Question

Ethanol and methyl ether are isomers, having the molecular formula \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\). Based on structure, which one would be expected to have the higher vapor pressure? The higher boiling point? The greater solubility in water? Explain why in each case.

Short answer

Ethanol (CH₃CH₂OH) and methyl ether (CH₃OCH₃) are isomers with the molecular formula \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\). Due to the presence of a hydroxyl group (OH) in ethanol, it can form hydrogen bonds, resulting in stronger intermolecular forces compared to methyl ether's weaker Van der Waals forces. Consequently, ethanol has a lower vapor pressure, higher boiling point, and greater solubility in water than methyl ether.

Step-by-step solution

Determine Molecular Structures

Ethanol has the molecular formula \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\) and the structure can be written as CH₃CH₂OH. It has a hydroxyl (OH) group attached, which signifies that it is an alcohol. Methyl ether has the molecular formula \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\) and the structure can be written as CH₃OCH₃. It has an ether (C-O-C) functional group, which differentiates it from an alcohol like ethanol.

Compare Vapor Pressure

Ethanol has a hydroxyl group (OH), which can form hydrogen bonds with other ethanol molecules. Therefore, ethanol molecules are more likely to stay together since they are attracted to one another through hydrogen bonding, causing a lower vapor pressure. Methyl ether, however, lacks the ability to form hydrogen bonds. The intermolecular forces between methyl ether molecules are weaker (mainly Van der Waals forces), making it easier for individual molecules to escape from the liquid phase to the vapor phase. Therefore, methyl ether has a higher vapor pressure than ethanol.

Compare Boiling Point

Ethanol, due to the hydrogen bonding between its molecules, has stronger intermolecular forces compared to methyl ether. Because of these stronger intermolecular forces, more energy is required to break the hydrogen bonds and make the ethanol molecules separate from each other. Consequently, ethanol has a higher boiling point compared to methyl ether, which has weaker Van der Waals forces.

Compare Solubility in Water

Both ethanol and methyl ether are polar compounds, so they would have some level of solubility in water, which is also a polar solvent. Ethanol possesses the ability to form hydrogen bonds with water, due to the presence of a hydroxyl group (OH). This results in strong interactions between water and ethanol molecules, leading to a high solubility in water. On the other hand, methyl ether can form weaker dipole-dipole interactions and dispersion forces with water molecules, which are not as strong as hydrogen bonds. As a result, its solubility in water is less than that of ethanol. To summarize: - Ethanol has a lower vapor pressure, higher boiling point, and greater solubility in water compared to methyl ether. - These differences in properties can be attributed to the differences in molecular structure, particularly the presence of a hydroxyl group in ethanol, which allows for hydrogen bonding, leading to stronger intermolecular forces.

Key concepts

Isomers

Let's start by understanding what isomers are. Isomers are molecules that share the same chemical formula but differ in their structural arrangement. This means they have the same types and numbers of atoms, but those atoms are arranged differently. A good analogy could be thinking of isomers as identical building blocks constructed in different shapes. Although they may look similar on paper in terms of their chemical formula, their differing structures can lead to vastly different properties.
Ethanol and methyl ether are great examples of isomers. They both have the formula \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\), but, due to their distinct structural arrangements, they exhibit various physical and chemical differences, such as variations in vapor pressure, boiling point, and solubility in water. Understanding how structural differences affect these properties is crucial, as these characteristics determine how the substances behave and interact under different conditions.

Vapor Pressure

Vapor pressure is an essential property that indicates how prone a liquid is to evaporate. It is the pressure of the vapor formed by the evaporating molecules in the space above the liquid. Lower vapor pressure implies a substance's molecules prefer staying in the liquid phase rather than escaping into the gas phase.
Ethanol and methyl ether demonstrate differing vapor pressures due to the nature of their molecular structures and intermolecular forces. Ethanol possesses a hydroxyl group (\(\mathrm{OH}\)), which allows it to form hydrogen bonds—a type of strong dipole-dipole attraction. These bonds hold ethanol molecules together more tightly, hindering their transition into the vapor phase, thus resulting in lower vapor pressure.
Conversely, methyl ether lacks such hydrogen bonding capability. Its intermolecular forces are weaker, composed mainly of Van der Waals forces. As a result, its molecules can easily escape into the vapor phase, giving methyl ether a higher vapor pressure compared to ethanol.

Boiling Point

The boiling point is the temperature at which a liquid turns into vapor. Substances with stronger intermolecular forces need more energy to break these bonds and transition from liquid to gas, resulting in a higher boiling point.
Ethanol exhibits a higher boiling point than methyl ether due largely to its ability to form hydrogen bonds. The presence of the \(\mathrm{OH}\) group in ethanol creates stronger intermolecular attractions, meaning more energy in the form of heat is necessary to separate these molecules and get them to boil.
Methyl ether, on the other hand, can't form these robust hydrogen bonds and relies primarily on weaker dispersion forces. Therefore, its molecules require less energy to move apart, resulting in a lower boiling point than that of ethanol.

Solubility in Water

Solubility in water refers to the capability of a substance to dissolve in water, a vital polar solvent. This property greatly depends on the molecule's polarity and its ability to interact with water molecules.
Ethanol's \(\mathrm{OH}\) group makes it highly capable of hydrogen bonding with water molecules, promoting ethanol's excellent solubility in water. The strong interaction between the hydrogen bonds in ethanol and water molecules leads to a homogeneous solution.
In contrast, methyl ether is less soluble in water. Although it is a polar compound, its lack of hydrogen bonding capability means it primarily relies on weaker dipole-dipole interactions to dissolve in water. This results in less effective interactions with water molecules and, consequently, lower solubility than ethanol.