Question

"Butyric acid" and "ethyl acetate" have the same molecular formula, \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\). Predict which isomer has the higher boiling point.

Short answer

Butyric acid has the higher boiling point due to stronger hydrogen bonding.

Step-by-step solution

Understanding Molecular Structure

To predict boiling points, we first need to understand the structure of butyric acid and ethyl acetate. Butyric acid is a carboxylic acid with the formula C₃H₇COOH, while ethyl acetate is an ester with the formula CH₃COOC₂H₅. Each has the molecular formula C₄H₈O₂.

Boiling Point Trends

Boiling points are influenced by the type of intermolecular forces present. Carboxylic acids, like butyric acid, exhibit hydrogen bonding due to the presence of a hydroxyl (-OH) group. Esters, such as ethyl acetate, have dipole-dipole interactions but cannot form hydrogen bonds as effectively as acids.

Comparison of Intermolecular Forces

Hydrogen bonds are stronger than dipole-dipole interactions. Therefore, molecules that can form hydrogen bonds generally have higher boiling points compared to those that can only exhibit dipole-dipole interactions. Thus, butyric acid is expected to have stronger intermolecular forces than ethyl acetate.

Predicting the Higher Boiling Point

Given that butyric acid can form hydrogen bonds, and ethyl acetate cannot, the stronger hydrogen bonding means butyric acid will have a higher boiling point compared to ethyl acetate.

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. This property greatly depends on the strength of interactions between molecules. The stronger the intermolecular forces, the more energy is required for molecules to overcome these attractions and change into a gaseous state.

For isomers with the same molecular formula, such as butyric acid and ethyl acetate, the boiling point is influenced by their molecular structure, which determines the types of intermolecular forces present. Butyric acid, being a carboxylic acid, can form hydrogen bonds due to its hydroxyl (-OH) group. Conversely, ethyl acetate, an ester, primarily experiences dipole-dipole interactions.

Since hydrogen bonding is generally stronger than dipole-dipole interactions, butyric acid exhibits a higher boiling point than ethyl acetate. This highlights how variations in molecular structure directly impact the boiling point of similar compounds.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring molecules. These forces are key to understanding the physical properties of substances, such as boiling points and melting points. There are several types of intermolecular forces, including hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Hydrogen bonding occurs when a hydrogen atom is strongly attracted to a fluorine, oxygen, or nitrogen atom in neighboring molecules. This is a particularly strong type of dipole-dipole interaction. In butyric acid, hydrogen bonds form due to the presence of the -OH group, leading to a higher boiling point.

On the other hand, dipole-dipole interactions occur between molecules that have permanent dipoles. In ethyl acetate, these interactions are weaker than hydrogen bonds, resulting in a lower boiling point than butyric acid. Understanding these forces helps predict and explain the behavior and potential interactions of different molecules.

Molecular Structure

Molecular structure refers to the arrangement of atoms in a molecule and the chemical bonds that hold them together. This structure plays a crucial role in determining a compound's physical and chemical properties, including its boiling point.

Isomers such as butyric acid and ethyl acetate share the same molecular formula (C_4H_8O_2) but differ in structure. Butyric acid, a carboxylic acid, contains a functional group (-COOH) capable of forming hydrogen bonds. This structural feature significantly influences its higher boiling point.

In contrast, ethyl acetate, as an ester, has an -COOC- linkage unable to engage in hydrogen bonding effectively. Instead, it primarily exhibits dipole-dipole interactions. This structural variation explains why, despite having the same molecular formula, these isomers have different boiling points. Thus, analyzing molecular structure is essential for predicting and understanding the physical characteristics of compounds.