Question

What is the strongest type of intermolecular attraction that exists in each of the following liquids? (a) \(C_{6} \mathrm{H}_{14}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{F}\) (c) \(\mathrm{CH}_{3}-\mathrm{OH}\) (d) \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\)

Short answer

(a) London dispersion forces, (b) Dipole-dipole interactions, (c) Hydrogen bonding, (d) Dipole-dipole interactions.

Step-by-step solution

Understanding the Molecule

Identify the type of atoms and structures in each molecule to determine what kind of forces they might experience.

Step A: Determine Forces in Hexane

For the molecule \(C_{6} \mathrm{H}_{14}\), which is hexane, the molecules are nonpolar. The strongest intermolecular forces present are London dispersion forces because it's a hydrocarbon without functional groups that allow for other interactions.

Step B: Determine Forces in Fluoroethane

For \(\mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{F}\), which is fluoroethane, the fluoride atom creates a polar bond with carbon. Therefore, the strongest intermolecular forces are dipole-dipole interactions due to the permanent dipole from the \(-\mathrm{F}\) group.

Step C: Determine Forces in Methanol

In \(\mathrm{CH}_{3}-\mathrm{OH}\), which is methanol, the presence of the \(\mathrm{OH}\) group allows for hydrogen bonding. This is because hydrogen is bonded directly to highly electronegative oxygen, leading to strong hydrogen bonding being the strongest intermolecular force here.

Step D: Determine Forces in Dimethyl Ether

For \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\), or dimethyl ether, the strongest forces are dipole-dipole interactions. The molecule is polar due to the presence of an oxygen atom, although it cannot participate in hydrogen bonding as there is no \(\mathrm{H}\) attached to oxygen.

Key concepts

London Dispersion Forces

London dispersion forces are a type of intermolecular force that occurs in all molecules, whether they are polar or nonpolar. However, they are the only type of intermolecular force present in nonpolar molecules, such as hydrocarbons like hexane \(C_{6}H_{14}\).
These forces are due to the temporary fluctuations in electron density within a molecule, resulting in a temporary dipole. When these fluctuations occur simultaneously between neighboring molecules, they attract each other temporarily.
Some key attributes of London dispersion forces include:

• Being the weakest type of intermolecular force compared to others like dipole-dipole and hydrogen bonding.
• Increasing in strength with larger and more polarizable molecules, where more electrons can create temporary dipoles easily.
• Present universally in molecules, though they're often overshadowed by stronger intermolecular forces if present.

In the case of hexane, with no functional groups capable of stronger interactions, London dispersion forces are the most significant intermolecular attraction.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in polar molecules where there is a permanent dipole moment due to uneven electron sharing. This typically happens when atoms in the molecule have noticeable differences in electronegativity.
For example, in fluoroethane \(C_{2}H_{5}-F\), the carbon-fluorine bond is highly polar because fluorine is significantly more electronegative than carbon, creating a large dipole.
Characteristics of dipole-dipole interactions include:

• Stronger than London dispersion forces due to the presence of permanent rather than temporary dipoles.
• The force strength depends on the magnitude of the dipole moment, which is determined by the difference in electronegativity between the atoms forming the dipole.
• These interactions contribute to the higher boiling and melting points of polar compounds compared to nonpolar compounds of similar size.

In polar molecules like fluoroethane, where there is no possibility of hydrogen bonding, dipole-dipole interactions are the dominant intermolecular force.

Hydrogen Bonding

Hydrogen bonding is a specific and particularly strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative elements such as nitrogen, oxygen, or fluorine. In methanol \(CH_{3}-OH\), the presence of an \(OH\) group allows for extensive hydrogen bonding.
Let's look at the characteristics of hydrogen bonding:

• The presence of an \(H-F\), \(H-O\), or \(H-N\) bond is required for hydrogen bonding to occur.
• Hydrogen bonds are considerably stronger than dipole-dipole and London dispersion forces.
• This strength is due to the substantial difference in electronegativity between the hydrogen and the highly electronegative atom to which it is bonded, leading to a significantly polarized bond.
• Hydrogen bonding significantly increases the boiling points and solubility in water for compounds, exemplified by the relatively high boiling point of methanol compared to other alcohols without hydrogen bonding capabilities.

In the case of methanol, the hydrogen in the -OH group forms hydrogen bonds with the oxygen of neighboring molecules, which represents the strongest type of intermolecular force present.