Question

Consider the compounds butanoic acid, pentanal, $n$ -hexane, and 1-pentanol. The boiling points of these compounds (in no specific order) are ${69}^{\circ }\mathrm{C},{103}^{\circ }\mathrm{C},{137}^{\circ }\mathrm{C}$, and ${164}^{\circ }\mathrm{C}$. Match the boil- ing points to the correct compound.

Short answer

The assigned boiling points for the compounds based on the strength of their intermolecular forces are as follows: $n$-Hexane has the lowest boiling point at ${69}^{\circ }$ C due to its weak London Dispersion forces. Pentanal, with a slightly stronger dipole-dipole interaction, has a higher boiling point at ${103}^{\circ }$ C. 1-Pentanol exhibits hydrogen bonding leading to a higher boiling point at ${137}^{\circ }$ C. Butanoic Acid, which can form two hydrogen bonds, has the highest boiling point at ${164}^{\circ }$ C.

Step-by-step solution

Understand Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules. They are weaker than the intra-molecular forces that keep a molecule together. However, they are still strong enough to affect physical properties such as boiling point, melting point, and density. In this case, these forces are playing a significant role in determining the boiling points of the four compounds in the given exercise.

Assign Boiling Points based on Intermolecular Forces

1. n-Hexane (Alkane): This molecule only possesses London Dispersion forces, the weakest type of intermolecular forces. Therefore, it would have the lowest boiling point. This matches it to the boiling point of ${69}^{\circ }\mathrm{C}$. 2. Pentanal (Aldehyde): Aldehydes possess dipole-dipole interactions due to the presence of a polar carbonyl group, which is stronger than London Dispersion forces. Therefore, it would have a higher boiling point than n-Hexane but lower than compounds with hydrogen bonding. This matches it to the boiling point of ${103}^{\circ }\mathrm{C}$. 3. 1-Pentanol (Alcohol): Alcohols possess hydrogen bonding, a significantly stronger type of intermolecular force, due to the presence of an -OH group. This should result in a higher boiling point than an aldehyde, but it would be lower than the boiling point of a carboxylic acid which can form two hydrogen bonds. This matches it to the boiling point of ${137}^{\circ }\mathrm{C}$. 4. Butanoic Acid (Carboxylic Acid): This molecule possesses the strongest type of intermolecular force - hydrogen bonding, and it can form two hydrogen bonds due to the presence of both a carbonyl and a hydroxyl group on the acid functional group, leading to a very high boiling point. This matches it to the boiling point of ${164}^{\circ }\mathrm{C}$.

Connect the Boiling Points to the Compounds

Based on the analysis of the functional groups and their intermolecular forces the matching is as follows: - n-Hexane: ${69}^{\circ }\mathrm{C}$ - Pentanal: ${103}^{\circ }\mathrm{C}$ - 1-Pentanol: ${137}^{\circ }\mathrm{C}$ - Butanoic Acid: ${164}^{\circ }\mathrm{C}$

Key concepts

Chemical Physical Properties

Understanding the chemical physical properties of substances is essential for predicting how they behave. These properties, which include boiling point, melting point, solubility, and density, arise from the distinct characteristics of molecules and their intermolecular interactions. Specifically, boiling point is closely tied to the strength of the forces between molecules. Substances with stronger intermolecular forces require more energy to separate molecules, leading to higher boiling points.

When considering substances like alkanes, aldehydes, alcohols, and carboxylic acids, their boiling points increase as the strength of their intermolecular forces increases. For instance, the alkane in our example, n-hexane, exhibits just weak London dispersion forces, which explains its relatively low boiling point. In contrast, butanoic acid, which can form dual hydrogen bonds due to its functional groups, exhibits a substantially higher boiling point, as it is much harder to separate its molecules. This correlation assists students in identifying the compounds based on their boiling points.

Functional Groups in Organic Compounds

Functional groups in organic compounds are specific groups of atoms that are responsible for the characteristic chemical reactions of those compounds. They play a critical role in determining the physical and chemical properties of organic molecules.

Each functional group, be it a hydroxyl (-OH), aldehyde (-CHO), or carboxyl (-COOH), introduces different reactivities and interactions. For example, a hydroxyl group makes alcohols capable of engaging in hydrogen bonding, significantly altering the compound's boiling point when compared to an alkane without such a group. Aldehydes, with their carbonyl functional group, have dipole-dipole interactions which are stronger than the London dispersion forces in alkanes but weaker than hydrogen bonds in alcohols and carboxylic acids. Recognizing these groups allows us to systematically explain the differences in boiling points among various organic substances.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction and is one of the strongest forms of intermolecular forces. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine, and is in close proximity to another electronegative atom.

This specific attraction leads to unique properties in substances capable of hydrogen bonding, such as the high boiling point of water and, as seen in our exercise, a significant difference in boiling points between compounds like 1-pentanol and butanoic acid, compared to n-hexane and pentanal. The ability to form hydrogen bonds dramatically increases the boiling point, because a greater amount of energy is required to overcome these strong attractions between molecules. In our example, 1-pentanol can form one hydrogen bond per molecule, whereas butanoic acid can form two, resulting in the highest boiling point among the given compounds.