Question

Ethanol and dimethyl ether have the same molecular formula, \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\). Ethanol boils at \(78.4^{\circ} \mathrm{C}\), whereas dimethyl ether boils at \(-23.7^{\circ} \mathrm{C}\). Their structural formulas are $$ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \quad \mathrm{CH}_{3} \mathrm{OCH}_{3} $$ ethanol \(\quad\) dimethyl ether Explain why the boiling point of the ether is so much lower than the boiling point of ethanol.

Short answer

The boiling point of dimethyl ether is lower than that of ethanol because it lacks hydrogen bonding, which results in weaker intermolecular forces, requiring less energy to reach the boiling point.

Step-by-step solution

Analyze Intermolecular Forces

Identify the types of intermolecular forces present in each compound. Ethanol has hydrogen bonding due to the presence of an -OH group. Dimethyl ether has dipole-dipole interactions and dispersion forces but lacks hydrogen bonding because it does not have an -OH group.

Compare the Strength of the Intermolecular Forces

Compare the strength of these intermolecular forces. Hydrogen bonds are stronger than dipole-dipole interactions and dispersion forces. Therefore, ethanol will have stronger intermolecular forces compared to dimethyl ether.

Correlate Intermolecular Forces to Boiling Points

Understand that stronger intermolecular forces lead to higher boiling points because more energy is required to overcome these forces during the phase change from liquid to gas. Since ethanol has stronger intermolecular forces due to hydrogen bonding, it has a higher boiling point compared to dimethyl ether.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a strong type of intermolecular force that greatly influences the physical properties of substances, such as their boiling points. When a hydrogen atom is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, it can form a hydrogen bond with another electronegative atom in a different molecule.

In the case of ethanol, represented as \( \mathrm{CH}_3\mathrm{CH}_2\mathrm{OH} \), the hydrogen atoms attached to the oxygen of the hydroxyl group \( -OH \) are capable of forming hydrogen bonds. This bonding creates a strong attraction between ethanol molecules, requiring more thermal energy to separate them, thus raising the boiling point. These bonds are not present in dimethyl ether (\( \mathrm{CH}_3\mathrm{OCH}_3 \)), which instead relies on weaker dipole-dipole interactions and dispersion forces, resulting in a much lower boiling point.

Boiling Point

The boiling point of a substance can provide insights into the intermolecular forces at play. It is the temperature at which a liquid turns into a vapor. At this point, the thermal energy overcomes the attractive forces between the molecules.

For a molecule with hydrogen bonding like ethanol, a high boiling point signifies that considerable energy is required to break the bonds holding the molecules together. In contrast, a lower boiling point – as seen with dimethyl ether – suggests that the intermolecular forces are weaker, meaning a lower amount of energy is sufficient for the molecules to enter the gaseous phase. This difference in boiling points between substances having similar molecular formulas, like ethanol and dimethyl ether, is directly attributed to the presence or absence of hydrogen bonding.

Chemical Structure

The chemical structure of a compound is the definitive factor determining the type and strength of intermolecular forces it can exhibit. The structure dictates how molecules are arranged and how they will interact with each other.

Although ethanol and dimethyl ether share the same molecular formula \( \mathrm{C}_2\mathrm{H}_6\mathrm{O} \), their structural formulas reveal very different arrangements of atoms. Ethanol possesses a hydroxyl group that facilitates the formation of hydrogen bonds, whereas dimethyl ether, with its ether group \( -O- \), does not.

The chemical structure also determines the molecule's polarity and the potential for dipole-dipole interactions, which are less significant compared to hydrogen bonds. Therefore, analyzing the structure is crucial to understand the intermolecular forces and resulting properties like boiling points.