Benzoic acid

Benzoic acid /bɛnˈz.ɪk/ is a white (or colorless) solid with the formula C6H5CO2H. It is the simplest aromatic 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[9] and serves as an intermediate in the biosynthesis of many secondary metabolites. Salts of benzoic acid are used as food preservatives. Benzoic acid is an important precursor for the industrial synthesis of many other organic substances. The salts and esters of benzoic acid are known as benzoates /ˈbɛnz.t/.

Benzoic acid
Skeletal formula
Ball-and-stick model
Preferred IUPAC name
Benzoic acid[1]
Systematic IUPAC name
Benzenecarboxylic acid
Other names
  • Carboxybenzene
  • E210
  • Dracylic acid
  • Phenylmethanoic acid
  • BzOH
  • 65-85-0 Y
3D model (JSmol)
Beilstein Reference
ECHA InfoCard 100.000.562
EC Number
  • 200-618-2
E number E210 (preservatives)
Gmelin Reference
MeSH benzoic+acid
RTECS number
  • DG0875000
CompTox Dashboard (EPA)
  • InChI=1S/C7H6O2/c8-7(9)6-4-2-1-3-5-6/h1-5H,(H,8,9) Y
  • InChI=1/C7H6O2/c8-7(9)6-4-2-1-3-5-6/h1-5H,(H,8,9)
  • O=C(O)c1ccccc1
Molar mass 122.123 g/mol
Appearance Colorless crystalline solid
Odor Faint, pleasant odor
Density 1.2659 g/cm3 (15 °C)
1.0749 g/cm3 (130 °C)[2]
Melting point 122 °C (252 °F; 395 K)[3]
Boiling point 250 °C (482 °F; 523 K)[3]
1.7 g/L (0 °C)
2.7 g/L (18 °C)
3.44 g/L (25 °C)
5.51 g/L (40 °C)
21.45 g/L (75 °C)
56.31 g/L (100 °C)[2][4]
Solubility Soluble in acetone, benzene, CCl4, CHCl3, alcohol, ethyl ether, hexane, phenyls, liquid ammonia, acetates
Solubility in methanol 30 g/100g (−18 °C)
32.1 g/100g (−13 °C)
71.5 g/100g (23 °C)[2]
Solubility in ethanol 25.4 g/100g (−18 °C)
47.1 g/100g (15 °C)
52.4 g/100g (19.2 °C)
55.9 g/100g (23 °C)[2]
Solubility in acetone 54.2 g/100g (20 °C)[2]
Solubility in olive oil 4.22 g/100g (25 °C)[2]
Solubility in 1,4-Dioxane 55.3 g/100g (25 °C)[2]
log P 1.87
Vapor pressure 0.16 Pa (25 °C)
0.19 kPa (100 °C)
22.6 kPa (200 °C)[5]
Acidity (pKa)
−70.28·10−6 cm3/mol
1.5397 (20 °C)
1.504 (132 °C)[2]
Viscosity 1.26 mPa (130 °C)
Molecular shape
1.72 D in dioxane
146.7 J/mol·K[5]
Std molar
entropy (So298)
167.6 J/mol·K[2]
−385.2 kJ/mol[2]
Std enthalpy of
combustion cH298)
−3228 kJ/mol[5]
Main hazards Irritant
Safety data sheet JT Baker
GHS pictograms [8]
GHS Signal word Danger
GHS hazard statements
H318, H335[8]
GHS precautionary statements
P261, P280, P305+351+338[8]
NFPA 704 (fire diamond)
Flash point 121.5 °C (250.7 °F; 394.6 K)[3]
571 °C (1,060 °F; 844 K)[3]
Lethal dose or concentration (LD, LC):
1700 mg/kg (rat, oral)
Related compounds
Hydroxybenzoic acids
Aminobenzoic acids,
Nitrobenzoic acids,
Phenylacetic acid
Related compounds
Benzyl alcohol,
Benzoyl chloride,
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references


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).[10]

Justus von Liebig and Friedrich Wöhler determined the composition of benzoic acid.[11] 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.[12]


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 abundant materials, and proceeds in high yield.[13]

toluene oxidation

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.

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%[14]

By hydrolysis

Like other nitriles and amides, 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.[15] 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.[16][17][18][19][20]

Oxidation of benzyl compounds

Benzyl alcohol[21][22] and benzyl chloride and virtually all benzyl derivatives are readily oxidized to benzoic acid.


Benzoic acid is mainly consumed in the production of phenol by oxidative decarboxylation at 300−400 °C:[23]

C6H5CO2H + 1/2 O2 → C6H5OH + CO2

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 plasticizers, such as the glycol-, diethyleneglycol-, and triethyleneglycol esters, are obtained by transesterification of methyl benzoate with the corresponding diol.[23] These plasticizers, which are used similarly to those derived from terephthalic acid ester, represent alternatives to phthalates.[23]

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[24] 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 into 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.[25] 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 and 0.1%. Foods in which benzoic acid may be used and maximum levels for its application are controlled by local food laws.[26][27]

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.[28]


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.[29][30] As the principal component of gum benzoin, 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.[31]

Niche and laboratory uses

In teaching laboratories, benzoic acid is a common standard for calibrating a bomb calorimeter.[32]

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 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).[33] Gum benzoin contains up to 20% of benzoic acid and 40% benzoic acid esters.[34]

In terms of its biosynthesis, benzoate is produced in plants from cinnamic acid.[35] A pathway has been identified from phenol via 4-hydroxybenzoate.[36]


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

Aromatic ring

benzoic acid aromatic ring reactions

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.[37]

Carboxyl group

Reactions typical for carboxylic acids apply also to benzoic acid.[23]

  • Benzoate esters are the product of the acid catalysed reaction with alcohols.
  • Benzoic acid amides are usually prepared from benzoyl chloride.
  • Dehydration to benzoic anhydride is induced with acetic anhydride or phosphorus pentoxide.
  • Highly reactive acid derivatives such as acid halides are easily obtained by mixing with halogenation agents like phosphorus chlorides or thionyl chloride.
  • Orthoesters 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.
benzoic acid group reactions

Safety and mammalian metabolism

It is excreted as hippuric acid.[38] Benzoic acid is metabolized by butyrate-CoA ligase into an intermediate product, benzoyl-CoA,[39] which is then metabolized by glycine N-acyltransferase into hippuric acid.[40] Humans metabolize toluene and benzoic acid which is excreted as hippuric acid.[41]

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.[33] 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.[42] The oral LD50 for rats is 3040 mg/kg, for mice it is 1940–2263 mg/kg.[33]

In Taipei, Taiwan, a city health survey in 2010 found that 30% of dried and pickled food products had benzoic acid.[43]


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