Question

Two molecules that contain the same number of each kind of atom but that have different molecular structures are said to be isomers of each other. For example, both ethyl alcohol and dimethyl ether (shown below) have the formula \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\) and are isomers. Based on considerations of intermolecular forces, which substance would you expect to be more volatile? Which would you expect to have the higher boiling point? Explain. \(\mathrm{CH}_{3} \stackrel{\text { dimethyl ether }}{\mathrm{CH}}_{3} \quad \mathrm{CH}_{3} \stackrel{\text { ethyl alcohol }}{\mathrm{CH}}_{2}-\mathrm{OH}\)

Short answer

Dimethyl ether is more volatile due to its weaker intermolecular forces (dipole-dipole interactions and London dispersion forces) compared to ethyl alcohol, which has stronger intermolecular forces (hydrogen bonding, dipole-dipole interactions, and London dispersion forces). Ethyl alcohol has a higher boiling point as stronger intermolecular forces require more energy to break the bonds between the molecules.

Step-by-step solution

Identify the types of intermolecular forces present in each molecule

First, let's look at the two isomers and identify the intermolecular forces present in each of them. For ethyl alcohol (C2H5OH): - Hydrogen bonding (present due to the presence of hydroxyl group \(\mathrm{OH}\)) - Dipole-dipole interactions - London dispersion forces For dimethyl ether (C2H6O): - Dipole-dipole interactions - London dispersion forces

Compare the strengths of intermolecular forces in the two isomers

Now that we've identified the intermolecular forces present in both isomers, let's compare their strengths to determine which of the two molecules will be more volatile. In general, hydrogen bonding is stronger than other types of intermolecular forces, such as dipole-dipole interactions and London dispersion forces. Since ethyl alcohol forms hydrogen bonds, it will have stronger intermolecular forces than dimethyl ether, which only has dipole-dipole interactions and London dispersion forces.

Determine the more volatile isomer and isomer with a higher boiling point

Finally, let's use our understanding of the intermolecular forces present in the two isomers to determine their volatility and boiling points. - More volatile isomer: The more volatile compound will have weaker intermolecular forces, as it will be easier for the molecules to escape from the liquid phase into the vapor phase. In this case, dimethyl ether will be more volatile, as it has weaker intermolecular forces compared to ethyl alcohol. - Isomer with a higher boiling point: Boiling points are generally higher for compounds with stronger intermolecular forces, as more energy is required to break the bonds between the molecules. Therefore, ethyl alcohol will have a higher boiling point than dimethyl ether due to its stronger intermolecular forces (hydrogen bonding).

Key concepts

Isomers

Isomers are molecules with the same molecular formula but different structural arrangements. This means that they have the same number of each kind of atom, but the atoms are connected differently. This difference in structural arrangement can lead to distinct physical and chemical properties, such as boiling points and reactivity. For instance, ethyl alcohol (C2H5OH) and dimethyl ether (C2H6O) are isomers with different intermolecular forces, affecting their volatility and boiling points.

It's fascinating how a small change in structure can lead to significant differences in behavior. Isomers underscore the complex interplay between structure and function at the molecular level.

Hydrogen Bonding

Hydrogen bonding is a potent type of intermolecular force that occurs when hydrogen is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. This creates a significant dipole moment. Molecules with hydrogen bonds like ethyl alcohol can interact strongly with each other, which leads to high boiling points and lower volatility.

Understanding hydrogen bonding is crucial when comparing molecules because it's often the strongest intermolecular force present, barring ionic bonds. This means that its presence can dominate a molecule's properties, such as solubility in water or melting and boiling points.

Boiling Point

The boiling point of a substance is the temperature at which its liquid form turns into a gas. Substances with stronger intermolecular forces, like hydrogen bonds, typically have higher boiling points because more energy is needed to separate the molecules.

For instance, in the case of ethyl alcohol vs. dimethyl ether, the stronger hydrogen bonding in ethyl alcohol results in a higher boiling point compared to dimethyl ether, which lacks hydrogen bonds and relies mainly on dipole-dipole interactions and London dispersion forces. The comparison between isomers with different intermolecular forces highlights the role these forces play in determining boiling point.

Volatility

Volatility refers to a substance's tendency to vaporize or turn into a gas at a given temperature. The more volatile a substance, the easier it is for it to evaporate. Volatility is influenced by the strength of intermolecular forces within a substance; weaker forces result in higher volatility.

Dimethyl ether is more volatile than ethyl alcohol, as its molecules experience weaker intermolecular forces, making it easier for them to escape into the vapor phase. The concept of volatility is important in many practical applications, such as the design of fuels and perfumes, where the desired vapor pressure and evaporation rate are determined by the volatility of the compounds used.