Which of the following has the highest boiling point?

Analysis

To determine which compound has the highest boiling point, we must look at the intermolecular forces present in each molecule. Boiling point is a measure of the energy required to overcome the forces holding molecules together in the liquid state.

Concept Explanation

1. Van der Waals forces (London Dispersion Forces): These exist in all molecules. The strength increases with the molecular mass and the length of the carbon chain.
2. Dipole-Dipole interactions: Present in polar molecules (like the chloroalkane in option B).
3. Hydrogen Bonding: This is the strongest intermolecular force among these options. It occurs when a hydrogen atom is bonded to a highly electronegative atom like Oxygen, Nitrogen, or Fluorine. Molecules capable of hydrogen bonding generally have significantly higher boiling points than those that are not.

Comparing the options:

• A (Ethanol, $CH_3CH_2OH$): Contains an -OH group, so it exhibits hydrogen bonding.
• B (1-Chloropropane, $CH_3CH_2CH_2Cl$): Polar, but lacks hydrogen bonding.
• C (Hexane, $CH_3CH_2CH_2CH_2CH_2CH_3$): Non-polar, relies only on Van der Waals forces. While it has a long chain, it lacks the strong dipole or hydrogen bonding of the alcohols.
• D (Propan-1-ol, $CH_3CH_2CH_2OH$): Contains an -OH group, so it exhibits hydrogen bonding.

Step-by-Step Solution

1. Identify the strongest force: Both A and D have -OH groups, meaning they both undergo hydrogen bonding.
2. Compare molecular size: When comparing molecules with the same functional group (alcohols), the boiling point increases as the carbon chain length increases because the Van der Waals forces become stronger as the molecule gets larger.
3. Evaluate the chain length: • Ethanol (A) has 2 carbons. • Propan-1-ol (D) has 3 carbons.
4. Conclusion: Because Propan-1-ol has a longer carbon chain than Ethanol, it has a larger surface area and higher molecular mass, leading to stronger Van der Waals forces in addition to its hydrogen bonding. Therefore, it requires more energy to boil.

Answer

The correct option is D.

Exam Tip

When comparing boiling points of organic compounds:

1. Check for Hydrogen Bonding first: Alcohols, carboxylic acids, and amines usually have higher boiling points than alkanes or halides of similar size.
2. Check the Carbon Chain: If the functional group is the same, the molecule with the longer carbon chain (higher molar mass) will always have the higher boiling point.