|IUPAC name||Benzoic acid,
benzene carboxylic acid
E210, dracylic acid
|Molar mass||122.12 g/mol|
|Appearance||Colourless crystalline solid|
|Density||1.32 g/cm3, solid|
122.4 °C (395 K)
249 °C (522 K)
|Solubility in water||Soluble (hot water)
3.4 g/l (25 °C)
|Solubility in methanol, diethylether||Soluble|
|Dipole moment||1.72 D in Dioxane|
|Flash point||121 °C (394 K)|
|Related carboxylic acid||phenylacetic acid,
|Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
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.
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).
In 1875 Salkowski discovered the antifungal abilities of benzoic acid, which were used for a long time in the preservation of benzoate containing fruits.
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 attractive.
U.S. production capacity is estimated to be 126 000 tonnes per year, 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. Even after the discovery of other synthesis methods, it was forbidden to use benzoic acid of other source than gum benzoin.
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 it is 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 percent. The effectivity 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.
Benzoic acid is used to make a large number of chemicals, important examples:
Benzoic acid is a constituent of Whitfield Ointment which is used for the treatment of fungal skin diseases such as tinea, ringworm and athlete's foot. It is also considered an effective treatment for acne.
Benzoic acid is purified by a method called recrystallisation. Process starts with crystalline material, removes all the impurities and forms new crystals.
Gum benzoin contains up to 20 percent of benzoic acid and 40 percent 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:
Electrophilic aromatic substitution reaction will take place mainly in 3-position to the electron-withdrawing 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 (eg 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 an unusual feature 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 owing to its high solubility in hot and poor solubility in cold water. The avoidance of organic solvents for the recrystallization makes this experiment particularly safe.
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 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.
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.
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