PICRIC ACID: STRUCTURE, SYNTHESIS, PROPERTIES AND USES - CHEMISTRY - 2025

The picric acid is an organic chemical highly nitrated the IUPAC name 2,4,6-trinitrophenol is. Its molecular formula is C ₆ H ₂ (NO ₂) ₃ OH. It is a very acidic phenol, and can be found as sodium, ammonium, or potassium picrate; that is, in its ionic form C ₆ H ₂ (NO ₂) ₃ ONa.

It is a solid with a strong bitter taste, and from there it derives its name, from the Greek word 'prikos', which means bitter. It is found as wet yellow crystals. Its drying or dehydration is dangerous, since it increases the unstable properties that make it explosive.

Picric acid molecule. Source: Iomesus

The picric acid molecule is shown above. In the image it is difficult to recognize the bonds and the atoms because this corresponds to the representation of its surface of Van der Waals. The next section addresses the molecular structure in greater detail.

From picric acid some intermediate compounds, various picrate salts and complexes of picric acid are synthesized.

Picric acid is used as a base for the synthesis of permanent yellow dyes. Some pathologists and researchers use it in the fixation or staining of tissue sections and other immunohistochemical processes.

It is very useful in the production of pharmaceutical products. In addition, it is used in the production of matches or matches and explosives. It is also used to etch metals, to make colored glass, and in the colorimetric determination of biological parameters such as creatinine.

On the other hand, picric acid is an irritant when it comes into contact with the skin, the respiratory, ocular and digestive mucosa. In addition to damaging the skin, it can severely affect the kidneys, blood, and liver, among other organs.

Structure

Structure and formal charges in picric acid. Source: Cvf-ps

The image above shows all the bonds and the structure itself of the picric acid molecule in greater detail. It consists of a phenol with three nitro substituents.

It can be seen that in the NO ₂ groups the nitrogen atom has a positive partial charge, and therefore demands the electron density of its surroundings. But, the aromatic ring also attracts the electrons towards itself, and before the three NO _{2 it} ends up giving up part of its own electronic density.

As a consequence of this, the oxygen of the OH group tends more to share one of its free electronic pairs to supply the electronic deficiency suffered by the ring; and in doing so, the C = O ⁺ -H bond is formed. This partial positive charge on oxygen weakens the OH bond, and increases acidity; that is, it will be released as the hydrogen ion, H ⁺.

Acid phenol

It is for this reason that this compound is an exceptionally strong (and reactive) acid, even more so than acetic acid itself. However, the compound is actually a phenol whose acidity surpasses that of the other phenols; due, as just mentioned, to the NO ₂ substituents.

Therefore, since it is a phenol, the OH group has priority and directs the enumeration in the structure. The three NO ₂ are located at carbons 2, 4 and 6 of the aromatic ring with respect to OH. This is where the IUPAC nomenclature for this compound is derived: 2,4,6-Trinitrophenol (TNP).

If the NO ₂ groups were absent, or if there were fewer of them in the ring, the OH bond would be weakened less, and therefore the compound would have less acidity.

Crystal structure

Picric acid molecules are arranged in such a way that they favor their intermolecular interactions; either for hydrogen bonding between OH and NO _{2 groups}, dipole-dipole forces, or electrostatic repulsions between electron-deficient regions.

The NO ₂ groups could be expected to repel each other and orient in the direction of neighboring aromatic rings. Also, the rings would not be able to line up one on top of the other because of increased electrostatic repulsions.

Product of all these interactions, picric acid manages to form a three-dimensional network that defines a crystal; whose unit cell corresponds to a crystalline system of the orthorhombic type.

Synthesis

Initially, it was synthesized from natural compounds such as animal horn derivatives, natural resins, among others. Starting in 1841, phenol has been used as a precursor to picric acid, following various routes or by various chemical procedures.

As already mentioned, it is one of the most acidic phenols. To synthesize it, the phenol first undergoes a sulfonation process, followed by a nitration process.

The sulfonation of the anhydrous phenol is carried out by treating the phenol with fuming sulfuric acid, occurring electrophilic aromatic substitutions of H by sulfonate groups, SO ₃ H, in the -orto and -para position with respect to the OH group.

This product, 2,4-phenoldisulfonic acid, undergoes the nitration process, treating it with concentrated nitric acid. In doing so, the two SO ₃ H groups are replaced by the nitro groups, NO ₂, and a third enters the other nitro position. The following chemical equation illustrates this:

Nitration of 2,4-phenoldisulfonic acid. Source: Gabriel Bolívar.

Direct phenol nitration

The phenol nitration process cannot be carried out directly, since high molecular weight tars are generated. This synthesis method requires a very careful control of temperature since it is very exothermic:

Direct phenol nitration. Source: akane700

Picric acid can be obtained by carrying out the direct nitration process of 2,4-dinitrophenol, with nitric acid.

Another way of synthesis is by treating benzene with nitric acid and mercuric nitrate.

Physical and chemical properties

Molecular weight

229.104 g / mol.

Physical appearance

Yellow mass or suspension of wet crystals.

Odor

It is odorless.

Taste

It is very bitter.

Melting point

122.5 ° C.

Boiling point

300 ° C. But, when melted, it explodes.

Density

1.77 g / mL.

Solubility

It is a moderately soluble compound in water. This is because their OH and NO _{2 groups} can interact with water molecules through hydrogen bonds; although the aromatic ring is hydrophobic, and therefore impairs its solubility.

Corrosiveness

Picric acid is generally corrosive to metals, except for tin and aluminum.

pKa

0.38. It is a strong organic acid.

Instability

Picric acid is characterized by unstable properties. It constitutes a risk for the environment, it is unstable, explosive and toxic.

It should be stored tightly closed to avoid dehydration, since picric acid is very explosive if allowed to dry. Great care must be taken with its anhydrous form, because it is very sensitive to friction, shock and heat.

Picric acid should be stored in cool, ventilated places, away from oxidizable materials. It is irritating on contact with the skin and mucous membranes, it should not be ingested, and it is toxic to the body.

Applications

Picric acid has been widely used in research, chemistry, industry, and the military.

Investigation

When used as a fixative for cells and tissues, it improves the results of staining them with acid dyes. It happens with trichrome staining methods. After fixing the tissue with formalin, a new fixation with picric acid is recommended.

This guarantees an intense and very bright coloring of the fabrics. You do not get good results with basic dyes. However, precautions should be taken as picric acid can hydrolyze DNA if left too long.

Organic chemistry

-In organic chemistry it is used as alkaline picrates to carry out identification and analysis of various substances.

-It is used in the analytical chemistry of metals.

-In clinical laboratories it is used in the determination of serum and urinary creatinine levels.

-It has also been used in some of the reagents that are used for the analysis of glucose levels.

In the industry

-At the photographic industry level, picric acid has been used as a sensitizer in photographic emulsions. It has been part of the production of products such as pesticides, strong insecticides, among others.

-Picric acid is used to synthesize other intermediate chemical compounds such as chloropicrin and picramic acid, for example. Some medicines and dyes for the leather industry have been made from these compounds.

-Picric acid was used in the treatment of burns, as an antiseptic and other conditions, before its toxicity became evident.

-Important component due to its explosive nature in the production of matches and batteries.

Military applications

-Because of the high explosiveness of picric acid, it has been used in ammunition plants for military weapons.

- Pressed and melted picric acid has been used in artillery shells, grenades, bombs and mines.

-The ammonium salt of picric acid has been used as an explosive, it is very powerful but less stable than TNT. For a time it was used as a component of rocket fuel.

Toxicity

It has been proven that it is very toxic to the human body and in general to all living beings.

It is recommended to avoid its inhalation and ingestion, due to its acute oral toxicity. It also causes mutation in microorganisms. It has toxic effects on wildlife, mammals and the environment in general.

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. Wikipedia. (2018). Picric acid. Recovered from: en.wikipedia.org
4. Purdue University. (2004). Picric acid explosion. Recovered from: chemed.chem.purdue.edu
5. Crystallography 365 project. (February 10, 2014). Less than mellow yellow - the structure of picric acid. Recovered from: crystallography365.wordpress.com
6. PubChem. (2019). Picric Acid. Recovered from: pubchem.ncbi.nlm.nih.gov
7. Baker, JR (1958). Picric Acid. Methuen, London, UK.