Alkynes are hydrocarbon molecules defined by the presence of at least one carbon-carbon triple bond. These molecules exist in various physical states, defined primarily by their boiling point and vapor pressure. The boiling point is the temperature at which a substance transitions from a liquid to a gas, occurring when the liquid’s vapor pressure equals the surrounding atmospheric pressure.
Vapor pressure is the pressure exerted by a gas that is in thermodynamic equilibrium with its condensed phases in a closed system. These two physical properties are inversely related: a higher boiling point corresponds to a lower vapor pressure. Understanding how the length of the carbon chain influences these properties requires examining the forces that hold the liquid together.
Intermolecular Forces and Phase Change
A change of state from a liquid to a gas, known as vaporization, requires energy to separate individual molecules from one another. This necessary energy directly relates to the strength of the attractive forces operating between the molecules, which are called intermolecular forces (IMFs). To boil a liquid, enough thermal energy must be introduced to overcome these molecular attractions.
Alkynes, like all hydrocarbons, are nonpolar molecules, meaning they lack a permanent separation of charge. Consequently, the sole attractive force governing their liquid-state behavior is the London Dispersion Force (LDF), a type of van der Waals force. These forces arise from the continuous, random motion of electrons, which momentarily creates an uneven distribution of charge.
This temporary imbalance results in an instantaneous dipole, which induces a corresponding dipole in a neighboring molecule. The resulting attraction keeps the alkyne molecules associated in the liquid state. When these intermolecular forces are stronger, more energy is required to pull the molecules apart, leading to a higher boiling temperature. Conversely, stronger forces suppress a molecule’s tendency to escape the liquid phase, resulting in a lower vapor pressure.
How Molecular Size Affects Attractive Forces
The primary factor determining the magnitude of the London Dispersion Forces in alkynes is the overall size and shape of the molecule. As the carbon chain length increases, the molecular mass of the alkyne increases, which corresponds to a greater number of electrons. The larger and more numerous the electron cloud becomes, the more easily it can be distorted or “polarized” by a neighboring molecule’s instantaneous dipole.
This greater ease of polarization significantly enhances the strength of the London Dispersion Forces. A longer alkyne chain also possesses a substantially increased surface area. This elongated, more linear shape allows for more extensive and effective contact between neighboring molecules in the liquid state.
While any single LDF interaction remains weak, the total attractive force becomes much greater because there are many more opportunities for these instantaneous dipoles to align and interact. The increased surface-to-surface interaction is the main structural reason why longer chains exhibit stronger intermolecular attraction. Increasing the chain length effectively increases the total attractive energy holding the liquid together.
The Combined Effect on Boiling Point and Vapor Pressure
The increase in chain length and the resulting strengthening of the London Dispersion Forces have a direct and predictable impact on the alkyne’s physical properties. To transition a longer-chain alkyne from a liquid to a gas, a greater input of thermal energy is necessary to overcome the stronger, cumulative intermolecular attraction. This requirement translates directly to an elevated boiling point.
For example, propyne (a three-carbon alkyne) boils at a much lower temperature than hexyne (a six-carbon alkyne) because the longer hexyne molecule requires more heat to break its stronger cohesive forces. The same mechanism explains the corresponding change in vapor pressure. Since the molecules are held more tightly within the liquid phase by stronger London Dispersion Forces, fewer molecules possess the necessary energy to escape into the gaseous state.
This reduced tendency results in a lower vapor pressure for the longer-chain alkynes. Therefore, increasing the chain length of an alkyne results in a higher boiling point and a lower vapor pressure.