Benzoic acid

Benzoic acid
General
Systematic name	Benzoic acid
Other names	Carboxybenzene, E210, dracylic acid
Molecular formula	C₇H₆O₂
SMILES	c1ccccc1C(=O)O
Molar mass	122.12 g/mol
Appearance	Colourless crystalline solid
CAS number	^*
Properties
Density and phase	1.32 g/cm³, solid
Solubility in water	Soluble (hot water) 3.4 g/l (25 °C)
In methanol, diethyl ether	Soluble
Melting point	122.4 °C (395 K)
Boiling point	249 °C (522 K)
Acidity (pK_a)	4.21
Structure
Molecular shape	planar
Crystal structure	Monoclinic
Dipole moment	1.72 D in Dioxane
Hazards
MSDS	External MSDS, External MSDS
Main hazards	Irritating
NFPA 704
Flash point	121°C (394 K)
R/S statement	R: , S:
RTECS number	DG0875000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related carboxylic acids	phenylacetic acid, hippuric acid, salicylic acid
Related compounds	benzene, benzaldehyde, benzyl alcohol, benzyl benzoate, benzoyl chloride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

Benzoic acid, C₆H₅COOH, is a colourless 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.

The weak acid and its salts are historically used as food preservative and as raw material for several chemical substances like benzoyl chloride and plasticizers.

History

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).

Justus von Liebig and Friedrich Wöhler determined the structure of benzoic acid in 1832. They also investigated how hippuric acid is related to benzoic acid.

In 1875 Salkowski discovered the antifungal abilities of benzoic acid which were used for a long time in the preservation of benzoate containing fruits.

Production

Industrial preparations

Starting materials containing an alkyl substituted benzene can be oxidised with potassium permanganate, chromium trioxide, nitric acid or oxygen (in the presence of a catalyst) to give benzoic acid.

Benzoic acid is produced commercially by partial oxidation of toluene with oxygen with catalytic amounts of cobalt or manganese naphthenate. The process uses cheap raw materials, proceeds in high yield, and it is environmentally attractive in that it avoids the use of stoichiometric amounts of chromium, manganese or other metals.

U.S. production capacity is estimated to be 126000 tonnes per year, much of which is consumed internally by the producers to prepare other industrial chemicals.

Historical preparations

The first industrial process was the reaction of benzotrichloride (trichloromethyl benzene) with calcium hydroxide in water, with 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 derivates. Because of this the only source for benzoic acid for human consumption was the 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.

Uses

Food preservative

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 intra cellular pKa changes to 5 or lower the anaerobic fermentation of glucose through phosphofructokinase is decreased by 95%. The effectivity of benzoic acid and benzoate is depending on the pKa 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.

There has been concern 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.

Synthesis

Benzoic acid is used to make a large number of chemicals, for example:

Benzoyl chloride

The chlorination of benzoic acid can be performed with thionyl chloride, phosgene or one of the chlorides of phosphorus. As a very reactive acid chloride, benzoyl chloride is the important starting material for several other benzoic acid derivates like benzyl benzoate and benzoyl peroxide.

Benzyl benzoate

Benzoic acid esters (for example benzyl benzoate) are also used as artificial flavours and insect repellents.

Benzoyl peroxide

Mixing sodium peroxide with benzoyl chloride gives benzoyl peroxide, which is a radical starter in polymerization reactions and also used in cosmetic products.

Terephthalic acid

With the Kolbe-Schmitt reaction it is possible to introduce a second carboxylic acid group. Under high pressure and alkaline conditions carbon dioxide reacts directly to the terephtalate. Terephtalic acid is the starting material for several polyester polymers like PET. (Most terephthalic acid is manufactured from p-xylene by direct oxidation with oxygen, described in the section industrial production of benzoic acid of this article.)

Benzoate plasticizers

The Glycol- diethylengylcol- and triethyleneglycol esters are obtained by transesterification of methyl benzoate with the corresponding glycol derivate. A second synthesis starts with the benzoylchloride and the glycol derivate. The use of these plasticizer is similar to those of the terephthalic acid ester.

Phenol

The decarboxylation reaction at 300-400°C lead to phenol. 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 than starting material for nylon synthesis.

Biology and health effects

Gum benzoin contains up to 20% of benzoic acid and 40% 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 IPCS suggests a provisional tolerable intake would be 5 mg/kg body weight per day.Chemical Profiles (updated 2002-04-03)Concise International Chemical Assessment Document 26: BENZOIC ACID AND SODIUM BENZOATE 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, where as mice die of an intake of 6000 mg/kg body weight. PMID 4672555 The LD₅₀ for rats is 1700 mg/kg, and for humans 500 mg/kg.

Chemistry

Reactions may occur in either the aromatic ring or the carboxylic group:

Aromatic ring

Substitution reactions at the aromatic ring are possible and are influenced by the effects of the carboxylic group. Electrophilic aromatic substitution reaction will take place mainly in 3-position to the electron-withdrawing carboxylic group.

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Carboxylic 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.
The most 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 or LiAlH₄.
The copper catalysed decarboxylation of benzoate to benzene is effected by heating with quinoline.

Laboratory preparations

Benzoic acid is cheaply and readily available, so the laboratory synthesis of benzoic acid is mainly of educational value. For all syntheses, benzoic acid can be purified by recrystallization owing to its high solubility in hot and poor solubility in cold water. Starting from:

Benzonitrile

Under alkaline or acidic conditions hydrolysis of the nitrile takes place. The reaction involves a carboxylic acid amide or imine, which is subsequently hydrolysed to the acid or salt.

Benzaldehyde

The disproportionation of benzaldehyde, in the presence of base, using the Cannizzaro reaction, giving a mixture of benzoate and benzyl alcohol. This disproportionation leads always to trace amounts of benzyl alcohol and benzoic acid in benzaldehyde which has to be removed by fractionated distillation.

Bromobenzene

Bromobenzene in diethyl ether is dropped onto magnesium turnings with a small amount of iodine, and stirred to produce the Grignard reagent, phenylmagnesium bromide (C₆H₅MgBr), in a free radical reaction. Once the Grignard is ready, it is added to dry ice (solid carbon dioxide), to give the conjugate base of benzoic acid. Dilute acid is added to reform benzoic acid.

Footnotes

External links

Organic acids | Carboxylic acids | Aromatic compounds

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