- The boiling point of propanal is 49°C, whereas the boiling point of propanol is 97°C. Explain this difference in boiling points.
- Explain why propane boils at 242°C, whereas ethanal, which has the same molecular weight, boils at 20°C.
The Correct Answer and Explanation is :
1. Boiling Point of Propanal vs. Propanol
The difference in boiling points between propanal (49°C) and propanol (97°C) can be explained by the types of intermolecular forces present in each compound.
- Propanal is an aldehyde, and it primarily exhibits dipole-dipole interactions and van der Waals forces. Aldehydes have a polar carbonyl group (C=O), which results in dipole-dipole interactions, but these forces are not as strong as those present in alcohols.
- Propanol, on the other hand, is an alcohol. Alcohols contain a hydroxyl group (-OH) that can form hydrogen bonds with other alcohol molecules. Hydrogen bonds are significantly stronger than dipole-dipole interactions and van der Waals forces. These strong intermolecular forces require more energy to break, leading to a higher boiling point for propanol.
Therefore, despite both compounds having similar molecular weights, the presence of hydrogen bonding in propanol leads to a higher boiling point compared to the dipole-dipole interactions in propanal.
2. Boiling Point of Propane vs. Ethanal
Propane (boiling point 242°C) and ethanal (boiling point 20°C) have the same molecular weight, but their boiling points differ significantly due to the types of intermolecular forces each compound exhibits.
- Propane is a non-polar molecule, and its only significant intermolecular force is van der Waals (London dispersion) forces. These forces arise from temporary dipoles induced in the molecules. While these forces are weak compared to hydrogen bonds, they still influence boiling points. However, propane’s large molecular size (C3H8) results in a relatively high boiling point for a non-polar molecule, as the London dispersion forces become stronger with the number of electrons and surface area.
- Ethanal (also known as acetaldehyde) is a polar molecule due to the presence of the carbonyl group (C=O), which leads to dipole-dipole interactions. However, ethanal does not exhibit hydrogen bonding, and the dipole-dipole interactions are weaker than hydrogen bonds. As a result, despite having the same molecular weight as propane, ethanal has a much lower boiling point because its intermolecular forces are not as strong as propane’s dispersion forces.
Thus, propane boils at a higher temperature than ethanal due to the stronger dispersion forces between its larger non-polar molecules compared to the weaker dipole-dipole interactions in ethanal.