Question

Propane, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\), is a gas at room temperature, while propanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), is a liquid. Why the difference?

Short answer

Propanol is a liquid at room temperature because it can form hydrogen bonds due to the hydroxyl group which result in stronger intermolecular forces, whereas propane is a gas due to weaker van der Waals forces.

Step-by-step solution

Understand the Molecular Structure

Examine the molecular structures of both propane and propanol to identify key differences. Propane is a hydrocarbon with only C-H and C-C bonds, while propanol contains an -OH (hydroxyl) group in addition to the C-H and C-C bonds.

Analyze Intermolecular Forces

Recognize that the different intermolecular forces in each compound affect their state at room temperature. Propane has only weak van der Waals forces (dispersion forces), whereas propanol has hydrogen bonding due to the presence of the hydroxyl group.

Relate Intermolecular Forces to Physical State

Identify that hydrogen bonds in propanol are much stronger than the van der Waals forces in propane. These stronger intermolecular forces give propanol a higher boiling point, making it a liquid at room temperature, while propane, with weaker forces, is a gas.

Key concepts

Hydrogen Bonding

Hydrogen bonding is an exceptionally strong type of intermolecular force that significantly influences a substance's physical properties, such as its boiling point. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences attraction to another electronegative atom in a nearby molecule.

In the context of our exercise, propanol ((CH_3CH_2CH_2OH)) exhibits hydrogen bonding due to the presence of the hydroxyl (-OH) group. This group is polar, as the electronegativity difference between the oxygen and hydrogen atoms leads to an uneven distribution of electrons. The oxygen atom has a partial negative charge, while the hydrogen atom has a partial positive charge. This polarity allows the hydrogen to form a hydrogen bond with the oxygen atom of another propanol molecule, leading to a network of strong intermolecular attractions that increase the compound's boiling point.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure. It is a critical physical property influenced by the strength of intermolecular forces within the substance. Molecules with stronger intermolecular forces require more energy to break free from their neighbors and transition into the gaseous state; thus, these substances have higher boiling points.

When comparing propane and propanol, the strength of their respective intermolecular forces is the primary reason for the difference in boiling points and therefore their state at room temperature. Since propanol can form hydrogen bonds, a considerable amount of energy is needed to overcome these attractions. Propane, on the other hand, cannot form such bonds and only has weak van der Waals forces, making it considerably easier to vaporize — which is why propane is a gas at room temperature, while propanol is a liquid.

Molecular Structure

Molecular structure determines the types of intermolecular forces that can occur between molecules, which in turn affect a substance’s properties. The structure is defined by the types of atoms present, how they are bonded together, and their spatial arrangement.

Propane ((CH_3CH_2CH_3)) and propanol ((CH_3CH_2CH_2OH)) have similar carbon backbones but differ significantly due to the hydroxyl group in propanol. This small structural difference accounts for the disparate intermolecular forces present in these two compounds. Propanol's molecular structure allows it to engage in hydrogen bonding, while propane's does not. In educational content, emphasizing the direct link between molecular structure and the presence of specific intermolecular forces helps students understand why substances behave differently under similar conditions.