Amines (Diazonium Salts)

Method Of Preparation Of Diazonium Salts

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Diazonium salts ($[ArN_2]^+X^-$) are important intermediates in organic synthesis, particularly for preparing aromatic compounds.

Preparation Of Diazonium Salts

Diazotization: Diazonium salts are prepared by the reaction of primary aromatic amines with nitrous acid ($HNO_2$) in the presence of a strong mineral acid (like $HCl$ or $H_2SO_4$) at low temperatures (0-5°C).

Reaction:

$$ArNH_2 + NaNO_2 + 2HX \xrightarrow{0-5^\circ C} [ArN_2]^+X^- + NaX + 2H_2O$$

Where:

• $ArNH_2$ is the primary aromatic amine.
• $NaNO_2$ is sodium nitrite, which reacts with the mineral acid ($HX$) to generate nitrous acid ($HNO_2$) in situ.
• $HX$ is the mineral acid (e.g., $HCl$).
• $[ArN_2]^+X^-$ is the diazonium salt.

Conditions:

• Low Temperature (0-5°C): Diazonium salts are unstable and decompose at higher temperatures. Hence, the reaction is carried out in an ice-salt bath.
• Acidic Medium: A strong mineral acid is required both to generate nitrous acid and to keep the amine protonated initially.

Example: Preparation of benzenediazonium chloride:

$$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} [C_6H_5N_2]^+Cl^- + NaCl + 2H_2O$$

Physical Properties

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Diazonium salts have specific physical properties related to their ionic nature and instability.

Physical Properties:

• State: Typically exist as crystalline solids.
• Stability: They are generally unstable, especially in the dry state. Dry diazonium salts are explosive and should not be isolated. They are usually prepared and used immediately in solution.
• Solubility: Soluble in water and some polar organic solvents.
• Temperature Sensitivity: Decompose rapidly at temperatures above 5-10°C.

Chemical Reactions

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Diazonium salts are versatile intermediates due to the facile displacement of the stable dinitrogen gas ($N_2$). Their reactions can be broadly categorized based on whether the nitrogen group is displaced or retained.

Reactions Involving Displacement Of Nitrogen

Description: In these reactions, the diazonium group ($[N_2]^+$) is replaced by various substituents. The release of highly stable $N_2$ gas makes these reactions thermodynamically favorable.

1. Replacement by Halogens (Sandmeyer Reaction):

• Reaction of diazonium salts with cuprous halides ($CuCl$, $CuBr$) or potassium halides ($KI$) replaces the diazonium group with a halogen atom.

$[ArN_2]^+X^- \xrightarrow{CuCl/HCl} ArCl + N_2$

$[ArN_2]^+X^- \xrightarrow{CuBr/HBr} ArBr + N_2$

• For iodine, reaction with $KI$ is sufficient.

$[ArN_2]^+X^- + KI \rightarrow ArI + N_2 + KX$

2. Replacement by Cyano Group (Sandmeyer Reaction):

• Reaction with cuprous cyanide ($CuCN$) introduces a cyano group ($ -CN $) onto the aromatic ring.

$[ArN_2]^+X^- \xrightarrow{CuCN/KCN} ArCN + N_2$

3. Replacement by Halogen (Gattermann Reaction):

• Reaction with copper powder in the presence of $HX$. Similar to Sandmeyer reaction but uses copper powder instead of cuprous salts.

$[ArN_2]^+X^- \xrightarrow{Cu/HX} ArX + N_2$

4. Replacement by Hydroxyl Group:

• Heating diazonium salt solution gently ($ \approx 273-283 K $) results in hydrolysis to form phenols.

$[ArN_2]^+X^- + H_2O \xrightarrow{warm} ArOH + N_2 + HX$

5. Replacement by Hydrogen (Deamination):

• Reduction of diazonium salts with hypophosphorous acid ($H_3PO_2$) or ethanol ($C_2H_5OH$) replaces the diazonium group with a hydrogen atom.

$[ArN_2]^+X^- + H_3PO_2 + H_2O \rightarrow Ar-H + N_2 + H_3PO_3 + HX$

$[ArN_2]^+X^- + C_2H_5OH \rightarrow Ar-H + N_2 + CH_3CHO + HX$

6. Replacement by Fluorine (Schiemann Reaction):

• Treatment of diazonium salts with fluoroboric acid ($HBF_4$) precipitates the less soluble diazonium fluoroborate ($[ArN_2]^+BF_4^-$).
• Thermal decomposition of the dry diazonium fluoroborate yields aryl fluorides.

$[ArN_2]^+Cl^- + HBF_4 \rightarrow [ArN_2]^+BF_4^-(s) + HCl$

$[ArN_2]^+BF_4^-(s) \xrightarrow{\Delta} ArF + N_2(g) + BF_3(g)$

Reactions Involving Retention Of Diazo Group

Description: In these reactions, the diazonium group itself acts as a leaving group ($N_2$ is liberated), but the remaining part of the molecule is attacked by something else, or the diazonium group plays a role in coupling.

1. Azo Coupling Reactions:

• Description: Diazonium salts undergo electrophilic aromatic substitution with activated aromatic compounds (like phenols and aromatic amines) in the pH range of 4-7. The diazonium ion acts as an electrophile.
• Products: Form brightly colored azo compounds, which are used as dyes.
• Conditions: Coupling occurs at the para position of the phenol or amine unless it is blocked, in which case it occurs at the ortho position. The reaction is carried out in weakly acidic to neutral or alkaline conditions depending on the coupling partner.

Example: Coupling of benzenediazonium chloride with phenol in alkaline medium:

$[C_6H_5N_2]^+Cl^- + C_6H_5OH \xrightarrow{pH \ 7-8} p-(OH)C_6H_4N=NC_6H_5$ (Yellow precipitate)

Example: Coupling with aniline in weakly acidic medium:

$[C_6H_5N_2]^+Cl^- + C_6H_5NH_2 \xrightarrow{pH \ 4-5} p-(H_2N)C_6H_4N=NC_6H_5$ (Butter yellow)

Importance Of Diazonium Salts In Synthesis Of Aromatic Compounds

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Diazonium salts are extremely important synthetic intermediates in organic chemistry, particularly for the preparation of aromatic compounds. Their utility stems from the fact that the diazonium group ($[N_2]^+$) is an excellent leaving group, easily displaced by a variety of nucleophiles.

Versatility: Diazonium salts serve as a versatile source of the aryl group ($Ar-$) which can be attached to a variety of functional groups that cannot be directly introduced onto the benzene ring by electrophilic substitution.

Key Synthetic Applications:

1. Introduction of Halogens: The Sandmeyer and Gattermann reactions allow for the introduction of $-Cl$, $-Br$, and $-I$ groups onto the benzene ring, which are often difficult to introduce directly or selectively via electrophilic substitution.
2. Introduction of Cyano Group: The Sandmeyer reaction with $CuCN$ introduces the nitrile group ($-CN$), which can be further converted to carboxylic acids or amines.
3. Introduction of Hydroxyl Group: The synthesis of phenols from amines via diazonium salts is a standard method, as direct hydroxylation of benzene is difficult.
4. Introduction of Fluorine: The Schiemann reaction is the primary method for introducing fluorine onto an aromatic ring.
5. Introduction of Hydrogen: Deamination reactions remove the amino group, effectively replacing it with hydrogen, which is useful in certain synthetic strategies.
6. Formation of Azo Compounds: Azo coupling reactions are crucial for the synthesis of azo dyes, a major class of synthetic dyes used extensively in the textile and food industries.

Synthetic Pathway: Diazotization provides a reliable way to convert an amino group (often introduced via nitration and reduction) into a wide range of other functional groups that are otherwise difficult to introduce directly onto the benzene ring.

Example Pathway: To convert aniline to phenol, then to anisole, or to introduce a halogen, the amino group is first converted to a diazonium salt, which then readily undergoes substitution.