Question

Pick the compound with the highest boiling point in each pair. Explair your reasoning. a. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{SH}\) b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) c. \(\mathrm{CH}_{4}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{3}\)

Short answer

\(\mathrm{CH}_{3}\mathrm{OH}\), \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\), and \(\mathrm{CH}_{3}\mathrm{CH}_{3}\) have the highest boiling points in their respective pairs due to stronger intermolecular forces.

Step-by-step solution

- Analyze Molecular Structures for the First Pair

Compare the molecular structures of \(\mathrm{CH}_{3}\mathrm{OH}\) and \(\mathrm{CH}_{3}\mathrm{SH}\). \(\mathrm{CH}_{3}\mathrm{OH}\) (methanol) can form hydrogen bonds due to the presence of -OH group which is more electronegative than the -SH group in \(\mathrm{CH}_{3}\mathrm{SH}\) (methanethiol), which is less capable of strong hydrogen bonding.

- Determine the Stronger Intermolecular Forces

Recognize that hydrogen bonds are stronger than other dipole interactions. Since \(\mathrm{CH}_{3}\mathrm{OH}\) can form hydrogen bonds and \(\mathrm{CH}_{3}\mathrm{SH}\) cannot, this implies that \(\mathrm{CH}_{3}\mathrm{OH}\) will have a higher boiling point.

- Analyze Molecular Structures for the Second Pair

Examine the molecular structures of \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\) (dimethyl ether) and \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) (ethanol). Both molecules have similar molar masses, but ethanol has a hydroxyl group, making it capable of hydrogen bonding.

- Evaluate Intermolecular Forces in the Second Pair

Since ethanol can participate in hydrogen bonding and dimethyl ether cannot, ethanol will exhibit stronger intermolecular forces and therefore have a higher boiling point.

- Analyze Molecular Structures for the Third Pair

Look at the structures of \(\mathrm{CH}_{4}\) (methane) and \(\mathrm{CH}_{3}\mathrm{CH}_{3}\) (ethane). Ethane has a larger molecular size and more electron distribution compared to methane.

- Assess Intermolecular Forces in the Third Pair

Greater molecular size and electron distribution lead to stronger London dispersion forces. Therefore, \(\mathrm{CH}_{3}\mathrm{CH}_{3}\) with its greater molecular size compared to \(\mathrm{CH}_{4}\), will have a higher boiling point.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between molecules. They play a crucial role in dictating the boiling points of compounds. Simple compounds like gases usually have weak intermolecular forces and thus low boiling points, while larger, more complex molecules typically exhibit stronger forces and higher boiling points. There are several types of intermolecular forces, including hydrogen bonds, London dispersion forces, and dipole-dipole interactions. Understanding these forces helps explain why different substances have different physical properties, such as boiling points.

When comparing two substances, the one with stronger intermolecular forces will generally have the higher boiling point. This is because more energy is required to overcome these forces during the transition from liquid to gas.

Hydrogen Bonding

Hydrogen bonding is a specific and particularly strong type of polar intermolecular force that occurs when hydrogen is covalently bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. This results in a significant charge difference across the molecule, allowing the hydrogen to be attracted to the electronegative atoms in other molecules.

Importance of Hydrogen Bonds

Hydrogen bonds are significantly stronger than other dipole-dipole interactions, yet they are weaker than covalent and ionic bonds. The presence of hydrogen bonding can explain why certain substances, like water, have unusually high boiling points for their molecular weight. In the context of the exercises, molecules with hydrogen bonding will have higher boiling points compared to similar molecules without this ability.

Molecular Structure

Molecular structure influences the boiling point of substances through its effect on intermolecular forces. The shape and size of molecules, as well as the types and locations of bonds, determine how molecules interact with each other. Larger molecules or those with more complex structures tend to have stronger London dispersion forces due to increased contact area and electron distribution.

Structure-Property Relationships

The 'structure-property relationship' is a foundational concept in chemistry, linking the 3D arrangement of atoms within a molecule to its observed properties. In comparing molecular structures, we note that linear or spherical molecules may pack closer, leading to higher intermolecular forces, and as a result, higher boiling points, compared to those with less efficient packing.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular force, but they are present in all molecules. They are caused by the temporary polarizations of the electron cloud in a molecule, leading to temporary dipole moments that induce attractions with nearby molecules. London dispersion forces increase with the size of the electron cloud, meaning larger atoms and molecules can generate stronger forces.

Role in Boiling Points

Even though they are the weakest, London dispersion forces are very significant for substances without permanent dipole moments or the ability to hydrogen bond. For instance, in nonpolar molecules like hydrocarbons, with similar molar masses, the one with a more extended structure will have stronger London dispersion forces and, as a consequence, a higher boiling point.