Question

Which should have the higher boiling point, ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH},\right.\) found in alcoholic beverages and fermented berries that are favored by wild bears) or ethanethiol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH},\right.\) a foul-smelling liquid found in the urine of rabbits that have feasted on cabbage)?

Short answer

Ethanol (\text{CH}_{3}\text{CH}_{2}\text{OH}) should have a higher boiling point than ethanethiol (\text{CH}_{3}\text{CH}_{2}\text{SH}) due to stronger hydrogen bonding.

Step-by-step solution

- Analyze Molecular Structure and Intermolecular Forces

Compare the molecular structures of ethanol and ethanethiol. Both molecules have similar molecular weights and structures, but the functional groups (OH in ethanol and SH in ethanethiol) differ. This affects the types of intermolecular forces present in each compound.

- Identify Intermolecular Forces

Ethanol has a hydroxyl group (OH), which can engage in hydrogen bonding, a strong type of intermolecular force. Ethanethiol has a sulfhydryl group (SH), which can also engage in hydrogen bonding, but it is much weaker due to sulfur being less electronegative than oxygen.

- Compare Boiling Points Based on Intermolecular Forces

Since ethanol can form stronger hydrogen bonds than ethanethiol, its intermolecular forces are stronger. Substances with stronger intermolecular forces have higher boiling points. Therefore, ethanol should have a higher boiling point than ethanethiol.

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which its liquid form transitions into a gas, which occurs when vapor pressure equals atmospheric pressure. Boiling point is not only a fundamental property of a substance but also an indicator of the strength of its intermolecular forces.

A higher boiling point suggests that the molecules are more strongly attracted to each other, requiring more energy to break these attractions and transition into the gas phase. Considering the substances ethanol and ethanethiol, a prediction can be made based on the type and strength of the intermolecular attractions.

In the case of ethanol \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\), the presence of hydrogen bonding elevates its boiling point because this specific interaction is much stronger compared to other types of intermolecular forces present in molecules like ethanethiol \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{SH}\), which has less effective hydrogen bonding due to the presence of sulfur.

Hydrogen Bonding

Hydrogen bonding is an exceptionally strong type of dipole-dipole interaction that occurs when a hydrogen atom is attached to a highly electronegative element such as oxygen, nitrogen, or fluorine, and the hydrogen is in turn attracted to an electronegative atom in another molecule.

In ethanol, the hydroxyl group (OH) contains oxygen, a highly electronegative atom, making hydrogen bonding very effective and contributing to the high boiling point of ethanol. This type of bonding is powerful because it significantly increases the amount of energy required to separate molecules from each other.

While ethanethiol also has hydrogen atoms, its bonds are inherently weaker due to sulfur being less electronegative than oxygen. As a result, the hydrogen bonds formed are not as strong as those in ethanol, leading to a lower boiling point for ethanethiol.

Molecular Structure

Understanding the molecular structure of a compound is critical in predicting its physical and chemical properties. Structurally, both ethanol and ethanethiol share a similar carbon backbone; however, the key difference lies in their functional groups, which dramatically influences intermolecular forces.

Alcohol groups (OH) in ethanol make it capable of strong hydrogen bonding, whereas thiol groups (SH) in ethanethiol result in subtler intermolecular interactions. These functional groups fundamentally affect their boiling points due to the types and strengths of intermolecular forces they can establish.

When two compounds have similar molecular weights and carbon chains, as in our exercise, it’s the nature of their functional groups that predominantly determines their boiling points. Thus, the molecular structure directly influences the intermolecular forces between molecules, which in turn affects the boiling point of the substance.