The picric acid is a highly nitrated organic chemical compound whose IUPAC name is 2,4,6-trinitrophenol. Its molecular formula is C6Htwo(NOTtwo)3OH. It is a very acidic phenol, and can be found as sodium, ammonium or potassium picrate; that is, in its ionic form C6Htwo(NOTtwo)3ONa.
It is a solid with a strong bitter taste, and from there it derives its name, from the Greek word 'prikos', which means bitter. Found as wet yellow crystals. Its desiccation or dehydration is dangerous, since it increases the unstable properties that make it explosive.
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 Van der Waals surface. The next section addresses the molecular structure in greater detail..
Some intermediate compounds, various picrate salts and complexes of picric acid are synthesized from picric acid..
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 irritating when it comes into contact with the skin, the respiratory, ocular and digestive mucosa. In addition to damaging the skin, it can seriously affect the kidneys, blood and liver, among other organs..
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In the upper image, all the bonds and the structure itself of the picric acid molecule are shown in greater detail. It consists of a phenol with three nitro substituents.
It can be seen that in the groups NOtwo the nitrogen atom has a positive partial charge, and therefore demands the electron density of its surroundings. But, the aromatic ring also attracts electrons towards itself, and before the three NOtwo 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 bond is formed+-H. This partial positive charge on oxygen weakens the O-H bond, and increases acidity; that is, it will be released as a hydrogen ion, H+.
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 substituentstwo.
Therefore, since it is a phenol, the OH group has priority and directs the enumeration in the structure. The three NOtwo they 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 groups were not there, NOtwo, or if there were a smaller number of them in the ring, the O-H bond would weaken less, and therefore the compound would have less acidity.
Picric acid molecules are arranged in such a way as to favor their intermolecular interactions; either for the formation of hydrogen bonds between OH and NO groupstwo, dipole-dipole forces, or electrostatic repulsions between electron-deficient regions.
It might be expected that the groups will NOTtwo they repelled each other and would 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.
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 through various chemical procedures.
As already mentioned, it is one of the most acidic phenols. To synthesize it, the phenol first undergoes a sulphonation process, followed by a nitration process..
The sulfonation of anhydrous phenol is carried out by treating the phenol with fuming sulfuric acid, producing electrophilic aromatic substitutions of H by sulfonate groups, SO3H, 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 groups SO3H are replaced by nitro groups, NOtwo, and a third enters the other nitro position. The following chemical equation illustrates this:
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 the temperature since it is very exothermic:
Picric acid can be obtained by carrying out the direct nitration process of 2,4-dinitrophenol, with nitric acid.
Another way of synthesis is treating benzene with nitric acid and mercuric nitrate..
229.104 g / mol.
Yellow mass or suspension of wet crystals.
It is odorless.
It's very bitter.
122.5 ° C.
300 ° C. But, when melted, it explodes.
1.77 g / mL.
It is a moderately water soluble compound. This is because its OH and NO groupstwo they can interact with water molecules through hydrogen bonds; although the aromatic ring is hydrophobic, and therefore impairs its solubility.
Picric acid is generally corrosive to metals, except tin and aluminum.
0.38. It is a strong organic acid.
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, should not be ingested, and is toxic to the body.
Picric acid has been widely used in research, chemistry, industry and the military..
When used as a fixative for cells and tissues, it improves the results of staining these with acid dyes. It happens with trichrome staining methods. After fixing the tissue with formalin, a new fixation with picric acid is recommended..
In this way, an intense and very bright coloring of the fabrics is guaranteed. You do not get good results with basic dyes. However, precautions should be taken, as picric acid can hydrolyze DNA if left too long..
-In organic chemistry it is used as alkaline picrates to carry out identification and analysis of various substances.
-It is used in 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 used for the analysis of glucose levels..
-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 chemical intermediates like chloropicrin and picramic acid, for example. Some medicines and dyes for the leather industry have been made from these compounds..
-Picric acid came to be 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 manufacture of matches and batteries.
-Due to the high explosiveness of picric acid, it has been used in ammunition plants for military weapons.
-Pressed and molten picric acid has been used in artillery shells, grenades, bombs, and mines.
-Picric acid ammonium salt 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.
It has been proven to be very toxic to the human body and in general to all living beings.
It is recommended to avoid 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.
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