Uses of benzoic acid

Benzoic acid, C₇H₆O₂ (or C₆H₅COOH), is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. This weak acid and its salts are used as a food preservative. Benzoic acid is an important precursor for the synthesis of many other organic substances.

Uses of benzoic acid

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Benzoic acid is used to make a large number of chemicals, important examples of which are:

• Benzoyl chloride, C₆H₅C(O)Cl, is obtained by treatment of benzoic with thionyl chloride, phosgene or one of the chlorides of phosphorus. C₆H₅C(O)Cl is an important starting material for several benzoic acid derivates like benzyl benzoate, which is used in artificial flavours and insect repellents.
• Benzoate plasticizers, such as the glycol-, diethylengylcol-, and triethyleneglycol esters, are obtained by transesterification of methyl benzoate with the corresponding diol. Alternatively these species arise by treatment of benzoylchloride with the diol. These plasticizers are used similarly to those derived from terephthalic acid ester.
• Phenol, C₆H₅OH, is obtained by oxidative decarboxylation at 300-400°C. The temperature required can be lowered to 200°C by the addition of catalytic amounts of copper(II) salts. The phenol can be converted to cyclohexanol, which is a starting material for nylon synthesis.

Food preservative

Benzoic acid and its salts are used as a food preservative, represented by the E-numbers E210, E211,E212, and E213. Benzoic acid inhibits the growth of mold, yeast^[7] and some bacteria. It is either added directly or created from reactions with its sodium, potassium, or calcium salt. The mechanism starts with the absorption of benzoic acid in to the cell. If the intracellular pH changes to 5 or lower, theanaerobic fermentation of glucose through phosphofructokinase is decreased by 95%. The efficacy of benzoic acid and benzoate is thus dependent on the pH of the food.^[8] Acidic food and beverage like fruit juice (citric acid), sparkling drinks (carbon dioxide), soft drinks (phosphoric acid), pickles (vinegar) or other acidified food are preserved with benzoic acid and benzoates.

Typical levels of use for benzoic acid as a preservative in food are between 0.05 – 0.1%. Foods in which benzoic acid may be used and maximum levels for its application are laid down in international food law.^[9][10]

Concern has been expressed that benzoic acid and its salts may react with ascorbic acid (vitamin C) in some soft drinks, forming small quantities of benzene.

Medicinal

Benzoic acid is a constituent of Whitfield's Ointment which is used for the treatment of fungal skin diseases such as tinea, ringworm, and athlete's foot.

Biology and health effects

Benzoic acid occurs naturally free and bound as benzoic acid esters in many plant and animal species. Appreciable amounts have been found in most berries (around 0.05%). Ripe fruits of several Vacciniumspecies (e.g., cranberry, V. vitis idaea; bilberry, V. macrocarpon) contain as much as 0.03-0.13% free benzoic acid. Benzoic acid is also formed in apples after infection with the fungus Nectria galligena. Among animals, benzoic acid has been identified primarily in omnivorous or phytophageous species, e.g., in viscera and muscles of the ptarmigan (Lagopus mutus) as well as in gland secretions of malemuskoxen (Ovibos moschatus) or Asian bull elephants (Elephas maximus).^[14]

Gum benzoin contains up to 20% of benzoic acid and 40% benzoic acid esters.^[15]

Benzoic acid is present as part of hippuric acid (N-Benzoylglycine) in urine of mammals, especiallyherbivores (Gr. hippos = horse; ouron = urine). Humans produce about 0.44 g/L hippuric acid per day in their urine, and if the person is exposed to toluene or benzoic acid it can rise above that level.^[16]

For humans, the WHO's International Programme on Chemical Safety (IPCS) suggests a provisional tolerable intake would be 5 mg/kg body weight per day.^[14] Cats have a significantly lower tolerance against benzoic acid and its salts than rats and mice. Lethal dose for cats can be as low as 300 mg/kg body weight.^[17] The oral LD₅₀ for rats is 3040 mg/kg, for mice it is 1940–2263 mg/kg.^[14]

In Taipei, Taiwan, a city health survey in 2010 found 30% of tested dried and pickled food products failed a test having too much benzoic acid, which is known to affect the liver and kidney^[18], along with more serious issues like excessive cyclamate.