Organic Compounds Containing Nitrogen — Revision Notes

⚡ 30-Second Revision

Amines — $RNH_2$ ($1^\circ$), $R_2NH$ ($2^\circ$), $R_3N$ ($3^\circ$). Basic due to lone pair on N.
Basicity Order (aqueous methylamines) — $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$.
Aromatic Amines — Less basic than aliphatic due to resonance.
Hofmann Bromamide Degradation — $RCONH_2 \xrightarrow{Br_2/NaOH} RNH_2$ (1 C less).
Gabriel Phthalimide Synthesis — For pure $1^\circ$ aliphatic amines from $R-X$.
Carbylamine Test — For $1^\circ$ amines, $RNH_2 + CHCl_3 + KOH \rightarrow RNC$ (foul smell).
Hinsberg's Test — $1^\circ$ amine \rightarrow alkali soluble sulfonamide; $2^\circ$ amine \rightarrow alkali insoluble sulfonamide; $3^\circ$ amine \rightarrow no reaction.
Nitro Compounds — $R-NO_2$. Aromatic nitro groups are meta-directing, deactivating.
Nitriles — $R-C\equiv N$. Hydrolysis to $RCOOH$, reduction to $RCH_2NH_2$.
Diazonium Salts — $Ar-N_2^+X^-$. Unstable at RT ($0-5^\circ C$ for preparation).
Sandmeyer Reaction — $Ar-N_2^+Cl^- \xrightarrow{Cu_2X_2/HX} Ar-X$ ($X=Cl, Br, CN$).
Gattermann Reaction — $Ar-N_2^+Cl^- \xrightarrow{Cu/HX} Ar-X$ ($X=Cl, Br$).
Coupling Reaction — $Ar-N_2^+Cl^- + Ar'-OH \rightarrow Ar-N=N-Ar'$ (azo dye).

2-Minute Revision

Organic compounds containing nitrogen are vital, primarily categorized into amines, nitro compounds, nitriles, and diazonium salts. Amines are basic due to the nitrogen's lone pair, with their basicity in aqueous solution influenced by inductive effects, solvation, and steric hindrance, leading to a specific order (e.

g., secondary > primary > tertiary for methylamines). Aromatic amines are weaker bases due to resonance. Key preparation methods for amines include reduction of nitro compounds, nitriles, and amides (Hofmann bromamide degradation for chain shortening), and Gabriel phthalimide synthesis for pure primary aliphatic amines.

Distinguishing tests like Carbylamine (for primary amines) and Hinsberg's (for primary, secondary, tertiary) are crucial. Nitro compounds are electron-withdrawing and meta-directing. Nitriles are versatile, converting to carboxylic acids or amines.

Aromatic diazonium salts, formed from primary aromatic amines at $0-5^\circ C$, are highly reactive intermediates. They undergo replacement reactions (Sandmeyer, Gattermann) to introduce halogens, cyano, or hydroxyl groups, and coupling reactions to form vibrant azo dyes.

Understanding these reactions, their reagents, and the factors affecting basicity is paramount for NEET.

5-Minute Revision

Organic compounds containing nitrogen are a cornerstone of organic chemistry, encompassing amines, nitro compounds, nitriles, and diazonium salts. Amines are classified as primary ($RNH_2$), secondary ($R_2NH$), or tertiary ($R_3N$) based on the number of organic groups attached to nitrogen.

Their basicity arises from the lone pair on nitrogen. In aqueous solution, the basicity order is a balance of inductive effect (electron-donating alkyl groups increase basicity), solvation effect (stabilization of the conjugate acid by hydrogen bonding with water), and steric hindrance.

For methylamines, the order is typically $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$. Aromatic amines, like aniline, are significantly less basic than aliphatic amines because the nitrogen's lone pair is delocalized into the aromatic ring via resonance, making it less available for protonation.

Key Reactions and Preparations:

Preparation of Amines — Reduction of nitro compounds ($R-NO_2 \xrightarrow{Sn/HCl} R-NH_2$), reduction of nitriles ($R-C\equiv N \xrightarrow{LiAlH_4} R-CH_2NH_2$), reduction of amides ($R-CONH_2 \xrightarrow{LiAlH_4} R-CH_2NH_2$).
Hofmann Bromamide Degradation — A specific method to prepare primary amines from amides with one carbon atom less: $R-CONH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O$.
Gabriel Phthalimide Synthesis — Ideal for preparing pure primary aliphatic amines, avoiding over-alkylation. Involves phthalimide, KOH, alkyl halide, and then hydrolysis or hydrazinolysis.
Carbylamine Reaction (Isocyanide Test) — A distinguishing test for primary amines (aliphatic and aromatic). Reaction with $CHCl_3$ and alcoholic KOH produces foul-smelling isocyanides ($R-NC$). Secondary and tertiary amines do not react.
Hinsberg's Test — Differentiates $1^\circ$, $2^\circ$, and $3^\circ$ amines using benzenesulphonyl chloride. $1^\circ$ amines form an alkali-soluble sulfonamide; $2^\circ$ amines form an alkali-insoluble sulfonamide; $3^\circ$ amines do not react with the reagent.

Diazonium Salts: Aromatic diazonium salts ($Ar-N_2^+X^-$) are formed by diazotization of primary aromatic amines with $NaNO_2/HCl$ at $0-5^\circ C$. They are highly unstable at room temperature. They are versatile for:

Replacement Reactions — Replacing the $-N_2^+$ group with $-Cl, -Br, -CN$ (Sandmeyer reaction with $Cu_2X_2/HX$ or Gattermann reaction with $Cu/HX$), $-I$ (with $KI$), $-F$ (Balz-Schiemann reaction), $-OH$ (warming with $H_2O$), or $-H$ (with $H_3PO_2$ or ethanol).
Coupling Reactions — Reaction with electron-rich aromatic compounds (phenols, anilines) to form brightly colored azo dyes ($Ar-N=N-Ar'$).

Nitro Compounds: The nitro group ($-NO_2$) is a strong electron-withdrawing group, deactivating aromatic rings towards electrophilic substitution and directing to the meta-position.

Nitriles: The $-C\equiv N$ group can be hydrolyzed to carboxylic acids or reduced to primary amines.

Prelims Revision Notes

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Amines Classification — Primary ($RNH_2$), Secondary ($R_2NH$), Tertiary ($R_3N$). Based on number of alkyl/aryl groups on nitrogen.
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Basicity of Amines — Due to lone pair on N. Electron-donating groups increase basicity. Electron-withdrawing groups decrease basicity.

* Gas Phase: $3^\circ > 2^\circ > 1^\circ > NH_3$ (only inductive effect). * Aqueous Phase (Methylamines): $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$ (inductive + solvation + steric). * Aqueous Phase (Ethylamines): $(C_2H_5)_2NH > (C_2H_5)_3N > (C_2H_5)NH_2 > NH_3$. * Aromatic Amines: Aniline is weaker base than aliphatic amines due to resonance delocalization of lone pair.

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Preparation of Amines

* Reduction of Nitro Compounds: $R-NO_2 \xrightarrow{Sn/HCl \text{ or } Fe/HCl \text{ or } H_2/Pd} R-NH_2$. * Ammonolysis of Alkyl Halides: $R-X + NH_3 \rightarrow RNH_2 + R_2NH + R_3N + R_4N^+X^-$ (mixture).

* Gabriel Phthalimide Synthesis: For pure $1^\circ$ aliphatic amines. Phthalimide $\xrightarrow{KOH} K^+\text{Phthalimide} \xrightarrow{R-X} N-R\text{Phthalimide} \xrightarrow{NH_2NH_2 \text{ or } H_2O/H^+} RNH_2$.

* Hofmann Bromamide Degradation: $RCONH_2 \xrightarrow{Br_2/NaOH} RNH_2$ (product has one carbon less). * Reduction of Nitriles: $R-C\equiv N \xrightarrow{LiAlH_4 \text{ or } H_2/Ni} R-CH_2NH_2$.

* Reduction of Amides: $RCONH_2 \xrightarrow{LiAlH_4} RCH_2NH_2$ (no carbon loss).

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Reactions of Amines

* Acylation: $RNH_2 + R'COCl \rightarrow RNHCOR'$ (amide). * Alkylation: $RNH_2 + R'X \rightarrow R_2NH \rightarrow R_3N \rightarrow R_4N^+X^-$. * Carbylamine Reaction (Isocyanide Test): $1^\circ$ amines only.

$RNH_2 + CHCl_3 + 3KOH \xrightarrow{\text{heat}} RNC$ (foul smell). * Hinsberg's Test: Reagent: Benzenesulphonyl chloride ($C_6H_5SO_2Cl$). * $1^\circ$ amine: Forms N-alkylbenzenesulphonamide (soluble in alkali).

* $2^\circ$ amine: Forms N,N-dialkylbenzenesulphonamide (insoluble in alkali). * $3^\circ$ amine: No reaction with reagent, but dissolves in HCl. * **Reaction with Nitrous Acid ($HNO_2$, from $NaNO_2/HCl$)**: * $1^\circ$ aliphatic amine: Forms alcohol ($R-OH$) with $N_2$ gas evolution.

* $1^\circ$ aromatic amine: Forms diazonium salt ($Ar-N_2^+X^-$) at $0-5^\circ C$. * $2^\circ$ amines (aliphatic/aromatic): Forms N-nitrosamines ($R_2N-NO$, yellow oily). * $3^\circ$ amines (aliphatic): Forms trialkylammonium nitrite salt.

* $3^\circ$ amines (aromatic): Undergo electrophilic substitution at para position to form p-nitrosoaniline derivative.

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Diazonium Salts ($Ar-N_2^+X^-$)

* Preparation: Diazotization of $1^\circ$ aromatic amine with $NaNO_2/HCl$ at $0-5^\circ C$. * Stability: Unstable above $5^\circ C$. * Replacement Reactions: * Sandmeyer: $Ar-N_2^+Cl^- \xrightarrow{Cu_2Cl_2/HCl} Ar-Cl$; $Ar-N_2^+Cl^- \xrightarrow{Cu_2Br_2/HBr} Ar-Br$; $Ar-N_2^+Cl^- \xrightarrow{CuCN/KCN} Ar-CN$.

* Gattermann: $Ar-N_2^+Cl^- \xrightarrow{Cu/HCl} Ar-Cl$; $Ar-N_2^+Cl^- \xrightarrow{Cu/HBr} Ar-Br$. * With $KI$: $Ar-N_2^+Cl^- \xrightarrow{KI} Ar-I$. * With $H_2O$: $Ar-N_2^+Cl^- \xrightarrow{H_2O, \text{warm}} Ar-OH$ (phenol).

* With $H_3PO_2$ (hypophosphorous acid) or ethanol: $Ar-N_2^+Cl^- \rightarrow Ar-H$. * Coupling Reactions: With phenols (mildly alkaline medium) or anilines (mildly acidic medium) to form azo dyes ($Ar-N=N-Ar'$).

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Nitro Compounds — $R-NO_2$. Aromatic nitro groups are strong deactivating and meta-directing groups for electrophilic substitution.
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Nitriles — $R-C\equiv N$. Hydrolysis to amides ($RCONH_2$) then carboxylic acids ($RCOOH$). Reduction to primary amines ($RCH_2NH_2$).

Vyyuha Quick Recall

To remember the basicity order of methylamines in aqueous solution: Don't Make Three Allies. (Dimethylamine > Methylamine > Trimethylamine > Ammonia)