Aliphatic Amines: structure, preparation, reactions, and basicity Why study aliphatic amines for NEET? Amines are ammonia-like molecules where hydrogen(s) are replaced by alkyl groups: 1° (R–NH2), 2° (R2NH), 3° (R3N). Their lone pair makes them basic and nucleophilic, so they form salts, react with acyl halides, and help build many bioactive molecules. NEET loves their preparation routes (Gabriel, Hoffmann bromamide), identification tests (Hinsberg, carbylamine), and the aqueous vs gas-phase basicity trap. Classification of amines by groups on nitrogen: primary (R–NH2), secondary (R2NH), tertiary (R3N) with simple alkyl examples. Structure, bonding, and naming (alkanamine) Nitrogen in amines is sp3-hybridised and adopts a trigonal pyramidal geometry with a non-bonding lone pair at the apex. Think of a three-legged stool with a cushion (lone pair) on top. The H–N–H/R angles are close to tetrahedral (about 107–109°), slightly adjusted by the lone pair’s extra repulsion. The lone pair is basic: it can pick up a proton (H+), forming an alkylammonium ion (RNH3+ / R2NH2+ / R3NH+). IUPAC naming: replace the parent alkane’s “-e” with “-amine” (alkanamine). For 2°/3° amines with different alkyls, name the longest chain as parent alkanamine and prefix the others with N- as substituents. Pyramidal nitrogen with a highlighted lone pair showing how it can accept a proton — the origin of basicity. Core terms One alkyl group on N: R–NH2. Alkanamine (e.g., methanamine) Primary amine (1°) alkylamine (1°) N-Substituted alkanamine dialkylamine Secondary amine (2°) Two alkyl groups on N: R2NH. Three alkyl groups on N: R3N. Tertiary amine (3°) trialkylamine Tertiary alkanamine Quaternary ammonium halide R4N+X−, formed by exhaustive alkylation of amines. ammonium salt (4°) Quaternary ammonium salt IUPAC class name for aliphatic amines; suffix “-amine”. alkylamine Alkanamine Alkyl halide + ammonia → mixture of amines via successive alkylations; excess NH3 favours 1°. Hofmann ammonolysis Phthalimide → N-alkyl phthalimide → 1° amine; does not give 2°/3°; fails for aryl halides. Gabriel phthalimide synthesis Hoffmann bromamide degradation Amide + Br2/NaOH → 1° amine with one fewer carbon (–CO unit lost). 1° amine + CHCl3 + alc. KOH, heat → isocyanide (R–NC), foul odour; test for 1° amine. Carbylamine (isocyanide) reaction Carbonyl compound + NH3/RNH2 → imine/iminium → reduction → amine. Reductive amination Tendency to accept H+; for amines, modulated by +I effect, steric inhibition to protonation, and solvation. Basicity N-Nitrosamine R2N–NO formed from 2° amines with nitrous acid; typically yellow oily liquids. Quick examples (IUPAC; common; SMILES) Methanamine (methylamine); methylamine; CN Ethanamine (ethylamine); ethylamine; CCN N-Methylethanamine; ethyl(methyl)amine; CCNC Trimethylamine; trimethylamine; N(C)(C)C Preparation of aliphatic amines — the high-yield routes Preparation methods vs selectivity (NEET focus) Route Key reagents/conditions Product class Selectivity/notes Method Hofmann ammonolysis (alkyl halide + NH3) Alkyl halide (R–X), excess NH3 (often ethanolic), heat 1°, 2°, 3°, R4N+ (mixture) Excess NH3 favours 1°; otherwise over-alkylation occurs Gabriel phthalimide synthesis Phthalimide/KOH → N-alkyl phthalimide → hydrazinolysis (NH2NH2) or hydrolysis Pure 1° only SN2 on 1° RX; fails for aryl halides and hinders 2° RX Reduction of nitro compounds Sn/HCl or Fe/HCl; or H2/Pd or Ni 1° R–NO2 → R–NH2 (aliphatic nitro gives 1° amine) Reduction of nitriles LiAlH4; or H2/Ni, Pd 1° R–C≡N → R–CH2–NH2 (adds –CH2NH2) Reduction of amides LiAlH4 (dry ether), then work-up 1° R–CONH2 → R–CH2–NH2 Hoffmann bromamide degradation Br2/NaOH (aq), heat 1° One fewer C than the amide: RCONH2 → RNH2 Reductive amination Carbonyl + NH3/RNH2; H2/Pt/Ni or NaBH3CN/NaBH(OAc)3 1°/2°/3° Imine/iminium reduced to amine; very versatile Phthalimide → potassium phthalimide → N-alkyl phthalimide → hydrazinolysis/hydrolysis → pure 1° alkylamine. Preferred for pure 1° aliphatic amines; no 2°/3° formed. Gabriel phthalimide synthesis — mechanism Strong base generates the imide anion, which undergoes SN2 with a 1° alkyl halide; hydrazinolysis/hydrolysis liberates the 1° amine. Deprotonation: Phthalimide (N–H) is deprotonated by KOH to give potassium phthalimide (N−). Base removes N–H proton KOH, ethanol or DMF SN2 alkylation: The imide anion attacks a 1° alkyl halide (backside attack), forming N-alkyl phthalimide. 1° RX; avoid 3° RX (E2 competes) and aryl halides (no SN2). N− → C–X; X− leaves NH2NH2/ethanol; or H3O+/OH−, heat Ring opening → amine + phthalhydrazide/phthalate Cleavage (hydrazinolysis or hydrolysis): Treat N-alkyl phthalimide with NH2NH2 (or acidic/basic hydrolysis) to liberate the 1° alkylamine (R–NH2). Phthalimide 1,3-Dihydro-1,3-dioxo-1H-isoindol-2-yl (parent: phthalimide) substrate (N–H acidic) nucleophile Potassium 2,3-dioxo-2,3-dihydro-1H-isoindol-1-ide Potassium phthalimide electrophile Haloalkane (e.g., bromoethane) Alkyl halide Hydrazine releases the amine (hydrazinolysis) Diazane Mechanism diagram of Gabriel synthesis in 3 panels on white background. Panel 1: KOH deprotonates phthalimide to give potassium phthalimide (show negative charge on N). Panel 2: SN2 attack on a 1° bromoethane; show curved arrow from N− to electrophilic carbon with Br leaving. Panel 3: hydrazinolysis opens the imide to release ethanamine. Label each intermediate; red arrows; clean 2D vector style; no internal text. gpt-image-2 2026-05-26T17:05:41.678Z Gabriel phthalimide synthesis: stepwise SN2 alkylation and hydrazinolysis producing a pure 1° alkylamine. Hofmann ammonolysis of alkyl halides Alkyl halides react with ammonia by nucleophilic substitution to form 1° amines, which can further react with more RX to give 2° and 3° amines and finally quaternary ammonium salts. Because each newly formed amine is more nucleophilic than ammonia, over-alkylation is common. Using a large excess of NH3 and controlled conditions increases the 1° amine yield. Outcome of ammonolysis (R–X + NH3) Primary: R–NH2 + HX (salt forms first; free amine after base work-up) Secondary: R2NH via further alkylation of 1° Tertiary: R3N via further alkylation of 2° Quaternary: R4N+X− (exhaustive alkylation) Reductions to 1° amines Several functional groups are reduced to aliphatic 1° amines: - Nitroalkanes: R–NO2 → R–NH2 using Sn/HCl or Fe/HCl (in situ H2) or catalytic H2/Pd, Ni. - Nitriles: R–C≡N → R–CH2–NH2 using LiAlH4 or catalytic H2/Ni. Note the extra –CH2 unit added. - Amides: R–CONH2 → R–CH2–NH2 with LiAlH4 (dry ether), then water work-up. Hoffmann bromamide degradation — one carbon lost Primary amide → 1° amine with one fewer carbon using Br2/NaOH (aq). Acetamide (example) substrate Ethanamide Bromine Dibromine electrophile at N Sodium hydroxide base Sodium hydroxide N-Bromination: Deprotonated amide (amide anion) is brominated at nitrogen to give N-bromoamide. Br2/NaOH (aq), 0–50 N− attacks Br2 Rearrangement: Base induces migration of R from carbonyl carbon to nitrogen with expulsion of Br− to form an isocyanate (R–N=C=O). Heat R migrates with pair (1,2-shift) Hydrolysis: The isocyanate hydrolyses to the 1° amine (R–NH2) and CO2 (as carbonate under basic conditions). Aqueous base Isocyanate + H2O → amine + CO2 Base-promoted halogenation at N, rearrangement (migration) with loss of CO from the amide to give a 1° amine (one C shorter). Hoffmann bromamide (bromamide) — mechanism sketch 2026-05-26T17:05:42.145Z Hoffmann bromamide degradation: N-bromination → rearrangement → isocyanate → hydrolysis to a 1° amine with one fewer carbon. Four-panel sequence showing ethanamide reacting with Br2/NaOH. Panel 1: N-bromoacetamide formation. Panel 2: rearrangement with migration of CH3 from carbonyl C to N (curved arrow). Panel 3: formation of methyl isocyanate. Panel 4: hydrolysis to methylamine + CO2/CO3(2−). Labels; red arrows; clean vector style. gpt-image-2 Reductive amination — make any class of amine Nucleophilic addition of NH3 (or RNH2) to the carbonyl, followed by dehydration to form an imine (Schiff base) or an iminium ion. Trace acid catalysis Mild hydride source or catalytic hydrogenation Reduction of the C=N (or C=N+) to C–N using H2/Pt/Ni or NaBH3CN/NaBH(OAc)3, furnishing the amine. Acetaldehyde (example) Ethanal carbonyl electrophile nucleophile/base Azane Ammonia Sodium cyanoborohydride Sodium cyanoborohydride reductant (selective) Reductive amination (general) A carbonyl compound condenses with NH3/amine to give an imine/iminium, which is then reduced to an amine. Reactions used to identify and transform aliphatic amines Hinsberg (1° amine) Sulfonamide from 1° amine has an acidic N–H; dissolves in alkali as its anion. 2° sulfonamide has no N–H; remains insoluble in alkali. Hinsberg (2° amine) 1° amine: sulfonamide soluble in alkali; 2°: sulfonamide insoluble; 3°: no sulfonamide (no reaction). Hinsberg test outcomes: 1° gives sulfonamide soluble in KOH; 2° gives insoluble sulfonamide; 3° amines do not form sulfonamides (often only form soluble ammonium salts under acidic work-up). Test for 1° amines only; foul-smelling isocyanide forms. Carbylamine (isocyanide) reaction 1° aliphatic (and aromatic) amines give R–NC with CHCl3/KOH, heat. 2°/3° do not. Reaction with nitrous acid (HNO2, made in situ from NaNO2 + HCl): 1° aliphatic amines give unstable aliphatic diazonium ions that decompose to alcohols with brisk N2 evolution (R–NH2 → R–OH). 2° amines form N-nitrosamines (R2N–NO, yellow oily). 3° aliphatic amines generally form soluble ammonium nitrite salts; no N–nitroso product as there is no N–H. For aromatic 1° amines (contrast only): stable diazonium salts form at 0– 5 C . Aliphatic diazonium ions are unstable and give alcohols. Aromatic contrast (diazotization) Other common reactions Salt formation: R–NH2 + HX → R–NH3+ X− (acid-base; enhances water solubility). Acylation: R–NH2 + CH3COCl/(CH3CO)2O/PhCOCl → amide (R–NH–CO–R′); 1°/2° amines react; 3° do not (no N–H). Alkylation: amines react with RX to give higher amines; over-alkylation leads to R4N+ X−. Basicity of aliphatic amines — the aqueous vs gas-phase trap Conjugate acid stability and solvation govern basicity in aqueous solution. Amine basicity in water Three factors compete: (a) +I effect of alkyl groups increases electron density on N, favouring protonation (3° has most +I). (b) Steric inhibition to protonation makes it harder for H+ to approach a crowded 3° amine. (c) Solvation of the conjugate acid R3NH+ matters in water: more N–H bonds allow more hydrogen bonding, stabilising RNH3+ > R2NH2+ > R3NH+. As a result, in aqueous solution of simple alkylamines: 2° > 1° > 3° > NH3, while in the gas phase (no solvation): 3° > 2° > 1° > NH3. Amine Basicity comparison — aqueous pKb and gas-phase intrinsic order Base Aqueous pKb (25°C, approx.) Relative basicity in water Gas-phase trend position NH3 (ammonia) 4.75 Least among listed CH3NH2 (methanamine; methylamine) ≈ 3.36 High (CH3)2NH (N-methylmethanamine; dimethylamine) ≈ 2.89 Highest (CH3)3N (trimethylamine) ≈ 4.19 Lower than 1°/2° 1 (most basic in gas phase) gpt-image-2 Side-by-side bar charts comparing basicity orders: Left panel aqueous with bars for NH3, MeNH2, Me2NH, Me3N (heights 2°>1°>3°>NH3). Right panel gas-phase with bars 3°>2°>1°>NH3. Neutral palette; clear labels; vector style. Basicity trend chart: aqueous (2° > 1° > 3° > NH3) vs gas-phase (3° > 2° > 1° > NH3). 2026-05-26T17:05:42.461Z Do not memorise only one basicity order. In water: 2° > 1° > 3° > NH3 due to solvation and sterics. In the gas phase: 3° > 2° > 1° > NH3 (no solvation). NEET has asked both. neet-alert neet-alert Gabriel phthalimide synthesis is for alkyl (1° RX via SN2). It fails for aryl halides (Ar–X does not undergo SN2 under these conditions). Classifying amines as 1°, 2°, 3° is based on the carbon bearing the –NH2 group, like in alcohols or alkyl halides. For amines, 1°/2°/3° classification is based on how many alkyl groups are directly attached to the nitrogen atom (R–NH2, R2NH, R3N), not on the carbon bearing the functional group. All aliphatic amines are stronger bases than ammonia in water, and the gas-phase order 3° > 2° > 1° > NH3 holds in aqueous solution too. In water, solvation and steric factors usually make 2° > 1° > 3° > NH3 for simple alkylamines. In the gas phase, without solvation, 3° > 2° > 1° > NH3. Amines are everywhere in biology and medicine. Neurotransmitters like adrenaline (epinephrine), noradrenaline, and dopamine are amines. Many drugs (antihistamines, local anaesthetics, decongestants) contain amine groups — their basicity controls absorption and salt formation. clinical Industrial and societal relevance Applications you should recognise Methyl- and ethylamines: intermediates for fertilizers, rubber accelerators, solvents. Diamines → nylons: e.g., hexane-1,6-diamine for nylon-6,6; caprolactam (from amines) for nylon-6. Ethylenediamine → EDTA (chelating agent). Pharmaceutical intermediates: antihistamines, amphetamines (CNS stimulants), many APIs contain amines. Dye and pigment manufacture: amino precursors for azo dyes (link to aromatic diazonium chemistry). Chemical defense context: synthesis of certain nerve agents passes through amine intermediates (handled under strict regulation). TREND Explaining the anomalous order of methyl and ethyl amines in water. Phase/Solvent Basicity of Amines Trend Organic Chemistry Amines Basicity Trend NEET High Yield Secondary is always first; Methyl is smaller (213) while Ethyl is larger (231). Order ( 1 , 2 , 3 ) Dominant Factor Here are a few prompt options tailored for a text-to-image AI (like Midjourney, DALL-E 3, or Stable Diffusion) to generate this specific NEET chemistry concept. I have broken them down by the specific scientific aspect you might want to highlight (Inductive Effect vs. Solvation/Aqueous trend), as NEET often tests the difference between gas phase and aqueous phase. Option 1: General Trend (Focus on Inductive Effect / Gaseous Phase) Best for: Explaining why alkyl groups increase basicity generally ( 3 > 2 > 1 ). > Prompt: professional scientific diagram, labeled textbook vector, chemical structures of Primary ( 1 ), Secondary ( 2 ), and Tertiary ( 3 ) amines arranged horizontally, electron donating alkyl groups marked with arrows pointing toward Nitrogen, highlighted lone pair electrons on the Nitrogen atom, high contrast black lines on white background, basicity trend indicator arrow, flat design, educational chemistry illustration, high resolution, sans-serif typography style. Option 2: Aqueous Phase Trend (Methyl Substituted - The "NEET Special") Best for: The specific exception trend ( 2 > 1 > 3 ) due to steric hindrance and solvation. > Prompt: educational chemistry vector chart, basicity of amines in aqueous solution, comparative structures of Methyl-substituted amines, visual representation of Hydrogen bonding with water molecules around the nitrogen, clear depiction of steric hindrance in tertiary amine, ordered arrangement showing Secondary ( 2 ) as most stable followed by Primary ( 1 ) then Tertiary ( 3 ), clean scientific diagram, distinct blue and black colors, white background, high contrast, textbook style. Option 3: Minimalist Trend Table (Comparison View) Best for: A clean summary table for quick revision. > Prompt: scientific trend table, basicity of amines, three distinct columns labeled Primary ( 1 ), Secondary ( 2 ), and Tertiary ( 3 ), skeletal chemical formulas, distinct lone pair dots on Nitrogen, vector graphics, high contrast black and red ink on white paper style, clean lines, no shadows, 2D flat illustration, chemistry study guide aesthetic. Tips for Best Results: Text Rendering: AI struggles to render specific text (like " CH 3NH 2 ") perfectly. It is usually better to generate the visual structures using the prompt and add the specific alphanumeric labels (Values, pK b data, or compound names) manually in Photoshop or Canva afterwards. Aspect Ratio: For a table/trend, use a landscape aspect ratio. If using Midjourney, add --ar 3:2 or --ar 16:9 to the end of the prompt. Vector Style: Using keywords like "flat design" and "no shading" ensures the image looks like a diagram and not a 3D render. Gas Phase 3 > 2 > 1 > NH 3 Inductive ( +I ) effect only Aqueous: Methyl substituted (CH 3) 2NH > CH 3NH 2 > (CH 3) 3N > NH 3 Steric vs. Solvation vs. Inductive (Order: 213) Aqueous: Ethyl substituted (C 2H 5) 2NH > (C 2H 5) 3N > C 2H 5NH 2 > NH 3 Steric vs. Solvation vs. Inductive (Order: 231) Solvation Factor (Aqueous) 1 > 2 > 3 Stability of substituted ammonium cation via H -bonding Inductive Factor (General) 3 > 2 > 1 Electron release by alkyl groups increases N basicity Steric Hindrance (Aqueous) 1 > 2 > 3 Bulky groups inhibit H + approach and solvation Alkyl vs. Ammonia vs. Aryl R-NH 2 > NH 3 > Ar-NH 2 Resonance delocalization in Ar-NH 2 reduces basicity Test REACTION VARIANTS Identifying amine class based on solubility of sulfonamides or gas evolution. Distinction Tests for Amines Organic Chemistry Amines Qualitative Analysis NEET High Yield Reagent Primary Amine Secondary Amine Tertiary Amine Hinsberg says: 1 is Soluble, 2 is Oily, 3 is Lazy; while Nitrous gives 1 the Gas, 2 the Oil, and 3 the Salt. Hinsberg Reagent ( C 6H 5SO 2Cl ) Forms sulfonamide soluble in alkali ( NaOH/KOH ) Forms sulfonamide insoluble in alkali Does not react (remains insoluble) Nitrous Acid ( HNO 2 / NaNO 2+HCl ) Evolution of N 2 gas with alcohol formation Formation of yellow oily N -nitrosoamine Formation of soluble nitrite salts Carbylamine Test ( CHCl 3 + alc. KOH ) Foul-smelling isocyanides ( R-NC ) produced No reaction No reaction Aromatic Amine + HNO 2 ( 273-278 K ) Formation of stable Diazonium salt ( ArN 2 +Cl - ) Formation of yellow oily liquid Formation of green p -nitroso derivative Hofmann Mustard Oil Test ( HgCl 2 + CS 2 ) Pungent smell of alkyl isothiocyanate ( R-NCS ) No reaction No reaction Liebermann Nitroso Test Negative Positive (Red color Blue/Green with NaOH ) Negative Reaction with Acetyl Chloride ( CH 3COCl ) Forms N -substituted amide Forms N,N -disubstituted amide No reaction (lacks replaceable H )