting the shapes of molecules, their reactivity, and how they might interact in a chemical reaction.

To draw a correct Lewis structure, one must count the total number of valence electrons, arrange the atoms with least electronegative in the center, form bonds (lines) to connect atoms, and then distribute remaining electrons to fulfill the octet rule (or duet rule for hydrogen). In the case of the exercise, the structures illustrate the electron pair donation and displacement, thereby serving as a guide to understanding the reaction process itself.

Chemical Reaction Mechanisms

A chemical reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs. It outlines how the reactants transform into products, detailing the breaking and formation of bonds, the order of steps, and the movement of electrons during the course of a reaction.

Understanding the mechanism is essential for predicting the outcome of a reaction and can help identify the intermediates, transition states, and rate-determining steps involved. In our exercise, the mechanism involves the transfer of an electron pair from a Lewis base to a Lewis acid, followed by bond reorganization that results in the displacement of another base. This can be represented symbolically through each step in the solution or visually through the drawn Lewis structures, highlighting the mechanism behind the Lewis acid-base reaction.