15.4: Physical Properties of Carboxylic Acids

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Learning Objectives

Compare the boiling points of carboxylic acids with alcohols of similar molar mass.
Compare the solubilities of carboxylic acids in water with the solubilities of comparable alkanes and alcohols in water.

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. They therefore have high boiling points compared to other substances of comparable molar mass.

The carboxyl group readily engages in hydrogen bonding with water molecules (Figure \(\PageIndex{1}\)). 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 carboxylic acids generally are soluble in such organic solvents as ethanol, toluene, and diethyl ether.

Figure \(\PageIndex{1}\): Hydrogen Bonding between an Acetic Acid Molecule and Water Molecules. Carboxylic acids of low molar mass are quite soluble in water.

Table 15.4.1 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.

Table \(\PageIndex{1}\): Physical Constants of Carboxylic Acids
Condensed Structural Formula	Name of Acid	Melting Point (°C)	Boiling Point (°C)	Solubility (g/100 g of Water)
HCOOH	formic acid	8	100	miscible
CH₃COOH	acetic acid	17	118	miscible
CH₃CH₂COOH	propion ic acid	–22	141	miscible
CH₃(CH₂)₂COOH	butyric acid	–5	163	miscible
CH₃(CH₂)₃COOH	valeric acid	–35	187	5
CH₃(CH₂)₄COOH	caproic acid	–3	205	1.1
C₆H₅COOH	benzoic acid	122	249	0.29

Key Takeaways

Carboxylic acids have high boiling points compared to other substances of comparable molar mass. Boiling points increase with molar mass.
Carboxylic acids having one to four carbon atoms are completely miscible with water. Solubility decreases with molar mass.