Question

In carbon compounds a given group of atoms can often be arranged in more than one way. This means that more than one structure may be possible for the same atoms. For example, both the molecules diethyl ether and 1 -butanol have the same number of each type of atom, but they have different structures and are said to be isomers of one another. diethy \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) 1-buta \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\) Which substance would you expect to have the larger vapor pressure? Why?

Short answer

Diethyl ether would have a larger vapor pressure than 1-butanol because it has weaker intermolecular forces, as it only has London dispersion forces, while 1-butanol also involves hydrogen bonding in addition to the London dispersion forces. Stronger intermolecular forces lead to lower vapor pressures, so the weaker intermolecular forces in diethyl ether result in a higher vapor pressure compared to 1-butanol.

Step-by-step solution

Understand vapor pressure and its relationship with intermolecular forces

Vapor pressure is the pressure exerted by a vapor when the vapor is in equilibrium with its liquid phase at a given temperature. The stronger the intermolecular forces between the molecules of a substance, the lower its vapor pressure. This is because stronger intermolecular forces hold the molecules together more tightly, making it more difficult for them to escape into the vapor phase.

Compare the structures of diethyl ether and 1-butanol

Diethyl ether has the following structure: \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\). In this structure, an oxygen atom is bonded to two ethyl groups. 1-butanol, on the other hand, has the following structure: \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\). In this structure, a hydroxyl group (-OH) is bonded to a butyl group.

Identify the intermolecular forces in diethyl ether and 1-butanol

In both diethyl ether and 1-butanol, the primary intermolecular forces are London dispersion forces, which are always present between nonpolar molecules or atoms. However, 1-butanol has an additional intermolecular force present due to the presence of the hydroxyl group (-OH) which is involved in hydrogen bonding. Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative element such as oxygen, nitrogen, or fluorine.

Compare the vapor pressures of diethyl ether and 1-butanol based on their intermolecular forces

Since 1-butanol has hydrogen bonding in addition to London dispersion forces, its overall intermolecular forces are stronger than those in diethyl ether, which only has London dispersion forces. As we know that stronger intermolecular forces lead to lower vapor pressures, we can conclude that diethyl ether would have a higher vapor pressure than 1-butanol.

State the conclusion

Based on the analysis of the intermolecular forces present in diethyl ether and 1-butanol due to their different structures, we can conclude that diethyl ether would have a larger vapor pressure than 1-butanol. This is because diethyl ether has weaker intermolecular forces than 1-butanol, as it only has London dispersion forces while 1-butanol also involves hydrogen bonding in addition to the London dispersion forces.

Key concepts

Intermolecular Forces

In the world of chemistry, understanding molecular interactions is key to predicting physical properties like vapor pressure. Intermolecular forces are attractions that occur between molecules, influencing how they interact with each other. These forces include:

• London Dispersion Forces: All molecules experience these forces. They arise due to temporary dipoles created when electrons move around a molecule. Though generally weak, they are stronger in larger molecules.
• Dipole-Dipole Forces: These occur in molecules with permanent dipoles, where there is an uneven distribution of electrons. Molecules align their positive and negative ends to enhance attraction.
• Hydrogen Bonding: A special type of dipole interaction occurring when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. This creates a significant pull between molecules, significantly affecting boiling and melting points.

Intermolecular forces strongly influence a substance's vapor pressure. Stronger intermolecular forces mean molecules need more energy to escape into the vapor phase, resulting in lower vapor pressure.

Isomers

Isomers are fascinating because they show how the same set of atoms can be re-arranged to form different molecules, each with unique properties. Consider diethyl ether and 1-butanol—two isomers with the same atomic composition:

• Diethyl Ether: In this molecule, oxygen is bonded to two ethyl groups, forming a structure that allows only London dispersion forces.
• 1-Butanol: This structure includes a hydroxyl group, which allows for both hydrogen bonding and London dispersion forces.

Despite having identical atomic compositions, these isomers differ in their functional groups and structure, leading to differences in their physical properties such as boiling points and vapor pressures. This concept vividly demonstrates that molecular structure greatly affects chemical behavior.

Hydrogen Bonding

Hydrogen bonding is a type of strong intermolecular force essential in the world of chemistry. Occurring when hydrogen atoms bond to electronegative elements like oxygen, nitrogen, or fluorine, it creates a strong intermolecular attraction. In the case of 1-butanol, the hydroxyl group (-OH) lends the molecule this high strength attraction.

• Strength: Hydrogen bonds are stronger than typical dipole-dipole forces, though still weaker than covalent or ionic bonds.
• Impact on Properties: Due to these strong interactions, compounds with hydrogen bonding, like 1-butanol, typically have higher boiling points and lower vapor pressures.

This influence of hydrogen bonds on physical properties is significant and explains why 1-butanol, with its potential for hydrogen bonding, has a lower vapor pressure compared to diethyl ether, which cannot form hydrogen bonds.

Carbon Compounds

Carbon compounds are the foundation of organic chemistry. Carbon's unique ability to form four covalent bonds makes it incredibly versatile, allowing for a multitude of configurations and functional groups.

• Diversity: Carbon can form chains, rings, and complex structures, making endless varieties like alkanes, alkenes, alcohols, and ethers.
• Functional Groups: Groups like hydroxyl (-OH) or ether (-O-) define the properties and reactivity of these compounds. For instance, the presence of the hydroxyl group in alcohols like 1-butanol enables hydrogen bonding.
• Structure and Reactivity: The configuration of carbon and surrounding atoms directly influences physical properties. This is evident in isomers like diethyl ether and 1-butanol, which have the same atoms but different properties due to their arrangement.

Understanding carbon compounds not only illuminates their physical and chemical behaviors but also helps explain the vast array of organic materials found in nature.