Benzoic acid () is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. The benzoyl group is often abbreviated "Bz" (not to be confused with "Bn," which is used for benzyl), thus benzoic acid is also denoted as BzOH, since the benzoyl group has the formula –. It is the simplest aromaticity carboxylic acid. The name is derived from gum benzoin, which was for a long time its only source.
Benzoic acid occurs naturally in many plants
History
Benzoic acid was discovered in the sixteenth century. The
dry distillation of
gum benzoin was first described by
Nostradamus (1556), and then by Alexius Pedemontanus (1560) and Blaise de Vigenère (1596).
Justus von Liebig and Friedrich Wöhler determined the composition of benzoic acid.
In 1875 Salkowski discovered the antifungal properties of benzoic acid, which explains the preservation of benzoate-containing Cloudberry.
Production
Industrial preparations
Benzoic acid is produced commercially by partial oxidation of
toluene with
oxygen. The process is catalyzed by
cobalt or
manganese . The process uses abundant materials, and proceeds in high yield.
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 Halogenation 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.
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.
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. This process usually gives a yield of around 65%.
By hydrolysis
Like other
and
,
benzonitrile and
benzamide can be hydrolyzed to benzoic acid or its conjugate base in acid or basic conditions.
From Grignard reagent
Bromobenzene can be converted to benzoic acid by "carboxylation" of the intermediate phenylmagnesium bromide.
This synthesis offers a convenient exercise for students to carry out a Grignard reaction, an important class of carbon–carbon bond forming reaction in organic chemistry.
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Oxidation of benzyl compounds
Benzyl alcohol and
benzyl chloride and virtually all benzyl derivatives are readily oxidized to benzoic acid.
Uses
Benzoic acid is mainly consumed in the production of
phenol 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.
Precursor to plasticizers
Benzoate
, such as the glycol-, diethyleneglycol-, and triethyleneglycol esters, are obtained by transesterification of
methyl benzoate with the corresponding
diol.
[ These plasticizers, which are used similarly to those derived from terephthalic acid ester, represent alternatives to .][
]
Precursor to sodium benzoate and related preservatives
Benzoic acid and its salts are used as food preservatives, represented by the E numbers E210, sodium benzoate, E212, and calcium benzoate. Benzoic acid inhibits the growth of mold, yeast
Typical Concentration of benzoic acid as a preservative in food are between 0.05 and 0.1%. Foods in which benzoic acid may be used and maximum levels for its application are controlled by local food laws.[ GSFA Online Food Additive Group Details: Benzoates (2006) ][ EUROPEAN PARLIAMENT AND COUNCIL DIRECTIVE No 95/2/EC of 20 February 1995 on food additives other than colours and sweeteners (Consleg-versions do not contain the latest changes in a 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 carcinogenic benzene.
Medicinal
Benzoic acid is a constituent of Whitfield's ointment which is used for the treatment of fungal skin diseases such as ringworm and athlete's foot.
Benzoic acid was used as an expectorant, analgesic, and antiseptic in the early 20th century.
Niche and laboratory uses
In teaching laboratories, benzoic acid is a common standard for calibrating a bomb calorimeter.[ Experiment 2: Using Bomb Calorimetry to Determine the Resonance Energy of Benzene ]
Biology and health effects
Benzoic acid occurs naturally as do its esters in many plant and animal species. Appreciable amounts are found in most berries (around 0.05%). Ripe fruits of several Vaccinium species (e.g., cranberry, V. vitis macrocarpon; bilberry, V. myrtillus) contain as much as 0.03–0.13% free benzoic acid. Benzoic acid is also formed in 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 ( Ovibos moschatus) or Asian bull elephants ( Elephas maximus).
In terms of its biosynthesis, benzoate is produced in plants from cinnamic acid.
Reactions
Reactions of benzoic acid can occur at either the aromaticity or at the carboxyl group.
Aromatic ring
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.
Carboxyl group
Reactions typical for apply also to benzoic acid.[
]
-
Benzoate are the product of the acid catalysed reaction with alcohols.
-
Benzoic acid are usually prepared from benzoyl chloride.
-
Dehydration to benzoic anhydride is induced with acetic anhydride or phosphorus pentoxide.
-
Highly reactive acid derivatives such as Acyl halide are easily obtained by mixing with halogenation agents like phosphorus chlorides or thionyl chloride.
-
can be obtained by the reaction of alcohols under acidic water free conditions with benzonitrile.
-
Reduction to benzaldehyde and benzyl alcohol is possible using DIBAL-H, LiAlH4 or sodium borohydride.
-
Decarboxylation to benzene may be effected by heating in quinoline in the presence of copper salts. Hunsdiecker decarboxylation can be achieved by heating the silver salt.
Safety and mammalian metabolism
It is excreted as hippuric acid.[ Substrate/Product] which is then metabolized by glycine N-acyltransferase into hippuric acid.[ Substrate/Product] Humans metabolize toluene which is also excreted as hippuric acid.
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.
In Taipei, Taiwan, a city health survey in 2010 found that 30% of dried and pickled food products had benzoic acid.
See also
External links
