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 found in milk, butter, and cheese. It is the first carboxylic acid—that is, the one with the lowest molecular weight—to exhibit some of the properties of fatty acids.
Ethanedioic (oxalic) acid is the simplest di-carboxylic acid (containing two carboxyl groups). It is formed by oxidation (burning) of carbohydrates. It 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 appears in the juices of berries (especially cranberry). It is used as a preservative in many foods. Salicylic acid (found in willow bark) reacts with acetic anhydride, a derivative of acetic acid, to give acetylsalicylic acid, otherwise known as 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. Metabolism is the process through which all living things turn food into energy and living tissue.
Citric, acetic, tartaric, and propionic acids are commonly used as food additives and are known as acidulants. They are used to prevent rancidity (spoilage), enhance flavors, and modify texture in foods.
Organic chemistry: Organic acids 85
B Grease becomes covered with soap anions
D Grease eventually forms small globules (micelles) suspended in the water
' Mutual repulsion between micelles
Soapsare alkali-metal salts of long-chain aliphatic acids. Usually, these are the sodium (in hard soaps) or potassium (soft soaps) salts of oleic acid, palmitic acid, or stearic acid. Diagram A (far left) illustrates the structure of a typical hard soap (top) and a simplified representation (bottom). Diagrams B to D (near left) show the detergent action of soap. In aqueous solution (water), the sodium cation separates from the main part of the soap molecule. The hydrophobic tail (still attached to the soap anion) embeds itself in grease (B). The grease is gradually forced away from the fabric as more soap anions embed themselves (C). Eventually, the grease separates from the fabric, forming a micelle (D). Micelles repel each other and so remain suspended in the water.