Diazotization Reaction Mechanism - Detailed Information with FAQs

Diazotization Reaction

Diazotization Reaction involves the formation of diazonium salts when aromatic amines are made to react with nitrous acid in presence of mineral acid. Water is obtained as a by-product or side product.

The reaction can be clarified as follows:-

Aromatic Amine + Nitrous Acid + Mineral Acid gives Diazonium Salt + Water

Aromatic amine is usually a derivative of aromatic hydrocarbon where the hydrogen of the aromatic ring is replaced by or substituted by an amino group. This amine group is nucleophilic and directly bonded to the aromatic ring compound. Nitrous acid is generally a nitrogen oxoacid a conjugate acid of nitrite. It is a monoprotic weak acid used in the gaseous phase. The structure of nitrous acid is as follows-

Mineral acids or inorganic acids are derived from one or more inorganic chemical compounds. Mineral acids when dissolved in water tend to produce hydrogen ions and conjugate base.

General diazotization reaction is as follows:-

$\mathrm{ArNH}_2+\mathrm{HNO}_2^{+} \mathrm{HX} \longrightarrow\mathrm{ArN}_2^{+} \mathrm{X}^{-}+\mathrm{H}_2 \mathrm{O}$

Diazotization of an Amine:

Diazotization of amines is a process where primary amines are converted to diazonium salts using nitrous acid and mineral acid respectively.

This reaction involves losing nitrogen from the aromatic amine generating a carbon cation further leading to the formation of a diazonium salt.

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Diazotization Mechanism

Now, let us have a look at the mechanism followed by aromatic primary amine to form diazonium salt.

• Formation of nitrosonium ion

The main reagent used in this reaction is either sodium

nitrite (NaNO₂) or Nitrous acid (HNO₂).

Sodium nitrite salt can be easily handled while Nitrous acid is an unstable liquid to be handled cautiously.

Secondly, mineral acid plays an important role in converting NaNO₂ to HNO₂ provided if NaNO₂ is used in the reaction.

The mineral acid generally used is HCl.

HCl converts NaNO₂ to HNO₂ further converting it into nitrosonium ion (NO⁺) which is a strong electrophile.

This electrophile is responsible for the formation of a diazonium salt. The formation of nitrosonium ion takes place by protonation of a hydroxyl group (OH^-) resulting in loss of a water molecule.

• Formation of diazonium ion

The reaction between amine and nitrosonium ion

accompanied by mineral acid is the next step.

1. The reaction between nitrosonium ion and aromatic amine leads to the formation of a new N-N bond.

2. Two proton transfers take place from nitrogen to oxygen followed by rearrangement of pi bonds.

i.e. N-N a pi bond is formed and an N-O bond is broken.

3. N-N triple bond is formed with the elimination of water molecule.

This is the final step in the diazotization reaction mechanism.

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Diazonium Coupling Reaction

We all know that dyes form an important part of our day to day lives. Dyes are nothing but diazobenzene derivatives. The structure of diazobenzene or azobenzene is such that the two benzene molecules are attached by the nitrogen-nitrogen double bond. The most important characteristic of diazonium salts is that they undergo coupling reactions. The azo group is the N-N bond present in the compound.

Aromatic diazo compounds are stable and crystalline chemical species. They can be synthesized by azo cooling reaction following electrophilic substitution reaction. The reaction takes place between the aryl diazonium salt and an aryl ring. The aryl ring attacks the diazonium cation forming azo compounds.

$\mathrm{ArN}_2^{+}+\mathrm{Ar}^{\prime} \mathrm{H}-\rightarrow \mathrm{ArN}=\mathrm{NAr}{ }^{\prime}+\mathrm{H}^{+}$

The diazo coupling chemical reactions take place at or near 0°C as the diazonium compounds or diazonium salts are unstable at the above-mentioned temperature.

The azo compound formation also takes place via the oxidation of hydrazines (R-NH-NH-R).

Condensation of nitro aromatic compounds with aniline result in the formation of azoxy intermediate which is further reduced to the azo compound.

$\mathrm{Ar}\left(\mathrm{NO}_2\right)+\mathrm{ArNH}_2 \rightarrow \mathrm{ArN}(\mathrm{O})=\mathrm{NAr}+\mathrm{H}_2 \mathrm{O}$

$\operatorname{ArN}(\mathrm{O})=\mathrm{NAr}+\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6 \rightarrow \operatorname{ArN}=\mathrm{NAr}^{+}+\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+\mathrm{H}_2 \mathrm{O}$

As a result of π-delocalisation of electrons, Aromatic diazo compounds express themselves in various colours, mostly red, orange and yellow. Hence, they are usually used as dyes and so are called azo dyes.

Diazotisation of Aniline

Aniline or Arenediazonium salt reacts with a highly reactive compound like phenol and or amines leading to the formation of coloured compounds called azo compounds. This reaction is termed a coupling reaction. This chemical reaction is used in the preparation of red azo dye. Preparation of aromatic diazonium salt generally takes place by the addition of cold aqueous sodium nitrite to primary aromatic amine in presence of an acid. The temperature required for this reaction to take place is 273 – 278K.

Reaction is illustrated as follows

Aliphatic Azo or Diazo compounds are less used than aromatic diazo compounds. Aliphatic azo or diazo compounds are used as radical initiators. Diethyldiazine, an aliphatic azo compound cleaves the C-N bond at increased temperature leading to the elimination of nitrogen gas generating radicals.

The aliphatic azo chemical should be handled with great care as they can explode due to their instability.

NCERT Chemistry Notes:

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Diazotization Titration

Converting primary amine the diazonium salt or compound is called diazotization titration.

A primary amine is made to react with sodium nitrite and mineral acid like HCl to obtain a diazonium salt compound.

Nitrous acid is formed when sodium nitrite and HCl react with each other.

$\mathrm{NaNO}_2+\mathrm{HCl} \rightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}$

The nitrous acid forms react with primary amine forming diazonium salt. The addition of ammonium sulphamate solution is added to remove excess nitrous acid formed.

$\mathrm{R}-\mathrm{NH}_2+\mathrm{HNO}_2 \rightarrow \mathrm{R}-\mathrm{N}=\mathrm{NH}+\mathrm{H}_2 \mathrm{O}$

Starch iodide paper is used in the detection of the endpoint. The end point is detected by formation of blue colour.

The starch iodide paper preparation involves immersion of filter paper in starch and potassium iodide solution.

The following reactions depict the end point raections:-

$\mathrm{KI}+\mathrm{HCl} \rightarrow \mathrm{KCl}+\mathrm{H}_2 \mathrm{O}$

$2 \mathrm{HI}+2 \mathrm{HNO}_2 \rightarrow \mathrm{I}_2+2 \mathrm{NO}+2 \mathrm{H}_2 \mathrm{O}$

$\mathrm{I}_2+$ starch mucilage $\rightarrow$ blue colour

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Some Solved Examples

Question 1: Products A and B formed in the following reactions are respectively :

1)

2)

3)

4)

Solution:

The reaction will be -

Hence, the answer is option (4).

Question 2: Reagent, 1-naphthylamine and sulphanilic acid in acetic acid is used for the detection of :
1)$\mathrm{N}_2 \mathrm{O}$
2)NO
3)$\mathrm{NO}_3^{-}$
4) (correct)$\mathrm{NO}_2^{-}$

Solution:

For the detection of NO₂^– , the following test is used -

1) Acetic acid

2) Sulphanilic acid

3) 1-Naphthylamine

Stepwise reactions-

Hence, the answer is option (4).

Question 3: The major product Z is obtained in the following reaction scheme is :

1)

2)

3)

4)

Solution:

The reaction will be -

Hence, the answer is option (3).