Question

Which would have the higher boiling point, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) ? Explain your answer.

Short answer

CH3CH2CH2CH2OH (butanol) would have a higher boiling point than CH3CH2OCH2CH3 (diethyl ether) because it can form stronger intermolecular forces in the form of hydrogen bonds.

Step-by-step solution

Understand intermolecular forces and boiling points

Before analyzing the specific compounds in this exercise, it's essential to understand the role of intermolecular forces in determining a substance's boiling point. Intermolecular forces are the interactions between different molecules. The stronger the intermolecular forces, the more energy is required to break them. Consequently, substances with stronger intermolecular forces have higher boiling points.

Analyze the first molecule - CH3CH2OCH2CH3

Now let's look at CH3CH2OCH2CH3, also known as diethyl ether. The oxygen atom in this molecule is bonded to carbon atoms and cannot form hydrogen bonds with other diethyl ether molecules, since hydrogen bonding requires a hydrogen atom directly bonded to a highly electronegative atom (N, O, or F). Therefore, the dominant intermolecular forces in diethyl ether are dispersion forces and dipole-dipole interactions. The latter arises from the difference in electronegativity between oxygen and carbon.

Analyze the second molecule - CH3CH2CH2CH2OH

This molecule is butanol, a type of alcohol. In alcohols, there's a hydroxyl group (OH) that can form strong hydrogen bonds with other alcohol molecules – as hydrogens in the OH group are directly bonded to oxygen, a highly electronegative atom. This ability to form hydrogen bonds on top of dispersion forces and dipole-dipole interactions means butanol has stronger overall intermolecular forces than diethyl ether.

Key concepts

Intermolecular Forces

Intermolecular forces are crucial in determining the physical properties of substances, including their boiling points. These forces are the attractions and repulsions between molecules that arise from various interactions.
The strength of these forces influences how much energy is needed to separate molecules from one another. Therefore, substances with strong intermolecular forces require more energy to vaporize, leading to higher boiling points.
There are several types of intermolecular forces, including:

• London dispersion forces: These are weak forces resulting from the temporary fluctuations in electron distribution around atoms or molecules, leading to momentary dipoles even in nonpolar species.
• Dipole-dipole interactions: These occur between molecules having permanent dipole moments due to differences in electronegativity in bonds.
• Hydrogen bonding: A particularly strong type of dipole-dipole interaction when hydrogen is bonded to highly electronegative atoms (such as N, O, or F).

Hydrogen Bonding

Hydrogen bonding occurs when hydrogen atoms are bonded directly to electronegative atoms like oxygen, nitrogen, or fluorine. This results in a strong dipole-dipole attraction due to the significant difference in electronegativity.
In the exercise, butanol (\( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH} \) ) exhibits hydrogen bonding because of its hydroxyl group (OH). The oxygen in the OH group pulls electron density away from hydrogen, resulting in a positive charge on hydrogen and a negative charge on oxygen.
This leads to strong attractions between hydrogen on one molecule and oxygen on another, significantly increasing the boiling point of butanol.

Key Points on Hydrogen Bonding:

• Hydrogen must be bonded directly to an electronegative atom.
• Creates strong intermolecular forces in addition to dipole-dipole interactions.
• Significantly raises boiling points compared to substances without hydrogen bonds.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in molecules that have a permanent dipole, meaning one part of the molecule is slightly positive and the other part is slightly negative. This polarity occurs due to the presence of polar bonds, where electronegative atoms attract electrons more strongly.
In the given exercise, diethyl ether (\( \text{CH}_3\text{CH}_2\text{OCH}_2\text{CH}_3 \) ) features dipole-dipole interactions. The oxygen atom bonded to carbon creates a polar bond due to oxygen's high electronegativity. Though weaker than hydrogen bonds, dipole-dipole interactions still affect boiling points.
Compared to diethyl ether, substances like butanol that can also form hydrogen bonds will have stronger total intermolecular forces, thus resulting in higher boiling points.

Key Characteristics of Dipole-Dipole Interactions:

• Involve molecules with permanent dipoles.
• Generally stronger than London dispersion forces but weaker than hydrogen bonds.
• Depend on molecular polarity and the presence of electronegative atoms.