benzoic acid

benzoic acid

benzoic acid, C6H5CO2H, crystalline solid organic acid that melts at 122°C; and boils at 249°C;. It is the simplest aromatic carboxylic acid (see aryl group and carboxyl group). In addition to being synthesized from a variety of organic compounds (e.g., benzyl alcohol, benzaldehyde, toluene, and phthalic acid), it may be obtained from resins, notably gum benzoin. It is used largely for making its salts and esters, most notably sodium benzoate, which is widely used as a preservative in foods and beverages and as a mild antiseptic in mouthwashes and toothpastes.
benzoic acid, 3,4,5-trihydroxy-, IUPAC name for gallic acid.
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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.

History

Benzoic acid was discovered in the 16th century. The dry distillation of gum benzoin first described by Nostradamus (1556), and subsequently by Alexius Pedemontanus (1560) and Blaise de Vigenère (1596).

Justus von Liebig and Friedrich Wöhler determined the structure of benzoic acid in 1832. They also investigated how hippuric acid is related to benzoic acid.

In 1875 Salkowski discovered the antifungal abilities of benzoic acid, which was used for a long time in the preservation of benzoate-containing cloudberry fruits.

Production

Industrial preparations

Benzoic acid is produced commercially by partial oxidation of toluene with oxygen. The process is catalyzed by cobalt or manganese naphthenates. The process uses cheap raw materials, proceeds in high yield, and is considered environmentally green.

U.S. production capacity is estimated to be 126,000 tonnes per year (139,000 tons), much of which is consumed domestically to prepare other industrial chemicals.

Laboratory synthesis

Benzoic acid is cheap and readily available, so the laboratory synthesis of benzoic acid is mainly practiced for its pedagogical value. It is a common undergraduate preparation.

For all syntheses, benzoic acid can be purified by recrystallization from water because of its high solubility in hot water and poor solubility in cold water. The avoidance of organic solvents for the recrystallization makes this experiment particularly safe. Other possible recrystallization solvents include acetic acid (anhydrous or aqueous), benzene, petroleum ether, and a mixture of ethanol and water.

By hydrolysis

Like any other nitrile or amide, benzonitrile and benzamide can be hydrolyzed to benzoic acid or its conjugate base in acid or basic conditions.

From benzaldehyde

The base-induced disproportionation of benzaldehyde, the Cannizzaro reaction, affords equal amounts of benzoate and benzyl alcohol; the latter can be removed by distillation.

From bromobenzene

Bromobenzene can be converted to benzoic acid by "carbonation" of the intermediate phenylmagnesium bromide:

C6H5MgBr + CO2 → C6H5CO2MgBr
C6H5CO2MgBr + HCl → C6H5CO2H + MgBrCl

From benzyl alcohol

Benzyl alcohol is refluxed with potassium permanganate or other oxidizing reagents in water. The mixture is hot filtered to remove manganese dioxide and then allowed to cool to afford benzoic acid.

Historical preparations

The first industrial process involved the reaction of benzotrichloride (trichloromethyl benzene) with calcium hydroxide in water, using iron or iron salts as catalyst. The resulting calcium benzoate is converted to benzoic acid with hydrochloric acid. The product contains significant amounts of chlorinated benzoic acid derivatives. For this reason, benzoic acid for human consumption was obtained by dry distillation of gum benzoin. Food-grade benzoic acid is now produced synthetically.

Alkyl substituted benzene derivatives give benzoic acid with the stoichiometric oxidants potassium permanganate, chromium trioxide, nitric acid.

Uses

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 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, the anaerobic 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. 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.

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.

Feedstock

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

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 Vaccinium species (e.g., cranberry, V. vitis idaea; bilberry, V. macrocarpon) contain as much as 300-1300 mg free benzoic acid per kg fruit. 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 male muskoxen (Ovibos moschatus) or Asian bull elephants (Elephas maximus).

Gum benzoin contains up to 20% of benzoic acid and 40% benzoic acid esters.

Benzoic acid is present as part of hippuric acid (N-Benzoylglycine) in urine of mammals, especially herbivores (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.

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. 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. The oral LD50 for rats is 3040 mg/kg, for mice it is 1940-2263 mg/kg.

Chemistry

Reactions of benzoic acid can occur at either the aromatic ring or the carboxyl group:

Aromatic ring

Electrophilic aromatic substitution reaction will take place mainly in 3-position to the electron-withdrawing carboxylic group; i.e. benzoic acid is meta directing.

The second substitution reaction (on the right) is slower because the first nitro group is deactivating. Conversely, if an activating group (electron-donating) was introduced (e.g., alkyl), a second substitution reaction would occur more readily than the first and the disubstituted product might not accumulate to a significant extent.

Carboxyl group

All the reactions mentioned for carboxylic acids are also possible for benzoic acid.

References

Further reading

  • Cosmetic Ingredient Review Expert Panel Bindu Nair (2001). "Final Report on the Safety Assessment of Benzyl Alcohol, Benzoic Acid, and Sodium Benzoate". Int J Tox 20 (Suppl. 3): 23–50.

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