Question

Diethyl ether has a boiling point of \(34.5^{\circ} \mathrm{C},\) and I-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

The difference in the boiling points of diethyl ether and I-butanol, despite having the same types and numbers of atoms, is due to the difference in their intermolecular forces. I-butanol has a higher boiling point because it has a hydrogen bond, a stronger intermolecular force, in addition to van der Waals and dipole-dipole interactions. Diethyl ether, on the other hand, only has van der Waals and dipole-dipole interactions.

Step-by-step solution

Understand the Types of Intermolecular Forces

Firstly, identify the types of intermolecular forces. The common types are: Van der Waals forces (London dispersion forces), dipole-dipole interactions, and hydrogen bonds. London dispersion forces occur in all molecules, dipole-dipole interactions happen in polar molecules, and hydrogen bonds occur in molecules where hydrogen is bonded to nitrogen, oxygen, or fluorine.

Identify the Intermolecular Forces for Each Compound

For Diethyl ether (CH3CH2OCH2CH3), there are van der Waals forces and dipole-dipole interactions, as it is a polar molecule with the oxygen atom creating a dipole. For I-butanol (CH3CH2CH2CH2OH), there are van der Waals forces, dipole-dipole interactions, and also a hydrogen bond due to the hydroxyl (OH) group. The hydrogen atom in the hydroxyl group can form a strong hydrogen bond with oxygen atoms in other molecules.

Relate the Identified Forces to the Boiling Points

Once the intermolecular forces have been identified, the relation to the boiling points can be explained. Intermolecular forces affect the energy required to change the state of the substance, hence a higher boiling point indicates stronger intermolecular forces. Diethyl ether has only van der Waals and dipole-dipole interactions, whereas I-butanol has an added hydrogen bond which is stronger. This leads to a higher boiling point in I-butanol compared to Diethyl ether.

Key concepts

Van der Waals Forces

Van der Waals forces, also known as London dispersion forces, play a fundamental role in the physical properties of molecules. They are the weakest type of intermolecular force and arise due to momentary fluctuations in electron density in atoms or molecules. This temporary uneven distribution of electrons leads to the formation of short-lived dipoles, which induce a corresponding dipole in nearby molecules. The attraction between these induced dipoles results in Van der Waals forces.

These forces are universal and present in all molecules, regardless of whether they are polar or non-polar. However, their strength can vary and is influenced by the size of the molecules and their molar mass; larger and heavier atoms or molecules generally have stronger Van der Waals forces. In the context of boiling points, substances with stronger Van der Waals forces require more energy to separate the molecules, resulting in a higher boiling point.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in polar molecules, where there is an uneven distribution of electrons resulting in a permanent dipole moment. A polar molecule has a partial positive charge on one end and a partial negative charge on the other, which leads to an electrostatic attraction between the oppositely charged parts of two different molecules.

In comparison to Van der Waals forces, dipole-dipole interactions are typically stronger due to the permanent nature of the dipoles. These interactions significantly influence the boiling points of substances. Molecules with strong dipole-dipole interactions require more thermal energy to overcome these forces during the phase transition from liquid to gas, thus exhibiting higher boiling points.

Hydrogen Bonds

Among the intermolecular forces, hydrogen bonds stand out for their strength and specificity. A hydrogen bond is a type of dipole-dipole interaction that occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine, is attracted to a lone pair of electrons on another electronegative atom. The result is a relatively strong bond that, although weaker than a covalent or ionic bond, is the strongest type of intermolecular force.

Hydrogen bonds substantially raise the boiling point of a compound because such bonds need significantly more energy to break. More interestingly, hydrogen bonding imparts unique properties to compounds, like the high boiling point of water and the secondary and tertiary structures of proteins. Water's cohesive and adhesive properties are also largely attributable to hydrogen bonding.

Physical Properties of Compounds

The physical properties of compounds, such as boiling point, melting point, and solubility, are directly affected by the type and strength of the intermolecular forces present. Boiling points are particularly telling as they represent the temperature at which a substance transitions from the liquid phase to the gas phase. A higher boiling point implies that more energy is needed to overcome the intermolecular forces holding the molecules together in the liquid.

For example, the reason why I-butanol has a noticeably higher boiling point than diethyl ether, despite having similar molecular weights and structures, is attributed to the intermolecular forces present. I-butanol can form hydrogen bonds due to its hydroxyl group, which requires more energy to break during the boiling process. In contrast, diethyl ether cannot form hydrogen bonds, and its boiling point is determined mainly by the presence of Van der Waals forces and dipole-dipole interactions.