Physical Properties of Alkynes

The physical properties of alkynes, such as boiling point, melting point, and solubility, are largely determined by their molecular structure, particularly the presence of the carbon-carbon triple bond, and the resulting intermolecular forces.

Boiling Point

• Comparison to Alkanes and Alkenes: Alkynes generally have boiling points that are slightly *higher* than alkanes and alkenes with the same number of carbon atoms. However, the difference is not dramatic.
• London Dispersion Forces: The primary intermolecular forces in alkynes (and alkanes, alkenes) are London dispersion forces (instantaneous dipole-induced dipole interactions). The slightly higher boiling points compared to alkanes and alkenes can be attributed to the more linear shape of alkynes, which allows for slightly better packing and thus stronger London Dispersion forces.
• Effect of Size: Boiling points increase with increasing molecular size (number of carbon atoms) due to stronger London dispersion forces.
• Branching: Branching in the carbon chain *decreases* the boiling point, similar to alkanes and alkenes, as it reduces the surface area available for London dispersion forces.

Melting Point

• Comparison to Alkanes and Alkenes: Melting point trends for alkynes can be somewhat more complex, similar to alkenes.
• Factors Affecting Melting Point: Melting points are influenced by how well molecules pack together in the solid state. Symmetry plays a role.
• Trends: There is no simple trend comparing alkynes to alkanes or alkenes. Melting points depend on how well the molecules pack.

Solubility

• Insoluble in Water: Alkynes are essentially *insoluble* in water due to their nonpolar nature. The dominant intermolecular forces are weak London dispersion forces. They cannot form hydrogen bonds with water.
• Soluble in Organic Solvents: Alkynes are soluble in most nonpolar organic solvents (e.g., hexane, diethyl ether).

Density

• Alkynes are generally less dense than water.

Summary Table (Illustrative Trends)