Reactions of Amines & Diazonium Salts
Decode the nucleophilic nature of the nitrogen lone pair. Master the distinction tests, the aniline protection trap, and the ultimate synthetic power of the diazonium group.
Module Focus: Nucleophilic Nitrogen & Leaving Groups
The chemistry of amines is dictated by the lone pair of electrons on nitrogen, making them strong nucleophiles. A large portion of NEET questions focuses on identifying whether an amine is $1^\circ$, $2^\circ$, or $3^\circ$ using specific chemical tests. Furthermore, when aromatic primary amines react with nitrous acid, they form Diazonium Salts containing the $-N_2^+$ group—the best leaving group in organic chemistry, enabling incredible synthetic transformations.
1. Distinction Tests for Amines
These two tests are universally used to differentiate between the classes of amines based on the number of replaceable hydrogen atoms attached to the nitrogen.
A. Carbylamine Reaction (Isocyanide Test)
Heating an amine with Chloroform ($CHCl_3$) and ethanolic KOH.
- Exclusively for $1^\circ$ Amines: Only primary aliphatic and aromatic amines give this test. $2^\circ$ and $3^\circ$ amines DO NOT react.
- Observation: Forms an isocyanide ($R-NC$), characterized by an intensely foul, offensive odor.
B. Hinsberg's Test
Uses Benzenesulfonyl chloride ($C_6H_5SO_2Cl$) to distinguish between $1^\circ$, $2^\circ$, and $3^\circ$ amines based on the solubility of the resulting sulfonamide in aqueous alkali ($NaOH$).
2. Reaction with Nitrous Acid ($HNO_2$)
Nitrous acid is highly unstable and is prepared in situ by reacting $NaNO_2$ with $HCl$. The reaction dictates whether a stable diazonium salt can be isolated.
They form aliphatic diazonium salts which are highly unstable even at low temperatures.
They instantly decompose to yield an Alcohol and massive effervescence of Nitrogen Gas ($N_2$).
They react at cold temperatures ($0 - 5^\circ\text{C}$ or $273 - 278\text{K}$) to form a stable Arenediazonium salt.
The diazonium ion is stabilized by resonance with the benzene ring.
3. Electrophilic Aromatic Substitution (EAS) of Aniline
The $-NH_2$ group is powerfully activating and strongly ortho/para directing due to the $+M$ effect of the nitrogen lone pair. This extreme reactivity leads to major synthetic problems.
If you add Bromine water ($Br_2/H_2O$) to aniline, the ring is so activated that it instantly exhaustively brominates at all ortho and para positions, yielding a white precipitate of 2,4,6-Tribromoaniline.
Direct nitration of aniline using conc. $HNO_3$ and conc. $H_2SO_4$ yields a surprising amount of m-Nitroaniline (47%), despite $-NH_2$ being an o/p director. Why?
Note: To get only p-Nitroaniline, you must use the Acetylation protection strategy described above!
4. Reactions of Diazonium Salts ($Ar-N_2^+X^-$)
The $-N_2^+$ group is an incredible leaving group (leaves as highly stable $N_2$ gas). This makes diazonium salts perfect for synthesizing a massive variety of substituted benzenes.
| Reaction Name | Reagents Added to $Ar-N_2^+Cl^-$ | Product Formed ($Ar-Z$) | Key Detail |
|---|---|---|---|
| Sandmeyer Reaction | $Cu_2Cl_2/HCl$ $Cu_2Br_2/HBr$ $CuCN/KCN$ |
Chlorobenzene Bromobenzene Cyanobenzene |
Uses Cuprous ($Cu^+$) salts. Better yield than Gattermann. |
| Gattermann Reaction | $Cu \text{ powder} / HCl$ $Cu \text{ powder} / HBr$ |
Chlorobenzene Bromobenzene |
Uses Copper powder instead of Cuprous salts. |
| Balz-Schiemann Reaction | 1. $HBF_4$ (Fluoroboric acid) 2. Heat ($\Delta$) |
Fluorobenzene | The only good way to introduce Fluorine to the ring. |
| Iodination | Warm with $KI_{(aq)}$ | Iodobenzene | Does not require any Copper catalyst! |
| Replacement by $-OH$ | Warm with $H_2O$ | Phenol | Temperature slightly > $5^\circ\text{C}$ required. |
Azo Coupling Reactions (Dyes)
The diazonium ion acts as a weak electrophile. It attacks highly activated aromatic rings (like Phenol or Aniline) at the para position to form extended conjugated systems called Azo compounds ($-N=N-$), which are intensely colored dyes.
Requires mildly alkaline medium (pH 9-10).
p-Hydroxyazobenzene
Requires mildly acidic medium (pH 4-5).
p-Aminoazobenzene
NEET Grand Test: Amines Part 2
15 High-Yield Questions testing Hinsberg solubility, EAS protection traps, and Diazonium name reactions.
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