Amines - Class 12 JEE Revision Notes

Definition and Classification of Amines

• Definition: Amines are organic derivatives of ammonia ($NH_3$) in which one or more hydrogen atoms are replaced by alkyl or aryl groups.
• General Structures:
  • Primary ($1^\circ$): $R-NH_2$
  • Secondary ($2^\circ$): $R_2NH$ or $R-NH-R'$
  • Tertiary ($3^\circ$): $R_3N$ or $R-N(R')-R''$
  • Quaternary ammonium salt: $[R_4N]^+X^-$
• Structural Classification Principle: The classification of an amine ($1^\circ$, $2^\circ$, or $3^\circ$) depends strictly on the number of carbon groups directly attached to the nitrogen atom, not on the nature or degree of the carbon atom attached to the functional group.
• Representative Examples:
  • $CH_3NH_2$ : Primary amine ($1^\circ$)
  • $(CH_3)_2NH$ : Secondary amine ($2^\circ$)
  • $(CH_3)_3N$ : Tertiary amine ($3^\circ$)
  • $[CH_3]_4N^+Cl^-$ : Quaternary ammonium salt
• Classification Based on the Nature of Carbon Groups:
  • Aliphatic Amines: Nitrogen is attached exclusively to alkyl groups. Examples include methylamine ($CH_3NH_2$), ethylamine ($C_2H_5NH_2$), and dimethylamine ($(CH_3)_2NH$).
  • Aromatic Amines: Nitrogen is directly attached to at least one aromatic benzene ring. Example: aniline ($C_6H_5NH_2$). Aniline is classified as an aromatic amine specifically because its nitrogen atom is bonded directly to the aromatic ring.

Nomenclature of Amines

• Common Nomenclature System:
  • Named by listing the alkyl groups attached to nitrogen in alphabetical order, followed by the suffix "amine".
  • Examples: methylamine, ethylamine, dimethylamine, diethylamine, ethylmethylamine.
• IUPAC Nomenclature System:
  • Named by replacing the final terminal "e" of the parent alkane name with the suffix "-amine".
  • Simple aliphatic examples: methanamine, ethanamine, propan-1-amine, propan-2-amine.
  • Substituents attached directly to the nitrogen atom are designated using the locant prefix $N-$ (e.g., $CH_3NHCH_2CH_3$ is named $N$-methylethanamine).

Structure and Hybridization of Amines

• Hybridization: The nitrogen atom in a simple amine is approximately $sp^3$ hybridized.
• Molecular Geometry and Electronic Configuration:
  • Nitrogen forms three $\sigma$ bonds and possesses one non-bonding lone pair of electrons.
  • The spatial arrangement of electron pairs gives amines an overall pyramidal geometry.
• Origin of Basicity: The unshared lone pair of electrons on the nitrogen atom enables amines to act as Lewis bases and Brønsted-Lowry bases.
• Pyramidal Inversion: Ordinary aliphatic amines undergo rapid pyramidal inversion (umbrella inversion) at room temperature.

Preparation Methods of Amines

• Reduction of Nitro Compounds:
  • Reaction Pathway: $R-NO_2 \rightarrow R-NH_2$
  • Suitable Reducing Agents: $H_2/Ni$, $Sn/HCl$, $Fe/HCl$, or $Zn/HCl$
  • Specific Example: Nitrobenzene is reduced to aniline ($C_6H_5NO_2 \rightarrow C_6H_5NH_2$).
  • Acidic Medium Behavior: When reduction is carried out in acidic medium (e.g., $Sn/HCl$ or $Fe/HCl$), the amine initially forms its corresponding ammonium salt: $C_6H_5NH_2 + HCl \rightarrow C_6H_5NH_3^+Cl^-$. Subsequent addition of a strong base liberates the free amine.
• Ammonolysis of Alkyl Halides:
  • Reaction Pathway: $R-X + NH_3 \rightarrow R-NH_2 + HX$ via nucleophilic substitution ($S_N2$ mechanism).
  • Sequential Reaction Problem: The formed primary amine acts as a nucleophile and undergoes further reaction with remaining alkyl halide, producing a mixture of secondary ($2^\circ$), tertiary ($3^\circ$) amines, and ultimately quaternary ammonium salts ($[R_4N]^+X^-$).
  • Analytical Evaluation: Ammonolysis is not a suitable method for synthesizing pure primary amines.
• Reduction of Nitriles:
  • Reaction Pathway: $R-C\equiv N \rightarrow R-CH_2NH_2$
  • Reagents: Lithium aluminium hydride ($LiAlH_4$) or catalytic hydrogenation ($H_2/Ni$).
  • Specific Example: Acetonitrile reduces to ethanamine ($CH_3CN \rightarrow CH_3CH_2NH_2$).
  • Carbon Count Conservation: The nitrile carbon atom is converted into the $CH_2$ group attached directly to $NH_2$; total carbon count is strictly preserved.
• Reduction of Amides:
  • Reaction Pathway: $RCONH_2 \rightarrow RCH_2NH_2$
  • Reagent: $LiAlH_4$
  • Specific Example: Acetamide reduces to ethanamine ($CH_3CONH_2 \rightarrow CH_3CH_2NH_2$).
  • Retention of Carbon Count: Both nitrile reduction and amide reduction produce primary amines with retention of the original carbon chain length.
• Gabriel Phthalimide Synthesis:
  • Reaction Sequence:
  1. Phthalimide reacts with potassium hydroxide ($KOH$) to generate potassium phthalimide.
  2. Potassium phthalimide undergoes nucleophilic substitution ($S_N2$) with a primary alkyl halide ($R-X$) to form $N$-alkylphthalimide.
  3. Alkaline or acidic hydrolysis of $N$-alkylphthalimide liberates a pure primary aliphatic amine ($R-NH_2$).
  • Selectivity & Scope: Designed exclusively for preparing pure primary aliphatic amines.
  • Limitations: Primary aromatic amines such as aniline cannot be prepared by this method because aryl halides do not undergo the necessary $S_N2$ nucleophilic substitution with the phthalimide anion. Best suited for primary alkyl halides.
• Hofmann Bromamide Degradation:
  • General Reaction Equation: $RCONH_2 + Br_2 + 4NaOH \rightarrow RNH_2 + Na_2CO_3 + 2NaBr + 2H_2O$
  • Specific Example: Acetamide yields methylamine ($CH_3CONH_2 \rightarrow CH_3NH_2$).
  • Structural Outcome: The product primary amine contains exactly one carbon atom less than the starting primary amide.
  • Reaction Comparison Cues:
  • $RCONH_2 \xrightarrow{Br_2/NaOH} RNH_2$ (carbon chain decreases by 1 carbon atom).
  • $RCN \xrightarrow{LiAlH_4} RCH_2NH_2$ (carbon chain length is strictly preserved).

Basicity of Amines and Factors Affecting Basicity

• Fundamental Origin of Basicity:
  • Amines behave as basic compounds because nitrogen possesses an unshared lone pair of electrons available for accepting a proton ($H^+$).
  • Protonation Equilibrium: $RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-$
  • Direct Protonation: $RNH_2 + H^+ \rightarrow RNH_3^+$
  • Rule: Greater availability of the nitrogen lone pair for protonation directly corresponds to higher basicity.
• Gas-Phase Basicity and Inductive Effect ($+I$ Effect):
  • Alkyl groups release electron density toward nitrogen via the inductive ($+I$) effect, increasing electron density on the nitrogen atom.
  • In the gas phase (where solvation is absent), basicity increases monotonically with alkyl substitution: $3^\circ > 2^\circ > 1^\circ > NH_3$
• Aqueous-Phase Basicity of Aliphatic Amines:
  • In aqueous solutions, basicity depends on a complex interplay between three factors: inductive effect ($+I$), steric hindrance, and hydration/solvation energy of the conjugate ammonium cation ($RNH_3^+$).
  • Solvation Factor: Extent of hydrogen bonding with water stabilizes the ammonium cation. Smaller, less sterically hindered ammonium ions ($1^\circ$ and $2^\circ$) undergo more effective solvation than bulky $3^\circ$ ammonium ions.
  • Standard JEE Aqueous Basicity Order for simple methyl and ethyl substituted amines: $2^\circ > 1^\circ > 3^\circ > NH_3$
  • Warning: Do not blindly apply the gas-phase order ($3^\circ > 2^\circ > 1^\circ$) in aqueous solutions. Always evaluate solvation, inductive, and steric effects simultaneously.
• Comparative Basicity: Aniline vs. Ammonia:
  • In aniline ($C_6H_5NH_2$), the nitrogen lone pair is delocalized into the aromatic $\pi$-system of the benzene ring through resonance interactions.
  • Because the lone pair is involved in resonance, it is significantly less available to accept a proton ($H^+$).
  • Result: Aniline is substantially weaker as a base than ammonia ($NH_3$).
• Effect of Ring Substituents on Aniline Basicity:
  • Electron-Donating Groups (EDGs) such as $-CH_3$ and $-OCH_3$ increase electron density on the ring and nitrogen, thereby increasing basicity relative to aniline.
  • Electron-Withdrawing Groups (EWGs) such as $-NO_2$, $-CN$, $-COOH$, and $-CHO$ withdraw electron density, thereby decreasing basicity relative to aniline.
  • Nitro group ($-NO_2$) strongly decreases basicity via combined strong inductive ($-I$) and resonance ($-M$) electron withdrawal.
• Positional Substituent Effects (Ortho, Meta, Para):
  • Substituents at the ortho and para positions interact strongly with the amino group via resonance ($\pm M$).
  • Substituents at the meta position cannot participate in direct resonance interaction with the amino group and exert their influence primarily via inductive ($\pm I$) effects.
  • JEE Strategy: Evaluate basicity by identifying group electronic nature (EDG/EWG), inductive vs resonance contribution, ring position, and steric effects.

Physical Properties of Amines

• Intermolecular Hydrogen Bonding:
  • Primary ($1^\circ$) and secondary ($2^\circ$) amines contain $N-H$ bonds, allowing them to form intermolecular hydrogen bonds.
  • Tertiary ($3^\circ$) amines lack $N-H$ bonds and cannot act as hydrogen-bond donors to themselves, although they can act as hydrogen-bond acceptors via nitrogen's lone pair.
• Boiling Point Trends:
  • For compounds of comparable molecular mass, primary and secondary amines have lower boiling points than corresponding alcohols because the $N-H\cdots N$ hydrogen bond is significantly weaker than the $O-H\cdots O$ hydrogen bond in alcohols.
• Solubility in Water:
  • Lower aliphatic amines are appreciably soluble in water because they readily form hydrogen bonds with water molecules.
  • Water solubility decreases systematically with increasing molecular weight as the hydrophobic hydrocarbon portion grows larger.
  • Aromatic amines (such as aniline) are substantially less soluble in water than small aliphatic amines due to the large non-polar benzene ring.

Chemical Reactions of Amines

• Reaction with Mineral Acids:
  • $RNH_2 + HCl \rightarrow RNH_3^+Cl^-$
  • The resulting alkylammonium salts are generally water-soluble ionic solids.
  • Treatment of the ammonium salt with a strong inorganic base regenerates the original free amine.
• Alkylation:
  • Amines react with alkyl halides via nucleophilic substitution: $RNH_2 + R'X \rightarrow RNHR' + HX$
  • Sequential alkylation proceeds until quaternary ammonium salts are formed: $R_3N + R'X \rightarrow [RR'_3N]^+X^-$
• Acylation:
  • Primary ($1^\circ$) and secondary ($2^\circ$) amines react with acid chlorides or acid anhydrides to yield amides.
  • Reaction with primary amine: $RNH_2 + R'COCl \rightarrow R'CONHR + HCl$
  • Reaction with secondary amine: $R_2NH + R'COCl \rightarrow R'CONR_2 + HCl$
  • Tertiary ($3^\circ$) amines do not undergo acylation because they lack an $N-H$ hydrogen atom required for the elimination of $HX$.
  • Essential Condition: Acylation strictly requires at least one $N-H$ bond in the starting amine.
• Carbylamine Reaction (Isocyanide Test):
  • Equation: $RNH_2 + CHCl_3 + 3KOH \rightarrow RNC + 3KCl + 3H_2O$
  • Reagents: Chloroform ($CHCl_3$) and alcoholic potassium hydroxide ($KOH$).
  • Specificity: Positive ONLY for primary amines (both aliphatic and aromatic primary amines).
  • Test Diagnostic: Secondary ($2^\circ$) and tertiary ($3^\circ$) amines give a negative test.
  • Product Characteristics: Yields an isocyanide ($RNC$) characterized by an extremely offensive, foul odor.
• Hinsberg Test:
  • Reagent: Benzenesulfonyl chloride ($C_6H_5SO_2Cl$).
  • Primary Amines ($1^\circ$):
  • React to form an $N$-alkylbenzenesulfonamide possessing an acidic $N-H$ proton.
  • Dissolves readily in aqueous alkali ($NaOH$ or $KOH$) to form a soluble salt.
  • Acidification of this alkaline solution reprecipitates the insoluble sulfonamide.
  • Secondary Amines ($2^\circ$):
  • React to form an $N,N$-dialkylbenzenesulfonamide lacking an acidic $N-H$ proton.
  • Insoluble in aqueous alkali.
  • Tertiary Amines ($3^\circ$):
  • Do not react with benzenesulfonyl chloride due to absence of $N-H$ bonds.
  • Remain insoluble in aqueous alkali, but dissolve readily in dilute aqueous mineral acid due to ammonium salt formation.
• Hinsberg Test Summary Table:
  • $1^\circ$ Amine $\rightarrow$ Forms sulfonamide with acidic $N-H$ $\rightarrow$ Soluble in alkali
  • $2^\circ$ Amine $\rightarrow$ Forms sulfonamide without $N-H$ $\rightarrow$ Insoluble in alkali
  • $3^\circ$ Amine $\rightarrow$ No reaction to form sulfonamide $\rightarrow$ Insoluble in alkali; dissolves in dilute acid
• Reaction with Nitrous Acid ($HNO_2$):
  • Generation of Reagent: Nitrous acid is unstable and generated in situ using sodium nitrite and hydrochloric acid: $NaNO_2 + HCl \rightarrow HNO_2 + NaCl$
  • Primary Aliphatic Amines:
  • React with $HNO_2$ to produce highly unstable aliphatic diazonium salts.
  • Diazonium salts decompose rapidly at room temperature, evolving nitrogen gas ($N_2$) quantitatively and forming alcohols alongside alkenes and alkyl halides.
  • Diagnostic Signal: Rapid evolution of $N_2$ gas bubbles.
  • Primary Aromatic Amines (Diazotization):
  • $C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5\,^\circ\text{C}} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$
  • Forms stable benzenediazonium chloride in aqueous solution.
  • Critical Condition: Must be conducted strictly at low temperatures ($0-5\,^\circ\text{C}$) to prevent thermal decomposition of the diazonium ion.
  • Secondary Amines (Aliphatic & Aromatic):
  • React with $HNO_2$ to form yellow, oily $N$-nitrosamines: $R_2NH + HNO_2 \rightarrow R_2N-N=O + H_2O$
  • Tertiary Amines:
  • Lack an $N-H$ bond; in cold aqueous acidic media, they dissolve to form soluble ammonium nitrite salts.

Reactions and Transformations of Diazonium Salts

• General Structure of Aryl Diazonium Salts: $Ar-N_2^+ X^-$ (e.g., benzenediazonium chloride $C_6H_5N_2^+Cl^-$).
• Temperature Sensitivity: Aryl diazonium salts are stable in cold aqueous solution ($0-5\,^\circ\text{C}$), but decompose at higher temperatures to yield phenols and nitrogen gas.
• Sandmeyer Reactions:
  • Substitution by Chlorine: $ArN_2^+Cl^- + CuCl \rightarrow ArCl + N_2$
  • Substitution by Bromine: $ArN_2^+Cl^- + CuBr \rightarrow ArBr + N_2$
  • Substitution by Cyanide: $ArN_2^+Cl^- + CuCN \rightarrow ArCN + N_2$
  • Key Reagents: Uses Copper(I) salts ($CuCl$, $CuBr$, $CuCN$).
• Gattermann Reactions:
  • Substitution by Chlorine: $ArN_2^+Cl^- \xrightarrow{HCl/Cu} ArCl + N_2$
  • Substitution by Bromine: $ArN_2^+Cl^- \xrightarrow{HBr/Cu} ArBr + N_2$
  • Sandmeyer vs Gattermann Distinction: Sandmeyer reactions employ pre-formed Copper(I) salts ($CuX$), whereas Gattermann reactions employ metallic copper powder ($Cu$) alongside halogen acid ($HX$).
• Replacement by Iodine:
  • $ArN_2^+Cl^- + KI \rightarrow ArI + N_2 + KCl$
  • Process Note: Does not require copper powder or copper salts; warming directly with potassium iodide ($KI$) is sufficient.
• Balz-Schiemann Reaction (Fluorine Insertion):
  • Reaction Pathway: $ArN_2^+Cl^- + HBF_4 \rightarrow ArN_2^+BF_4^- \xrightarrow{\text{heat}} ArF + BF_3 + N_2$
  • Reagents: Fluoroboric acid ($HBF_4$) followed by dry heating of the isolated diazonium fluoroborate intermediate.
• Replacement by Hydroxyl Group ($-OH$):
  • $ArN_2^+Cl^- + H_2O \xrightarrow{\text{warm}} ArOH + N_2 + HCl$
  • Provides a direct synthetic route from aniline to phenol.
• Replacement by Hydrogen ($-H$) (Deamination):
  • $ArN_2^+ \xrightarrow{H_3PO_2/H_2O} ArH + N_2 + H_3PO_3$
  • Reducing Agent: Hypophosphorous acid ($H_3PO_2$) or ethanol ($CH_3CH_2OH$).
  • Synthetic Utility: Allows temporary introduction of the amino group for ring orientation followed by complete removal.
• Electrophilic Coupling Reactions (Azo Dye Formation):
  • Mechanism: Aryl diazonium ions act as weak electrophiles and react with highly activated aromatic systems such as phenols and aromatic amines.
  • Structural Feature: Products contain the extended conjugated azo linkage $-N=N-$ ($Ar-N=N-Ar'$).
  • Optical Properties: Azo compounds exhibit intense colors (yellow, orange, red) and serve as industrial dyes.
  • Coupling with Phenol:
  • Diazonium salt reacts with phenol in weakly alkaline medium (pH 9–10) to form $p$-hydroxyazobenzene (orange dye).
  • Substitution occurs strictly at the para position relative to the $-OH$ group.
  • Coupling with Aniline:
  • Diazonium salt reacts with aniline in weakly acidic medium (pH 4–5) to form $p$-aminoazobenzene (yellow dye).
  • Substitution occurs strictly at the para position relative to the $-NH_2$ group.

Ring Substitution and Protection Reactions of Aniline

• Direct Bromination of Aniline:
  • Equation: $C_6H_5NH_2 + 3Br_2 \rightarrow C_6H_2Br_3NH_2 + 3HBr$
  • Product: 2,4,6-tribromoaniline (isolable as a white precipitate).
  • Activation Factor: The $-NH_2$ group strongly activates the aromatic ring through resonance ($+M$). Bromination occurs rapidly at all available ortho and para positions simultaneously without requiring a Lewis acid catalyst such as $FeBr_3$.
• Protecting the Amino Group via Acetylation:
  • Reaction Equation: $C_6H_5NH_2 + CH_3COCl \rightarrow C_6H_5NHCOCH_3 + HCl$
  • Conversion: Aniline is converted into acetanilide.
  • Chemical Rationale: In acetanilide, the lone pair of nitrogen is delocalized into the adjacent carbonyl group ($C=O$) via resonance ($-NH-C(=O)CH_3$). This decreases electron donation into the aromatic ring, moderating its activating strength.
  • Protection Synthetic Sequence:
  1. $C_6H_5NH_2 + CH_3COCl \rightarrow C_6H_5NHCOCH_3$ (Acetylation / Protection)
  2. $C_6H_5NHCOCH_3 + E^+ \rightarrow \text{para-substituted acetanilide}$ (Controlled Monosubstitution)
  3. $\text{para-substituted acetanilide} + H_2O/H^+ \rightarrow \text{para-substituted aniline}$ (Hydrolysis / Deprotection)
• Effect of Acidic Medium on Aniline Orientational Directivity:
  • $C_6H_5NH_2 + HCl \rightarrow C_6H_5NH_3^+Cl^-$
  • In strongly acidic media, aniline undergoes protonation to form the anilinium cation ($-NH_3^+$).
  • Directivity Shift: While free $-NH_2$ is strongly activating and ortho/para-directing, the positively charged anilinium ion ($-NH_3^+$) lacks a lone pair and acts as a strongly deactivating, meta-directing substituent.

Reagent Mapping and Reaction Flowcharts

• Summary Reagent Map:
  • $Sn/HCl$ or $Fe/HCl$: Converts $-NO_2 \rightarrow -NH_2$
  • $LiAlH_4$: Converts $-CN \rightarrow -CH_2NH_2$
  • $LiAlH_4$: Converts $-CONH_2 \rightarrow -CH_2NH_2$
  • $Br_2/KOH$: Converts $-CONH_2 \rightarrow -NH_2$ (Hofmann degradation; loses 1 carbon)
  • $CHCl_3 + \text{alcoholic } KOH$: Carbylamine test (selective for $1^\circ$ amines)
  • $C_6H_5SO_2Cl$: Hinsberg reagent (differentiates $1^\circ$, $2^\circ$, $3^\circ$ amines)
  • $NaNO_2 + HCl, 0-5\,^\circ\text{C}$: Diazotization ($ArNH_2 \rightarrow ArN_2^+Cl^-$)
  • $CuCl$: Converts $ArN_2^+ \rightarrow ArCl$ (Sandmeyer)
  • $CuBr$: Converts $ArN_2^+ \rightarrow ArBr$ (Sandmeyer)
  • $CuCN$: Converts $ArN_2^+ \rightarrow ArCN$ (Sandmeyer)
  • $KI$: Converts $ArN_2^+ \rightarrow ArI$
  • $HBF_4, \text{heat}$: Converts $ArN_2^+ \rightarrow ArF$ (Balz-Schiemann)
  • $H_2O, \text{heat}$: Converts $ArN_2^+ \rightarrow ArOH$
  • $H_3PO_2$: Converts $ArN_2^+ \rightarrow ArH$
• Primary Reaction Flowchart from Aniline:
  • Starting Material: Aniline ($C_6H_5NH_2$)
  • Step 1: $C_6H_5NH_2 \xrightarrow{NaNO_2/HCl,\, 0-5\,^\circ\text{C}} C_6H_5N_2^+Cl^-$
  • Step 2 Derivatives from Benzenediazonium Chloride ($C_6H_5N_2^+Cl^-$):
  • $\xrightarrow{CuCl} C_6H_5Cl$
  • $\xrightarrow{CuBr} C_6H_5Br$
  • $\xrightarrow{CuCN} C_6H_5CN$
  • $\xrightarrow{KI} C_6H_5I$
  • $\xrightarrow{HBF_4,\, \text{heat}} C_6H_5F$
  • $\xrightarrow{H_2O,\, \text{heat}} C_6H_5OH$
  • $\xrightarrow{H_3PO_2} C_6H_6$
  • $\xrightarrow{\text{Phenol / Aniline}} \text{Azo Dyes } (Ar-N=N-Ar')$

Key Named Reactions and JEE Traps

• Primary Named Reactions List:
  1. Gabriel phthalimide synthesis
  2. Hofmann bromamide degradation
  3. Carbylamine reaction
  4. Hinsberg test
  5. Diazotization
  6. Sandmeyer reaction
  7. Gattermann reaction
  8. Balz-Schiemann reaction
  9. Azo coupling
• Common JEE Conceptual Traps:
  1. Ammonolysis of alkyl halides does not yield a single pure primary amine; it produces complex mixtures of $1^\circ$, $2^\circ$, $3^\circ$ amines and quaternary ammonium salts.
  2. Gabriel phthalimide synthesis yields primary aliphatic amines exclusively; it cannot produce aniline or aromatic amines.
  3. Hofmann bromamide degradation always reduces the carbon chain length by exactly one carbon atom.
  4. Carbylamine test gives a positive result exclusively with primary amines ($1^\circ$ aliphatic or aromatic); secondary and tertiary amines do not react.
  5. Tertiary amines do not form sulfonamides in the Hinsberg test due to the complete absence of $N-H$ bonds.
  6. Aniline is less basic than ammonia because the nitrogen lone pair is delocalized into the aromatic ring by resonance.
  7. Aqueous basicity of aliphatic amines does not strictly follow inductive effect ($+I$); solvation energy and steric factors alter the basicity order to $2^\circ > 1^\circ > 3^\circ > NH_3$.
  8. Diazotization reactions must be maintained strictly at $0-5\,^\circ\text{C}$ to prevent rapid thermal decomposition of diazonium salts.
  9. Reaction of aniline with bromine water ($Br_2/H_2O$) yields 2,4,6-tribromoaniline white precipitate directly without requiring any Lewis acid catalyst.
  10. Sandmeyer reaction requires pre-formed Copper(I) salts ($CuCl$, $CuBr$, $CuCN$), whereas Gattermann uses metallic copper powder with $HX$.
  11. $KI$ replaces diazonium group with iodine directly upon warming without needing any copper catalyst.
  12. Balz-Schiemann reaction specifically introduces fluorine onto the aromatic ring via heating of the isolated diazonium fluoroborate salt ($ArN_2^+BF_4^-$).
  13. Primary ($1^\circ$) and secondary ($2^\circ$) amines undergo acylation; tertiary ($3^\circ$) amines cannot form amides because they lack an $N-H$ bond.

JEE Problem-Solving Strategy and Rapid Cues

• Rapid Cues (If You See This, Think This):
  • $CHCl_3 + \text{alcoholic } KOH + 1^\circ \text{ amine} \rightarrow$ Carbylamine test / Isocyanide formation
  • $C_6H_5SO_2Cl \rightarrow$ Hinsberg test for amine degree differentiation
  • $Br_2 + KOH + \text{primary amide} \rightarrow$ Hofmann bromamide degradation (1 fewer carbon)
  • $NaNO_2 + HCl + 0-5\,^\circ\text{C} \rightarrow$ Diazotization reaction
  • $ArN_2^+ + CuCl \rightarrow ArCl$
  • $ArN_2^+ + CuBr \rightarrow ArBr$
  • $ArN_2^+ + CuCN \rightarrow ArCN$
  • $ArN_2^+ + KI \rightarrow ArI$
  • $ArN_2^+ + HBF_4, \text{heat} \rightarrow ArF$
  • $ArN_2^+ + H_2O, \text{heat} \rightarrow ArOH$
  • $ArN_2^+ + H_3PO_2 \rightarrow ArH$
  • $\text{Aniline} + Br_2\text{ water} \rightarrow$ 2,4,6-tribromoaniline white precipitate
  • $\text{Aniline} + \text{acid} \rightarrow$ Anilinium salt ($-NH_3^+$ is meta-directing)
  • $\text{Aniline} + CH_3COCl \rightarrow$ Acetanilide (protective moderation of reactivity)
• Step-by-Step JEE Problem-Solving Strategy:
  • Step 1: Classify the substrate as $1^\circ$, $2^\circ$, or $3^\circ$ amine, and determine whether it is aliphatic or aromatic.
  • Step 2: Identify the precise reagent ($Br_2/KOH$, $CHCl_3/KOH$, $C_6H_5SO_2Cl$, $NaNO_2/HCl$, Cu salts, $KI$, $HBF_4$, $H_3PO_2$).
  • Step 3: Check temperature and reaction conditions, specifically enforcing $0-5\,^\circ\text{C}$ for diazonium ion stability.
  • Step 4: Track carbon atom count meticulously (deduct 1 carbon for Hofmann bromamide degradation; preserve carbon count for nitrile/amide reductions).
  • Step 5: For basicity comparison questions, systematically evaluate resonance delocalization, inductive effects, hydration/solvation stability, and steric hindrance.
• Top 10 Facts to Memorize First:
  1. Aniline is less basic than $NH_3$.
  2. Aqueous aliphatic amine basicity commonly follows 2^\circ > 1^\circ > 3^\circ > NH_3$.\n 3. Hofmann bromamide degradation results in one carbon atom less.\n 4. Gabriel phthalimide synthesis produces pure primary aliphatic amines.\n 5. Carbylamine test gives a positive result exclusively with primary amines.\n 6. Hinsberg test distinguishes primary, secondary, and tertiary amines.\n 7. Diazotization requires NaNO_2/HCl$at$0-5\,^\circ\text{C}.\n 8. CuCl$,$CuBr$, and$CuCN perform Sandmeyer substitutions.\n 9. HBF_4$$ with heat introduces aryl fluorine via Balz-Schiemann reaction.
  3. Aniline reacting with bromine water yields 2,4,6-tribromoaniline as a white precipitate.