| Benzoic acid | |
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Benzoic acid crystals
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Benzoic acid |
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Other names
Benzenecarboxylic acid, |
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| Identifiers | |
| CAS number | 65-85-0 |
| PubChem | 243 |
| ChemSpider | 238 |
| UNII | 8SKN0B0MIM |
| EC number | 200-618-2 |
| DrugBank | DB03793 |
| KEGG | D00038 |
| MeSH | benzoic+acid |
| ChEBI | CHEBI:30746 |
| ChEMBL | CHEMBL541 |
| RTECS number | DG0875000 |
| Beilstein Reference | 636131 |
| Gmelin Reference | 2946 |
| 3DMet | B00053 |
| Jmol-3D images | Image 1 Image 2 |
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| Properties | |
| Molecular formula | C7H6O2 |
| Molar mass | 122.12 g mol−1 |
| Appearance | Colorless crystalline solid |
| Density | 1.27 g/cm3[1] |
| Melting point |
122.38 °C, 396 K, 252 °F (source[2]) |
| Boiling point |
250 °C, 523 K, 482 °F ([1]) |
| Solubility in water | 2.9 g/L[1] |
| Acidity (pKa) | 4.21 |
| Refractive index (nD) | 1.5397 |
| Structure | |
| Crystal structure | Monoclinic |
| Molecular shape | planar |
| Dipole moment | 1.72 D in Dioxane |
| Hazards | |
| MSDS | JT Baker |
| EU Index | Not listed |
| Main hazards | Irritating |
| NFPA 704 |
1
2
0
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| Flash point | 121.5 °C (250.7 °F)[1] |
| Autoignition temperature |
570 °C (1,058 °F)[1] |
| Related compounds | |
| Related carboxylic acids | Hydroxybenzoic acids Aminobenzoic acids, Nitrobenzoic acids, Phenylacetic acid |
| Related compounds | Benzaldehyde, Benzyl alcohol, Benzoyl chloride, Benzylamine, Benzamide |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Benzoic acid (pronunciation: /bɛnˈzoʊ.ɪk/), 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. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis of many other organic substances. The salts and esters of benzoic acid are known as benzoates (pronunciation: /ˈbɛnzoʊ.eɪt/).
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Benzoic acid was discovered in the sixteenth century. The dry distillation of gum benzoin was first described by Nostradamus (1556), and subsequently by Alexius Pedemontanus (1560) and Blaise de Vigenère (1596).[3]
Pioneer work in 1830 through a variety of experiences based on amygdalin, obtained from bitter almonds (the fruit of Prunus dulcis) oil by Pierre Robiquet and Antoine Boutron-Charlard, two French chemists, had produced benzaldehyde [4] but they failed in working out a proper interpretation of the structure of amygdalin that would account for it, and thus missed the identification of the benzoyl radical C7H5O. This last step was achieved some few months later (1832) by Justus von Liebig and Friedrich Wöhler, who determined the composition of benzoic acid.[5] These latter 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.[6]
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.
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, acetone, petroleum ether, and a mixture of ethanol and water.[7]
Like any other nitrile or amide, benzonitrile and benzamide can be hydrolyzed to benzoic acid or its conjugate base in acid or basic conditions.
The base-induced disproportionation of benzaldehyde, the Cannizzaro reaction, affords equal amounts of benzoate and benzyl alcohol; the latter can be removed by distillation.
Bromobenzene can be converted to benzoic acid by "carbonation" of the intermediate phenylmagnesium bromide:[8]
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.
Benzoic acid can be prepared by oxidation of benzyl chloride in the presence of alkaline KMnO4:
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 is the most commonly used chemical standard to determine the heat of capacity of a bomb calorimeter.[9]
Benzoic acid is used to make a large number of chemicals; important examples of which are:
Benzoic acid and its salts are used as a food preservatives, represented by the E-numbers E210, E211, E212, and E213. Benzoic acid inhibits the growth of mold, yeast[10] 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.[11] 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.[12][13]
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.[14]
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.[15][16] As the principal component of benzoin resin, benzoic acid is also a major ingredient in both tincture of benzoin and Friar's balsam. Such products have a long history of use as topical antiseptics and inhalant decongestants.
Benzoic acid was used as an expectorant, analgesic, and antiseptic in the early 20th century.[17]
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 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 Rock Ptarmigan (Lagopus muta) as well as in gland secretions of male muskoxen (Ovibos moschatus) or Asian bull elephants (Elephas maximus).[18]
Gum benzoin contains up to 20% of benzoic acid and 40% benzoic acid esters.[19]
Cryptanaerobacter phenolicus is a bacterium species that produces benzoate from phenol via 4-hydroxybenzoate[20]
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.[21]
For humans, the World Health Organization's International Programme on Chemical Safety (IPCS) suggests a provisional tolerable intake would be 5 mg/kg body weight per day.[18] 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.[22] The oral LD50 for rats is 3040 mg/kg, for mice it is 1940–2263 mg/kg.[18]
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,[23] along with more serious issues like excessive cyclamate.
Benzoic acid activates bromelain.[24]
Reactions of benzoic acid can occur at either the aromatic ring or the carboxyl group:
Electrophilic aromatic substitution reaction will take place mainly in 3-position due 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.[25] 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.