Question

Why do aldehydes and ketones have lower boiling points than alcohols with similar molecular masses? Why are their boiling points higher than those of alkanes with similar molecular masses?

Short answer

Aldehydes and ketones have dipole-dipole forces, weaker than alcohol's hydrogen bonds. Alkanes only have Van der Waals forces, weaker than the carbonyl's dipole-dipole.

Step-by-step solution

Understand the Composition

To answer the question, we need to identify the different functional groups involved: aldehydes, ketones, alcohols, and alkanes. Aldehydes and ketones have carbonyl groups (-C=O), alcohols have hydroxyl groups (-OH), and alkanes have only C-H bonds without functional groups.

Identify Intermolecular Forces

Alcohols can form hydrogen bonds due to their hydroxyl group, which significantly increases the boiling point. Aldehydes and ketones, having carbonyl groups, can only engage in dipole-dipole interactions, which are weaker than hydrogen bonds. Alkanes can only engage in Van der Waals interactions, which are even weaker.

Compare Boiling Points

Boiling point is influenced by the strength of intermolecular forces. Since alcohols can form strong hydrogen bonds, they have higher boiling points than aldehydes and ketones of similar molar masses. Aldehydes and ketones, in turn, have stronger dipole-dipole interactions than the Van der Waals forces in alkanes, leading to higher boiling points compared to similar molar mass alkanes.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules. These forces are crucial in determining the physical properties of compounds, such as their boiling points. Different types of intermolecular forces vary in strength:

• Hydrogen bonds — the strongest among the common intermolecular forces.
• Dipole-dipole interactions — moderate in strength and occur between polar molecules.
• Van der Waals forces — the weakest, found in all molecules, but most significant in nonpolar compounds like alkanes.

The stronger the intermolecular forces, the higher the boiling point of a substance because more energy is required to separate the molecules during the phase change from liquid to gas.

Functional Groups

Functional groups define the chemical characteristics and reactions of organic molecules. They play a significant role in determining the boiling points of compounds:

• Alcohols — contain hydroxyl groups (-OH), which enable hydrogen bonding.
• Aldehydes and Ketones — possess carbonyl groups (-C=O), which participate in dipole-dipole interactions.
• Alkanes — lack functional groups, engaging only in weak Van der Waals forces.

Alcohols generally have higher boiling points than aldehydes and ketones of similar molecular masses because hydrogen bonds in alcohols are stronger than the dipole-dipole interactions in carbonyl-containing compounds.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that significantly raises boiling points. It occurs between a hydrogen atom bonded to a highly electronegative atom (like oxygen) and another electronegative atom with a lone pair of electrons. In alcohols:

• The presence of an -OH group allows the formation of hydrogen bonds both within and between molecules.
• This not only increases boiling points but also contributes to unique properties like solubility in water.

Hydrogen bonding is absent in aldehydes and ketones due to the different nature of their functional groups, leading to comparatively lower boiling points.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in polar molecules where partial positive and negative charges attract each other. These interactions are stronger than Van der Waals forces but weaker than hydrogen bonds:

• Aldehydes and ketones have a carbonyl group, resulting in a polar bond due to differences in electronegativity between carbon and oxygen.
• These compounds engage in dipole-dipole interactions, making their boiling points higher than nonpolar alkanes.

Although weaker than hydrogen bonds, these interactions still significantly influence the boiling points, explaining why aldehydes and ketones boil at higher temperatures than similar mass alkanes but lower than alcohols.