Aromatic Amines: Aniline & Its Derivatives Meet aniline — the aromatic amine that launched a dye revolution Aniline (IUPAC: benzenamine; common: aniline) is the simplest aromatic amine, formula C6H7N. It kick-started the modern dye industry when William H. Perkin accidentally discovered the first synthetic dye, “mauveine,” from aniline in 1856. From cushions (polyurethanes) to medicines (paracetamol), aniline stands at the center of useful chemistry for NEET and beyond. Aromatic amine An amine where the –NH2 (or –NR2) group is directly attached to an aromatic ring (e.g., aniline). Benzenamine (C6H5–NH2); the parent aromatic amine and the ‘mother compound’ for many dyes and drugs. Benzenamine Phenylamine Aniline Basicity (of amines) Tendency to accept a proton (form R–NH3+). More available lone pair = stronger base. Lone-pair delocalization Resonance donation of nitrogen’s lone pair into the aromatic π system, reducing its availability for protonation. Acetanilide Aniline protected as an amide (C6H5–NHCOCH3); less activating than –NH2; used to control EAS. N-phenylacetamide p-Methyl aniline (para-methyl benzenamine); slightly more basic than aniline due to +I effect of –CH3. p-Toluidine Para-nitro substituted aniline; very weak base due to strong –NO2 electron withdrawal. p-Nitroaniline p-Aminobenzenesulfonic acid; internal zwitterion (–NH3+ and –SO3− in the same molecule). Sulfanilic acid p-Aminobenzoic acid; precursor to folic acid in bacteria; target of sulfa drugs. PABA Polymers formed from diisocyanates (e.g., MDI, TDI) and polyols; used in foams and elastomers. Polyurethane (MDI/TDI) Key terms at a glance Structure and resonance of aniline: why the ring flattens the amine Nitrogen in aniline is formally sp3 with a lone pair, but that lone pair can overlap with the benzene π system by resonance. This partial delocalization pulls electron density from N into the ring, giving the –NH2 a partial sp2-like character and making the overall geometry flatter than a typical aliphatic amine. The resonance also makes the ring strongly activated toward electrophilic aromatic substitution (o/p-directing), while simultaneously making the nitrogen’s lone pair less available to pick up a proton — hence weaker basicity than alkyl amines. Five-structure resonance scheme of aniline on white background. Show starting neutral form with N lone pair, then four ring-delocalized forms placing negative charge at ortho/para carbons and positive charge on N. Use curved arrows in red. Label o/p positions. Vector chemistry diagram style; no inside text captions. gpt-image-2 2026-05-26T17:05:42.554Z Resonance in aniline: five canonical forms showing lone-pair donation from N into the ring, with negative charge at o/p positions. Basicity: aniline is much less basic than alkyl amines Because the lone pair is partially tied up in resonance with the ring, it is less available to bind H+. Hence aniline is a weaker base than alkyl amines. At 25 C (aqueous), pKb(aniline) ≈ 9.4, while pKb(methylamine) ≈ 3.4. Lower pKb means a stronger base; so methylamine is far more basic than aniline. Aliphatic vs aromatic amine basicity (qualitative, NEET-level) Compound pKb (25 °C, aq.) Basicity ranking Reason Examples Methylamine (CH3NH2) ≈ 3.4 Stronger base +I effect; lone pair fully localized on N Aniline (C6H5NH2) ≈ 9.4 Weaker base Lone pair delocalized into aromatic ring (resonance) High-yield: aniline is a much weaker base than methylamine. Expect direct pKb comparison questions. neet-alert False. In aniline, the N lone pair delocalizes into the ring, reducing availability for protonation. Result: aniline is far less basic (pKb ≈ 9.4) than alkyl amines like methylamine (pKb ≈ 3.4). Aniline should be as basic (or more basic) than alkyl amines because both have –NH2. Substituent effects on aniline’s basicity Electron-withdrawing groups (–NO2, –CN, –SO3H, –CHO): decrease basicity by pulling electron density from N and the ring. Electron-donating groups (–CH3, –OCH3, –OH): slightly increase basicity by pushing electron density toward the ring/N. Typical order (para): p-toluidine > aniline > p-nitroaniline. Ortho effect: any ortho substituent often makes aniline less basic than the para isomer due to steric hindrance and poorer solvation of the protonated form. How ring substituents tune –NH2 basicity Physical properties to remember State: Colorless to slightly yellow liquid (bp ~ 184 C ); turns brown on air-oxidation. Odor: Characteristic amine-like. Solubility: Poor in water (hydrophobic ring); soluble in organic solvents. Basicity: Weaker than aliphatic amines; forms anilinium salts with strong acids. Preparation of aniline (benzenamine) Reduction of nitrobenzene: Fe/HCl or Sn/HCl (acidic) gives aniline; catalytic hydrogenation H2/Ni also works. In strongly alkaline reduction (Zn dust + NaOH), coupling can occur to form azobenzene — not good for making aniline. Hofmann bromamide (from benzamide): Benzamide (C6H5CONH2) on Br2/NaOH rearranges (loss of CO2) to give aniline. Gabriel synthesis does NOT work for aniline: Aryl halides do not undergo SN2; so this method is for aliphatic primary amines, not aryl amines. Core methods Hofmann bromamide: Benzamide to aniline Benzamide rearranges under Br2/NaOH to form an isocyanate, which hydrolyses to aniline with loss of CO2 (one-carbon loss). Br2, NaOH (aq), heat Electrophilic bromination at nitrogen N-bromination of benzamide under basic conditions to form N-bromobenzamide; deprotonation generates a reactive N-bromoamide anion. Hofmann rearrangement: migration of the aryl group from carbonyl carbon to nitrogen with loss of Br−; forms phenyl isocyanate intermediate. Basic, warm 1,2-shift (aryl migration) with rearrangement Nucleophilic addition of water to isocyanate; subsequent decarboxylation yields aniline. Aqueous workup Hydrolysis and loss of CO2 Benzamide substrate Benzamide electrophile Bromine Bromine Benzenamine product Aniline Core reactions of aniline At the nitrogen Acid–base: With HCl, aniline forms anilinium chloride (C6H5NH3+Cl−). Acylation (protection): With acetic anhydride (or acetyl chloride) forms acetanilide (C6H5–NHCOCH3), which reduces over-activation of the ring (–NHCOCH3 is less activating than –NH2). The –NH2 group strongly activates benzene and directs incoming electrophiles to ortho/para. Bromination — instant white precipitate of 2,4,6-tribromoaniline The –NH2 group is so activating that even bromine water suffices. Without any Lewis acid, aniline rapidly forms 2,4,6-tribromoaniline as a white precipitate. Bromination (no catalyst needed) Strong o/p activation causes substitution at both ortho and para positions, giving the tribromo product directly. On aniline, halogenation is exceptionally fast; protection may be used to control substitution. Three-panel vector diagram: Panel 1 aniline + Br2/H2O; Panel 2 sigma-complex at ortho/para; Panel 3 product 2,4,6-tribromoaniline (white ppt) with labels. Red arrows for electron flow; clean white background. gpt-image-2 2026-05-26T17:05:42.927Z Mechanism sketch of aniline bromination showing o/p attack and formation of 2,4,6-tribromoaniline precipitate. For aniline, no catalyst is needed. Br2 in water suffices due to very strong activation by –NH2. Tribromoaniline forms only with a Lewis acid catalyst like FeBr3. Nitration — the classic trap and how protection fixes it Direct nitration (HNO3/H2SO4) of aniline is problematic because the basic –NH2 gets protonated to –NH3+, which is meta-directing and deactivating. Result: significant meta-nitroaniline is formed. Strategy: Protect the amine as acetanilide first; nitration then occurs predominantly at para (also some ortho). Finally, hydrolyse to p-nitroaniline. gpt-image-2 Split diagram: Left shows aniline in nitration acid forming anilinium (–NH3+) meta-directing → meta product. Right shows acetanilide nitration giving mostly para-nitroacetanilide; deprotection to p-nitroaniline. Clear arrows; textbook vector style. Why aniline gives meta nitration directly and how acetanilide protection restores o/p control. 2026-05-26T17:05:43.180Z Sulfonation — formation of sulfanilic acid (zwitterion) Heating aniline with concentrated H2SO4 introduces –SO3H para to –NH2 (after rearrangement and work-up), furnishing sulfanilic acid (p-aminobenzenesulfonic acid). It exists as a zwitterion in the solid state, containing –NH3+ and –SO3− within the same molecule. Sulfonation of activated rings; sulfanilic acid is industrially important. gpt-image-2 Clean line structure of p-aminobenzenesulfonic acid with formal charges indicated: + on NH3, − on SO3. Include a small bracket showing ‘zwitterion’. Vector style on white. Structure of sulfanilic acid illustrating internal salt (zwitterion) with –NH3+ and –SO3−. 2026-05-26T17:05:43.580Z Friedel–Crafts on aniline? Why it fails (and how protection helps) Friedel–Crafts alkylation/acylation fails on free aniline because the Lewis acid (e.g., AlCl3) coordinates strongly to the basic –NH2, forming anilinium–AlCl4−, which deactivates the ring and prevents EAS. Protecting the –NH2 as an amide (acetanilide) reduces basicity and prevents strong complexation, allowing controlled electrophilic substitution. Fails on aniline due to Lewis acid–amine complexation; works after acyl protection (e.g., acetanilide). The opposite happens: AlCl3 (or similar) binds to –NH2, deactivating the ring. Use acetanilide protection to proceed. Friedel–Crafts reactions work smoothly on aniline because the ring is very activated. Reaction Conditions Major product(s) Notes EAS on aniline: products and conditions Bromination Br2/H2O, rt 2,4,6-Tribromoaniline (white ppt) No Lewis acid needed Nitration (direct) HNO3/H2SO4 Meta-nitroaniline (significant) –NH2 protonates → –NH3+ (meta-directing) Nitration (protected) Acetanilide, then HNO3/H2SO4 p-Nitroacetanilide (major), then p-nitroaniline after hydrolysis Protection restores o/p control Sulfonation Conc. H2SO4 (heat, then work-up) Sulfanilic acid (zwitterion) Industrial importance Friedel–Crafts AlCl3 (Lewis acid) Fails on aniline Protect –NH2 first Tests and character reactions Primary amines (like aniline) + CHCl3 + alcoholic KOH → isocyanides (foul smell) — positive carbylamine test. Primary amines form sulfonamides. Note: Aniline forms sulfonamide that is typically not soluble in alkali (resonance-stabilized) — a classic nuance vs aliphatic amines. Hinsberg nuance: Aliphatic 1° amide sulfonamides dissolve in alkali (acidic N–H), but aromatic 1° (aniline) sulfonamide is often insoluble due to strong resonance — a potential NEET trap. tip Diazonium gateway (covered in detail in NTCH18/03) Aniline reacts with nitrous acid at 0– 5 C to form benzenediazonium chloride, a versatile intermediate for substitutions (Sandmeyer, Balz–Schiemann) and for making azo dyes via coupling. Full mechanisms and scope are in NTCH18/03; the essential NEET equations and visuals are included here for continuity. Diazotization: primary aromatic amine to aryl diazonium chloride (stable only at 0– 5 C ). Mechanism map: aniline to benzenediazonium chloride at 0– 5 C via nitrous acid, with resonance stabilization of the diazonium. Sandmeyer (chlorination) — one of many substitutions available from diazonium salts. Cu(I) salts mediate substitution of diazonium with halides/cyano. Product map: benzenediazonium can yield chlorobenzene, bromobenzene, iodobenzene, phenol, benzonitrile, nitrobenzene, etc. — a versatile hub. Azo coupling at pH 4–5 to form brightly colored azo dyes. Electrophilic diazonium couples with activated aromatics (phenols, anilines) to form azo dyes. A diazonium salt coupling with phenol/amine to make an azo dye — the basis of many vibrant textile colors. Alkyl diazonium salts have stability similar to aryl diazonium salts. Alkyl diazonium salts are highly unstable (decompose immediately), whereas aryl diazonium salts are isolable only at 0– 5 C . Diazonium salts are stable only at 0– 5 C . Warmer conditions cause rapid decomposition (often to phenols). Keep cold. Temperature control is optional during diazotization and subsequent steps. Azo dyes changed the world: coupling diazonium with electron-rich rings gives intensely colored compounds used in textiles, leather, paper, even food colorants. remember Important aniline-derived compounds and uses Acetanilide (N-phenylacetamide) Amide (–NHCOCH3) Former analgesic; protecting group for –NH2 in EAS Sulfanilic acid (p-aminobenzenesulfonic acid) –NH2 and –SO3H (zwitterion) Precursor to sulfa drugs; dye intermediates PABA (p-aminobenzoic acid) –NH2 and –COOH (para) Folic acid precursor; target of sulfonamide antibiotics p-Aminophenol Phenolic –OH and –NH2 (para) Intermediate to paracetamol (acetaminophen) MDI/TDI (diisocyanates from anilines) –NCO groups on aromatic rings Polyurethane foams (cushions, mattresses, interiors) From aniline to industry and medicine Compound Key functional group(s) Primary use/importance Synthesis of paracetamol: acylation of p-aminophenol with acetic anhydride to give paracetamol (acetaminophen). 2026-05-26T17:05:44.159Z gpt-image-2 One-step reaction scheme: p-aminophenol + acetic anhydride → paracetamol + acetic acid. Label functional groups (–OH, –NH2 → –NHCOCH3). Clean vector style, red arrows, white background. neet-alert High-yield facts: (1) Aniline basicity ≪ methylamine; (2) Direct nitration gives meta (protonation trap); (3) Use acetanilide to control EAS; (4) Br2/H2O → 2,4,6-tribromoaniline (white ppt) with no catalyst. Aryl halides do not undergo SN2. Gabriel synthesis is for aliphatic primary amines, not aryl amines like aniline. Gabriel phthalimide synthesis can be used to make aniline from chlorobenzene.