Properties of carboxylic acids

Acidity. The most important property of carboxylic acids, and the one that is responsible for naming them such, is their acidity. Carboxylic acids are said to be strong acids, even though they are much weaker than the most important mineral acids – sulfuric (H2SO4), nitric (HNO3), and hydrochloric (HCl).

Solubility. The solubility of carboxylic acids in water is similar to that of alcohols, aldehydes, and ketones. Acids with fewer than about five carbons dissolve in water; those with a higher molecular weight are insoluble owing to the larger hydrocarbon portion, which is hydrophobic. The sodium, ammonium, and potassium salts of carboxylic acids, however, are generally quite soluble in water.

Boiling point. Carboxylic acids have much higher boiling points than hydrocarbons, alcohols, ethers, aldehydes, or ketones of similar molecular weight. Carboxylic acids with higher molecular weights are solids at room temperature (e.g., benzoic and palmitic acids). Virtually all salts of carboxylic acids are solids at room temperature, as can be expected for ionic compounds.

Odour. Unbranched-chain carboxylic acids (fatty acids) that are liquids at room temperature, especially those from propanoic (C3) to decanoic (C10) acid, have very foul, disagreeable odours. An example is butanoic (butyric) acid (C4), which is the main ingredient in stale perspiration and thus the chief cause of “locker-room” odour.

Classes of carboxylic acids

Saturated aliphatic acids

Formic acid, HCOOH, is found not only in ants but also in the droplets on the tiny hairs of the stinging nettle plant (in the family Urticaceae), and the acidity of this compound causes the stinging sensation felt when these hairs are touched.

Acetic acid, CH3COOH, has been known to humankind for thousands of years (at least in water solution). It is the compound that gives the sourness to vinegar and is produced by the bacterial oxidation of ethanol in wine. Household vinegar contains about five percent acetic acid. Acetic acid is important in the metabolic processes of humans and, indeed, of all animals and plants.

The even-numbered fatty acids from 4 to 10 carbon atoms are mostly found in milkfats (cow’s milk, goat’s milk, some hard cheeses). The higher even-numbered saturated acids, from C12 to C18 (lauric, myristic, palmitic, and stearic), are present in the fats and oils of many animals and plants, with palmitic and stearic acids being the most prevalent. Lauric acid (C12) is the main acid in coconut oil (45 - 50 %) and palm kernel oil (45 - 55 %). Nutmeg butter is rich in myristic acid (C14), which constitutes 60 - 75 % of the fatty-acid content. Palmitic acid (C16) constitutes between 20 and 30 % of most animal fats and is also an important constituent of most vegetable fats (35 - 45 % of palm oil). Stearic acid (C18) is also present in most fats but usually in smaller amounts than palmitic. Cocoa butter is unusually rich in stearic acid (35 %).

Even-numbered saturated fatty acids higher than C18 are much less common in fats but do occur in some waxes (e.g. beeswax). Odd-numbered fatty acids have been found only in trace amounts in natural compounds, but many have been produced synthetically in the laboratory.

Unsaturated aliphatic acids

A number of acids important in organic chemistry contain carbon-carbon double bonds. There exist О±,ОІ-unsaturated acids, in which the double bond is between the second and third carbons of the chain, as well as unsaturated acids, in which the double bond occurs in other positions. Although many of these latter acids occur in nature, they are less easy to synthesize than О±,ОІ-unsaturated acids. Many unsaturated acids occur in fats.

Unsaturated acids exhibit chemical properties expected of compounds that contain both a COOH group and one or more carbon-carbon double bonds. Like all carboxylic acids, they are acidic; can be reduced to alcohols; can be converted to acid derivatives; and, like other compounds containing double bonds, can undergo the normal double-bond addition reactions and oxidation-reduction reactions.

Aromatic acids

Aromatic acids include compounds that contain a COOH group bonded to an aromatic ring. The simplest aromatic acid is benzoic acid. Some other important aromatic acids include the following: salicylic acid, gallic acid, phenylacetic acid, mandelic acid, cinnamic acid and many others.

Polycarboxylic acids

Unbranched-chain dicarboxylic acids contain two COOH groups. As a result they can yield two kinds of salts. Because one COOH group is still present in the compound, it has the properties of both a salt and an acid. The two most important unsaturated dicarboxylic acids are fumaric and maleic acids. Malic acid is found in many fruits, including apples. Fumaric acid is found in bolete mushrooms, lichen and Iceland moss. Human skin naturally produces fumaric acid when exposed to sunlight.

Amino acids

Compounds containing both a carboxyl group and an amino group are called amino acids. Twenty of these are found in proteins, all of which are О±-amino acids.