The boiling point is the specific temperature required to change a substance from a liquid state into a gaseous state. This transformation happens when the liquid’s vapor pressure equals the pressure exerted by the surrounding atmosphere. Achieving this phase change requires energy to overcome the forces holding the molecules together. Generally, a direct relationship exists between the length of a carbon chain and the temperature at which it boils, meaning longer chains have higher boiling points.
Understanding Intermolecular Forces
The physical state of a substance is determined by the forces acting between its molecules, known as intermolecular forces (IMFs). Boiling requires enough energy to completely separate these molecules, allowing them to escape into the gas phase. Stronger intermolecular attractions mean that more thermal energy is necessary to overcome them, resulting in a higher boiling point.
In simple carbon-chain molecules, such as alkanes, the primary force is the London Dispersion Force (LDF). LDFs are temporary, attractive forces present between all molecules, occurring because the electrons surrounding an atom or molecule are constantly shifting. This motion occasionally causes a temporary imbalance in electron distribution, creating a momentary, weak dipole. This temporary dipole can then induce a corresponding dipole in a neighboring molecule, leading to a brief attraction. The ease with which a molecule’s electron cloud can be distorted is called polarizability, and it directly influences the strength of the LDFs.
How Chain Length Increases Boiling Point
The length of a carbon chain directly affects the magnitude of the London Dispersion Forces, which explains the rise in boiling points. Longer chains possess a greater number of electrons and a larger overall molecular surface area. This larger surface area allows for a more extensive area of contact between adjacent molecules in the liquid phase.
More points of contact between molecules mean that the cumulative London Dispersion Forces are significantly stronger. Consider a short chain like methane (one carbon) compared to a long chain like octane (eight carbons). The longer octane chain has many more opportunities for the temporary electron imbalances to interact with its neighbors, making the total attraction greater.
These stronger cumulative forces require a higher input of thermal energy to break apart the molecules and turn the liquid into a gas. As a result, the boiling point increases in a nearly linear fashion as the carbon chain is extended. For example, the boiling point of a straight-chain alkane rises by about 20–30 °C for every carbon atom added to the chain.
The Effect of Molecular Branching
The rule that longer chains have higher boiling points requires a qualification when considering molecules with the same number of carbon atoms but different shapes, known as isomers. Molecular shape, specifically the presence of branching, introduces a significant effect on the boiling point. Branching involves side chains of carbon atoms coming off the main backbone.
A molecule with a straight chain has the largest possible surface area for interaction. Branching causes the molecule to become more compact and spherical, which reduces the total surface area available for contact with neighboring molecules. This reduced contact area means there are fewer sites for the London Dispersion Forces to act between molecules.
For instance, n-pentane, a straight-chain molecule with five carbons, has a higher boiling point (36.1 °C) than its isomer, neopentane (2,2-dimethylpropane). Neopentane has a highly branched, spherical structure and boils at a much lower temperature (10 °C). The compact shape of the branched isomer prevents the molecules from packing as tightly together as the linear chains. This looser packing results in weaker net intermolecular forces, meaning less energy is needed to achieve boiling.