Organic Compounds Containing Nitrogen revision notes

• IUPAC System: Primary amines are named as alkanamines (e.g., $CH_3NH_2$ is methanamine). For 2° and 3° amines, the locant 'N' designates the substituent attached to the nitrogen atom (e.g., $CH_3NHCH_2CH_3$ is N-methylethanamine).
• Arylamines: The simplest arylamine is $C_6H_5NH_2$. Its common and accepted IUPAC name is aniline, though in the strict IUPAC system, it is benzenamine.
• Isomerism in Amines: Amines exhibit structural isomerism, including chain isomerism, position isomerism, and functional isomerism (1°, 2°, and 3° amines are functional isomers of each other). Secondary and tertiary amines also exhibit metamerism (e.g., $C_4H_{11}N$ can be diethylamine or methylpropylamine). JEE Tip

• The nitrogen atom in amines is trivalent and carries one unshared pair of electrons.
• The nitrogen orbitals are $sp^3$ hybridised, resulting in a pyramidal geometry.
• Bond Angle Anomaly: Due to the presence of the unshared electron pair (lone pair-bond pair repulsion), the $C-N-H$ or $C-N-C$ bond angle is less than the standard tetrahedral angle of 109.5°. For example, in trimethylamine, the angle is 108°. JEE Tip

• Reduction of Nitro Compounds: Nitro compounds (and nitroalkanes) are reduced to amines by passing hydrogen gas in the presence of finely divided Ni, Pd, or Pt, or by reduction with metals in an acidic medium. Reduction with iron scrap and $HCl$ is preferred because the $FeCl_2$ formed gets hydrolysed to release $HCl$, meaning only a small amount of initial $HCl$ is needed. JEE Tip

• Reduction of Nitriles: Nitriles treated with $LiAlH_4$ or catalytic hydrogenation produce primary amines, used for the ascent of the amine series (adding one carbon atom).
• Reduction of Amides: Amides yield amines upon reduction with $LiAlH_4$.
• Gabriel Phthalimide Synthesis: Phthalimide reacts with ethanolic KOH to form a potassium salt, which is heated with an alkyl halide followed by alkaline hydrolysis to yield primary aliphatic amines. JEE Tip (Aromatic primary amines cannot be prepared this way).
• Hoffmann Bromamide Degradation Reaction: An amide is treated with $Br_2$ in aqueous or ethanolic $NaOH$ to form a primary amine. The alkyl/aryl group migrates from the carbonyl carbon to the nitrogen atom. The resulting amine has one carbon less than the starting amide. JEE Tip

• Physical State & Odour: Lower aliphatic amines are gases with a fishy odour. Primary amines with 3 or more carbons are liquids, and higher ones are solids. Aniline and other arylamines are usually colourless but turn coloured upon storage due to atmospheric oxidation.

• Boiling Points: Primary and secondary amines engage in intermolecular hydrogen bonding (absent in 3° amines). Order of boiling points for isomeric amines: Primary > Secondary > Tertiary. JEE Tip

Because of the unshared electron pair on nitrogen, amines behave as nucleophiles and Lewis bases.

• Basic Character & Salt Formation: Amines react with mineral acids to form soluble ammonium salts, which regenerate the parent amine when treated with a strong base ($NaOH$). This property is used to separate amines from non-basic, water-insoluble organic compounds.
• Reaction with Carboxylic Acids: Amines react with carboxylic acids to form acid-base salts at room temperature.
• Alkylation: Amines react with alkyl halides to form substituted amines.
• Acylation: 1° and 2° aliphatic/aromatic amines react with acid chlorides, anhydrides, and esters via nucleophilic substitution to form amides. The reaction uses a stronger base like pyridine to remove the formed $HCl$ and shift the equilibrium to the right. Benzoylation occurs similarly using benzoyl chloride ($C_6H_5COCl$).
• Carbylamine Reaction (Isocyanide Test): Heating aliphatic and aromatic primary amines with chloroform ($CHCl_3$) and ethanolic $KOH$ forms foul-smelling isocyanides (carbylamines). JEE Tip 2° and 3° amines do not show this reaction.

General formula: $ArN_2^+X^-$ ($X^-$ can be $Cl^-$, $Br^-$, $HSO_4^-$, $BF_4^-$). Aliphatic diazonium salts are highly unstable, whereas arene diazonium salts are stable for a short time at low temperatures (273-278 K) due to resonance stabilization.

• Preparation: Diazotisation of aniline with $NaNO_2$ and $HCl$ at 273-278 K.

• Amine Basicity Constants: The larger the $K_b$ value (or smaller the $pK_b$ value), the stronger the base. Ammonia has a $pK_b$ of 4.75. Aliphatic amines range from 3.00 to 4.22. Aniline is much weaker with a $pK_b$ of 9.38.
• Factors Governing Amine Basicity in Aqueous Phase: The basic strength of aliphatic amines in water is dictated by a subtle interplay of three factors: Inductive Effect (+I), Solvation Effect (hydrogen bonding stabilizes the substituted ammonium cation), and Steric Hindrance of the alkyl groups.
• Conjugation and Basicity: Aromatic amines are weaker bases than ammonia because the lone pair of electrons on the nitrogen atom is in conjugation with the benzene ring (delocalised over 5 resonating structures), making it less available for protonation. The anilinium ion only has two Kekule structures, making aniline more stable than its protonated form.
• Substituent Effects on Basicity: Electron-releasing groups ($-OCH_3$, $-CH_3$) increase the basic strength of aromatic amines, whereas electron-withdrawing groups ($-NO_2$, $-SO_3H$, $-COOH$, $-X$) decrease it.

• Aqueous Basicity Order Anomaly: While gas-phase basicity strictly follows the +I effect (3° > 2° > 1° > $NH_3$), the aqueous phase order is anomalous due to a delicate balance of solvation and steric hindrance. For methylamines: $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$. For ethylamines: $(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3$. JEE Tip
• Gabriel Phthalimide Exception: This method CANNOT be used to prepare aromatic primary amines (like aniline). Reason: Aryl halides do not undergo nucleophilic substitution with the phthalimide anion under these conditions.
• Direct Nitration of Aniline Anomaly: Even though $-NH_2$ is an ortho/para-directing and powerfully activating group, direct nitration using strongly acidic medium unexpectedly produces a significant amount (47%) of meta-nitroaniline. Reason: Aniline gets protonated to form the anilinium ion, which is a meta-directing and deactivating group.
• Friedel-Crafts Reaction Failure: Aniline does not undergo Friedel-Crafts alkylation or acetylation. Reason: Aniline forms a salt with the Lewis acid catalyst ($AlCl_3$). The nitrogen atom acquires a positive charge, acting as a strong deactivating group that halts further electrophilic substitution. JEE Tip
• Bond Angle Anomaly: The geometry of amines is $sp^3$ hybridised, but the $C-N-H$ or $C-N-C$ bond angle (e.g., 108° in trimethylamine) is less than the standard tetrahedral angle (109.5°). Reason: Lone pair-bond pair repulsions push the bonded orbitals closer together.
• Acetanilide Reactivity Anomaly: The activating effect of the $-NHCOCH_3$ group is less than that of the free amino ($-NH_2$) group. Reason: The lone pair of electrons on the nitrogen is pulled into resonance with the carbonyl oxygen of the acetyl group, making it less available to donate into the benzene ring.
• Diazonium Salt Stability Anomaly: Primary aliphatic amines form highly unstable diazonium salts that immediately decompose into nitrogen gas and alcohol. In contrast, primary aromatic amines form arene diazonium salts which are stable for a short time at low temperatures (273-278 K). Reason: The arene diazonium ion is stabilized by resonance with the benzene ring.
• Carboxylic Acid Reaction Anomaly: Unlike acid chlorides and anhydrides which readily form amides via acylation, amines reacting directly with carboxylic acids at room temperature only form acid-base salts. Heating is required to eventually form the amide.
• Solubility Anomaly: While lower aliphatic amines are readily soluble in water due to hydrogen bonding, higher amines are essentially insoluble. Reason: The hydrophobic alkyl part becomes too large and disrupts the hydrogen bonding network.

• Reactivity of Alkyl Halides in Ammonolysis: $RI > RBr > RCl$.
• Boiling Point Order (Isomeric Amines): Primary > Secondary > Tertiary (Due to decreasing intermolecular H-bonding).
• Boiling Point Comparison (Different Functional Groups): Alcohols > Amines > Alkanes of comparable molar mass.
• Basic Strength in Gas Phase: 3° Amine > 2° Amine > 1° Amine > $NH_3$.
• Basic Strength Order (General Aqueous): Alkylamines > Ammonia > Arylamines.

• Amine Basicity Constant ($K_b$): $K_b = [RNH_3^+][OH^-] / [RNH_2]$ $pK_b = -\log K_b$
• Hoffmann Bromamide Degradation: $R-CONH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O$ (Conceptual application; results in loss of one carbon).
• Carbylamine Reaction: $R-NH_2 + CHCl_3 + 3KOH$ (alc) $\rightarrow R-NC + 3KCl + 3H_2O$.
• Diazotisation: $C_6H_5NH_2 + NaNO_2 + 2HCl$ (273-278 K) $\rightarrow C_6H_5N_2^+Cl^- + NaCl + 2H_2O$.
• Reduction by Hypophosphorous Acid: $ArN_2^+Cl^- + H_3PO_2 + H_2O \rightarrow ArH + N_2 + H_3PO_3 + HCl$.
• Reduction by Ethanol: $ArN_2^+Cl^- + CH_3CH_2OH \rightarrow ArH + N_2 + CH_3CHO + HCl$.

• Anomalous base strength trends of primary, secondary, and tertiary amines in aqueous media vs gas phase.
• Protection of aniline via acetylation before electrophilic substitutions (nitration, bromination).
• Distinguishing 1°, 2°, and 3° amines using Hinsberg's reagent and Carbylamine tests.
• Hoffmann bromamide degradation (mechanism, intermediate, and step-down carbon count).
• Transformations using diazonium salts (Sandmeyer vs Gattermann, unique syntheses of fluorobenzene/cyanobenzene, and specific reductions to benzene).