1. 16.2 Properties of Carboxylic Acids
Carboxylate salts are often used as preservatives and flavor enhancers in soups and seasonings. Sodium propionate, a preservative, is added to cheeses, bread, and other bakery items to inhibit the spoilage of the food by microorganisms.
Learning Goal Describe the boiling points, solubilities, dissociation, and neutralization of carboxylic acids.

2. Polarity of Carboxylic Acids
Carboxylic acids are strongly polar because they have two polar groups:
a hydroxyl group (—OH) and
a carbonyl group (C = O).

3. Boiling Points of Carboxylic Acids
The polar carboxyl groups allow carboxylic acids to form multiple hydrogen bonds with other carboxylic acid molecules.
This effect of hydrogen bonds gives carboxylic acids higher boiling points than alcohols, ketones, and aldehydes of similar molar mass.

4. Carboxylic Acids, Dimers
The higher boiling points of carboxylic acids can also be attributed to the formation of dimers between two carboxylic acids.
Two hydrogen bonds form between their carboxyl groups, which effectively double the mass of the molecule, increasing the temperature required to reach the boiling point.

5. Study Check
Match each of the compounds 2-butanol, pentane, and propanoic acid with the boiling points 141°C, 100°C, and 36°C. (They have about the same molar mass.)

6. Solution
Match each of the compounds 2-butanol, pentane, and propanoic acid with the boiling points 141°C, 100°C, and 36°C. (They have about the same molar mass.)
Boiling points increase due to
increased formation of hydrogen bonds.
increased dipole–dipole attractions.
increasing molar mass.

7. Solution
Match each of the compounds 2-butanol, pentane, and propanoic acid with the boiling points 141°C, 100°C, and 36°C. (They have about the same molar mass.)
Pentane, an alkane, cannot form hydrogen bonds. It has the lowest boiling point, 36°C.
2-Butanol, an alcohol, can form hydrogen bonds, so it has a higher boiling point, 100°C.
Propanoic acid, a carboxylic acid, can form dimers increasing the effective molar mass. It therefore has the highest boiling point, 141°C.

8. Solubility in Water Carboxylic acids
form hydrogen bonds with many water molecules.
with one to five carbon atoms are very soluble in water.
are less soluble as the number of carbons increases.
Acetic acid forms hydrogen bonds with water molecules.

9. Acidity of Carboxylic Acids
are weak acids.
dissociate in water to produce carboxylate ions and hydronium ions.
can lose a proton because two oxygen atoms in a carboxylate ion stabilize negative charge.

10. Properties of Carboxylic Acids

11. Study Check
Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion. butanoic acid

12. Solution
Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion. + H2O + H3O+
carboxylate ion

13. Neutralization of Carboxylic Acids
Carboxylate salts are a product of the neutralization of a carboxylic acid with a strong base such as NaOH or KOH. The carboxylate ion is named by replacing the ic acid ending of the acid name with ate.

14. Neutralization of Carboxylic Acids
Carboxylate salts
are ionic compounds with strong attractions between positively charged metal ions such as Li+, Na+, and K+ and the negatively charged carboxylate ion.
are solids at room temperature.
have high melting points.
are usually soluble in water.

15. Carboxylic Acids, Preservatives
Sodium propionate, a preservative, is added to bread, cheeses, and bakery items to inhibit the spoilage of the food by microorganisms.
Sodium benzoate is added to juices, margarine, relishes, salads, and jams to inhibit the growth of mold and bacteria.

16. Carboxylic Acids, Preservatives
Monosodium glutamate (MSG) is added to meats, fish, vegetables, and bakery items to enhance flavor, although it may cause headaches in some people.

17. Study Check
Write the equation for the reaction of propanoic acid with NaOH.

18. Solution
Write the equation for the reaction of propanoic acid with NaOH.

19. Chemistry Link to Health: Carboxylic Acids in Metabolism
At the pH of the aqueous environment in the cells,
the carboxylic acids are dissociated.
it is actually the carboxylate ions that take part in the reactions of the citric acid cycle.
For example, in water, succinic acid is in equilibrium with its carboxylate ion, succinate.

20. Chemistry Link to Health: Carboxylic Acids in Metabolism
Carboxylates are part of the metabolic processes within our cells. For example,
during glycolysis, a molecule of glucose is broken down into two molecules of pyruvate, the carboxylate salt of pyruvic acid.
during strenuous exercise when oxygen levels are low (anaerobic), pyruvate is reduced to give lactic acid or the lactate ion.

21. Chemistry Link to Health: Carboxylic Acids in Metabolism
In the citric acid cycle (Krebs cycle),
di- and tricarboxylates are oxidized and decarboxylated (loss of CO2) to produce energy for cells.
citrate, with six carbons, is oxidized to a five-carbon
α-ketoglutarate:

22. Chemistry Link to Health: Carboxylic Acids in Metabolism
The citric acid cycle continues as
α-ketoglutarate loses CO2 to give a four-carbon succinate.
a series of reactions then converts succinate to oxaloacetate.