Question

"Propionic acid" and "methyl acetate" have the same molecular formula, \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}_{2}\). Predict which isomer has the higher boiling point.

Short answer

Propionic acid has the higher boiling point due to hydrogen bonding.

Step-by-step solution

Identifying Functional Groups

Propionic acid has a carboxylic acid functional group (-COOH), while methyl acetate has an ester functional group (-COOCH3). Carboxylic acids are known for having stronger intermolecular forces due to hydrogen bonding.

Understanding Hydrogen Bonding

Carboxylic acids, like propionic acid, can form hydrogen bonds, which are strong dipole-dipole attractions between a hydrogen atom attached to an electronegative atom (such as oxygen) and another electronegative atom.

Role of Intermolecular Forces

Hydrogen bonding significantly increases the boiling point due to the additional energy required to break these strong bonds during the phase change from liquid to gas. Esters generally cannot form hydrogen bonds with themselves.

Comparing Boiling Points

Since propionic acid can form hydrogen bonds due to its carboxylic acid functional group, it has stronger intermolecular forces compared to methyl acetate, which leads to a higher boiling point for propionic acid.

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that have certain properties and reactions. They play a significant role in determining the chemical behavior of the molecules. In the case of the isomers propionic acid and methyl acetate, different functional groups define their unique characteristics.

Propionic acid contains a carboxylic acid group (-COOH), known for its specific properties, especially in the presence of hydrogen bonding. This group consists of a carbon atom double-bonded to an oxygen atom (carbonyl) and a hydroxyl group (-OH) attached to the same carbon.

On the other hand, methyl acetate contains an ester group (-COOCH3). While esters are commonly used as solvents or flavorings, they differ from carboxylic acids because they lack the hydrogen atom that participates in hydrogen bonding with other molecules.

Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules, determining many physical properties, including boiling points and melting points. There are different types of intermolecular forces, such as van der Waals forces, dipole-dipole interactions, and hydrogen bonding.

In propionic acid, the presence of a carboxylic acid group allows for hydrogen bonding. This type of intermolecular force occurs when a hydrogen atom covalently bonded to a highly electronegative atom like oxygen experiences an attraction to another electronegative atom.

Esters like methyl acetate generally do not form hydrogen bonds with themselves because of lacking the hydrogen attached to an electronegative atom within the group. Thus, they rely more on dipole-dipole interactions and van der Waals forces, which are typically weaker than hydrogen bonds.

Boiling Point

The boiling point is the temperature at which a substance transitions from a liquid to a gas, and it is heavily influenced by intermolecular forces. Stronger intermolecular forces require more energy to overcome, resulting in a higher boiling point.

Due to the ability of propionic acid to form hydrogen bonds, it has stronger intermolecular forces compared to methyl acetate. This is because breaking these hydrogen bonds necessitates a greater amount of energy input during the phase change.

In contrast, methyl acetate can't form hydrogen bonds, having weaker intermolecular forces primarily because it relies on dipole-dipole attractions and van der Waals forces. Thus, methyl acetate's boiling point is lower, highlighting the relationship between functional groups, intermolecular forces, and boiling points.