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.
 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.</p>
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<h3>Laboratory preparations</h3>
<p>Starting from:</p>
<p><b>Benzonitrile</b></p>
<p>Under alkaline or acidic conditions hydrolysis of the nitrile takes place. The reaction involves a carboxylic acid amide or imin, which is subsequently hydrolysed to the acid or salt.</p>
<p><b>Benzaldehyde</b></p>
<p>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 <strong>benzoic acid</strong> in benzaldehyde which has to be removed by fractionated distillation.</p>
<p><b>Bromobenzene</b></p>
<p>First step of the reaction is the Grignard reagent phenylmagnesium, which is formed by the reaction of bromobenzene with magnesium. The Grignard reagent reacts with carbon dioxide (mostly applied as dry ice) to produce magnesium benzoate.</p>
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<h2>Uses</h2>
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<h3>Food preservative</h3>
<p>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 used either directly or as 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 phosphorfructokinase is decreased by 95%. The effectivity of benzoic acid and benzoate is depending on the pKa of the food.<span class=)
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.
Synthesis
Benzoic acid is used to make a large number of chemicals, for example:
Benzoyl chloride
The chlorination of benzoic acid can be done with thionyl chloride, phosgene or one of the chlorides of phosphorus. As 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 presure and alkaline conditions carbondioxide reacts directly to the terephtalate. Terephtalic acid is the starting material for several polyester polymers like PET. (Most of 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 temperatures 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.
Most of the benzoic acid is used for sodium benzoate, benzoate plasticizers, phenol synthesis and benzoyl chloride 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. 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. 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
 or by coupling reagents used in peptide synthesis like DCC and DMAP.</li>
<li>The more active benzoic anhydride is formed by dehydration using acetic anhydride or phosphorus pentoxide.</li>
<li>The most reactive acid derivatives such as acid halides are easily obtained by mixing with halogenation agents like phosphorus chlorides or thionyl chloride.</li>
<li>Orthoesters can be obtained by the reaction of alcohols under acidic water free conditions with benzonitrile.</li>
<li>Reduction to benzaldehyde and benzyl alcohol is possible using DIBAL-H or LiAlH<sub>4</sub>.</li>
<li>The copper catalysed decarboxylation of benzoate to benzene is effected by heating with quinoline.</li>
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