Which of the following carboxylic acids has the highest boiling point?

Decanoic acid

The correct answer is option 4. Decanoic acid.

The boiling points of carboxylic acids increase with increasing molecular weight. This trend arises from the fact that larger molecules have more electrons, leading to stronger London dispersion forces (also known as van der Waals forces) between molecules. These intermolecular forces require more energy to overcome, thus raising the boiling point of the compound.

Let us apply this understanding to the carboxylic acids provided:

1. Heptanoic acid \((C_7H_{15}COOH)\): This compound has the lowest molecular weight among the options given, approximately 130 g/mol.

2. Octanoic acid \((C_8H_{17}COOH)\): With a slightly higher molecular weight than heptanoic acid, around 144 g/mol, octanoic acid has a higher boiling point due to stronger London dispersion forces.

3. Nonanoic acid \((C_9H_{19}COOH)\): Nonanoic acid has a molecular weight of approximately 158 g/mol, higher than both heptanoic acid and octanoic acid, indicating a higher boiling point than both.

4. Decanoic acid \((C_{10}H_{21}COOH)\): Decanoic acid is the largest molecule among the options provided, with a molecular weight of approximately 172 g/mol. It has the highest boiling point among the given carboxylic acids due to the strongest London dispersion forces resulting from its larger size and greater number of electrons.

Conclusion:

The order of increasing boiling points among the carboxylic acids given is: Heptanoic acid < Octanoic acid < Nonanoic acid < Decanoic acid.

Therefore, Decanoic acid has the highest boiling point among the options provided.