Organic Acids

Carboxylic acids are liquids at room temperature and have sharp or unpleasant odors.  One example is vinegar.

Long-chain acids are waxlike solids.  The carboxyl group is polar and forms hydrogen bonds to other carboxyl groups or other molecules.

Carboxylic acids have relatively high boiling points, and those with lower molecular masses are soluble in water.

Soluble carboxylic acids behave as weak acids, donating protons to water molecules.  The portion of the acid left after proton loss is called a carboxylate ion.

The simplest carboxylic acid is propenoic acid (a.k.a. acrylic acid) used in the manufacture of polymers.  Two isomers exist for the simplest dicarboxylic acid, fumaric acid and maleic acid.  The addition of small amounts of maleic acid to fats and oils prevents them from becoming rancid.

Some carboxylic acids are called metabolic acids: those which are intermediate compounds in metabolic reactions.  Fumaric acid is a molecule involved in fermentation under low oxygen stress as a conserved metabolic response in many species.  Succinic acid is the four-carbon unsubstituted diacid.  It is another metabolic hub.  

Hydroxyacids are those carboxylic acids with alcohol substituents, which occur naturally in many foods.  Glycolic acid is present in sugar cane and sugar beets.  Lactic acid is present in sauerkraut and dill pickles.  Both malic and tartaric acid occur naturally in fruits, including apples and grapes.

Citric acid is a tricarboxylic acid used widely in beverages and in foods.  In jams, jellies and preserves it produces tartness and supports gelation.  In fresh fruit salads, it prevents oxidation (browning).  In frozen fruits, it prevents deterioration of color and flavor.  In seafood, it retards microbial growth.

Keto acids contain a carbonyl group within a carbon chain.  Pyruvic acid, with three carbon atoms, would be a key metabolic intermediate of this classification.

These molecules, unsubstituted up until 4 carbon backbone, are completely miscible with water.  In order of increasing carbon chain length we have: formic, acetic, propionic, butyric, valeric, and caproic acids. The pKa of formic acid (3.75) is significantly lower than acetic (4.75) and the rest (4.8-4.9).

Works Cited

Stoke, H. S. General, Organic and Biological Chemistry, 6th Ed. Cengage Learning > K-12 > Brooks/Cole, 2013. Chapter 5: Carboxylic Acids, Esters, and Other Acid Derivatives, p.163-206.