Diazonium Salts — Revision Notes

⚡ 30-Second Revision

Diazotization — $Ar-NH_2 xrightarrow{NaNO_2/HCl, 0-5^circ C} Ar-N_2^+ Cl^-$ (Arenediazonium salt)
Sandmeyer Reactions

- $Ar-N_2^+ Cl^- xrightarrow{CuCl/HCl} Ar-Cl$ - $Ar-N_2^+ Cl^- xrightarrow{CuBr/HBr} Ar-Br$ - $Ar-N_2^+ Cl^- xrightarrow{CuCN/KCN} Ar-CN$

Gattermann Reactions

- $Ar-N_2^+ Cl^- xrightarrow{Cu/HCl} Ar-Cl$ - $Ar-N_2^+ Cl^- xrightarrow{Cu/HBr} Ar-Br$

Balz-Schiemann Reaction — $Ar-N_2^+ Cl^- xrightarrow{HBF_4} Ar-N_2^+ BF_4^- xrightarrow{Delta} Ar-F$
Replacement by Iodine — $Ar-N_2^+ Cl^- xrightarrow{KI} Ar-I$
Replacement by Hydroxyl — $Ar-N_2^+ Cl^- xrightarrow{H_2O, Delta} Ar-OH$
Replacement by Hydrogen (Reduction) — $Ar-N_2^+ Cl^- xrightarrow{H_3PO_2/H_2O \text{ or } CH_3CH_2OH} Ar-H$
Replacement by Nitro — $Ar-N_2^+ Cl^- xrightarrow{NaNO_2/Cu} Ar-NO_2$
Coupling with Phenols — $Ar-N_2^+ Cl^- + C_6H_5OH xrightarrow{OH^-} Ar-N=N-C_6H4-OH(p)$ (Orange dye)
Coupling with Anilines — $Ar-N_2^+ Cl^- + C_6H_5NH_2 xrightarrow{H^+} Ar-N=N-C_6H4-NH_2(p)$ (Yellow dye)
Stability — Aromatic diazonium salts are stable at $0-5^circ C$ (resonance); Aliphatic are highly unstable.

2-Minute Revision

Diazonium salts, primarily aromatic ones, are formed by diazotization of primary aromatic amines using $NaNO_2/HCl$ at $0-5^circ C$. This low temperature is crucial to prevent decomposition. The general structure is $Ar-N_2^+ X^-$, where $N_2^+$ is an excellent leaving group.

Their reactions fall into two main categories: replacement and coupling. Replacement reactions involve the loss of $N_2$ gas and its substitution by other groups. Key examples include Sandmeyer reactions (using $CuCl$, $CuBr$, $CuCN$ for $Cl$, $Br$, $CN$ respectively), Gattermann reactions (using $Cu$ powder for $Cl$, $Br$), Balz-Schiemann reaction ($HBF_4/Delta$ for $F$), and replacement by $I$ ($KI$), $OH$ ($H_2O/Delta$), or $H$ ($H_3PO_2$ or $CH_3CH_2OH$).

Coupling reactions retain the nitrogen atoms, forming brightly colored azo dyes. These are electrophilic aromatic substitutions where the diazonium ion attacks activated aromatic rings like phenols (in weakly alkaline medium) or anilines (in weakly acidic medium).

Remember the specific reagents and conditions for each transformation.

5-Minute Revision

Diazonium salts are pivotal intermediates in organic synthesis, derived from primary aromatic amines through a process called diazotization. This reaction involves treating the amine with sodium nitrite ($NaNO_2$) and a mineral acid ($HCl$) at a strictly controlled low temperature of $0-5^circ C$.

The active species is nitrous acid ($HNO_2$), generated *in situ*, which then forms the electrophilic nitrosonium ion ($NO^+$). Aromatic diazonium salts ($Ar-N_2^+ X^-$) are relatively stable at these low temperatures due to resonance stabilization, unlike their aliphatic counterparts which decompose instantly.

The key to their versatility is the dinitrogen ($N_2$) group, an exceptionally good leaving group.

Their reactions are broadly classified into two types:

1
Replacement Reactions (Loss of $N_2$) — The $-N_2^+$ group is replaced by various atoms or groups, with the evolution of nitrogen gas. Examples include:

* Halogenation: Sandmeyer reaction ($CuCl/HCl \rightarrow Ar-Cl$; $CuBr/HBr \rightarrow Ar-Br$; $CuCN/KCN \rightarrow Ar-CN$) and Gattermann reaction ($Cu/HCl \rightarrow Ar-Cl$; $Cu/HBr \rightarrow Ar-Br$).

For iodine, $KI$ is used ($Ar-I$). For fluorine, the Balz-Schiemann reaction ($HBF_4$, then heat $ightarrow Ar-F$) is employed. * Hydroxyl Group: Warming with water ($H_2O/Delta$) yields phenols ($Ar-OH$).

* Hydrogen: Reduction with hypophosphorous acid ($H_3PO_2$) or ethanol ($CH_3CH_2OH$) replaces $-N_2^+$ with $-H$, forming arenes ($Ar-H$). * Nitro Group: Reaction with $NaNO_2/Cu$ forms nitroarenes ($Ar-NO_2$).

1
Coupling Reactions (Retention of Nitrogen) — The diazonium ion acts as a weak electrophile, attacking activated aromatic rings to form azo compounds, which are brightly colored dyes. These are electrophilic aromatic substitution reactions.

* With Phenols: Occurs in a weakly alkaline medium, typically at the *para*-position, forming *p*-hydroxyazobenzene (orange dye). * With Anilines: Occurs in a weakly acidic medium, also typically at the *para*-position, forming *p*-aminoazobenzene (yellow dye).

Example: Convert aniline to bromobenzene.

1
Diazotization — Aniline $xrightarrow{NaNO_2/HCl, 0-5^circ C}$ Benzenediazonium chloride.
2
Sandmeyer Reaction — Benzenediazonium chloride $xrightarrow{CuBr/HBr}$ Bromobenzene.

Remember the specific conditions and reagents for each reaction to avoid common pitfalls in NEET.

Prelims Revision Notes

1
Diazotization — Primary aromatic amines ($Ar-NH_2$) react with $NaNO_2$ and $HCl$ at $0-5^circ C$ to form arenediazonium salts ($Ar-N_2^+ Cl^-$). Nitrous acid ($HNO_2$) is generated *in situ*. Low temperature is critical to prevent decomposition.
2
Stability — Aromatic diazonium salts are stable at $0-5^circ C$ due to resonance stabilization. Aliphatic diazonium salts are highly unstable and decompose immediately to carbocations.
3
Replacement Reactions (Loss of $N_2$)

* Sandmeyer Reaction: Uses cuprous salts. $Ar-N_2^+ Cl^- xrightarrow{CuCl/HCl} Ar-Cl$; $Ar-N_2^+ Cl^- xrightarrow{CuBr/HBr} Ar-Br$; $Ar-N_2^+ Cl^- xrightarrow{CuCN/KCN} Ar-CN$. * Gattermann Reaction: Uses copper powder.

$Ar-N_2^+ Cl^- xrightarrow{Cu/HCl} Ar-Cl$; $Ar-N_2^+ Cl^- xrightarrow{Cu/HBr} Ar-Br$. * Balz-Schiemann Reaction: For fluorine. $Ar-N_2^+ Cl^- xrightarrow{HBF_4} Ar-N_2^+ BF_4^- xrightarrow{Delta} Ar-F$.

* Iodine: $Ar-N_2^+ Cl^- xrightarrow{KI} Ar-I$ (no copper salt needed). * Hydroxyl: $Ar-N_2^+ Cl^- xrightarrow{H_2O, Delta} Ar-OH$ (phenol formation). * Hydrogen (Reduction): $Ar-N_2^+ Cl^- xrightarrow{H_3PO_2/H_2O \text{ or } CH_3CH_2OH} Ar-H$.

* Nitro: $Ar-N_2^+ Cl^- xrightarrow{NaNO_2/Cu} Ar-NO_2$.

1
Coupling Reactions (Retention of $N_2$) — Electrophilic aromatic substitution, forming azo dyes ($Ar-N=N-Ar'$).

* With Phenols: In weakly alkaline medium ($OH^-$), forms *p*-hydroxyazobenzene (orange dye). * With Anilines: In weakly acidic medium ($H^+$), forms *p*-aminoazobenzene (yellow dye).

1
Key Takeaways — Memorize reagents and products for each reaction. Understand the role of temperature. Differentiate between Sandmeyer and Gattermann. Recognize the synthetic utility of diazonium salts for preparing diverse aromatic compounds from amines.

Vyyuha Quick Recall

To remember the key replacement reactions of diazonium salts, think of 'CHIF-H-NO':

CuCl/HCl $ightarrow$ Chloro (Sandmeyer) HBr/CuBr $ightarrow$ Bromo (Sandmeyer) Iodide (KI) $ightarrow$ Iodo Fluoroboric acid ($HBF_4$) $ightarrow$ Fluoro (Balz-Schiemann) Hypophosphorous acid ($H_3PO_2$) or Hydroxy (water) $ightarrow$ Hydrogen or Hydroxyl Nitrite ($NaNO_2/Cu$) $ightarrow$ Nitro

For coupling reactions, remember: Phenols need PH-alkaline, Anilines need Acidic (weakly).