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 boiling points to the correct compound.

Short answer

The boiling points of the compounds can be matched as follows: 1. \(n\)-hexane: \(69^{\circ}\mathrm{C}\) 2. Pentanal: \(103^{\circ}\mathrm{C}\) 3. 1-pentanol: \(137^{\circ}\mathrm{C}\) 4. Butanoic acid: \(164^{\circ}\mathrm{C}\)

Step-by-step solution

Molecular weight and formula

Firstly, we determine the molecular formula of each compound: 1. Butanoic acid: \(\text{C}_4\text{H}_8\text{O}_2\) 2. Pentanal: \(\text{C}_5\text{H}_{10}\text{O}\) 3. \(n\)-hexane: \(\text{C}_6\text{H}_{14}\) 4. 1-pentanol: \(\text{C}_5\text{H}_{12}\text{O}\)

Intermolecular forces

Analysis of intermolecular forces in each compound: 1. Butanoic acid: Carboxylic acids can form hydrogen bonds and have dispersion forces. 2. Pentanal: Aldehydes have dipole-dipole interactions due to the presence of a carbonyl group and dispersion forces. 3. \(n\)-hexane: Alkanes only have dispersion forces. 4. 1-pentanol: Alcohols can form hydrogen bonds and have dispersion forces.

Strength of intermolecular forces and boiling points

The boiling points generally increase with stronger intermolecular forces and higher molecular weight. 1. Butanoic acid: Strong hydrogen bonding and moderate molecular weight suggest a higher boiling point. 2. Pentanal: Dipole-dipole interactions are weaker than hydrogen bonding, and its molecular weight is less than that of butanoic acid. Therefore, it should have a lower boiling point than butanoic acid. 3. \(n\)-hexane: With only dispersion forces and the lowest molecular weight, it is expected to have the lowest boiling point. 4. 1-pentanol: Hydrogen bonding makes its boiling point higher than that of hexane and pentanal, but, due to its lower molecular weight, its boiling point will be lower than that of butanoic acid.

Matching boiling points

Now, we can match the boiling points to the correct compound: 1. \(n\)-hexane: \(69^{\circ}\mathrm{C}\) 2. Pentanal: \(103^{\circ}\mathrm{C}\) 3. 1-pentanol: \(137^{\circ}\mathrm{C}\) 4. Butanoic acid: \(164^{\circ}\mathrm{C}\)

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring molecules. The strength of these forces plays a critical role in determining the boiling points of compounds.
Butanoic acid and 1-pentanol both exhibit hydrogen bonding, which is a particularly strong type of intermolecular force. Hydrogen bonds form when a hydrogen atom bonds to a highly electronegative atom, such as oxygen, causing the boiling point to increase.
Pentanal, an aldehyde, has weaker dipole-dipole interactions because of its polar carbonyl group. These forces are stronger than dispersion forces but weaker than hydrogen bonds.
n-Hexane, a nonpolar alkane, only experiences London dispersion forces. These are the weakest intermolecular forces, as they result from temporary dipoles that occur when electrons randomly shift within an atom or molecule. Hence, n-hexane has a lower boiling point compared to other compounds with stronger forces.

Molecular Weight

Molecular weight, also known as molecular mass, is the sum of the atomic masses of the atoms in a molecule. It is measured in atomic mass units (amu).
Generally, compounds with higher molecular weights have higher boiling points, as larger molecules require more energy to change from a liquid to a gas. For instance, butanoic acid and 1-pentanol have relatively high molecular weights due to their larger structures. This contributes to their higher boiling points relative to lighter molecules like pentanal and n-hexane.
However, it's important to note that molecular weight is not the sole determinant of boiling points. It works in tandem with intermolecular forces, meaning a heavier molecule with weak forces might still have a lower boiling point than a lighter molecule with strong forces.

Organic Compounds

Organic compounds are typically structured from carbon and hydrogen atoms and may include other elements like oxygen and nitrogen. These compounds are classified into different groups based on their functional groups, which influence their chemical properties and reactions.
Butanoic acid includes a carboxylic acid group, known for its ability to form hydrogen bonds, thereby having higher boiling points. 1-Pentanol features an alcohol group with an -OH structure, also enabling strong hydrogen bonding.
Pentanal belongs to the aldehyde group and has a carbonyl group, responsible for its moderate intermolecular forces and boiling points. Conversely, n-hexane, an alkane, lacks a polar group and relies solely on dispersion forces.
Understanding these groupings helps predict the physical behaviors of organic compounds, such as boiling points, which are crucial for applications in both chemical experimentation and industrial processes.