Many carboxylic acids are colorless liquids with disagreeable odors. The carboxylic acids with 5 to 10 carbon atoms all have “goaty” odors (explaining the odor of Limburger cheese). These acids are also produced by the action of skin bacteria on human sebum (skin oils), which accounts for the odor of poorly ventilated locker rooms. The acids with more than 10 carbon atoms are waxlike solids, and their odor diminishes with increasing molar mass and resultant decreasing volatility.

Carboxylic acids exhibit strong hydrogen bonding between molecules, forming dimers in the gas phase, as shown below.   They therefore have high boiling points compared to other substances of comparable molar mass: the entity that actually enters the gas phase, carboxylic acid dimer, is actually twice as large as the carboxylic acid molecule!

Carboxylic Acid dimer, public domain image

The carboxyl group readily engages in hydrogen bonding with water molecules (Figure 15.3 “Hydrogen Bonding between an Acetic Acid Molecule and Water Molecules”). The acids with one to four carbon atoms are completely miscible with water. Solubility decreases as the carbon chain length increases because dipole forces become less important and dispersion forces become more predominant. Hexanoic acid [CH₃(CH₂)₄COOH] is barely soluble in water (about 1.0 g/100 g of water). Palmitic acid [CH₃(CH₂)₁₄COOH], with its large nonpolar hydrocarbon component, is essentially insoluble in water. The larger carboxylic acids generally are soluble in such organic solvents as ethanol, toluene, and diethyl ether.

Figure 15.3 Hydrogen Bonding between an Acetic Acid Molecule and Water Molecules. Carboxylic acids of low molar mass are quite soluble in water.

Table 15.2 “Physical Constants of Carboxylic Acids” lists some physical properties for selected carboxylic acids. The first six are homologs. Notice that the boiling points increase with increasing molar mass, but the melting points show no regular pattern.