Alkanoic acids have higher boiling points than alkanols because

Analysis

The question asks for a comparison of the boiling points of alkanoic acids (carboxylic acids) and alkanols (alcohols) and the underlying reason for the difference.

Concept Explanation

Boiling point is a measure of the energy required to overcome intermolecular forces. Both alkanols and alkanoic acids exhibit hydrogen bonding, which is a strong intermolecular force.

1. Alkanols (R-OH): These molecules form hydrogen bonds between the hydrogen atom of one hydroxyl group and the oxygen atom of another.
2. Alkanoic Acids (R-COOH): These molecules contain a carboxyl group. They can form hydrogen bonds in a way that creates dimers (two molecules linked together). Because the carboxyl group contains both a carbonyl group (C=O) and a hydroxyl group (O-H), the hydrogen bonding in alkanoic acids is more extensive and stable than in simple alkanols.

Because the hydrogen bonding in alkanoic acids is more effective and results in the formation of stable dimers, more energy (heat) is required to break these bonds, leading to a higher boiling point compared to alkanols of similar molecular mass.

Step-by-Step Solution

1. Identify the intermolecular forces: Both classes of compounds use hydrogen bonding.
2. Compare the strength: Alkanoic acids form dimers due to the interaction between the C=O and O-H groups of adjacent molecules. This creates a more stable, "double" hydrogen-bonded structure.
3. Relate to boiling point: Stronger intermolecular forces require more thermal energy to break. Since the dimerized hydrogen bonds in alkanoic acids are stronger/more extensive than the simple hydrogen bonds in alkanols, the boiling point of the acid is higher.
4. Evaluate the options: • A is incorrect; alkanoic acids are less volatile (higher boiling point means lower volatility). • B is incorrect; both are polar, but the hydrogen bonding in acids is more effective. • C is incorrect; boiling point is determined by intermolecular forces, not just molecular size. • D is correct; the formation of stable dimers via hydrogen bonding makes the bonds effectively stronger.

Conclusion

Exam Tip: Whenever you see a question about boiling points in organic chemistry, always look for the strength of intermolecular forces in this order: Hydrogen bonding > Dipole-dipole interactions > Van der Waals forces (London dispersion forces). Since both have hydrogen bonds, look for which one allows for more extensive or stable bonding!