Preparation, Chemical Reactions — Revision Notes

⚡ 30-Second Revision

Diazotization: — $\text{Ar-NH}_2 + \text{NaNO}_2 + \text{HCl} \xrightarrow{0-5^\circ\text{C}} \text{Ar-N}_2^+\text{Cl}^-$ (Nitrosonium ion $\text{NO}^+$ is active electrophile).
Stability: — Aromatic diazonium salts stable at $0-5^\circ\text{C}$ (resonance stabilized); Aliphatic are highly unstable.
Replacement by Halogens/CN:

- Sandmeyer: $\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{CuX}/\text{HX}} \text{Ar-X}$ ($\text{X} = \text{Cl}, \text{Br}, \text{CN}$) - Gattermann: $\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{Cu powder}/\text{HX}} \text{Ar-X}$ ($\text{X} = \text{Cl}, \text{Br}$) - Balz-Schiemann (for F): $\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{HBF}_4} \text{Ar-N}_2^+\text{BF}_4^- \xrightarrow{Delta} \text{Ar-F}$ - Iodine: $\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{KI}} \text{Ar-I}$

Replacement by H: — $\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{H}_3\text{PO}_2/\text{H}_2\text{O} \text{ or } \text{CH}_3\text{CH}_2\text{OH}} \text{Ar-H}$
Replacement by OH: — $\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{H}_2\text{O}/Delta} \text{Ar-OH}$
Coupling Reactions (Azo Dyes):

- With Phenols: $\text{Ar-N}_2^+ + \text{Phenol} \xrightarrow{\text{OH}^-/\text{pH 9-10}} \text{Ar-N=N-Ar'-OH}$ - With Anilines: $\text{Ar-N}_2^+ + \text{Aniline} \xrightarrow{\text{H}^+/\text{pH 4-5}} \text{Ar-N=N-Ar'-NH}_2$

2-Minute Revision

Diazonium salts are crucial intermediates in organic synthesis, particularly aromatic ones. They are formed via diazotization, where a primary aromatic amine reacts with sodium nitrite and a mineral acid (like $\text{HCl}$) at a very low temperature ($0-5^\circ\text{C}$). This low temperature is vital as the diazonium salts are unstable and decompose to phenols at higher temperatures. Aliphatic diazonium salts are extremely unstable and decompose immediately.

The utility of diazonium salts stems from the excellent leaving group ability of the nitrogen molecule ($\text{N}_2$). They undergo two main types of reactions: replacement reactions and coupling reactions.

In replacement reactions, the diazonium group is substituted by various atoms or groups. Key examples include Sandmeyer (using $\text{CuX}$ for $\text{Cl}$, $\text{Br}$, $\text{CN}$), Gattermann (using $\text{Cu}$ powder for $\text{Cl}$, $\text{Br}$), Balz-Schiemann (for $\text{F}$ using $\text{HBF}_4$ then heat), and replacement by $\text{I}$ (using $\text{KI}$), $\text{H}$ (using $\text{H}_3\text{PO}_2$ or ethanol), or $\text{OH}$ (warming with water).

Coupling reactions involve the diazonium ion acting as an electrophile to react with activated aromatic compounds (phenols in alkaline medium, anilines in acidic medium) to form brightly colored azo dyes, retaining the $-\text{N}=\text{N}-$ linkage.

Mastering these reagents, conditions, and products is key for NEET.

5-Minute Revision

Diazonium salts, especially aromatic ones, are indispensable in organic chemistry. Their preparation, known as diazotization, involves treating a primary aromatic amine (e.g., aniline) with sodium nitrite ($\text{NaNO}_2$) and a strong mineral acid (e.

g., $\text{HCl}$) at a precisely controlled low temperature of $0-5^\circ\text{C}$. The active species is the nitrosonium ion ($\text{NO}^+$), which is generated in situ. This low temperature is critical because aromatic diazonium salts are resonance-stabilized but still highly reactive and decompose rapidly at higher temperatures to form phenols and nitrogen gas.

Aliphatic diazonium salts, lacking this resonance stabilization, are too unstable to be isolated and decompose instantly.

The reactions of diazonium salts are broadly categorized:

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Replacement Reactions (Loss of $\text{N}_2$): — The diazonium group is replaced by another atom or group, with $\text{N}_2$ gas being evolved.

* Halogenation: * Sandmeyer Reaction: For $\text{Cl}$, $\text{Br}$, $\text{CN}$. Uses $\text{CuCl}/\text{HCl}$, $\text{CuBr}/\text{HBr}$, or $\text{CuCN}/\text{KCN}$. Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{CuCl}/\text{HCl}} \text{C}_6\text{H}_5\text{Cl}$.

* Gattermann Reaction: For $\text{Cl}$, $\text{Br}$. Uses $\text{Cu}$ powder in $\text{HCl}$ or $\text{HBr}$. Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{Cu powder}/\text{HCl}} \text{C}_6\text{H}_5\text{Cl}$.

(Sandmeyer generally gives better yields). * Balz-Schiemann Reaction (for F): Involves $\text{HBF}_4$ to form $\text{Ar-N}_2^+\text{BF}_4^-$, followed by heating. Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{HBF}_4} \text{C}_6\text{H}_5\text{N}_2^+\text{BF}_4^- \xrightarrow{Delta} \text{C}_6\text{H}_5\text{F}$.

* Iodine: Direct reaction with $\text{KI}$. Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{KI}} \text{C}_6\text{H}_5\text{I}$. * Replacement by Hydrogen: Using mild reducing agents like hypophosphorous acid ($\text{H}_3\text{PO}_2$) or ethanol.

Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{H}_3\text{PO}_2/\text{H}_2\text{O}} \text{C}_6\text{H}_6$. * Replacement by Hydroxyl Group (Phenol formation): Warming with water.

Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{H}_2\text{O}/Delta} \text{C}_6\text{H}_5\text{OH}$.

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Coupling Reactions (Retention of Diazo Group): — The diazonium ion acts as a weak electrophile, attacking activated aromatic compounds to form azo dyes.

* With Phenols: In mildly alkaline medium (pH 9-10). Forms p-hydroxyazobenzene (orange dye). Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{C}_6\text{H}_5\text{OH} \xrightarrow{\text{OH}^-/\text{pH 9-10}} \text{C}_6\text{H}_5\text{N=N-C}_6\text{H}_4\text{-OH}$.

* With Anilines: In mildly acidic medium (pH 4-5). Forms p-aminoazobenzene (yellow dye). Example: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{H}^+/\text{pH 4-5}} \text{C}_6\text{H}_5\text{N=N-C}_6\text{H}_4\text{-NH}_2$.

For NEET, focus on recognizing the reagents and products for each reaction, understanding the stability differences, and the role of temperature and pH in specific reactions.

Prelims Revision Notes

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Diazotization: — Converts primary aromatic amines to diazonium salts. Reagents: $\text{NaNO}_2 + \text{HCl}$ (or $\text{HBr}$, $\text{H}_2\text{SO}_4$). Temperature: $0-5^\circ\text{C}$ (ice-cold). Key intermediate: Nitrosonium ion ($\text{NO}^+$). Example: Aniline to Benzenediazonium chloride.
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Stability: — Aromatic diazonium salts are stable at $0-5^\circ\text{C}$ due to resonance. Aliphatic diazonium salts are highly unstable and decompose immediately to carbocations and $\text{N}_2$.
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**Replacement Reactions (Loss of $\text{N}_2$):**

* Halogens (Cl, Br): * Sandmeyer: $\text{CuCl}/\text{HCl}$ (for Cl), $\text{CuBr}/\text{HBr}$ (for Br). Higher yield. * Gattermann: $\text{Cu}$ powder/$\text{HCl}$ (for Cl), $\text{Cu}$ powder/$\text{HBr}$ (for Br).

Lower yield. * Fluorine (Balz-Schiemann): $\text{HBF}_4$ (forms $\text{Ar-N}_2^+\text{BF}_4^-$), then heat ($\Delta$). * Iodine: $\text{KI}$ (potassium iodide). * Cyanide (CN): $\text{CuCN}/\text{KCN}$ (Sandmeyer).

* Hydrogen (H): $\text{H}_3\text{PO}_2$ (hypophosphorous acid) or $\text{CH}_3\text{CH}_2\text{OH}$ (ethanol). * Hydroxyl (OH): Warm with $\text{H}_2\text{O}$. Forms phenol. * **Nitro ($\text{NO}_2$):** $\text{NaNO}_2/\text{Cu}$ powder.

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Coupling Reactions (Retention of $-\text{N}=\text{N}-$ group): — Form azo dyes.

* With Phenols: Mildly alkaline medium (pH 9-10). Product: p-hydroxyazobenzene (orange dye). * With Anilines: Mildly acidic medium (pH 4-5). Product: p-aminoazobenzene (yellow dye).

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Key Point: — Nitrogen gas ($\text{N}_2$) is an excellent leaving group in replacement reactions.

Vyyuha Quick Recall

To remember the Sandmeyer reagents for Cl, Br, CN: 'Sand-Copper-Halide-Cyanide' (Sandmeyer uses Copper(I) salts for Halides and Cyanide).

For Gattermann: 'Gatter-Copper-Powder' (Gattermann uses Copper powder).

For Balz-Schiemann (Fluorine): 'Balz-Schiemann-Fluoroborate-Heat' (Fluorine via Fluoroboric acid and heating).

For Coupling reactions pH: 'Phenol-Alkaline, Aniline-Acidic' (Phenols couple in alkaline, Anilines in acidic medium).