SolutionsPhysical and Bond Properties
1. Boiling points
Acetic acid and 1-propanol have the same molecular mass (60). Why does the acid boil at 118 °C and the alcohol at 97 °C?
Carboxylic acids form dimers in which two molecules are held together by two hydrogen bonds (each OH with the C=O of the other). This association, stronger than in alcohols, partly survives even in the vapour, and the molecule behaves as if it had a larger mass.
2. Solubility in water
Look at the table: why are the first four acids miscible with water while decanoic acid hardly dissolves (0.2 g/L)?
The CO2H group is hydrophilic and fits into the hydrogen-bond network of water; the carbon chain is hydrophobic. In short acids the polar group dominates; as the chain grows, the non-polar part takes over and solubility drops.