Organic acids

Organic acids are a group of compounds characterized by the carboxyl group, which is itself made up of a carbonyl group and a hydroxyl group. A carbonyl group contains a carbon atom and an oxygen atom, connected by a double bond. A hydroxyl group is a specific arrangement of atoms in which a hydrogen atom is bonded to an oxygen atom. Organic acid compounds are commonly called carboxylic acids.

Organic acids are weak compared to mineral acids. In solution they dissociate (break up) less completely, producing a lower concentration of hydrogen ions than do mineral acids. (The hydrogen ions react with other substances and cause the burning or corroding for which acids are known.)

The most common type of carboxylic acid attaches itself to an alkyl group (a compound consisting solely of carbon and hydrogen). Other carboxylic acids attach themselves to a ring system (carbon atoms in the shape of a ring) or to another carboxylic group (becoming dicarboxylic acids). Some types contain carbon-carbon double bonds. Such carboxylic acids are called unsaturated.

Carboxylic acids can react to form salts, esters, anhydrides (oxygen compounds that unite with water to form an acid or base), acid halides (acids containing one of the halogens—fluorine, chlorine, bromine, iodine, or astatine), and amides (a type of nitrogen-oxygen compound). Carboxylic acids can also be reduced to form aldehydes and alcohols.

Soaps are salts of one of the alkali metals such as lithium, sodium, or potassium, combined with an organic (fatty) acid. The alkali metal salt is usually sodium hydroxide, called lye or caustic soda. The fatty acid comes from animal fat or a vegetable oil, such as palm oil or olive oil.

In water, the soap molecule has a hydrophobic (“water-fearing”) end and a hydrophilic (“water-loving”) end. Soaps work by surrounding dirt or grease particles with many soap molecules. The hydrophobic ends dissolve in the grease or dirt particles. The hydrophilic ends point outward, dissolving in the water. Thus, the soap molecules form an emulsion. This is a suspension of the oil or fat in the water. The molecules are “suspended” rather than dissolved. These soap molecules enclose the dirt or grease in a water-soluble envelope called a micelle. In other words, soap molecules package dirt in droplets that can be taken into solution and washed away.

Organic acids come in different forms. All contain at least one carboxyl group (the pink squares). The diagram at left illustrates the main types: saturated aliphatic, such as methanoic and ethanoic acid (blue boxes); unsaturated aliphatic, such as 2-prope-noic acid (purple box); dicarboxylic, such as eth-anedioic acid (pale green box); aromatic, such as benzoic acid (darker green box); and polyfunctional (containing one or more radical groups in addition to the carboxyl group), such as 2-hydroxypropa-noic acid (pale brown box).

Carboxylic acids

Carboxylic acids are important in nature. All amino acids, the basic building blocks of protein, are carboxylic. A number of higher-molecular-weight acids are isolated from animal and vegetable sources (as triglycerides) for industry. Lower acids can also be prepared synthetically (artificially). Methanoic (formic) acid is the simplest monocarboxylic acid (containing one carboxyl group). It occurs in the venom of bees and ants. Ethanoic (acetic) acid is produced naturally by some organisms. It is also the active ingredient in vinegar. Propanoic (propionic) acid is the first carboxylic acid—that is, the one with the lowest molecular weight—to exhibit some of the properties of fatty acids. It is found in milk, butter, and cheese.

Ethanedioic (oxalic) acid is the simplest dicarboxylic acid (containing two carboxyl groups). It is formed by oxidation (burning) of carbohydrates, and occurs naturally in many plants, notably rhubarb. Many other fatty acids (with between 14 and 22 carbon atoms) are oxidized in animals and plants to provide energy.

Other examples of carboxylic acids are benzoic acid and salicylic acid, both of which occur naturally. Benzoic acid, found in the juice of berries (especially cranberry), is used as a food preservative. Salicylic acid, found in willow bark, is used to make acetylsalicylic acid (aspirin).

Polyfunctional acids include tartaric acid, found in grapes, and lactic acid, responsible for the acidity of sour milk. Lactic acid is also formed in muscle tissue during exercise. Citrus fruits owe their sharp flavor to citric acid. Malic acid gives apples their tartness. All of these acids are key biochemical helpers in the metabolism of sugars and fats. Citric, acetic, tartaric, and propionic acids, known as acidu-lants, are common food additives. They are used to prevent spoilage, enhance flavors, and modify texture in foods.

The venom of ants (seen attacking a wasp, left) and the bark of willow trees both contain organic acids. Methanoic acid is in the ants’ venom, and salicylic acid in willow bark.
During vigorous activity lactic acid accumulates in muscles as a result of incomplete oxidation of glycogen (the energy source). This incomplete oxidation is itself due to a lack of oxygen. When activity stops, some lactic acid is oxidized (to carbon dioxide and water) and some is converted back to glycogen.