BENZOIC ACID: STRUCTURE, PROPERTIES, PRODUCTION, USES - CHEMISTRY - 2024

The benzoic acid is the simplest of all aromatic acid, with the molecular formula C ₆ H ₅ COOH. It owes its name to the fact that its main source for a long time was benzoin, a resin obtained from the bark of various trees of the genus Styrax.

It is found in many plants, especially fruits, such as apricot and blueberry. It is also produced in bacteria as a by-product of the metabolism of the amino acid phenylalanine. It is also generated in the intestine by the bacterial (oxidative) processing of polyphenols present in some foods.

Source: Norsci, from Wikimedia Commons

As shown in the image above, C ₆ H ₅ COOH is, unlike many acids, a solid compound. Its solid consists of light, white and filiform crystals, which give off an almond aroma.

These needles have been known since the 16th century; for example, Nostradamus in 1556 describes its dry distillation from benzoin gum.

One of the main capacities of benzoic acid is to inhibit the growth of yeast, mold and some bacteria; for which it is used as a food preservative. This action is pH dependent.

Benzoic acid has several medicinal actions, being used as a component of pharmaceutical products that are used in the treatment of skin diseases such as ringworm and athlete's foot. It is also used as an inhalant decongestant, expectorant, and pain reliever.

A high proportion of industrially produced benzoic acid is destined for the production of phenol. Likewise, a part of it is destined for the production of glycol benzoates, used in the manufacture of plasticizers.

Although benzoic acid is not a particularly toxic compound, it does possess some harmful actions for health. For this reason, the WHO recommends a maximum intake dose of 5 mg / Kg of body weight / day, which is equivalent to a daily intake of 300 mg of benzoic acid.

Structure of benzoic acid

Source: Jynto and Ben Mills via Wikipedia

The upper image shows the structure of benzoic acid with a bar and sphere model. If the number of black spheres is counted, it will be found that there are six of them, that is, six carbon atoms; two red spheres correspond to the two oxygen atoms of the carboxyl group, –COOH; and finally, the white spheres are the hydrogen atoms.

As can be seen, on the left is the aromatic ring, whose aromaticity is illustrated by the broken lines in the center of the ring. And on the right, the –COOH group, responsible for the acidic properties of this compound.

Molecularly, C ₆ H ₅ COOH has a flat structure, due to the fact that all its atoms (with the exception of hydrogens) have sp ² hybridization.

On the other hand, the highly polar -COOH group allows a permanent dipole to exist in the structure; dipole that could be observed at first glance if its electrostatic potential map were available.

This fact has as a consequence that C ₆ H ₅ COOH can interact with itself through dipole-dipole forces; specifically, with the special hydrogen bonds.

If you look at the –COOH group you will find that the oxygen in C = O can accept a hydrogen bond; while oxygen from OH, donates them.

Crystal and hydrogen bonds

Benzoic acid can form two hydrogen bonds: it receives and accepts one simultaneously. Therefore, it is found forming dimers; that is, its molecule is "related" to another.

These pairs or dimers, C ₆ H ₅ COOH-HOOCC ₆ H ₅, are the structural base that defines the solid resulting from its arrangement in space.

These dimers compose a plane of molecules that, given their strong and directional interactions, manage to establish an ordered pattern in the solid. Aromatic rings also participate in this ordering through interactions by dispersal forces.

As a result, the molecules build a monoclinic crystal, the exact structural characteristics of which can be studied by instrumental techniques, such as X-ray diffraction.

It is from here then that a pair of flat molecules can be arranged in space, predominantly by hydrogen bonding, to give rise to those white and crystalline needles.

Physical and chemical properties

Chemical names

Acid:

-benzoic

-carboxylic benzene

-dracilic

-carboxybenzene

-benzeneoform

Molecular formula

C ₇ H ₆ O ₂ or C ₆ H ₅ COOH.

Molecular weight

122.123 g / mol.

Physical description

Solid or in the form of crystals, usually white in color, but can be beige in color if it contains certain impurities. Its crystals are scaly or needle-shaped (see first image).

Odor

It smells like almonds and is pleasant.

Taste

Tasteless or slightly bitter. The taste detection limit is 85 ppm.

Boiling point

480 ° F to 760 mmHg (249 ° C).

Melting point

252.3 ° F (121.5-123.5 ° C).

ignition point

250 ° F (121 ° C).

Sublimation

It can sublimate from 100ºC.

Water solubility

3.4 g / L at 25 ° C.

Solubility in organic solvents

-1 g of benzoic acid is dissolved in a volume equal to: 2.3 mL of cold alcohol; 4.5 ml of chloroform; 3 mL of ether; 3 mL of acetone; 30 mL of carbon tetrachloride; 10 mL of benzene; 30 mL of carbon disulfide; and 2.3 mL of turpentine oil.

-It is also soluble in volatile and fixed oils.

-It is slightly soluble in petroleum ether.

-Its solubility in hexane is 0.9 g / L, in methanol 71.5 g / L, and in toluene 10.6 g / L.

Density

1.316 g / mL at 82.4 ° F, and 1.2659 g / mL at 15 ° C.

Vapor density

4.21 (in relation to the air taken as reference = 1)

Vapor pressure

1 mmHg at 205 ° F and 7.0 x 10 ^-4 mmHg at 25 ° C.

Stability

A solution with a concentration of 0.1% in water is stable for at least 8 weeks.

Decomposition

It decomposes on heating, emitting acrid and irritating smoke.

Viscosity

1.26 cPoise at 130 ° C.

Heat of combustion

3227 KJ / mol.

Heat of vaporization

534 KJ / mol at 249 ° C.

pH

About 4 in water.

Surface tension

31 N / m at 130 ° C.

pKa

4.19 at 25 ° C.

Refractive index

1.504 - 1.5397 (ηD) at 20 ° C.

Reactions

-In contact with bases (NaOH, KOH, etc.) it forms benzoate salts. For example, if it reacts with NaOH, it forms sodium benzoate, C ₆ H ₅ COONa.

-Reacts with alcohols to form esters. For example, its reaction with ethyl alcohol produces the ethyl ester. Some esters of benzoic acid serve as plasticizers.

-Reacts with phosphorus pentachloride, PCl ₅, to form benzoyl chloride, an acid halide. Benzoyl chloride can react with ammonium (NH ₃) or an amine such as methylamine (CH ₃ NH ₂) to form benzamide.

-The reaction of benzoic acid with sulfuric acid produces the sulfonation of the aromatic ring. The functional group -SO ₃ H replaces a hydrogen atom in the meta position of the ring.

-It can react with nitric acid, with the use of sulfuric acid as a catalyst, forming meta-nitrobenzoic acid.

-In the presence of a catalyst, such as ferric chloride, FeCl ₃, benzoic acid reacts with halogens; for example, it reacts with chlorine to form meta-chlorobenzoic acid.

Production

Some production methods for this compound are listed below:

-Most of benzoic acid is produced industrially by oxidation of toluene with oxygen present in the air. The process is catalyzed by cobalt naphthenate, at a temperature of 140-160 ° C and at a pressure of 0.2 - 0.3 MPa.

-Toluene, on the other hand, can be chlorinated to produce benzotrichloride, which is subsequently hydrolyzed to benzoic acid.

-The hydrolysis of benzonitrile and benzamide, in an acidic or alkaline environment, can give rise to benzoic acid and its conjugate bases.

-Benzyl alcohol in an oxidation mediated by potassium permanganate, in an aqueous medium, produces benzoic acid. The reaction occurs by heating or reflux distillation. Once the process is finished, the mixture is filtered to eliminate manganese dioxide, while the supernatant is cooled to obtain benzoic acid.

-The benzotrichloride compound is reacted with calcium hydroxide, using iron or iron salts as catalysts, initially forming calcium benzoate, Ca (C ₆ H ₅ COO) ₂. Then this salt by reaction with hydrochloric acid is converted into benzoic acid.

Applications

Industrial

-Used in the production of phenol by oxidative decarboxylation of benzoic acid at temperatures of 300 - 400 ºC. For what purpose? Because phenol can be used in the synthesis of Nylon.

-From it, benzoate glycol is formed, a chemical precursor of diethylene glycol ester and triethylene glycol ester, substances used as plasticizers. Perhaps the most important application for plasticizers is adhesive formulations. Some long chain esters are used to soften plastics like PVC.

-Used as a rubber polymerization activator. In addition, it is an intermediate in the manufacture of alkyd resins, as well as additives for applications in the recovery of crude oil.

-In addition, it is used in the production of resins, colorants, fibers, pesticides and as a modifying agent for polyamide resin for the production of polyester. It is used to maintain the aroma of tobacco.

-It is a precursor of benzoyl chloride, which is a starting material for the synthesis of compounds such as benzyl benzoate, used in the elaboration of artificial flavors and insect repellants.

Medicinal

-It is a component of Whitfield ointment that is used for the treatment of skin diseases caused by fungi such as ringworm and athlete's foot. Whitfield's ointment is made up of 6% benzoic acid and 3% salicylic acid.

-It is an ingredient in benzoin tincture that has been used as a topical antiseptic and inhalant decongestant. Benzoic acid was used as an expectorant, analgesic, and antiseptic until the beginning of the 20th century.

-Benzoic acid has been used in the experimental therapy of patients with residual nitrogen accumulation diseases.

Food preservation

Benzoic acid and its salts are used in the preservation of food. The compound is capable of inhibiting the growth of mold, yeast, and bacteria, through a pH-dependent mechanism.

They act on these organisms when their intracellular pH falls to a pH lower than 5, almost totally inhibiting the anaerobic fermentation of glucose for the production of benzoic acid. This antimicrobial action requires a pH between 2.5 to 4 for a more effective action.

-Used to preserve foods such as fruit juices, carbonated drinks, sodas with phosphoric acid, pickles and other acidified foods.

Inconvenient

It can react with ascorbic acid (vitamin C) present in some beverages, producing benzene, a carcinogenic compound. Because of this, we are looking for other compounds with the ability to preserve food that do not have benzoic acid problems.

Others

-It is used in an active packaging, being present in ionomer films. Benzoic acid is released from them, capable of inhibiting the growth of species of the Penicillium and Aspergillus genera in microbial media.

-It is used as a preservative agent for the aroma of fruit juice and perfume. It is also used with this application in tobacco.

-Benzoic acid is used as a selective herbicide to control broad-leaved weeds and grasses in soybeans, cucumbers, melons, peanuts and woody ornamental plants.

Toxicity

-In contact with the skin and eyes, it can cause redness. Inhalation can cause respiratory tract irritation and coughing. Ingesting large amounts of benzoic acid can cause gastrointestinal disorders, which can lead to liver and kidney damage.

-Benzoic acid and benzoates can release histamine which can cause allergic reactions and irritation of the eyes, skin and mucous membranes.

-It lacks cumulative, mutagenic or carcinogenic effect, since it is rapidly absorbed in the intestine, being eliminated in the urine without accumulating in the body.

-The maximum dose allowed according to WHO is 5 mg / Kg of body weight / day, approximately 300 mg / day. Acute toxicity dose in man: 500 mg / kg.

References

1. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 ^th edition.). Wiley Plus.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Chemical Book. (2017). Benzoic acid. Recovered from: chemicalbook.com
4. PubChem. (2018). Benzoic acid. Recovered from: pubchem.ncbi.nlm.nih.gov
5. Wikipedia. (2018). Benzoic acid. Recovered from: en.wikipedia.org
6. Dadachanji, Dinshaw. (May 18, 2018). Chemical Properties of Benzoic Acid. Sciencing. Recovered from: sciencing.com
7. Ministry of Labor and Social Affairs Spain. (sf). Benzoic acid. International Chemical Safety Cards. Recovered from: insht.es