Benzoic acid

Benzoic acid

Benzoic acid crystals
Names
IUPAC name Benzoic acid
Other names Carboxybenzene; E210; Dracylic acid; Phenylmethanoic acid; Benzenecarboxylic acid
Identifiers
CAS Number	65-85-0 Y
3DMet	B00053
Beilstein Reference	636131
ChEBI	CHEBI:30746 Y
ChEMBL	ChEMBL541 Y
ChemSpider	238 Y
DrugBank	DB03793 Y
EC Number	200-618-2
Gmelin Reference	2946
Jmol 3D model	Interactive image Interactive image
KEGG	D00038 Y
MeSH	benzoic+acid
PubChem	243
RTECS number	DG0875000
UNII	8SKN0B0MIM Y
InChI InChI=1S/C7H6O2/c8-7(9)6-4-2-1-3-5-6/h1-5H,(H,8,9) Y Key: WPYMKLBDIGXBTP-UHFFFAOYSA-N Y InChI=1/C7H6O2/c8-7(9)6-4-2-1-3-5-6/h1-5H,(H,8,9) Key: WPYMKLBDIGXBTP-UHFFFAOYAD
SMILES O=C(O)c1ccccc1 c1ccc(cc1)C(=O)O
Properties
Chemical formula	C7H6O2
Molar mass	122.12 g·mol−1
Appearance	Colorless crystalline solid
Odor	faint, pleasant odor
Density	1.2659 g/cm3 (15 °C) 1.0749 g/cm3 (130 °C)
Melting point	122.41 °C (252.34 °F; 395.56 K)
Boiling point	249.2 °C (480.6 °F; 522.3 K) 370 °C (698 °F; 643 K) decomposes
Solubility in water	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)
Solubility	soluble in acetone, benzene, CCl4, CHCl3, alcohol, ethyl ether, hexane, phenyls, liquid ammonia, acetates
Solubility in methanol	30 g/100 g (-18 °C) 32.1 g/100 g (-13 °C) 71.5 g/100 g (23 °C)
Solubility in ethanol	25.4 g/100 g (-18 °C) 47.1 g/100 g (15 °C) 52.4 g/100 g (19.2 °C) 55.9 g/100 g (23 °C)
Solubility in acetone	54.2 g/100 g (20 °C)
Solubility in olive oil	4.22 g/100 g (25 °C)
Solubility in 1,4-Dioxane	55.3 g/100 g (25 °C)
log P	1.87
Vapor pressure	0.16 Pa (25 °C) 0.19 kPa (100 °C) 22.6 kPa (200 °C)
Acidity (pKa)	4.202
Refractive index (nD)	1.5397 (20 °C) 1.504 (132 °C)
Viscosity	1.26 mPa (130 °C)
Structure
Crystal structure	Monoclinic
Molecular shape	planar
Dipole moment	1.72 D in Dioxane
Thermochemistry
Specific heat capacity (C)	146.7 J/mol·K
Std molar entropy (So298)	167.6 J/mol·K
Std enthalpy of formation (ΔfHo298)	-385.2 kJ/mol
Std enthalpy of combustion (ΔcHo298)	3228 kJ/mol
Vapor pressure	{{{value}}}
Related compounds
Related carboxylic acids	Hydroxybenzoic acids Aminobenzoic acids, Nitrobenzoic acids, Phenylacetic acid
Related compounds	Benzaldehyde, Benzyl alcohol, Benzoyl chloride, Benzylamine, Benzamide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Benzoic acid /bɛnˈzoʊ.ɪk/, C₇H₆O₂ (or C₆H₅COOH), is a colorless crystalline solid and a simple aromatic carboxylic acid. The name is derived from gum benzoin, which was for a long time its only known source. Benzoic acid occurs naturally in many plants and it serves as an intermediate in the biosynthesis of many secondary metabolites. Salts of benzoic acid are used as food preservatives and 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 /ˈbɛnzoʊ.eɪt/.

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).

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 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. 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.

It is also one of the chemical compounds found in castoreum. This compound is gathered from the castor sacs of the North American Beaver.

Production

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 cheap raw materials, and proceeds in high yield.

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.

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. The solubility of benzoic acid in over 40 solvents with references to original sources can be found as part of the Open Notebook Science Challenge.

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 benzaldehyde

The base-induced disproportionation of benzaldehyde, the Cannizzaro reaction, affords equal amounts of benzoate and benzyl alcohol; the latter can be removed by distillation.

From bromobenzene

Bromobenzene can be converted to benzoic acid by "carbonation" 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.

From benzyl alcohol

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.

From benzyl chloride

Benzoic acid can be prepared by oxidation of benzyl chloride in the presence of alkaline KMnO₄:

C₆H₅CH₂Cl + 2 KOH + 2 [O] → C₆H₅COOH + KCl + H₂O

Historical preparation

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.

Uses

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

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-, diethylenegylcol-, and triethyleneglycol esters, are obtained by transesterification of methyl benzoate with the corresponding diol. Alternatively these species arise by treatment of benzoylchloride with the diol. These plasticizers are used similarly to those derived from terephthalic acid ester.

Precursor to sodium benzoate and related preservatives

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 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. 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 controlled by international food 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 benzene.

See also: Benzene in soft drinks

Medicinal

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. 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.

Benzoyl chloride

Benzoic acid is a precursor to benzoyl chloride, C₆H₅C(O)Cl by treatment with thionyl chloride, phosgene or one of the chlorides of phosphorus. is an important starting material for several benzoic acid derivates like benzyl benzoate, which is used in artificial flavours and insect repellents.

Niche and laboratory uses

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

Biology and health effects

Benzoic acid is relatively nontoxic. It is excreted as hippuric acid. Benzoic acid is metabolized by butyrate-CoA ligase into an intermediate product, benzoyl-CoA, which is then metabolized by glycine N-acyltransferase into hippuric acid.

Benzoic acid occurs naturally as do its 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 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).

Gum benzoin contains up to 20% of benzoic acid and 40% benzoic acid esters.

Cryptanaerobacter phenolicus is a bacterium species that produces benzoate from phenol via 4-hydroxybenzoate

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 World Health Organization'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.

In Taipei, Taiwan, a city health survey in 2010 found that 30% of dried and pickled food products had too much benzoic acid, which may affect the liver and kidney, along with more serious issues like excessive cyclamate.

Reactions

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

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 accumulate to a significant extent.

Carboxyl group

All the reactions mentioned for carboxylic acids are also possible for benzoic acid.

• Benzoic acid esters are the product of the acid catalysed reaction with alcohols.
• Benzoic acid amides are more easily available by using activated acid derivatives (such as benzoyl chloride) or by coupling reagents used in peptide synthesis like DCC and DMAP.
• The more active benzoic anhydride is formed by dehydration using 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.
• The copper catalysed decarboxylation of benzoate to benzene may be effected by heating in quinoline. Also, Hunsdiecker decarboxylation can be achieved by forming the silver salt and heating. Benzoic acid can also be decarboxylated by heating with an alkali hydroxide or calcium hydroxide.

External links

• International Chemical Safety Card 0103
• SIDS Initial Assessment Report for Benzoic Acid from the Organisation for Economic Co-operation and Development (OECD)
• ChemicalLand