Understanding Melting and Boiling Points of Organic Compounds

Expert reviewed • 04 March 2025 • 3 minute read

Introduction

Understanding how different functional groups affect the melting and boiling points of organic compounds is crucial for predicting their physical properties and behavior. This article explores how intermolecular forces influence these properties across various organic families.

Intermolecular Forces in Organic Compounds

The primary intermolecular forces that affect melting and boiling points are:

Hydrogen Bonding
Dipole-Dipole Interactions
Van der Waals Forces (London Dispersion Forces)

Comparing Functional Groups

Alkanes (R-H)

Weakest intermolecular forces (only van der Waals)
Boiling points increase with molecular mass
Example: Butane ( $C_4H_{10}$ ) boils at -0.5°C

Alcohols (R-OH)

Strong hydrogen bonding between -OH groups
Higher boiling points than alkanes of similar mass
Example: Butanol ( $C_4H_9OH$ ) boils at 117.7°C

Carboxylic Acids (R-COOH)

Strongest hydrogen bonding (can form dimers)
Highest boiling points among similar-sized molecules
Example: Butanoic acid ( $C_3H_7COOH$ ) boils at 163.5°C

Amines (R-NH₂)

Moderate hydrogen bonding capability
Boiling points between alcohols and alkanes
Example: Butylamine ( $C_4H_9NH_2$ ) boils at 77.8°C

Amides (R-CONH₂)

Strong hydrogen bonding network
High melting and boiling points
Example: Butanamide ( $C_3H_7CONH_2$ ) melts at 115°C

Trends in Boiling Points

For compounds of similar molecular mass: Carboxylic Acids > Amides > Alcohols > Amines > Alkanes

HSC Chemistry