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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.
Additional recommended knowledge
Benzoic acid was discovered in the 16th century. The dry distillation of gum benzoin was first described by Nostradamus (1556), and subsequently by Alexius Pedemontanus (1560) and Blaise de Vigenère (1596).
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.
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.
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.
Benzoic acid is used to make a large number of chemicals, important examples of which are:
Benzoic acid is purified by recrystallisation of the crude product. This involves dissolving the material and allowing it to recrystallize (or re-solidify), leaving any impurities in solution and allowing the pure material to be isolated from the solution. 
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.
Reactions of benzoic acid can occur at either the aromatic ring or the carboxylic group:
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.
All the reactions mentioned for carboxylic acids are also possible for benzoic acid.
Benzoic acid is cheap and readily available, so the laboratory synthesis of benzoic acid is mainly practiced for its pedogical value. It is a common undergraduate preparation and a convenient property of the compound is that its melting point equals its molecular weight (122). 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.
Bromobenzene in diethyl ether is stirred with magnesium turnings to produce phenylmagnesium bromide (C6H5MgBr). This Grignard reagent is slowly added to dry-ice (solid carbon dioxide) to give benzoate. Dilute acid is added to form benzoic acid.
From benzyl alcohol
Benzyl alcohol is refluxed with potassium permanganate or other oxidizing reagents in water. The mixture hot filtered to remove manganese oxide and then allowed to cool to afford benzoic acid.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Benzoic_acid". A list of authors is available in Wikipedia.|