Problem 56 - Chemistry The Molecular Nature of Matter

Question

Pyridine, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N},\) is a fishy-smelling compound used as an intermediate in making insecticides. It is a weak base. Write a chemical equation showing its reaction with water.

Step-by-Step Solution

Answer

Pyridine reacts with water as follows: \( \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N} + \mathrm{H}_2\mathrm{O} \longleftrightarrow \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NH}^+ + \mathrm{OH}^- \).

Step 1: Write the chemical formula of pyridine

Firstly, write down the chemical formula of pyridine, which is \( \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N} \).

Step 2: Write the chemical formula of water

Next, write down the chemical formula of water, which is \( \mathrm{H}_2\mathrm{O} \).

Step 3: Represent the reaction of pyridine as a weak base

Since pyridine is a weak base, it will accept a proton (\( \mathrm{H}^+ \) ion) from water. Represent this by writing a reversible reaction equation: \[ \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N} + \mathrm{H}_2\mathrm{O} \longleftrightarrow \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NH}^+ + \mathrm{OH}^- \].

Key Concepts

Pyridine

Pyridine is an organic compound with the chemical formula \( \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N} \). It is a heterocyclic aromatic compound, meaning it has a ring structure that includes at least one atom that is not carbon—in this case, nitrogen. The nitrogen atom in pyridine is basic because it has a pair of electrons that can be donated to an acid. However, unlike stronger bases such as sodium hydroxide (\( \mathrm{NaOH} \) ), pyridine is considered a weak base. This means it does not fully ionize in solution and is less likely to donate its electrons in proton transfer reactions. In the context of the given exercise, when pyridine reacts with water, it accepts a proton to become \( \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NH}^+ \) and generates hydroxide ions (\( \mathrm{OH}^- \) ). The smell often associated with pyridine is typically described as fishy or unpleasant, which can be noticeable even in low concentrations.

Weak Bases in Chemistry

In chemistry, a weak base is one that does not fully dissociate into its ions in an aqueous solution. This is in contrast to a strong base, which disassociates completely. The strength of a base is typically discussed in the context of its ability to accept protons from acids. Weak bases like pyridine have limited ability to bond with protons, which makes their reactions reversible rather than driving to completion.

When a weak base reacts with water, the equilibrium lies far to the left with the majority of the base remaining unreacted. This is because only a small portion of the base reacts to form the conjugate acid and hydroxide ion. This behavior is quantified by the base dissociation constant (\( K_b \) ), which is a measure of how readily a base reacts with water to accept a proton. A higher \( K_b \) value indicates a stronger base, capable of more extensively accepting protons.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They illustrate the substances involved in a reaction, their phase (solid, liquid, gas, or aqueous), and the direction in which the reaction proceeds. A balanced chemical equation shows the conservation of mass, indicating that atoms are neither created nor destroyed during the process. It ensures that the number of atoms for each element in the reactants is equal to that in the products.

The equation \( \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N} + \mathrm{H}_2\mathrm{O} \longleftrightarrow \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NH}^+ + \mathrm{OH}^- \) represents the reaction of pyridine with water. The use of the double arrow (\longleftrightarrow) signifies that the reaction is in equilibrium and can proceed in both the forward and reverse directions. Writing a clear and balanced chemical equation is essential for understanding the stoichiometry of the reaction which is fundamental for calculations in chemistry.

Proton Transfer Reactions

Proton transfer reactions are a subclass of acid-base reactions. They involve the transfer of a proton (\( \mathrm{H}^+ \) ) from an acid to a base. Acidity and basicity in a given reaction are comparative; substances act as acids if they can donate a proton, and as bases if they can accept a proton. This proton transfer is what constitutes an acid-base reaction.

In the equation for the reaction between pyridine and water, pyridine acts as the base and water as the acid. Proton transfer reactions are often reversible, as indicated by the reversible arrow in the equation \( \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N} + \mathrm{H}_2\mathrm{O} \longleftrightarrow \mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NH}^+ + \mathrm{OH}^- \). This also symbolizes the establishment of an equilibrium in which both the forward (base accepting a proton) and reverse (conjugate acid donating a proton) reactions occur. Understanding proton transfer reactions is crucial in organic chemistry, as they are fundamental to many reaction mechanisms and processes.