Question

The following pairs of compounds have about the same molar mass. Predict which compound in each pair has the higher boiling point. (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) or \(\mathrm{HCOOCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\) or \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\)

Short answer

(a)  ext{CH}_{3} ext{COOH}; (b)  ext{CH}_{3} ext{CH}_{2} ext{CONH}_{2}.

Step-by-step solution

Understanding Intermolecular Forces

Boiling points are influenced by the strength of intermolecular forces. Compounds that can form hydrogen bonds generally have higher boiling points than those that cannot. Additionally, polar compounds with dipole-dipole interactions tend to have higher boiling points compared to nonpolar compounds with only London dispersion forces.

Analyzing Compound (a)

The compounds mathrm{CH}_{3} ext{COOH} and  ext{HCOOCH}_{3} are compared. mathrm{CH}_{3} ext{COOH} has hydrogen bonding due to the presence of the -OH group.  ext{HCOOCH}_{3}, being an ester, lacks this capability as it does not contain an -OH or -NH group to participate in hydrogen bonding. Therefore, mathrm{CH}_{3} ext{COOH} will have a higher boiling point.

Analyzing Compound (b)

The compounds  ext{CH}_{3} ext{CH}_{2} ext{CONH}_{2} and  ext{CH}_{3} ext{COOCH}_{3} are compared.  ext{CH}_{3} ext{CH}_{2} ext{CONH}_{2} contains an amide group ( ext{CONH}_{2}) which can form hydrogen bonds due to the N-H group. Conversely,  ext{CH}_{3} ext{COOCH}_{3} is an ester and cannot form hydrogen bonds. Therefore,  ext{CH}_{3} ext{CH}_{2} ext{CONH}_{2} will have a higher boiling point.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that significantly impacts the properties of compounds. This force occurs when a hydrogen atom, covalently bonded to a highly electronegative element like oxygen, nitrogen, or fluorine, interacts with a lone pair of electrons on a neighboring electronegative atom.

The presence of hydrogen bonds makes a compound unique. It increases the intermolecular forces, making molecules "stick" together more. This stickiness requires more energy to separate the molecules into a gas, leading to higher boiling points.

For example, in \(\mathrm{CH}_3\mathrm{COOH}\) (acetic acid), the -OH group can form hydrogen bonds. This makes it exhibit a higher boiling point when compared to compounds like esters (e.g., \(\mathrm{HCOOCH}_3\)), which lack such hydrogen bonding ability.

Boiling Point Comparison

Boiling point is a critical property that indicates how strongly molecules interact with one another in a liquid. It depends largely on the types of intermolecular forces present.

• **Hydrogen Bonding**: Compounds with hydrogen bonds, like alcohols and carboxylic acids, generally have high boiling points.
• **Dipole-Dipole Interactions**: Polar compounds exhibit these interactions, which also increase boiling points.
• **London Dispersion Forces**: Present in all molecules, but more significant in nonpolar compounds. These forces are weaker than hydrogen bonds, resulting in lower boiling points.

In comparing molecules such as \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CONH}_2\) with -NH groups, these engage in hydrogen bonding, thereby giving the compound a higher boiling point than an ester like \(\mathrm{CH}_3\mathrm{COOCH}_3\), which lacks such hydrogen bonding capability.

Polar and Nonpolar Compounds

The polarity of molecules determines the type of intermolecular forces they experience, thereby influencing boiling points.

Polar compounds have an uneven distribution of electron density, resulting in a permanent dipole moment. This allows them to engage in dipole-dipole interactions, which are relatively strong.

Nonpolar compounds, on the other hand, have a more uniform electron distribution, and their interactions are primarily via London dispersion forces, which are weaker in nature.

• Polar compounds like \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CONH}_2\) not only have dipole-dipole interactions but also benefit from hydrogen bonding, leading to stronger intermolecular forces and higher boiling points.
• For nonpolar compounds, the absence of these interactions is why their boiling points tend to be lower.

Understanding these differences helps in predicting and comparing the boiling points of various compounds.