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 \(97.2^{\circ} \mathrm{C}\) 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 (\(97.2^{\circ} \mathrm{C}\)) is higher than that of isopropyl alcohol (\(82.5^{\circ} \mathrm{C}\)) due to stronger intermolecular forces in propyl alcohol. Both molecules can form hydrogen bonds due to their -OH groups, but propyl alcohol's straight-chain structure leads to stronger dispersion forces compared to isopropyl alcohol's branched structure. The cumulative effect of the hydrogen bonding and stronger dispersion forces makes the overall intermolecular forces in propyl alcohol stronger, resulting in a higher boiling point.

Step-by-step solution

Identify the molecular structure of each compound

: Propyl alcohol has the chemical formula \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH}\) and has a straight-chain structure. Isopropyl alcohol has the chemical formula \(\left(\mathrm{CH}_3\right)_2\mathrm{CHOH}\) and has a branched structure.

Determine the types of intermolecular forces present in each compound

: Both propyl alcohol and isopropyl alcohol have polar -OH (hydroxyl) groups, which make both molecules polar. For the -OH hydroxyl group, hydrogen bond is the dominant intermolecular force. Due to its straight-chain structure, propyl alcohol has stronger dispersion forces between its molecules compared to the branched structure of isopropyl alcohol.

Relate boiling point to intermolecular forces

: The boiling point of a substance is related to the strength of its intermolecular forces. The stronger the intermolecular forces, the higher the boiling point, as more energy is required to overcome these forces and transform the substance into the gas phase.

Compare the intermolecular forces in propyl alcohol and isopropyl alcohol

: While both propyl alcohol and isopropyl alcohol can form hydrogen bonds due to their -OH groups, the straight-chain structure of propyl alcohol leads to stronger dispersion forces between its molecules compared to isopropyl alcohol's branched structure. The cumulative effect of the hydrogen bonding and stronger dispersion forces makes the overall intermolecular forces in propyl alcohol stronger than those in isopropyl alcohol.

Conclude and explain the difference in boiling points

: The stronger intermolecular forces in propyl alcohol are responsible for its higher boiling point (\(97.2^{\circ} \mathrm{C}\)) compared to isopropyl alcohol (\(82.5^{\circ} \mathrm{C}\)), even though both compounds have the same molecular formula \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\).

Key concepts

Boiling Point

Boiling point refers to the temperature at which a liquid turns into a gas. It is a crucial property that reflects the strength of intermolecular forces within a substance. Substances with strong intermolecular forces will have higher boiling points since more energy is required to break these forces for the phase transition.

In the case of propyl alcohol versus isopropyl alcohol, both these alcohols share the same molecular formula \(C_3H_8O\), meaning they contain the same number of carbon, hydrogen, and oxygen atoms. However, they differ in their structures. The molecular arrangement impacts their respective boiling points significantly.

Propyl alcohol exhibits a boiling point of \(97.2^{\circ}\text{C}\), which is higher than that of isopropyl alcohol at \(82.5^{\circ}\text{C}\). This discrepancy primarily stems from the differences in intermolecular forces. With stronger intermolecular forces in play, particularly dispersion forces, propyl alcohol demands more heat for vaporization compared to isopropyl alcohol.

Hydrogen Bonding

Hydrogen bonding is a type of attractive interaction between a hydrogen atom, which is covalently bonded to a more electronegative atom like oxygen or nitrogen, and another electronegative atom. This is a significant force in determining the physical properties of molecules.

Both propyl alcohol and isopropyl alcohol feature hydroxyl (-OH) groups, enabling them to engage in hydrogen bonding. This makes both molecules polar, further contributing to their intermolecular forces.

Hydrogen bonds are generally stronger than van der Waals forces or London dispersion forces. In our case, while both alcohols can form hydrogen bonds, it is essential to recognize the influence of these bonds alongside other types of forces, such as dispersion forces, which will tell the entire story of their boiling points.

Molecular Structure

The molecular structure denotes the arrangement of atoms within a molecule. Different structures can drastically change a molecule's properties.

For propyl alcohol, the structure is a straight chain, represented chemically as \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\). This linear structure allows for more significant interactions between the molecules through dispersion forces. In contrast, isopropyl alcohol has a branched structure, \(\left(\text{CH}_3\right)_2\text{CHOH}\), which reduces the molecule's ability to interact as closely with others.

The straight-chain structure in propyl alcohol results in stronger dispersion forces because the molecules can pack more tightly and align orderly, leading to increased surface contact. This stacking strength in propyl alcohol results in a higher boiling point by needing more energy to separate these molecules during the transition from liquid to vapor.