Question

Hexylamine and triethylamine have the same molar mass. The boiling point of hexylamine is \(129^{\circ} \mathrm{C}(402 \mathrm{~K}),\) whereas that of triethylamine is only \(89^{\circ} \mathrm{C}(362 \mathrm{~K})\). Explain these observations.

Short answer

Hexylamine's hydrogen bonding increases its boiling point compared to triethylamine.

Step-by-step solution

Understand the Compounds

Hexylamine and triethylamine have the same molar mass but different structures. Hexylamine is a primary amine with the formula \( ext{C}_6 ext{H}_{13} ext{NH}_2 \) and triethylamine is a tertiary amine with the formula \( ( ext{C}_2 ext{H}_5)_3 ext{N} \).

Identify Boiling Point Factors

Boiling point is influenced by molecular interactions such as hydrogen bonding, dipole-dipole interactions, and van der Waals forces. We need to consider these interactions for both compounds.

Analyze Hexylamine

Hexylamine, as a primary amine, can form hydrogen bonds due to the presence of a \( ext{NH}_2 \) group. This hydrogen bonding significantly increases its boiling point.

Analyze Triethylamine

Triethylamine, a tertiary amine, lacks hydrogen atoms bonded to the nitrogen, preventing hydrogen bonding. It mainly relies on dipole-dipole interactions and van der Waals forces, which are weaker than hydrogen bonds.

Compare Boiling Points

Hexylamine's ability to hydrogen bond leads to a higher boiling point of \( 129^{ ext{o}} ext{C} \) (\( 402 ext{ K} \)), compared to triethylamine's boiling point of \( 89^{ ext{o}} ext{C} \) (\( 362 ext{ K} \)), which cannot form hydrogen bonds.

Key concepts

Molecular Interactions

Molecular interactions are forces that occur between molecules. These forces are crucial in determining properties like boiling points. There are several types of molecular interactions, including:

• Hydrogen Bonding: This is a strong type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. It plays a significant role in influencing boiling points.
• Dipole-Dipole Interactions: These occur in polar molecules where positive and negative charges attract each other, though they're generally weaker than hydrogen bonds.
• Van der Waals Forces: These are even weaker interactions that arise from temporary changes in electron density in molecules, affecting non-polar substances predominantly.

Hexylamine and triethylamine have different molecular interactions affecting their boiling points. Hexylamine's ability to form hydrogen bonds due to its structure gives it a higher boiling point than triethylamine.

Hydrogen Bonding

Hydrogen bonding is a special, stronger form of dipole-dipole interaction. It occurs when hydrogen is covalently bonded to electronegative atoms like nitrogen, as in the case of hexylamine. The hydrogen atom has a partial positive charge and can form a bond with a neighboring electronegative atom. This interaction significantly raises the boiling point.

In hexylamine, the presence of the \(\text{NH}_2\) group allows for the formation of hydrogen bonds between molecules. This means the molecules are more tightly held together when compared to a molecule like triethylamine, which lacks hydrogen atoms bonded to nitrogen and thus cannot form hydrogen bonds. Hence, the increased energy required to break these interactions results in a higher boiling point for hexylamine.

Primary and Tertiary Amines

Amines are classified based on the number of organic substituents attached to the nitrogen atom.

• Primary Amines: These have one organic substituent and two hydrogen atoms connected to nitrogen. Hexylamine is a primary amine. The presence of the \(\text{NH}_2\) group allows for hydrogen bonding, impacting its boiling point significantly.
• Tertiary Amines: These contain three organic groups attached to nitrogen, with no hydrogen atoms bonded to nitrogen. Triethylamine is a tertiary amine. It cannot form hydrogen bonds, relying instead on weaker interactions like dipole-dipole and van der Waals forces for intermolecular attraction.

The structural differences between primary and tertiary amines explain the variance in their boiling points. Primary amines like hexylamine can engage in hydrogen bonding, while tertiary amines cannot, leading to lower boiling points for tertiary amines.