Propyl alcohol $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)$ and isopropyl alcohol \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}\right],\) whose space- filling models are shown, have boiling points of 97.2 and $82.5^{\circ} \mathrm{C}$, respectively. Explain why the boiling point of propyl alcohol is higher, even though both have the molecular formula, $\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}$.

Boiling point is the temperature at which a substance changes from liquid to gas. When the temperature reaches the boiling point, the substance's vapor pressure equals the atmospheric pressure. The boiling point is mainly influenced by the type and strength of the intermolecular forces in the compound, which need to be overcome for a molecule to escape the liquid phase.

Intermolecular forces are the forces between different molecules. In this case, we have to identify the forces in propyl alcohol and isopropyl alcohol. Both compounds are alcohols, with an -OH group in their structures. This means that they will have hydrogen bonding, which is a strong intermolecular force. They will also have London dispersion forces or van der Waals forces, which are forces between non-polar molecules or parts of molecules.

Hydrogen bonding is the strongest intermolecular force in both propyl alcohol and isopropyl alcohol due to the presence of the -OH group. However, it's important to understand that the boiling point of a substance also depends on the strength of hydrogen bonding and the number of hydrogen bonds a molecule can form. The main difference between propyl alcohol and isopropyl alcohol in terms of their molecular structure is that propyl alcohol is a linear molecule while isopropyl alcohol has a more compact branched structure. In propyl alcohol, its linear structure allows the molecules to align more closely, allowing more hydrogen bonding between the molecules. In isopropyl alcohol, the compact structure makes it difficult for as many hydrogen bonds to form between molecules.

London dispersion forces are the second type of intermolecular forces present in both compounds. These forces depend on the surface area of the molecule, with larger surface areas leading to stronger dispersion forces. The linear structure of propyl alcohol creates a larger surface area than isopropyl alcohol's compact structure, which means stronger London dispersion forces in propyl alcohol.

Propyl alcohol has a higher boiling point than isopropyl alcohol primarily because its linear structure allows it to form more hydrogen bonds in addition to having a larger surface area for London dispersion forces. This means that stronger intermolecular forces in propyl alcohol need to be overcome for the substance to change from a liquid to a gas, requiring a higher temperature than isopropyl alcohol. This is why the boiling point of propyl alcohol is higher (97.2°C) compared to isopropyl alcohol (82.5°C).