Question

Identify the intermolecular forces (dipole-dipole, London dispersion, hydrogen bonding) that influence the properties of the following compounds: (a) Ethane, \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) (b) Ethanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (c) Chloroethane, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\)

Short answer

Ethane: London dispersion. Ethanol: London dispersion, dipole-dipole, hydrogen bonding. Chloroethane: London dispersion, dipole-dipole.

Step-by-step solution

Identify the Molecular Structure and Polarity

For each compound, examine its molecular structure. Ethane, \( \mathrm{CH}_3 \mathrm{CH}_3 \), is a non-polar molecule with only carbon and hydrogen atoms. Ethanol, \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH} \), contains an -OH (hydroxyl) group, making it polar. Chloroethane, \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Cl} \), has a chlorine atom which induces polarity.

Evaluate Presence of London Dispersion Forces

London dispersion forces are present in all molecules because they originate from temporary dipoles. Ethane, ethanol, and chloroethane all exhibit London dispersion forces due to their electron clouds.

Determine Dipole-Dipole Interactions

Dipole-dipole forces only occur in polar molecules. Ethanol and chloroethane are polar, which enables them to exhibit dipole-dipole interactions. Ethane, being non-polar, does not participate in these interactions.

Analyze Hydrogen Bonding Possibility

Hydrogen bonding occurs in molecules where hydrogen is directly bonded to nitrogen, oxygen, or fluorine. In ethanol, the -OH group (oxygen bonded to hydrogen) allows for hydrogen bonding. Neither ethane nor chloroethane has the necessary functional groups for hydrogen bonding.

Key concepts

Dipole-Dipole Interactions

Dipole-dipole interactions occur in polar molecules where there is an uneven distribution of electrons between atoms. These intermolecular forces arise because of the partial positive and negative charges generated within the molecule.
The positive end of one polar molecule is attracted to the negative end of another polar molecule. This interaction is stronger than London dispersion forces because it involves permanent dipoles. Here are a few key points to help you understand better:

• Only polar molecules have dipole-dipole interactions.
• The strength of these interactions depends on the magnitude of the charge and the distance between interacting molecules.
• Examples of polar molecules are ethanol and chloroethane, both of which exhibit dipole-dipole interactions.

Remember, non-polar molecules like ethane do not have dipole-dipole interactions because they lack the permanent separation of charge within the molecule.

London Dispersion Forces

London dispersion forces are a type of intermolecular force present in all molecules, whether polar or non-polar. These forces are the result of momentary changes in electron density, which create temporary dipoles. Even though they are the weakest type of intermolecular force, their impact is significant, especially in larger molecules with more electrons. Here’s what to keep in mind about London dispersion forces:

• They increase with the number of electrons in a molecule; larger molecules have stronger dispersion forces.
• The dynamic nature of electron clouds means these forces are constantly in flux.
• All given compounds (ethane, ethanol, and chloroethane) have London dispersion forces due to their electron clouds.

The universal presence of London dispersion forces makes them incredibly important when assessing the boiling and melting points of non-polar substances.

Hydrogen Bonding

Hydrogen bonding, one of the strongest types of intermolecular forces, occurs when hydrogen is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. This results in a significant dipole where hydrogen carries a partial positive charge.
Hydrogen bonds form between this partially positive hydrogen and the lone electron pair of an electronegative atom in another molecule. Here are some essential features of hydrogen bonding:

• Significant in stabilizing the structure of proteins and DNA.
• Water's unique properties, like high boiling and melting points, arise from hydrogen bonding.
• Ethanol exhibits hydrogen bonding because of the presence of an -OH group (oxygen bonded to hydrogen), forming strong interactions with other ethanol molecules or water.

In comparison, compounds like ethane and chloroethane lack the -OH, -NH, or -FH group necessary for hydrogen bonding.