Question

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, \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\) .

Short answer

The boiling point of propyl alcohol is higher than isopropyl alcohol because of the stronger intermolecular forces, specifically the London dispersion forces, present in propyl alcohol due to its linear molecular structure. The linear structure allows for more surface-to-surface contact between propyl alcohol molecules, resulting in overall stronger intermolecular attractions and therefore a higher boiling point compared to the branched structure of isopropyl alcohol.

Step-by-step solution

Identify the different molecular structures

Propyl alcohol has a linear molecular structure (CH3CH2CH2OH) while isopropyl alcohol has a branched molecular structure [(CH3)_2CHOH]. These different molecular structures lead to differences in the intermolecular forces between the molecules.

Identify the intermolecular forces present in both alcohols

Both propyl alcohol and isopropyl alcohol have hydrogen bonding due to the presence of the hydroxyl group (OH). They also have London dispersion forces (also called van der Waals forces) due to the presence of non-polar hydrocarbon chains in both molecules.

Determine the strength of intermolecular forces in each alcohol

In propyl alcohol, due to its linear molecular structure, the London dispersion forces are stronger than in isopropyl alcohol since there are more opportunities for surface-to-surface contact between the molecules. This will lead to overall stronger intermolecular attractions in propyl alcohol. In contrast, the branched structure of isopropyl alcohol does not allow it to have as important surface-to-surface contact, leading to weaker London dispersion forces.

Relate the strength of intermolecular forces to boiling points

Boiling points are related to the strength of intermolecular forces between molecules: the stronger the intermolecular forces, the more energy is required to separate the molecules and turn the liquid into a gas, resulting in a higher boiling point. Since propyl alcohol has stronger intermolecular forces due to the more effective London dispersion forces, it has a higher boiling point compared to isopropyl alcohol.

Conclude the explanation

The difference in the boiling points of propyl alcohol and isopropyl alcohol is due to the difference in their molecular structure, which affects the strength of London dispersion forces between the molecules. Propyl alcohol has stronger intermolecular forces due to its linear structure, which results in a higher boiling point compared to the branched structure of isopropyl alcohol.

Key concepts

Intermolecular Forces

The concept of intermolecular forces explains why different molecules exhibit varying degrees of attraction to each other. These forces are responsible for the way substances state of matter (solid, liquid, or gas) at given temperatures and pressures. In the context of alcohols, such as propyl alcohol and isopropyl alcohol, intermolecular forces dictate their boiling points.

As a step up in understanding, consider that these forces are not bonds within the molecule but rather the attractions between separate molecules. They encompass hydrogen bonds, London dispersion forces, and dipole-dipole interactions. When saturated alcohols like those in our exercise are compared, the types and strengths of their intermolecular forces largely determine their physical properties, including boiling points.

London Dispersion Forces

London dispersion forces are a type of van der Waals force, which are the weakest of intermolecular attractions, but they are present in all molecules. They arise due to the temporary uneven distribution of electrons that create temporary dipoles within the molecules. Factors such as molecular size and shape can intensify these forces.

For instance, linear molecules like propyl alcohol have a larger surface area, which enhances the London dispersion forces because there is more area over which the temporary dipoles can interact. This is a key point because these interactions are directly connected to why propyl alcohol requires more heat, hence a higher temperature (higher boiling point), to overcome these forces during the phase transition from liquid to gas.

Molecular Structure of Alcohols

Alcohols are organic compounds characterized by the presence of a hydroxyl group (-OH) attached to a saturated carbon atom. The molecular structure, namely whether the alcohol is linear or branched, plays a significant role in determining its physical properties.

In our example, propyl alcohol has a straight-chain structure, which allows molecules to pack closely together, increasing the London dispersion forces. Meanwhile, isopropyl alcohol's branched structure causes its molecules to be less closely packed, leading to weaker London dispersion forces and, consequently, a lower boiling point. Understanding the molecular structure provides a predictive insight into the boiling points and how heat energy influences these alcohols when transitioning from liquid to gas.

Hydrogen Bonding

Hydrogen bonding represents the strongest type of intermolecular force in alcohols and is critical for explaining their relatively high boiling points. A hydrogen bond is a special dipole-dipole interaction that occurs when a hydrogen atom is bonded to an electronegative atom like oxygen and is attracted to another electronegative atom nearby.

In both propyl alcohol and isopropyl alcohol, the hydroxyl (-OH) group allows for hydrogen bonding. These intermolecular bonds are much stronger than London dispersion forces and significantly influence the boiling point of alcohols. Each molecule can form multiple hydrogen bonds, thus stabilizing the liquid phase and requiring more energy (higher temperature) to break these interactions for boiling to occur. It is these hydrogen bonds, combined with the London dispersion forces, that account for the higher boiling point of propyl alcohol compared to isopropyl alcohol.