Question

Consider the following five compounds

The boiling points of these five compounds are $\mathbf{9}\mathbf{.}\mathbf{5}\mathbf{°}\mathit{C}\mathbf{,}\mathbf{}\mathbf{36}\mathbf{°}\mathit{C}\mathbf{,}\mathbf{}\mathbf{69}\mathbf{°}\mathit{C}\mathbf{,}\mathbf{76}\mathbf{°}\mathit{C}\mathbf{}\mathit{a}\mathit{n}\mathit{d}\mathbf{}\mathbf{117}\mathbf{°}\mathit{C}$. Which compound boils at 36^oC? Explain.

Short answer

Pentane boils at $36.1°C$.

Step-by-step solution

Trends that affect boiling point

General trends that affect boiling point:

a. Strength of intermolecular forces

The greater the intermolecular forces, the higher is the boiling point. We need to take into consideration all intermolecular forces (ionic, hydrogen bonding, dipole-dipole interaction, and Van der Waals dispersion force).

b. Length of carbon-carbon chain

As the length of the carbon-carbon chain increases, the boiling point also increases.

c. Branching

Branching decreases the boiling point.

Molecular weight

All molecules have a relatively similar molecular weight.

Pentane, hexane, and neopentane are alkanes. The only intermolecular forces in alkanes are Van der Waals dispersion forces. Butane-1-ol is alcohol, and alcohol contains the hydroxyl group which is capable of hydrogen bonding. Butyraldehyde is an aldehyde, in which charge separation between oxygen and carbon leads to dipole-dipole interactions.

Strength of intermolecular forces: Ionic - Hydrogen bonding - dipole-dipole - Van der Waals dispersion forces

Therefore, we can conclude that alcoholhas the highest boiling point, followed by aldehyde and alkanes have the lowest boiling point.

If we compare pentane and hexane, hexane has a longer carbon chain and therefore higher boiling point.

Neopentane is a branched form of pentane, which means that it has a lower boiling point.

Hence Pentane is boiled at $36.1°C.$