Question

Indicate whether the following compounds have boiling points that are generally higher or lower than that of a hydrocarbon of comparable molecular mass. (a) aldehydes (b) ketones

Short answer

Aldehydes and ketones have higher boiling points than hydrocarbons of similar molecular mass due to stronger dipole-dipole interactions.

Step-by-step solution

Understand Boiling Points

The boiling point of a compound is influenced by the strength of the intermolecular forces present. Stronger intermolecular forces lead to higher boiling points. Common forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Examine Intermolecular Forces in Hydrocarbons

Hydrocarbons are composed of only carbon and hydrogen atoms. They primarily exhibit London dispersion forces, which are generally weaker compared to other types of intermolecular forces. This often results in lower boiling points compared to similar-sized molecules with stronger forces.

Investigate Aldehydes

Aldehydes have the general formula RCHO. The carbonyl group (C=O) in aldehydes creates a dipole moment, leading to dipole-dipole interactions which are stronger than the London dispersion forces in hydrocarbons. Therefore, aldehydes generally have higher boiling points than hydrocarbons of similar molecular mass.

Investigate Ketones

Ketones have the general formula RCOR'. The carbonyl group in ketones, like in aldehydes, results in a significant dipole, enabling dipole-dipole interactions. These interactions are typically stronger than the London dispersion forces in hydrocarbons, so ketones also tend to have higher boiling points than hydrocarbons of comparable molecular mass.

Key concepts

Intermolecular forces

Intermolecular forces are the attractions between molecules. They play a crucial role in determining the boiling points of compounds. When these forces are strong, the molecules require more energy to separate and transition from liquid to gas, leading to a higher boiling point.

There are several types of intermolecular forces:

• London Dispersion Forces: Present in all molecules, these forces are the weakest. They arise from temporary shifts in electron density that create instantaneous dipoles.


• Dipole-Dipole Interactions: Occur in molecules with permanent dipoles, such as aldehydes and ketones. These interactions are stronger than London dispersion forces.


• Hydrogen Bonding: This is a special type of dipole-dipole interaction and is the strongest. It occurs when hydrogen is bonded to electronegative atoms like oxygen, nitrogen, or fluorine.

Understanding these forces helps predict the physical properties of compounds, especially their boiling points.

Aldehydes

Aldehydes are organic compounds with the formula RCHO, where R can be a hydrogen atom or a carbon-containing substituent. The critical feature of aldehydes is the presence of a carbonyl group (C=O), which is positioned at the end of the carbon chain.

The carbonyl group in aldehydes contributes significantly to their properties:

• Dipole Moment: Aldehydes possess a permanent dipole due to the carbon-oxygen double bond. This dipole enhances intermolecular forces through dipole-dipole interactions.


• Boiling Points: Due to stronger dipole-dipole interactions than hydrocarbons, aldehydes typically have higher boiling points than hydrocarbons of similar molecular mass.

Aldehydes are often more reactive than their structural relatives, ketones, making them of particular interest in chemical synthesis and reactions.

Ketones

Ketones are another class of organic compounds featuring a carbonyl group (C=O) but positioned within the carbon chain rather than at the end, which is the distinctive feature of aldehydes. Their general formula is RCOR', where R and R' are carbon-containing groups.

Ketones also exhibit properties stemming from their carbonyl group:

• Dipole-Dipole Interactions: Like aldehydes, ketones have a permanent dipole due to their carbonyl group. This results in dipole-dipole interactions that are stronger than London dispersion forces.


• Boiling Points: Due to the presence of a strong dipole, ketones generally have higher boiling points than hydrocarbons of similar molecular weight.

Ketones are used widely in industry, especially as solvents and intermediates in the synthesis of other chemicals. Understanding their properties, like boiling points, is essential when considering their practical applications.