Aromatic diazonium salts and their synthetic applications:

Primary aliphatic amines react with the nitrous acid through a reaction called diazotization to give highly unstable aliphatic diazonium salt. Aliphatic diazonium salts decompose to give carbocations even at very low temperature. So, these are highly unstable molecules. The carbocations further produce mixture of products such as alkenes, alkyl halides and alcohols by elimination of H+, reaction with water and reaction with alkyl halides respectively. So this reaction is of little importance because of mixture of products. However, the loss of N2 as a neutral molecule is quantitative and can be useful in analytical procedures. Since nitrous acid is highly unstable, it can be produced in-situ by treating sodium nitrate with the aqueous solution of a strong acid. The preparation and reactions of aliphatic diazonium salts are shown as:

Aromatic diazonium salts

Aromatic diazonium salts made from primary amines are far more stable than the aliphatic diazonium salts because they do not decompose when the temperature of reaction mixture is maintained at 0-5℃. These are very important synthetically because the diazonium group N2+ can be replaced by a variety of other functional groups.

Preparation of aryl diazonium salts

Aryl diazonium salts are prepared by the reaction of of aromatic primary amines with nitrous acid at about 0-5℃. The nitrous acid is generated in-situ by the reaction of sodium nitrite with mineral acid(HCl or H2SO4)within the reaction mixture. i.e.

The reaction of primary aromatic amine with nitrous acid to generate aryl diazonium salt is called diazotization reaction. The solution of aryl diazonium salt is immediately used for further reaction without isolation because it decomposes even at ice bath temperature. The aryl diazonium salts are highly unstable and explosive substances, therefore, it should not be isolated in the solid form, from the solution.

Mechanism of formation of diazonium salt

Diazotization of an aromatic primary amine begins with the nucleophilic attack of amine on the nitrosonium ion. In the presence of strong acid, nitrous acid generates nitrosonium ion. Nitrosonium ion react with amine to form an unstable N-nitrosoammonium ion as an intermediate which loses a Proton to form N-nitrosoamine, which in turn, tautomerize by a Proton shift to a diazenol which loses water in the presence of acid to form the diazonium ion which takes up Cl to form the diazonium salt. i.e.

Stability of aryl diazonium salts

Benzene diazonium chloride is unstable and is therefore seldom isolated in the solid form. However, Arenediazonium salts are more stable as compared to aliphatic diazonium salts. The electronic structure of aryl diazonium salts is written as:

Hence, the diazonium cation is resonance stabilized, the positive charge being distributed over the two nitrogen atoms. Observed greater stability of arenediazonium salts as compared to aliphatic diazonium salts is also due to the resonance involving the benzene ring. i.e.

Hence, the C-N Bond in aryl diazonium salt acquires the partial double bond character and is stronger than C-N Bond in alkyl diazonium salt. The aliphatic diazonium salts are unstable because -R group is incapable of participating in resonance. In alkaline solutions, the diazonium ion undergoes structural change yielding diazohydroxide and the following equilibria exist.

In acidic solutions, the formation of diazonium ion is favored while in strongly alkaline solutions the diazohydroxide that is produced gives anti and syn-diazotate ion. i.e.

Synthetic applications of aryl diazonium salts

Aryl diazonium salts are highly reactive compounds and are valuable synthetic intermediates for the synthesis of wide variety of aromatic compounds. The reactions of diazonium salts may be divided into two main types:

  • Reactions in which diazonium group is replaced by a number of other atoms or groups.
  • Reactions in which the two nitrogen atoms are retained in the product

Replacement reactions:

Specific examples are as follows:

1. Replacement by -OH:
Aqueous solution of of Benzene diazonium salt reacts with water slowly to form phenol with the evolution of N2 gas. However, if the solution is acidified usually with H2SO4 and strongly heated, hydrolysis of the diazonium salt is accelerated and phenol is formed.

This is a useful reaction for preparing substituted phenols which are not obtained by electrophilic aromatic substitution of phenol. The following example shows its use in organic synthesis.

2. Replacement by -I:
Aqueous solution of diazonium salts react with potassium iodide to give aryl iodide. i.e

An example is the synthesis of p-iodonitrobenzene.

3. Replacement by -F
The diazonium group can be replaced by fluorine by treating the diazonium salt with fluoroboric acid(HBF4). The diazonium fluoroborate that precipitates is isolated, dried and heated to yield an aryl fluoride.

4. The Sandmeyer Reaction
The replacement of of the diazonium group by -Cl, -Br or -CN is called Sandmeyer reaction. Cold aryl diazonium salts react with cuprous chloride, cuprous bromide or cuprous cyanide to form aryl chloride, aryl bromide or aryl nitrile, respectively. i.e.

Specific examples are as follows:

5. Replacement by nitro group
When an alkaline or neutral solution of an aromatic diazonium compound is treated with sodium nitrite in the presence of cuprous oxide, a nitro compound is produced. i.e.

6. Deamination through diazotization
Elimination of the the amino group from an aromatic amine is called deamination. Deamination can be achieved by the diazotising the primary aromatic amine and then replacing the diazonium group by hydrogen by treating with the reducing agent such as hypophosphorous acid, H3PO2. i.e.

This reaction provides a method for removing an -NH2 or an -NO2 group from an aromatic ring. This reaction allows the use of -NH2 group to direct the orientation of an aromatic electrophilic substitution reaction, followed by its removal(deamination). For example, we want to prepare 1,3,5-tribromobenzene. Direct bromination of benzene will not give this product because bromine is an ortho- para director. However, if we begin our synthesis with aniline, we can easily prepare 1,3,5-tribromobenzene by the following scheme:

7. Synthesis of biaryls
(Replacement of diazonium group by phenyl group). The synthesis of unsymmetrical biaryls involves the replacement of the diazonium group by an aromatic ring. The concentrated diazonium salt solution is made basic with Sodium Hydroxide mixed with an aromatic compound and stirred in the cold to get the biaryl. The reaction is called Gomberg-backmann reaction and proceeds through free radical mechanism.

Arylation occurs mainly at the para position even nitrobenzene undergoes a arylation at the para position. However, if the para position is already occupied. The arylation occurs at ortho position.

8. Reduction to arylhydrazines
Aromatic diazonium salts are reduced with the loss of Nitrogen, to arylhydrazines. The reducing agents commonly used for this purpose are Zinc, and acetic acid, SnCl2 and sodium sulphite. Phenylhydrazine produced in this reaction is used as a reagent in the identification of carboxyl compounds and sugars.

Reactions in which the two nitrogen atoms are retained in the product

  • These are also called diazo-coupling reactions. Diazonium ion are weak electrophiles. They react with highly reactive aromatic compounds such as phenols and tertiary aromatic amines to form highly colored derivatives known as azo compounds. Reactions in which the diazonium ion is joined or coupled to the ring of phenol or an aromatic amine are called diazo coupling reactions. Coupling with phenols is carried out in slightly alkaline solutions whereas with aromatic tertiary amines is carried out in slightly acidic solutions. Coupling takes place almost exclusively at the para position. However, if the para position is blocked the coupling takes place at the ortho .position.
  • Azo compounds are usually intensely colored because the azo linkage -N=N- brings the two aromatic rings into conjugation. This gives an extended system of delocalised pi electrons and allows absorption of light in visible region. Azo compounds because of their intense colors and because they can be synthesized from relatively inexpensive compounds are used extensively as dyes. Azo dyes almost always contain one or more SO3-Na+ groups to confirm water stability on the dye and assist in binding the dye to the surfaces of polar fibers(cotton, wool or nylon). Many dyes are made by coupling reactions of naphthols and naphthylamines.
  • Azo compounds are also used as acid-base indicators. For example, methyl orange which is prepared from diazotized sulfanilic acid and N,N-dimethyl aniline.
  • Methyl red is an acid dye prepared by coupling the diazotised aminobenzoic acid with dimethyl aniline.
  • Congo red contains two azo groups. It is obtained by coupling tetrazotised benzidine with two molecules of 4-amino naphthalenesulphonic acid.

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