Question

Explain why propanol has higher boiling point than that of the hydrocarbon. butane?

Short answer

Propanol can form hydrogen bonds, which are stronger than butane's London dispersion forces, leading to propanol's higher boiling point.

Step-by-step solution

Boiling Point and Intermolecular Forces

Understand that the boiling point of a substance is influenced by the strength of its intermolecular forces. The stronger these forces, the higher the energy required to overcome them, leading to a higher boiling point.

Examining Propanol's Intermolecular Forces

Propanol (C3H7OH) is an alcohol and contains an -OH (hydroxyl) group. This group is capable of hydrogen bonding, which is a strong type of dipole-dipole interaction between the hydrogen atom of one molecule and the oxygen atom of another.

Examining Butane's Intermolecular Forces

Butane (C4H10) is a hydrocarbon with only London dispersion forces, the weakest type of van der Waals forces, due to its nonpolar nature. It lacks any functional group that would engage in hydrogen bonding.

Comparing the Strength of Intermolecular Forces

Hydrogen bonds in propanol provide a stronger form of intermolecular attraction than the London dispersion forces in butane. Consequently, more energy is required to break these bonds in propanol, resulting in a higher boiling point than butane.

Key concepts

Intermolecular Forces

Intermolecular forces are the attraction forces that occur between molecules and dictate many physical properties, like boiling points.
These forces are weaker than intramolecular forces, which keep atoms bonded within a molecule.

There are several types of intermolecular forces, each varying in strength and effect on a substance's properties:

• Hydrogen bonds: very strong among intermolecular forces and greatly impact boiling points.
• Dipole-dipole interactions: occur between polar molecules where positive and negative ends attract.
• London dispersion forces: weakest type, present in all molecules but most significant in nonpolar substances.

Understanding the types and strengths of intermolecular forces in a substance helps explain why some substances boil at higher temperatures than others.

Propanol

Propanol is an alcohol with the chemical formula C₃H₇OH. It is an organic compound containing a hydroxyl (-OH) group, which is crucial for its physical and chemical characteristics.
The hydroxyl group is capable of forming hydrogen bonds with other propanol molecules, significantly increasing its intermolecular attraction.

This ability to form hydrogen bonds means:

• Propanol has stronger intermolecular forces than hydrocarbons like butane.
• The molecule's boiling point is higher as more energy is needed to break these bonds.
• This results in propanol being a liquid at room temperature, unlike some other similar-sized molecules.

These characteristics are why propanol can easily participate in reactions and is frequently used as a solvent.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction, occurring when hydrogen is bonded to a highly electronegative atom, like oxygen.
This creates a strong attraction between the hydrogen in one molecule and the electronegative atom in another molecule.

The key features of hydrogen bonding include:

• Highly electronegative atoms like oxygen or nitrogen, which pull electron density toward themselves.
• A significant impact on boiling and melting points due to the additional energy needed to overcome these strong bonds.
• A pronounced effect on the physical state of the substance, like making it liquid at room temperature.

In propanol, the hydrogen bonds formed due to the hydroxyl group are the main reason for its higher boiling point compared to substances like butane, which lack this capacity.

London Dispersion Forces

London dispersion forces are a type of van der Waals force, considered the weakest of the intermolecular forces.
These forces arise from temporary attractions between induced dipoles in atoms or molecules.

Key aspects of London dispersion forces include:

• Present in all atoms and molecules, but their influence is more noticeable in nonpolar compounds.
• Their strength increases with the size and mass of the molecule, as larger electron clouds are more easily polarized.
• They are the principal force in hydrocarbons, like butane, making them have relatively low boiling points.

Because butane relies solely on London dispersion forces, it does not require much energy to break these forces, explaining its lower boiling point compared to alcohols like propanol.