Question

Explain the relative boiling points of these two compounds: \(\mathrm{HCHO}\) at \(-21^{\circ} \mathrm{C}\) and \(\mathrm{CH}_{3} \mathrm{CHO}\) at \(20^{\circ} \mathrm{C}\).

Short answer

The difference in boiling points between the compounds formaldehyde (\(\mathrm{HCHO}\)) and acetaldehyde (\(\mathrm{CH}_{3} \mathrm{CHO}\)) is attributed to different levels of intermolecular forces. Both have dipole-dipole interactions, but the larger acetaldehyde also contains more London dispersion forces due to its larger and more complex molecular structure, resulting in a higher boiling point than formaldehyde.

Step-by-step solution

Identify Molecules and Their Structures

The first step involves identifying the molecules in question. \(\mathrm{HCHO}\) is known as formaldehyde, and its structure is of a carbon atom double-bonded to an oxygen atom and two hydrogen atoms. \(\mathrm{CH}_{3} \mathrm{CHO}\) is known as acetaldehyde, and its structure consists of a carbon atom double-bonded to an oxygen atom, a single hydrogen, and CH3 group.

Identify Intermolecular Forces

Next, identify the types of intermolecular forces in both the compounds. Both of these compounds feature dipole-dipole forces due to the presence of polar covalent bonds. However, acetaldehyde also features more London dispersion forces (LDF) as it has a longer chain, which allows it to have more points of contact and electron interactions.

Explain the Difference

Now, explain the boiling points using the information above. Both the molecules have dipole-dipole interactions due to the presence of polar bonds, but acetaldehyde (\(\mathrm{CH}_{3} \mathrm{CHO}\)) has stronger intermolecular forces due to additional dispersion forces. This results in a higher boiling point. In contrast, formaldehyde (\(\mathrm{HCHO}\)), having only dipole-dipole forces and being a smaller molecule comparatively, exhibits a lower boiling point.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that hold molecules close to one another. These forces are crucial in determining the physical properties of substances, like boiling point, melting point, and solubility. There are many types of intermolecular forces, including hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Each of these forces varies in strength, and their presence or absence can significantly influence how a molecule behaves.

Intermolecular forces are generally weaker than the strong covalent bonds within molecules. However, they still play a vital role in the physical characteristics of substances. The stronger the intermolecular forces present in a compound, the more energy (heat) is required to break these interactions, and therefore, the higher the boiling point.

• Hydrogen Bonding: A strong type of intermolecular interaction, found when hydrogen is bonded to highly electronegative atoms like oxygen.
• Dipole-Dipole Interactions: Occur between molecules with permanent dipoles.
• London Dispersion Forces: The weakest forces, present in all molecules, but more prominent in larger, more electron-rich molecules.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipoles. A molecule with a permanent dipole has regions of positive and negative charge, because of the difference in electronegativity between the atoms forming the bonds within the molecule.

In the context of acetaldehyde (\( \mathrm{CH_3CHO} \)) and formaldehyde (\( \mathrm{HCHO} \)), both have polar bonds because of the double bond between the carbon and the oxygen. This results in a partial negative charge on the oxygen and a partial positive charge on the carbon. These opposite charges on adjacent molecules attract, leading to dipole-dipole interactions.

The strength of dipole-dipole interactions is not as strong as hydrogen bonding, but they are still more powerful than London dispersion forces. In the case of formaldehyde and acetaldehyde, both experience dipole-dipole interactions, but because acetaldehyde is a larger molecule, it is also capable of more London dispersion forces. This means that, while dipole-dipole forces contribute significantly to intermolecular attractions, the effect of additional forces (like in acetaldehyde) cannot be overlooked.

London Dispersion Forces

London dispersion forces (LDF) are the weakest type of van der Waals forces. They arise from temporary fluctuations in electron density, which create temporary dipoles even in non-polar molecules. These temporary dipoles can induce neighboring dipoles, leading to an attraction between molecules.

Even though they are the weakest intermolecular force, London dispersion forces become significant in large atoms or long-chain molecules, which have more electrons and hence a greater probability of forming temporary dipoles. In the case of acetaldehyde (\( \mathrm{CH_3CHO} \)), the presence of a longer carbon chain compared to formaldehyde (\( \mathrm{HCHO} \)) allows for more London dispersion forces.

These forces increase the overall intermolecular interactions between molecules, including within acetaldehyde, resulting in a higher boiling point compared to smaller, less electron-rich molecules like formaldehyde. Essentially, the size and shape of acetaldehyde give it access to more dispersion forces, contributing to its higher boiling point.