Propyl alcoholC₃H₈O.
and isopropyl alcohol
, whose space-filling models are shown, have boiling points of
and
, respectively. Explain why the boiling point of propyl alcohol is higher, even though both have the molecular formula of
.
The Correct Answer and Explanation is:
Answer:
Propyl alcohol (1-propanol) has a higher boiling point than isopropyl alcohol (2-propanol) even though both share the same molecular formula, C₃H₈O. This difference in boiling point is due to differences in molecular structure and the resulting intermolecular forces.
Explanation
Propyl alcohol and isopropyl alcohol are structural isomers, meaning they have the same molecular formula but different arrangements of atoms. Propyl alcohol is a straight-chain alcohol, while isopropyl alcohol has a branched structure. This structural variation plays a key role in determining their physical properties, including boiling points.
The boiling point of a substance depends largely on the strength of intermolecular forces that must be overcome for a molecule to transition from liquid to gas. In alcohols, hydrogen bonding is the dominant intermolecular force due to the presence of the hydroxyl (-OH) group. However, hydrogen bonding is not the only force at play. Van der Waals forces, particularly dispersion forces, also contribute significantly.
In the case of propyl alcohol, its straight-chain structure allows molecules to pack more closely together. This tighter packing leads to stronger Van der Waals interactions between the molecules. As a result, more energy is required to overcome these interactions and turn the liquid into a gas, leading to a higher boiling point.
On the other hand, isopropyl alcohol has a branched structure that disrupts close molecular packing. The branched configuration reduces the surface area available for intermolecular contact, thereby weakening the dispersion forces. Even though isopropyl alcohol still exhibits hydrogen bonding, its overall intermolecular attractions are weaker than those in propyl alcohol due to the branching. This makes it easier for the molecules to escape into the gas phase, resulting in a lower boiling point.
In conclusion, the higher boiling point of propyl alcohol is primarily due to its linear structure, which enhances intermolecular attractions compared to the more compact and branched isopropyl alcohol. This demonstrates how molecular structure can significantly influence physical properties.
