Question

Diethyl ether has a boiling point of \(34.5^{\circ} \mathrm{C},\) and 1-butanol has a boiling point of \(117^{\circ} \mathrm{C}\): Both of these compounds have the same numbers and types of atoms. Explain the difference in their boiling points.

Short answer

1-butanol has a higher boiling point due to hydrogen bonding, whereas diethyl ether does not have hydrogen bonds.

Step-by-step solution

Understanding Molecular Structure

Diethyl ether and 1-butanol have the same molecular formula, which suggests they have identical types and numbers of atoms. However, the arrangement of these atoms is different in each molecule. Diethyl ether consists of an ether group, while 1-butanol contains a hydroxyl (-OH) group, impacting their boiling points.

Evaluate Intermolecular Forces

Examine the types of intermolecular forces present in each compound. Diethyl ether primarily has London dispersion forces and weak dipole-dipole interactions due to its ether bond. On the other hand, 1-butanol can form hydrogen bonds due to its hydroxyl group, which are significantly stronger than the forces present in diethyl ether.

Relate Molecular Forces to Boiling Points

Recognize that stronger intermolecular forces result in higher boiling points. Hydrogen bonds present in 1-butanol require more energy (higher temperature) to break than the dipole-dipole interactions and London dispersion forces in diethyl ether.

Conclusion

Summarize the evidence. The higher boiling point of 1-butanol (117°C) compared to diethyl ether (34.5°C) is largely due to the presence of hydrogen bonding in 1-butanol, which increases the energy needed to change it from a liquid to a gas.

Key concepts

Molecular Structure

Molecular structure refers to the arrangement of atoms within a molecule. Even with the same number and type of atoms, the way these atoms are connected can lead to vastly different properties. In the case of diethyl ether and 1-butanol, they share the same molecular formula but differ in structure. Diethyl ether features an ether group, which is two ethyl groups connected by an oxygen atom to form an R-O-R' structure. This arrangement provides limited opportunity for strong intermolecular interactions. On the other hand, 1-butanol includes a hydroxyl group (-OH). This addition drastically affects its physical properties, particularly its boiling point. The structure is such that the -OH group can participate in hydrogen bonding with other molecules.
The chemical makeup does not change, but how the atoms are arranged influences the strength of intermolecular forces, and as such, influences their boiling points.

Intermolecular Forces

Intermolecular forces are the forces that act between molecules, playing a crucial role in determining boiling and melting points. In general, stronger intermolecular forces lead to higher boiling points. Diethyl ether and 1-butanol have different types of intermolecular forces acting between their molecules, which explains the significant difference in their boiling points.
Diethyl ether primarily relies on London dispersion forces , which are weak, temporary attractions that arise due to fluctuations in electron distribution within molecules. It also experiences weak dipole-dipole interactions, thanks to the presence of an ether functional group. These are not strong enough to significantly elevate its boiling point. In contrast, 1-butanol can form strong hydrogen bonds due to its hydroxyl group. This type of force is one of the strongest intermolecular interactions, greatly enhancing the boiling point.

Hydrogen Bonds

Hydrogen bonds are a specific type of strong dipole-dipole attraction that occurs when a hydrogen atom is bonded to an electronegative atom like oxygen, nitrogen, or fluorine. In 1-butanol, the hydroxyl group's hydrogen is capable of forming hydrogen bonds with oxygen atoms in nearby molecules.
These bonds have a significant impact on a substance's boiling point. Because breaking hydrogen bonds requires additional energy, substances that can establish hydrogen bonds typically have higher boiling points. For example, 1-butanol, with its capabilities for hydrogen bonding, has a markedly higher boiling point compared to diethyl ether. This explains why more heat energy is necessary to convert 1-butanol from a liquid to a gas phase.

London Dispersion Forces

London dispersion forces are a type of weak intermolecular force that are present in all molecules, whether polar or nonpolar. These arise from the fluctuations of electrons within atoms or molecules, which create temporary dipoles, leading to a fleeting attraction between molecules. While universally present, they are the only type of intermolecular force in nonpolar molecules.
In diethyl ether, these dispersion forces play a significant role. Although present, they are weak compared to hydrogen bonds, leading to a lower boiling point than 1-butanol. Diethyl ether's reliance on these forces and minimal polar interactions via its ether group, contributes to its substantially lower boiling point of 34.5°C. Understanding these interactions helps us appreciate why boiling points can differ so drastically, based solely on molecular interactions.