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What is diphenylamine?

The diphenylamine is an organic compound with the chemical formula (C 6 H 5 ) 2 NH. Its very name indicates that it is an amine, as well as its formula (-NH 2 ) makes it clear. On the other hand, the term “diphenyl” refers to the presence of two aromatic rings linked to nitrogen. Consequently, diphenylamine is an aromatic amine.

Within the world of organic compounds , the word aromatic is not necessarily related to the existence of their odors, but rather to characteristics that define their chemical behavior against certain species.

Diphenylamine

In the case of diphenylamine, its aromaticity and the fact that its solid has a distinctive aroma coincide. However, the basis or the mechanisms that govern its chemical reactions can be explained by its aromatic character, but not by its pleasant aroma.

Its chemical structure, basicity, aromaticity and intermolecular interactions are the variables responsible for its properties: from the color of its crystals to its applicability as an antioxidant agent.

Diphenylamine

In the upper images the chemical structures of diphenylamine are represented. The black spheres correspond to the carbon atoms, the white ones to the hydrogen atoms and the blue ones to the nitrogen atom.

The difference between the two images is the model of how they represent the molecule graphically. The lower one highlights the aromaticity of the rings with the black dotted lines and, likewise, the flat geometry of these rings is evident.

Neither image shows the lone pair of unshared electrons on the nitrogen atom. These electrons “wander” through the conjugated π system of the double bonds in the rings. This system forms a kind of circulating cloud that allows intermolecular interactions; that is, with other rings of another molecule.

This means that the unshared pair of nitrogen walks through both rings, distributing their electron density equally in them, and then they return to the nitrogen, to repeat the cycle again.

In this process, the availability of these electrons decreases, resulting in a decrease in the basicity of diphenylamine (its tendency to donate electrons as a Lewis base).

Diphenylamine is an oxidizing agent capable of fulfilling a series of functions, and among them are the following:

  • During storage, apples and pears undergo a physiological process called scald, related to the production of conjugated triene, which leads to damage to the skin of the fruit. The action of diphenylamine allows to increase the storage period, reducing the damage of the fruits to 10% of that observed in its absence.
  • By combating oxidation, diphenylamine and its derivatives lengthen the operation of engines by preventing the thickening of used oil.
  • Diphenylamine is used to limit the action of ozone in the manufacture of rubber.
  • Diphenylamine is used in analytical chemistry for the detection of nitrates (NO  ), chlorates (ClO  ) and other oxidizing agents.
  • It is an indicator used in testing for nitrate poisoning.
  • When RNA is hydrolyzed for one hour, it reacts with diphenylamine; this allows its quantification.
  • Diphenylamine is used topically in veterinary medicine in the prevention and treatment of screwworm manifestations in farm animals.
  • Some of the diphenylamine derivatives belong to the category of non-steroidal anti-inflammatory drugs. Likewise, they can have pharmacological and therapeutic effects such as antimicrobial, analgesic, anticonvulsant and anticancer activity.

Preparation of diphenylamine

Diphenylamine occurs naturally in onions, corianders, green and black tea leaves, and citrus peels. Synthetically, there are many routes that lead to this compound, such as:

Thermal deamination of aniline

It is prepared by thermal deamination of aniline (C 6 H 5 NH 2 ) in the presence of oxidizing catalysts.

If the aniline in this reaction does not incorporate an oxygen atom into its structure, why does it oxidize? Because the aromatic ring is an electron-attracting group, unlike the H atom, which donates its low electron density to the nitrogen in the molecule.

        2C 6 H 5 NH 2            => (C 5 H 5 ) 2 NH + NH 3

Also, aniline can react with the aniline hydrochloride salt (C 6 H 5 NH + Cl  ) under heating at 230 ° C for twenty hours.

6 H 5 NH 2 + C 6 H 5 NH + Cl  => (C 5 H 5 ) 2 NH

Reaction with phenothiazine

Diphenylamine produces several derivatives when combined with different reagents. One of these is phenothiazine, which when synthesized with sulfur is a precursor of derivatives with pharmaceutical action.

(C 6 H 5 ) 2 NH + 2S => S (C 6 H 4 ) NH + H 2 S

Diphenylamine

Diphenylamine properties

Diphenylamine is a white crystalline solid that, depending on its impurities, can acquire bronze, amber or yellow tones. It has a pleasant flowery aroma, has a molecular weight of 169.23 g / mol and a density of 1.2 g / mL.

The molecules of these solids interact by Van der Waals forces, among which are the hydrogen bonds formed by nitrogen atoms (NH — NH) and the stacking of aromatic rings, their “electronic clouds” resting on each other. .

As aromatic rings take up a lot of space, they interfere with hydrogen bonds, without also considering the rotations of the N-ring bonds. This means that the solid does not have a very high melting point (53 ºC).

However, in the liquid state the molecules are further apart and the efficiency of the hydrogen bonds is improved. Likewise, diphenylamine is relatively heavy, requiring a lot of heat to pass into the gas phase (302 ºC, its boiling point ). This is also due in part to the weight and interactions of the aromatic rings.

Solubility and basicity

It is very insoluble in water (0.03 g / 100 g of water) due to the hydrophobic character of its aromatic rings. Instead, it is very soluble in organic solvents such as benzene, carbon tetrachloride (CCl 4 ), acetone, ethanol, pyridine, acetic acid, etc.

Its acidity constant (pKa) is 0.79, which refers to the acidity of its conjugated acid (C 6 H 5 NH + ). The proton added to the nitrogen has a tendency to detach, because the pair of electrons with which it is bonded can walk through the aromatic rings. Thus, the high instability C 6 H 5 NH + reflects the low basicity of diphenylamine.

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