Importance in Synthetic Organic Chemistry — Revision Notes

⚡ 30-Second Revision

Diazotization: — $ArNH_2 xrightarrow{NaNO_2/HCl, 0-5^circ C} ArN_2^+Cl^-$ (Aryl diazonium salt)
Stability: — Aryl diazonium salts stable only at $0-5^circ C$. Alkyl diazonium salts are highly unstable.
Sandmeyer Reaction: — $ArN_2^+Cl^- xrightarrow{CuCl/HCl} ArCl$; $ArN_2^+Br^- xrightarrow{CuBr/HBr} ArBr$; $ArN_2^+Cl^- xrightarrow{CuCN/KCN} ArCN$
Gattermann Reaction: — $ArN_2^+Cl^- xrightarrow{Cu/HCl} ArCl$; $ArN_2^+Br^- xrightarrow{Cu/HBr} ArBr$
Balz-Schiemann Reaction: — $ArN_2^+Cl^- xrightarrow{HBF_4} ArN_2^+BF_4^- xrightarrow{Delta} ArF$
Replacement by Iodine: — $ArN_2^+Cl^- xrightarrow{KI} ArI$
Replacement by Hydroxyl: — $ArN_2^+Cl^- xrightarrow{H_2O, Delta} ArOH$ (Phenol)
Replacement by Hydrogen: — $ArN_2^+Cl^- xrightarrow{H_3PO_2/H_2O \text{ or } CH_3CH_2OH} ArH$ (Benzene)
Azo Coupling: — $ArN_2^+ + \text{Activated ArH} \rightarrow Ar-N=N-Ar'$ (Azo dye)

- Phenol: Alkaline medium - Amine: Weakly acidic medium

2-Minute Revision

Aryl diazonium salts, formed from primary aromatic amines via diazotization ($NaNO_2/HCl$ at $0-5^circ C$), are incredibly versatile synthetic intermediates. Their utility stems from the excellent leaving group ability of the dinitrogen molecule ($N_2$).

This allows for the replacement of the diazonium group with a wide range of substituents. Key replacement reactions include the Sandmeyer reaction (using $CuX$ for Cl, Br, CN), Gattermann reaction (using $Cu$ powder for Cl, Br), Balz-Schiemann reaction ($HBF_4$ then heat for F), and direct replacement by I ($KI$), OH ($H_2O/Delta$), or H ($H_3PO_2$ or $CH_3CH_2OH$).

Beyond substitution, diazonium salts are crucial for synthesizing vibrant azo dyes through coupling reactions with activated aromatic compounds like phenols (alkaline medium) or amines (weakly acidic medium).

Remember, aryl diazonium salts are only stable at low temperatures, unlike highly unstable alkyl diazonium salts.

5-Minute Revision

Diazonium salts, specifically aryl diazonium salts, are indispensable tools in organic synthesis, acting as a gateway to a vast array of substituted aromatic compounds. They are generated through diazotization, where a primary aromatic amine reacts with nitrous acid (prepared in situ from $NaNO_2$ and $HCl$) at a strictly controlled low temperature of $0-5^circ C$.

This low temperature is critical because aryl diazonium salts are thermally unstable, decomposing rapidly at higher temperatures due to the expulsion of stable nitrogen gas ($N_2$). Alkyl diazonium salts, lacking resonance stabilization, are even more unstable and decompose immediately, making them synthetically useless for direct substitution.

The synthetic power of aryl diazonium salts lies in two main types of reactions:

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Replacement Reactions: — The diazonium group ($-\text{N}_2^+$) is an exceptional leaving group. It can be replaced by:

* Halogens: Chlorine and bromine via Sandmeyer reaction ($CuCl/HCl$ or $CuBr/HBr$) or Gattermann reaction ($Cu$ powder/$HCl$ or $HBr$). Fluorine via Balz-Schiemann reaction ($HBF_4$ followed by heating).

Iodine via simple warming with $KI$. * Cyano Group: Via Sandmeyer reaction ($CuCN/KCN$). This is crucial for extending the carbon chain and subsequent conversion to carboxylic acids or amides.

* Hydroxyl Group: By warming an aqueous solution of the diazonium salt ($H_2O/Delta$), yielding a phenol. * Hydrogen Atom: By reduction with hypophosphorous acid ($H_3PO_2$) or ethanol ($CH_3CH_2OH$), effectively removing the amino group and forming the parent hydrocarbon.

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Coupling Reactions: — The diazonium cation acts as a weak electrophile, attacking electron-rich aromatic compounds (activated aromatic rings) to form brightly colored azo dyes. Phenols couple in alkaline medium, while aromatic amines couple in weakly acidic medium. This reaction is fundamental to the dye industry.

Example Conversions:

Aniline $ightarrow$ Chlorobenzene: Diazotization $ightarrow$ Sandmeyer ($CuCl/HCl$)
Aniline $ightarrow$ Phenol: Diazotization $ightarrow$ Warm with $H_2O$
Aniline $ightarrow$ Benzonitrile $ightarrow$ Benzoic Acid: Diazotization $ightarrow$ Sandmeyer ($CuCN/KCN$) $ightarrow$ Hydrolysis ($H_3O^+$)

Mastering these reactions, their specific reagents, and conditions is key for NEET success.

Prelims Revision Notes

Diazonium Salts: Importance in Synthetic Organic Chemistry (NEET Revision)

1. Formation (Diazotization):

Reactants: — Primary aromatic amine ($ArNH_2$) + Sodium nitrite ($NaNO_2$) + Mineral acid ($HCl$ or $H_2SO_4$).
Conditions: — Very low temperature, $0-5^circ C$ (ice bath).
Product: — Aryl diazonium salt ($ArN_2^+X^-$).
Mechanism (Simplified): — $NaNO_2 + HCl \rightarrow HNO_2 + NaCl$. $HNO_2$ reacts with $ArNH_2$ to form $ArN_2^+X^-$.

2. Stability:

Aryl Diazonium Salts: — Relatively stable only at $0-5^circ C$ due to resonance stabilization. Decompose rapidly at higher temperatures.
Alkyl Diazonium Salts: — Extremely unstable, decompose immediately to carbocations, not synthetically useful for direct substitution.

3. Synthetic Utility (Key Reactions):

A. Replacement Reactions (Loss of $N_2$):

* Halogenation: * **Chlorine ($Cl$):** Sandmeyer reaction ($ArN_2^+Cl^- xrightarrow{CuCl/HCl} ArCl$) or Gattermann reaction ($ArN_2^+Cl^- xrightarrow{Cu/HCl} ArCl$). * **Bromine ($Br$):** Sandmeyer reaction ($ArN_2^+Br^- xrightarrow{CuBr/HBr} ArBr$) or Gattermann reaction ($ArN_2^+Br^- xrightarrow{Cu/HBr} ArBr$).

* **Fluorine ($F$):** Balz-Schiemann reaction ($ArN_2^+Cl^- xrightarrow{HBF_4} ArN_2^+BF_4^- xrightarrow{Delta} ArF$). * **Iodine ($I$):** Direct reaction ($ArN_2^+Cl^- xrightarrow{KI} ArI$). No copper catalyst needed.

* **Cyano Group ($CN$):** Sandmeyer reaction ($ArN_2^+Cl^- xrightarrow{CuCN/KCN} ArCN$). Useful for chain extension. * **Hydroxyl Group ($OH$):** Warming with water ($ArN_2^+Cl^- xrightarrow{H_2O, Delta} ArOH$).

Forms phenols. * **Hydrogen ($H$):** Reduction ($ArN_2^+Cl^- xrightarrow{H_3PO_2/H_2O \text{ or } CH_3CH_2OH} ArH$). Deamination.

B. Coupling Reactions (Retention of $N_2$ as Azo Linkage):

* Reaction Type: Electrophilic aromatic substitution. Diazonium cation is a weak electrophile. * Coupling Components: Activated aromatic compounds (phenols, aromatic amines). * Products: Azo dyes (contain $-\text{N}=\text{N}-$ linkage, highly colored).

* Conditions: * With Phenols: Alkaline medium (e.g., $NaOH$). Forms p-hydroxyazobenzene (orange dye). * With Aromatic Amines: Weakly acidic medium (e.g., Aniline, N,N-dimethylaniline). Forms p-aminoazobenzene (yellow dye).

4. Key Takeaways for NEET:

Memorize reagents and products for all named reactions.
Understand the critical role of $0-5^circ C$ for aryl diazonium salt stability.
Differentiate between aryl and alkyl diazonium salt stability and utility.
Practice multi-step conversions involving diazonium salts.

Vyyuha Quick Recall

Don't Stop Going Back, Instead Hydrolyze Hydrogen, Couple Nitrogen!

Don't: Diazotization (Formation of diazonium salts)
Stop: Sandmeyer (Cl, Br, CN with $CuX$)
Going: Gattermann (Cl, Br with $Cu$ powder)
Back: Balz-Schiemann (F with $HBF_4$)
Instead: Iodine (with $KI$)
Hydrolyze: Hydroxyl (with $H_2O/Delta$ to form phenol)
Hydrogen: Hydrogen (with $H_3PO_2$ or $CH_3CH_2OH$ to form benzene)
Couple Nitrogen: Coupling reactions (to form azo dyes, retaining the nitrogen linkage)