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What are diazonium salts?

The diazonium salts are organic compounds in which there ionic interactions between the azo group (-N + ) and an anion X  (Cl  , F  , CH 3 COO  , etc.). Its general chemical formula is RN + X  , and in this the side chain R can be either an aliphatic group or an aryl group; that is, an aromatic ring.

The lower image represents the structure of the arenediazonium ion. The blue spheres correspond to the azo group, while the black and white spheres make up the aromatic ring of the phenyl group. The azo group is very unstable and reactive, because one of the nitrogen atoms has a positive charge (–N + ≡N).

Diazonium salts

However, there are resonance structures that delocalize this positive charge, for example, on the neighboring nitrogen atom: –N = N + . This originates when a pair of electrons forming a bond is directed to the nitrogen atom on the left.

Likewise, this positive charge is capable of being delocalized by the Pi system of the aromatic ring. As a consequence, aromatic diazonium salts are more stable than aliphatic ones, since the positive charge cannot be delocalized along a carbon chain (CH 3 , CH 2 CH 3 , etc.).

Diazonium salts

These salts derive from the reaction of a primary amine with an acid mixture of sodium nitrite (NaNO 2 ).

Secondary (R 2 NH) and tertiary (R 3 N) amines originate other nitrogenous products such as N-nitrosoamines (which are yellowish oils), amine salts (R 3 HN + X  ) and N-nitrosoammonium compounds.

The upper image illustrates the mechanism by which the formation of diazonium salts, or also known as the diazotization reaction, is governed.

The reaction starts with phenylamine (Ar – NH 2 ), which performs a nucleophilic attack on the N atom of the nitrosonium cation (NO + ). This cation is produced by the NaNO 2 / HX mixture , where X is generally Cl; that is, HCl.

The formation of the nitrosonium cation releases water into the medium, which removes a proton from the positively charged nitrogen.

Then, this same water molecule (or another acidic species other than H 3 O + ) gives up a proton to oxygen, delocalizing the positive charge on the less electronegative nitrogen atom).

Now, the water again deprotonates the nitrogen, thus producing the diazohydroxide molecule (the penultimate in the sequence).

Since the medium is acidic, the diazohydroxide undergoes dehydration from the OH group; To counteract the electronic vacancy, the free pair of N forms the triple bond of the azo group.

Properties

In general, diazonium salts are colorless and crystalline, soluble and stable at low temperatures (less than 5 ºC).

Some of these salts are so sensitive to mechanical impact that any physical manipulation could detonate them. Finally, they react with water to form phenols.

Displacement reactions

Diazonium salts are potential releasers of molecular nitrogen, the formation of which is the common denominator in displacement reactions. In these, a species X displaces the unstable azo group, escaping as N 2 (g).

Sandmeyer reaction

ArN + + CuCl => ArCl + N 2 + Cu +

ArN + + CuCN => ArCN + N 2 + Cu +

Gatterman reaction

ArN + + CuX => ArX + N 2 + Cu +

Unlike the Sandmeyer reaction, the Gatterman reaction has metallic copper in place of its halide; that is, the CuX is generated in situ .

Schiemann reaction

[ArN + ] BF  => ArF + BF 3 + N 2

The Schiemann reaction is characterized by the thermal decomposition of benzenediazonium fluoroborate.

Gomberg Bachmann reaction

 [ArN + ] Cl  + C 6 H 6 => Ar − C 6 H 5 + N 2 + HCl

Other displacements

ArN + + KI => ArI + K + + N 2

 [ArN + ] Cl  + H 3 PO 2 + H 2 O => C 6 H 6 + N 2 + H 3 PO 3 + HCl

 ArN + + H 2 O => ArOH + N 2 + H +

ArN + + CuNO 2 => ArNO 2 + N 2 + Cu +

Redox reactions

Diazonium salts can be reduced to arylhydrazines, using a SnCl 2 / HCl mixture :

ArN + => ArNHNH 2

They can also be reduced to arylamines in stronger reductions with Zn / HCl:

ArN + => ArNH 2 + NH 4 Cl

Photochemical decomposition

[ArN + ] X  => ArX + N 2

Diazonium salts are sensitive to decomposition by incidence of ultraviolet radiation, or very close wavelengths.

Azo coupling reactions

ArN + + Ar′H → ArN 2 Ar ′ + H +

These reactions are perhaps the most useful and versatile of the diazonium salts. These salts are weak electrophiles (the ring delocalizes the positive charge of the azo group). In order for them to react with aromatic compounds, they then need to be negatively charged, thus giving rise to azos compounds.

The reaction proceeds with an efficient yield between a pH of 5 and 7. In acidic pH the coupling is lower because the azo group is protonated, making it impossible to attack the negative ring.

Also, at basic pH (greater than 10) the diazonium salt reacts with OH  to produce diazohydroxide, which is relatively inert.

Structures of this type of organic compound have a very stable conjugated Pi system, whose electrons absorb and emit radiation in the visible spectrum.

Consequently, azo compounds are characterized by being colorful. Due to this property they have also been called azo dyes.

Diazonium salts

The top image illustrates the concept of azo coupling with methyl orange as an example. In the middle of its structure, the azo group can be seen serving as the linker of the two aromatic rings.

Which of the two rings was the electrophile at the start of the coupling? The one on the right, because the sulfonate group (-SO 3 ) removes electron density from the ring, making it even more electrophilic.

Applications

Diazonium salts

One of its most commercial applications is the production of colorants and pigments, also encompassing the textile industry in the dyeing of fabrics. These azo compounds anchor to specific molecular sites on the polymer, staining it colors.

Due to its photolytic decomposition, it is (less than before) used in the reproduction of documents. How? The areas of the paper covered by a special plastic are removed and then a basic solution of phenol is applied to them, coloring the letters or the design blue.

In organic syntheses they are used as starting points for many aromatic derivatives.

Finally, they are having applications in the field of smart materials. In these they are covalently bound to a surface (of gold, for example), allowing it to give a chemical response to external physical stimuli.

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