Question

Order the following compounds by increasing boiling point: butane, butanol, butanone (A) Butanol \(<\) butane \(<\) butanone (B) Butane \(<\) butanone \(<\) butanol (C) Butanone \(<\) butane \(<\) butanol (D) Butane \(<\) butanol \(<\) butanone

Short answer

Butane < butanone < butanol

Step-by-step solution

Identify functional groups

First, identify the functional groups present in each compound.

Evaluate intermolecular forces

Determine the types of intermolecular forces each compound can form. Higher intermolecular forces result in higher boiling points.

Compare intermolecular forces

Butane is an alkane and has only London dispersion forces. Butanol is an alcohol and can form hydrogen bonds in addition to London dispersion forces. Butanone is a ketone and can form dipole-dipole interactions and London dispersion forces, but not hydrogen bonds.

Rank compounds by their boiling points

Rank the compounds from lowest to highest boiling point based on the strength and types of intermolecular forces.

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that have certain chemical properties regardless of the other atoms present in a molecule. They play a pivotal role in determining the physical and chemical properties of compounds. Identifying the functional groups in a compound can give insight into its reactivity and boiling point.

For example:

• Butane: An alkane with no functional groups.
• Butanol: An alcohol with an -OH (hydroxyl) group.
• Butanone: A ketone with a carbonyl (C=O) group.

These functional groups influence the type and strength of intermolecular forces that a molecule can exhibit.

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules, and they significantly affect the boiling points of substances. Generally, stronger intermolecular forces lead to higher boiling points because more energy is required to separate the molecules.

There are three primary types of intermolecular forces:

• London Dispersion Forces: Present in all molecules, stronger in larger and more polarizable molecules.
• Dipole-Dipole Interactions: Occur between molecules with permanent dipoles.
• Hydrogen Bonds: Strong interactions between molecules that have hydrogen atoms bonded to electronegative atoms like oxygen, nitrogen, or fluorine.

Understanding these forces helps predict and explain the boiling points of different substances.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. This significantly increases the boiling point of a substance.

For example, butanol can form hydrogen bonds because it has an -OH group. The hydrogen atom in the -OH group of one butanol molecule can interact strongly with the oxygen atom of another butanol molecule. This strong intermolecular force translates into a higher boiling point for butanol compared to molecules that cannot form hydrogen bonds.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular force and exist in all molecules, whether they are polar or nonpolar. These forces are caused by the momentary distribution of electrons in atoms or molecules, which creates a temporary dipole that induces another temporary dipole in a neighboring molecule.

Alkanes like butane have only London dispersion forces because they are nonpolar. Though these forces are generally weak, they can become significant in larger and more massive molecules. However, in smaller molecules like butane, they are relatively weak and result in a lower boiling point compared to compounds with stronger intermolecular forces.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipoles. These interactions are stronger than London dispersion forces but weaker than hydrogen bonds.

For instance, butanone, a ketone, has a carbonyl group (C=O) that is polar, giving the molecule a permanent dipole. The partial positive charge on the carbon atom and the partial negative charge on the oxygen atom allow butanone molecules to attract each other through dipole-dipole interactions. This results in a medium-strength boiling point, higher than that of alkanes like butane but lower than alcohols like butanol, which can also form hydrogen bonds.