Question

Suggest a reason why trimethylamine, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N},\) has a lower boiling point (BP \(3{ }^{\circ} \mathrm{C}\) ) than dimethylamine, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (BP \(8^{\circ} \mathrm{C}\) ), despite having a larger number of atoms.

Short answer

Dimethylamine has a higher boiling point than trimethylamine because it can form hydrogen bonds, which are stronger intermolecular forces than the van der Waals forces present in trimethylamine.

Step-by-step solution

Understand Molecular Structure

Look at the molecular structure of both trimethylamine, \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{N},\) and dimethylamine, \(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{NH}.\) Note that trimethylamine has three methyl groups attached to the nitrogen while dimethylamine has two methyl groups and one hydrogen.

Analyze Intermolecular Forces

Recognize that the presence of a hydrogen atom directly bonded to the nitrogen in dimethylamine allows for hydrogen bonding, a strong type of intermolecular force. Trimethylamine lacks hydrogen atoms bonded to the electronegative nitrogen and thus cannot engage in hydrogen bonding.

Correlate Boiling Point with Intermolecular Forces

Understand that the ability to form hydrogen bonds has a significant impact on the boiling point of a compound. Since dimethylamine can form hydrogen bonds due to the hydrogen attached to the nitrogen, it has a higher boiling point than trimethylamine, which cannot form hydrogen bonds.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of intermolecular force that occurs in molecules containing hydrogen attached to highly electronegative elements such as nitrogen, oxygen, or fluorine. This type of bonding is characterized by the attraction between a hydrogen atom in one molecule and an electronegative atom in another molecule.

For a molecule to participate in hydrogen bonding, it must have a hydrogen atom bonded to an electronegative atom, making it a 'hydrogen bond donor'. Another molecule with a lone pair of electrons on an electronegative atom acts as a 'hydrogen bond acceptor'. The resulting interaction is much stronger than other dipole-dipole interactions and Van der Waals forces.

Since hydrogen bonds considerably increase a molecule's boiling point, molecules that can form hydrogen bonds typically have higher boiling points compared to those that cannot. This is because the intermolecular forces need greater energy, such as heat, to be overcome for the molecules to transition from liquid to gas.

Molecular Structure

Understanding molecular structure is crucial when studying how intermolecular forces affect physical properties like boiling points. The molecular structure dictates how atoms are arranged in a molecule and the types of bonds between them.

The arrangement can have a substantial effect on a molecule's ability to interact with other molecules. For instance, the shape of the molecule can influence its polarity, which is a separation of electric charge leading to a molecule having an electric dipole. A polar molecule with an uneven distribution of charge will exhibit stronger intermolecular forces than a nonpolar molecule with a symmetrical distribution of electrons.

In the case of amines, the number and placement of hydrogen atoms attached to the nitrogen can determine whether the molecule is capable of hydrogen bonding or not, which in turn affects boiling points.

Trimethylamine

Trimethylamine ((CH_3)_3N) is an organic compound known primarily for its unpleasant fishy odor. Structurally, it consists of a nitrogen atom surrounded by three methyl groups (-CH_3).

The key point to understand about trimethylamine's molecular structure in the context of boiling point is that it lacks hydrogen atoms attached to the electronegative nitrogen atom, which would allow for hydrogen bonding. As a result, the main intermolecular forces at play are Van der Waals forces, which are generally weaker than hydrogen bonds. This explains why trimethylamine has a relatively low boiling point, as less energy is required to break these intermolecular forces during the phase change from liquid to gas.

Dimethylamine

Dimethylamine ((CH_3)_2NH) is another amine that is structurally similar to trimethylamine but has a significant difference—it bears one less methyl group and one hydrogen atom bonded to the nitrogen.

This hydrogen allows dimethylamine to act as a hydrogen bond donor, engaging in hydrogen bonding with other molecules. These bonds are much stronger than the Van der Waals forces that dominate in trimethylamine. Therefore, dimethylamine exhibits a higher boiling point compared to trimethylamine because more energy is required to disrupt these hydrogen bonds during the transition from liquid to the gas phase.

This capacity to form hydrogen bonds makes dimethylamine not just more associated in the liquid phase, leading to a higher boiling point, but also more soluble in water than trimethylamine.