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

The correct answer is Option (4) → Decanoic acid

All four are straight-chain aliphatic carboxylic acids, so each has the same –COOH group and the same intermolecular hydrogen bonding. They differ only in the length of the carbon chain. A longer chain means a larger molecular mass and surface area, so the van der Waals forces between the molecules are stronger and the boiling point is higher:

• Heptanoic acid — C₆H₁₃COOH, 7 carbon atoms: lowest boiling point of the four.

• Octanoic acid — C₇H₁₅COOH, 8 carbon atoms.

• Nonanoic acid — C₈H₁₇COOH, 9 carbon atoms.

• Decanoic acid — C₉H₁₉COOH, 10 carbon atoms: the longest chain and the highest boiling point.

Decanoic acid is the only one of the four above nine carbon atoms: it is a wax-like solid of low volatility, while the other three are liquids at room temperature.

NCERT (2026-27 edition):
8.8 Physical Properties – “Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and are practically odourless due to their low volatility.”