Amines — Revision Notes

⚡ 30-Second Revision

Classification: — 1° (RNH$_2$), 2° (R$_2$NH), 3° (R$_3$N).
Basicity: — Lone pair on N. Aliphatic > NH$_3$ > Aromatic. Aqueous order for methyl: 2° > 1° > 3°.
Hoffmann Bromamide: — R-CONH$_2$ \xrightarrow{Br_2/NaOH} R-NH$_2$ (1 carbon less).
Gabriel Phthalimide: — For pure 1° aliphatic amines. Phthalimide \xrightarrow{KOH} K-phthalimide \xrightarrow{R-X} N-alkylphthalimide \xrightarrow{Hydrolysis} R-NH$_2$.
Carbylamine Test: — 1° amines only. R-NH$_2$ + CHCl$_3$ + KOH \xrightarrow{\Delta} R-NC (foul smell).
Hinsberg Test: — 1° amine (soluble in KOH), 2° amine (insoluble in KOH), 3° amine (no reaction).
**Nitrous Acid (NaNO$_2$/HCl, 0-5°C):**

* 1° Aliphatic: R-OH + N$_2$ gas. * 1° Aromatic: Ar-N$_2^+$Cl$^-$ (stable diazonium salt). * 2°: R$_2$N-N=O (yellow oily nitrosoamine). * 3°: No characteristic reaction (aliphatic), p-nitroso compound (aromatic).

2-Minute Revision

Amines are ammonia derivatives, classified as primary (RNH$_2$), secondary (R$_2$NH), or tertiary (R$_3$N) based on alkyl/aryl substitution. Their basicity stems from the lone pair on nitrogen. Aliphatic amines are generally stronger bases than ammonia due to the electron-donating inductive effect of alkyl groups.

Aromatic amines (like aniline) are weaker bases because the lone pair is delocalized into the aromatic ring via resonance. In aqueous solution, solvation effects influence basicity, leading to orders like 2° > 1° > 3° for methylamines.

Key preparation methods include reduction of nitro compounds (for aromatic amines), nitriles, and amides (using LiAlH$_4$). Gabriel phthalimide synthesis is specific for pure primary aliphatic amines, while Hoffmann bromamide degradation produces primary amines with one less carbon.

Distinguishing tests are crucial: the carbylamine reaction is unique to primary amines, producing a foul-smelling isocyanide. The Hinsberg test uses benzenesulphonyl chloride to differentiate based on solubility in alkali.

Reaction with nitrous acid (NaNO$_2$/HCl) also yields distinct products for 1°, 2°, and 3° amines, including N$_2$ gas for 1° aliphatic, stable diazonium salts for 1° aromatic, and yellow oily nitrosoamines for 2° amines.

5-Minute Revision

Amines, organic derivatives of ammonia, are categorized as primary (1°), secondary (2°), or tertiary (3°) depending on the number of alkyl or aryl groups attached to the nitrogen. The nitrogen atom is sp$^3$ hybridized, possessing a lone pair of electrons that confers basic and nucleophilic properties.

Basicity is a critical concept: aliphatic amines are stronger bases than ammonia due to the +I effect of alkyl groups, which increases electron density on nitrogen. Aromatic amines, like aniline, are weaker bases because the lone pair is delocalized into the aromatic ring via resonance, making it less available for protonation.

In aqueous solution, the basicity order for aliphatic amines is influenced by both inductive and solvation effects, often resulting in 2° > 1° > 3° (e.g., for methylamines).

Key preparation methods include:

1
Reduction of Nitro Compounds: — Ar-NO$_2$ \xrightarrow{Sn/HCl} Ar-NH$_2$ (e.g., nitrobenzene to aniline).
2
Reduction of Nitriles: — R-C≡N \xrightarrow{LiAlH_4 \text{ or } H_2/Ni} R-CH$_2$NH$_2$.
3
Reduction of Amides: — R-CONH$_2$ \xrightarrow{LiAlH_4} R-CH$_2$NH$_2$.
4
Gabriel Phthalimide Synthesis: — A specific method for pure 1° aliphatic amines. Phthalimide \xrightarrow{KOH} K-phthalimide \xrightarrow{R-X} N-alkylphthalimide \xrightarrow{Hydrolysis} R-NH$_2$.
5
Hoffmann Bromamide Degradation: — R-CONH$_2$ \xrightarrow{Br_2/NaOH} R-NH$_2$ (amine with one less carbon).

Important reactions and distinguishing tests:

Carbylamine Reaction: — Only 1° amines (aliphatic or aromatic) react with CHCl$_3$ and alcoholic KOH to form foul-smelling isocyanides (R-NC).
Hinsberg Test: — Uses benzenesulphonyl chloride. 1° amines form sulphonamides soluble in KOH. 2° amines form sulphonamides insoluble in KOH. 3° amines do not react.
**Reaction with Nitrous Acid (NaNO$_2$/HCl at 0-5°C):**

* 1° Aliphatic amines: Form unstable diazonium salts, decomposing to alcohols and N$_2$ gas. * 1° Aromatic amines: Form stable aromatic diazonium salts (Ar-N$_2^+$Cl$^-$), crucial for dye synthesis. * 2° amines: Form yellow oily N-nitrosoamines. * 3° amines: Form soluble salts (aliphatic) or p-nitroso compounds (aromatic).

For aromatic amines like aniline, the -NH$_2$ group is a powerful activating and ortho-para directing group for electrophilic substitution. However, its strong activation often leads to polysubstitution, necessitating protection (e.g., by acetylation to form acetanilide) before reactions like nitration or bromination.

Prelims Revision Notes

Amines: NEET Revision Notes

1. Classification & Structure:

1° Amine: — R-NH$_2$ (one R group, two H on N). Ex: Methylamine.
2° Amine: — R$_2$NH (two R groups, one H on N). Ex: Dimethylamine.
3° Amine: — R$_3$N (three R groups, no H on N). Ex: Trimethylamine.
Nitrogen is sp$^3$ hybridized, pyramidal geometry, with a lone pair.

2. Basicity of Amines:

Due to lone pair on N, acts as Lewis base (electron donor) and Brønsted base (proton acceptor).
Factors:

* Inductive Effect (+I): Alkyl groups donate electrons, increasing electron density on N, enhancing basicity. (3° > 2° > 1° > NH$_3$ in gas phase). * Resonance Effect: In aromatic amines (e.g.

, aniline), lone pair is delocalized into ring, reducing availability, thus decreasing basicity. (Aliphatic > NH$_3$ > Aromatic). * Solvation Effect (Aqueous Solution): Stability of conjugate acid (RNH$_3^+$) by H-bonding with water.

More H-bonds, more stable, stronger base. * Methylamines: (CH$_3$)$_2$NH (2°) > CH$_3$NH$_2$ (1°) > (CH$_3$)$_3$N (3°) > NH$_3$. * Ethylamines: (C$_2$H$_5$)$_2$NH (2°) > (C$_2$H$_5$)$_3$N (3°) > C$_2$H$_5$NH$_2$ (1°) > NH$_3$.

3. Preparation Methods:

Reduction of Nitro Compounds: — Ar-NO$_2$ \xrightarrow{Sn/HCl \text{ or } Fe/HCl \text{ or } H_2/Pd} Ar-NH$_2$.
Ammonolysis of Alkyl Halides: — R-X + NH$_3$ \rightarrow R-NH$_2$ (can form 2°, 3°, 4° salts; excess NH$_3$ favors 1°).
Reduction of Nitriles: — R-C≡N \xrightarrow{LiAlH_4 \text{ or } H_2/Ni} R-CH$_2$NH$_2$ (1° amine, same carbons).
Reduction of Amides: — R-CONH$_2$ \xrightarrow{LiAlH_4} R-CH$_2$NH$_2$ (1° amine, same carbons).
Gabriel Phthalimide Synthesis: — For pure 1° aliphatic amines only. Phthalimide \xrightarrow{KOH} K-phthalimide \xrightarrow{R-X} N-alkylphthalimide \xrightarrow{Hydrolysis} R-NH$_2$.
Hoffmann Bromamide Degradation: — R-CONH$_2$ \xrightarrow{Br_2/NaOH} R-NH$_2$ (1° amine, one carbon less).

4. Chemical Reactions & Distinguishing Tests:

Acylation: — 1° and 2° amines react with acid chlorides/anhydrides to form amides. 3° amines do not react.
Carbylamine Reaction (Isocyanide Test): — Only 1° amines (aliphatic & aromatic) react with CHCl$_3$ + alc. KOH to form foul-smelling isocyanides (R-NC).
**Hinsberg Test (Benzenesulphonyl chloride, C$_6$H$_5$SO$_2$Cl):**

* 1° Amine: Forms N-alkylbenzenesulphonamide (soluble in KOH due to acidic H). * 2° Amine: Forms N,N-dialkylbenzenesulphonamide (insoluble in KOH, no acidic H). * 3° Amine: No reaction.

**Reaction with Nitrous Acid (NaNO$_2$/HCl, 0-5°C):**

* 1° Aliphatic Amine: Forms unstable diazonium salt, decomposes to R-OH + N$_2$ gas (effervescence). * 1° Aromatic Amine: Forms stable diazonium salt (Ar-N$_2^+$Cl$^-$), used in coupling reactions. * 2° Amine: Forms yellow oily N-nitrosoamine (R$_2$N-N=O). * 3° Aliphatic Amine: Forms soluble N-nitrosated ammonium salt. * 3° Aromatic Amine: Undergoes electrophilic substitution at para-position to form p-nitroso compound.

Electrophilic Substitution in Aniline: — NH$_2$ is strong activating, ortho-para director. Requires acetylation (protection) to control reactivity (e.g., for monobromination or nitration).

Vyyuha Quick Recall

Hoffmann Gabriel Carbylamine Nitrous Hinsberg: How Good Chemists Never Hesitate!

Hoffmann: Amide \rightarrow Amine (-1C)
Gabriel: Phthalimide \rightarrow 1° Aliphatic Amine
Carbylamine: 1° Amine \rightarrow Foul Smell (Isocyanide)
Nitrous: Distinguishes 1°, 2°, 3° Amines (N$_2$ gas, stable diazonium, yellow oil)
Hinsberg: Distinguishes 1°, 2°, 3° Amines (Soluble, Insoluble, No reaction)