From the smell of fish to life-saving drugs — the chemistry of nitrogen-containing organic compounds. With interactive basicity comparison, distinguishing tests, diazonium chemistry, and 20+ real-life examples.
Before diving in, here's the entire chapter on one page. How the lone pair on nitrogen drives basicity, and how amines are converted to diazonium salts and beyond.
Amines (R-NH₂) are Lewis bases (lone pair on nitrogen). Basicity order depends on substituent effects: alkyl groups (+I) increase basicity, aromatic ring (resonance) decreases basicity. Amines react with acids (salt), acid chlorides (amide), HNO₂ (diazonium for 1°, N-nitrosamine for 2°, no reaction for 3°), and give carbylamine test (1° only). Diazonium salts are versatile intermediates in organic synthesis.
Amines: Organic derivatives of ammonia (NH₃) where one or more hydrogens are replaced by alkyl or aryl groups.
General formula: R-NH₂, R₂NH, R₃N, Ar-NH₂, etc.
The nitrogen has a lone pair of electrons — the key to basicity and nucleophilicity.
Note: The 1°/2°/3° here refers to the number of R groups attached to nitrogen, NOT the carbon skeleton (unlike alcohols).
Amines classified by number of R groups (1°/2°/3°/quaternary) and by R group nature (aliphatic vs aromatic). Lone pair on nitrogen is the key feature → basicity and nucleophilicity. Aromatic amines are less basic (resonance).
IUPAC: alkane → alkanamine (drop -e, add -amine)
For secondary and tertiary amines, prefix with N-alkyl or N,N-dialkyl.
Examples:
IUPAC: Named as derivatives of aniline (or benzeneamine).
Examples:
| Common Name | IUPAC Name | Formula |
|---|---|---|
| Methylamine | Methanamine | CH₃NH₂ |
| Ethylamine | Ethanamine | C₂H₅NH₂ |
| Dimethylamine | N-Methylmethanamine | (CH₃)₂NH |
| Trimethylamine | N,N-Dimethylmethanamine | (CH₃)₃N |
| Aniline | Aniline (benzeneamine) | C₆H₅NH₂ |
| N-Methylaniline | N-Methylaniline | C₆H₅NHCH₃ |
| Ethylenediamine | Ethane-1,2-diamine | H₂N-CH₂-CH₂-NH₂ |
Mistake: "N,N-dimethylmethanamine is the same as 2-methylmethanamine."
Correction: N,N-dimethyl means both methyl groups are on the NITROGEN. 2-methyl would mean on carbon 2 of the chain. For (CH₃)₃N, it's N,N-dimethylmethanamine (all three methyls, one as main chain + two as N-substituents). N-locants are always used for substituents on nitrogen.
IUPAC: alkanamine. N- prefix for substituents on nitrogen. Aniline for aromatic amines. Multiple NH₂ → diamine, triamine. Positional locants for ring substituents.
Hybridization: Nitrogen in amines is sp³ hybridized.
Geometry: Pyramidal (trigonal pyramidal), like NH₃. Bond angles ~107° (slightly less than tetrahedral 109.5° due to lone pair).
Lone pair: One sp³ orbital contains the lone pair. This is the site of basicity and nucleophilicity.
Inversion: Amines undergo rapid inversion at room temperature (except when constrained in rings). So chiral amines without a stereogenic center are usually not resolvable.
The nitrogen has a lone pair of electrons that can accept a proton (H⁺) → Lewis base / Brønsted base.
R₃N + H⁺ ⇌ R₃NH⁺ (conjugate acid)
The basicity depends on availability of the lone pair:
Nitrogen in amines: sp³ hybridized, pyramidal geometry, one lone pair. The lone pair determines basicity (Brønsted base) and nucleophilicity. Alkyl groups increase basicity (+I effect); aryl groups decrease it (resonance).
R-NO₂ + H₂/Ni (or Fe/HCl or Sn/HCl) → R-NH₂
Ar-NO₂ + H₂/Pd (or Sn/HCl or Fe/HCl) → Ar-NH₂
Example: Ph-NO₂ + 3H₂ → Ph-NH₂ + 2H₂O (aniline from nitrobenzene)
R-CN + LiAlH₄ (or H₂/Ni or Na(Hg)/EtOH) → R-CH₂-NH₂
Product: primary amine with one extra carbon (chain extension).
Example: CH₃CN + 4[H] → CH₃CH₂NH₂ (ethylamine from acetonitrile)
R-CONH₂ + LiAlH₄ (or H₂/Ni, high T) → R-CH₂-NH₂
Example: CH₃CONH₂ + LiAlH₄ → CH₃CH₂NH₂ (acetamide → ethylamine)
R-CONH₂ + Br₂ + 4KOH → R-NH₂ + K₂CO₃ + 2KBr + 2H₂O
Product: primary amine with one carbon less than the amide (chain shortening).
Example: CH₃CH₂CONH₂ + Br₂ + 4KOH → CH₃CH₂NH₂ + K₂CO₃ + 2KBr + 2H₂O (propanamide → ethylamine)
Key feature: Reduces chain length by one carbon. Useful for going from carboxylic acid → primary amine (via amide).
Mechanism: Br₂ + KOH → KOBr (potassium hypobromite). Amide → N-bromoamide → isocyanate → amine (via decarboxylation).
Used for pure primary amines (no 2°/3° contamination).
Result: pure primary amine. Aromatic primary amines cannot be made by this method (aryl halides don't undergo SN2 with phthalimide).
Carbonyl + NH₃ (or amine) + H₂/Ni → amine
R₂C=O + NH₃ → imine (Schiff base) → reduction → 1° amine
R₂C=O + R'-NH₂ → imine → reduction → 2° amine
Useful for making mixed amines from carbonyls.
Ar-NO₂ + Sn/HCl → Ar-NH₂
Common for making aniline and substituted anilines.
Side reactions possible: when other reducible groups are present (halogen, etc.), use selective reagents.
Key routes: nitro reduction (→ amine), nitrile reduction (→ 1° amine), amide reduction (→ amine), Hoffmann bromamide (amide → 1° amine, chain shortened by 1 C), Gabriel (pure 1° amine from phthalimide + R-X), reductive amination (carbonyl + NH₃ → amine).
Amines have H-bonding (N-H ··· N) — but weaker than O-H bonds. Order for similar MW:
Carboxylic acids > Alcohols > Amines > Aldehydes/Ketones > Ethers > Alkanes
Among amines with same MW: 1° > 2° > 3° (more N-H bonds available for H-bonding)
Example: C₂H₅NH₂ (BP 17°C) > (CH₃)₂NH (BP 7°C) > (CH₃)₃N (BP 3°C)
Note: Amines have lower BP than corresponding alcohols (N-H bonds are weaker than O-H).
Lower amines (methanamine, ethanamine, diethylamine, etc.) are soluble in water — they form H-bonds with water.
Solubility decreases with increasing carbon chain length (hydrophobic effect).
Aromatic amines (aniline) are moderately soluble in water.
Most amines are also very soluble in organic solvents (alcohol, ether, benzene).
Lower aliphatic amines have fishy smell (like trimethylamine in fish).
Aromatic amines (aniline) are less smelly but toxic (absorb through skin).
Some diamines (putrescine, cadaverine) have extremely foul odors — produced by bacterial decomposition of proteins (putrefaction).
BP order: amine > ethers > alkanes (for similar MW). 1° > 2° > 3° amines (more N-H = more H-bonding). Lower amines water-soluble (H-bonding). Fishy smell from trimethylamine. Aromatic amines toxic.
Brønsted basicity: Ability to accept a proton (H⁺).
Lewis basicity: Ability to donate electron pair.
For amines: R₃N + H₂O ⇌ R₃NH⁺ + OH⁻
Stronger base → more R₃NH⁺ formed → higher K_b (or lower pK_b).
Alternatively: use pKa of the conjugate acid (R₃NH⁺). Lower pKa of conjugate acid = stronger base.
Choose a compound to see its pKa (of conjugate acid) and basicity rank.
(C₂H₅)₂NH > (C₂H₅)₃N ≈ C₂H₅NH₂ > NH₃ > C₆H₅NH₂ (aniline) > C₆H₅NHCH₃ > C₆H₅N(CH₃)₂
In aqueous solution, the order is somewhat anomalous due to solvation effects:
In gas phase (no solvation effect):
(C₂H₅)₃N > (C₂H₅)₂NH > C₂H₅NH₂ > NH₃ (alkyl groups stabilize cation by +I)
Aniline (Ph-NH₂) is much less basic than ammonia (NH₃) or aliphatic amines. Reasons:
pKa of aniline conjugate acid = 4.6 (very weak base). NH₃ pKa = 9.25. So aniline is ~10⁵ times weaker base.
Electron-donating groups (EDG) increase basicity:
Electron-withdrawing groups (EWG) decrease basicity:
Substitution on nitrogen:
Basicity = ability to donate lone pair to H⁺. Aliphatic amines > NH₃ > aniline. In water, 2° > 3° ≈ 1° (solvation effects). In gas phase, 3° > 2° > 1° > NH₃ (+I effect). Aniline much less basic (resonance). EWG decreases basicity, EDG increases.
R-NH₂ + HCl → R-NH₃⁺Cl⁻
R-NH₂ + H₂SO₄ → (R-NH₃⁺)₂SO₄²⁻
Demonstrates basic nature. Amine salts are water-soluble (unlike free amines, which are often less soluble).
Reverse: base (NaOH) regenerates the free amine: R-NH₃⁺Cl⁻ + NaOH → R-NH₂ + NaCl + H₂O
This is the basis of extraction of amines from mixtures.
R-NH₂ + R-X → R-NH-R' + HX (2° amine, contaminated with 3°, quaternary)
With excess CH₃I: R-NH₂ + 3CH₃I → R-N⁺(CH₃)₃ I⁻ (quaternary ammonium iodide)
Full methylation → quaternary salt. Then Ag₂O/H₂O → quaternary ammonium hydroxide → Hofmann elimination on heating → alkene + tertiary amine (Hofmann's exhaustive methylation test for ring size).
R-NH₂ + R'-COCl (or R'-CO-O-CO-R') → R-NH-CO-R' + HCl
Product: N-substituted amide (also called N-alkylamide).
Or with acid anhydride: R-NH₂ + (R'CO)₂O → R-NH-CO-R' + R'COOH
Use: Forms crystalline amides — used for identification. Also used as a "protecting group" in synthesis (amide is less reactive than free amine).
R-NH₂ + CHCl₃ + 3KOH (ethanolic) → R-NC + 3KCl + 3H₂O
Product: alkyl isocyanide (carbylamine) — has foul, offensive odor.
Positive for: Primary amines (1°) only. Aliphatic AND aromatic 1° amines give positive test.
Negative for: 2° and 3° amines.
Purpose: Distinguishes 1° amines from 2°/3° amines.
Reagent: NaNO₂ + HCl (forms HNO₂ in situ). Different products for 1°, 2°, 3°:
Primary (aliphatic): R-NH₂ + HNO₂ → R-OH + N₂↑ + H₂O
Product: alcohol + nitrogen gas. Quantitative N₂ evolution is used for structure determination (Van Slyke method for amino acids).
Primary (aromatic): Ar-NH₂ + HNO₂ (0-5°C, HCl) → Ar-N₂⁺Cl⁻ + 2H₂O
Product: diazonium salt (Ar-N₂⁺Cl⁻). Very useful intermediate for aromatic substitution (Sandmeyer, coupling, etc.).
Secondary: R₂NH + HNO₂ → R₂N-NO + H₂O
Product: N-nitrosamine (yellow oily liquid). No N₂ gas evolved.
Tertiary (aliphatic): R₃N + HNO₂ → no reaction (only forms soluble nitrite salt which hydrolyzes back)
Tertiary (aromatic): Ar-NR₂ + HNO₂ → p-nitroso-Ar-NR₂ (ring nitrosation)
R-NH₂ + R'₂C=O → R-N=CR'₂ + H₂O
Product: Schiff base (imine) — C=N double bond with R on N.
Used in reductive amination (with H₂/Ni for subsequent reduction) and in biochemistry (Schiff base intermediates in enzyme reactions).
Ar-NH-R (N-alkylaniline) + heat (HCl) → alkyl group migrates to para position of ring
Example: Ph-NH-CH₃ → p-CH₃-C₆H₄-NH₂ (p-toluidine)
The -NH₂ group is strongly activating (donates electron density via resonance) and ortho/para directing.
Hinsberg's test: Reaction with benzenesulfonyl chloride (Hinsberg reagent, Ph-SO₂Cl)
Carbylamine test: Only 1° amines give foul-smelling isocyanide.
HNO₂ test: 1° gives N₂ (aliphatic) or diazonium (aromatic). 2° gives N-nitrosamine. 3° gives no reaction (aliphatic) or ring nitrosation (aromatic).
| Test | 1° | 2° | 3° |
|---|---|---|---|
| Carbylamine (CHCl₃/KOH) | ✓ Foul smell (isocyanide) | ✗ | ✗ |
| Hinsberg (PhSO₂Cl) | Soluble in base | Insoluble | No reaction |
| HNO₂ (aliphatic) | N₂ gas evolved | Yellow oil (N-nitrosamine) | No reaction |
| HNO₂ (aromatic) | Diazonium salt | N-Nitrosamine | Ring nitrosation |
| Liebermann (nitroso) | No (if pure 1°) | Yes (2° amine) | Yes (3° amine) |
Amines react at nitrogen (lone pair): salt formation (with acids), alkylation (with R-X), acylation (with acid chloride → amide), carbylamine (1° only, foul smell), HNO₂ (1° → N₂ or diazonium, 2° → N-nitrosamine, 3° → no reaction/ring). Hinsberg test distinguishes 1°/2°/3°.
Diazonium salt: Ar-N₂⁺X⁻ — aromatic ring with -N₂⁺ group and a counter anion (usually Cl⁻ or HSO₄⁻).
Preparation: Diazotization of primary aromatic amine:
Ar-NH₂ + NaNO₂ + 2HCl (0-5°C) → Ar-N₂⁺Cl⁻ + NaCl + 2H₂O
Conditions: 0-5°C (low temperature to prevent decomposition), excess HCl.
Example: Aniline + NaNO₂/HCl (0°C) → benzenediazonium chloride (Ph-N₂⁺Cl⁻)
Stability: Aromatic diazonium salts are relatively stable at low T (0-5°C) but decompose on warming to give phenol + N₂.
1. Replacement of -N₂⁺ by other groups:
2. Coupling Reactions:
Ar-N₂⁺Cl⁻ + Ar'-H (with electron-donating group like -OH or -NH₂) → Ar-N=N-Ar' (azo dye) + HCl
Example: Benzenediazonium chloride + phenol (in alkaline medium) → p-hydroxyazobenzene (orange dye)
Benzenediazonium chloride + aniline → p-aminoazobenzene (yellow dye)
These azo dyes are used industrially for coloring textiles, foods, indicators (methyl orange, congo red).
3. Reduction:
Ar-N₂⁺ + SnCl₂/HCl → Ar-NH-NH₂ (aryl hydrazine)
The discovery that Prontosil (an azo dye) worked as an antibiotic in 1932 was a landmark in medicine — it led to the sulfa drug era and eventually to modern antibiotics. The azo group was actually just a "carrier" that released the active sulfanilamide in the body!
Diazonium salts (Ar-N₂⁺X⁻) prepared by diazotization of 1° aromatic amines at 0-5°C. Reactions: replacement of -N₂⁺ by OH (phenol), H (H₃PO₂), Cl (Sandmeyer), Br (Sandmeyer), I (KI), F (Balz-Schiemann), CN (Sandmeyer). Coupling with phenol/aniline → azo dyes. These are extremely versatile intermediates in aromatic chemistry.
"Aliphatic > Ammonia > Aniline"
Alkyl groups (+I) increase basicity. Aryl groups (resonance) decrease basicity.
"EDG ↑, EWG ↓" for substituted anilines.
"Only 1° gives smell"
CHCl₃/KOH + 1° amine → isocyanide (foul smell). 2° and 3° do NOT react.
"1° Soluble, 2° Insoluble, 3° No reaction"
With benzenesulfonyl chloride. 1° gives N-alkylsulfonamide (soluble in base). 2° gives N,N-dialkylsulfonamide (insoluble). 3° has no H → no reaction.
"1° gives N₂, 2° gives oil, 3° nothing"
Primary → N₂ gas + alcohol. Secondary → N-nitrosamine (yellow oil). Tertiary → no reaction.
"HOC-BrIF-CA" — replacement reactions:
Hypophosphorous (H₃PO₂), OH (H₂O), Cl (Sandmeyer), Br (Sandmeyer), I (KI), F (Balz-Schiemann), CN (Sandmeyer), Azo (coupling).
"Hoffmann shortens, Gabriel purifies"
Hoffmann bromamide: chain shortened by 1 C. Gabriel phthalimide: pure 1° amine (no 2°/3° contamination).
"DANS" — Dopamine, Adrenaline, Noradrenaline, Serotonin — all neurotransmitters with amine groups.
"Aniline's lone pair is busy"
The lone pair participates in resonance with the ring → less available for protonation → less basic. That's why aniline doesn't react with water (unlike aliphatic amines).
"C=N-R is a Schiff base"
Formed from carbonyl + primary amine (R-NH₂). Named after Hugo Schiff.
Use "Aliphatic > Ammonia > Aniline" for basicity order. "1° Soluble, 2° Insoluble, 3° No reaction" for Hinsberg. "HOC-BrIF-CA" for diazonium reactions. Hoffmann shortens by 1 C, Gabriel gives pure 1° amine.
Q1. Arrange in increasing order of basicity: NH₃, CH₃NH₂, (CH₃)₂NH, (CH₃)₃N, C₆H₅NH₂. Explain.
C₆H₅NH₂ < NH₃ < (CH₃)₃N < CH₃NH₂ < (CH₃)₂NH
(In aqueous solution — solvation effects matter.)
Aniline is weakest (lone pair delocalized into ring).
Aliphatic amines more basic than NH₃ (+I effect from methyl groups).
In water: 2° > 1° > 3° because more N-H bonds → more H-bonding with water → stabilizes the conjugate acid.
Q2. Give one test to distinguish between primary, secondary, and tertiary amines.
Reaction with CHCl₃ + KOH (ethanolic). Only primary amine gives the offensive-smelling isocyanide (carbylamine).
With benzenesulfonyl chloride (PhSO₂Cl):
1° → N-alkylsulfonamide (soluble in base)
2° → N,N-dialkylsulfonamide (insoluble in base)
3° → no reaction (no N-H)
1° (aliphatic) → N₂ gas; 2° → yellow oil (nitrosamine); 3° → no reaction.
Q3. Why is aniline less basic than ammonia? Give reasons.
In aniline, the lone pair on nitrogen is delocalized into the aromatic ring via resonance (the lone pair can form a double bond with ring carbons in some resonance structures). This makes the lone pair less available for protonation.
Due to resonance, the nitrogen adopts some sp² character → more s-character → electrons held more tightly → less basic.
The resonance structures of aniline are stabilized by the aromatic ring → lower energy → the molecule prefers to stay as aniline rather than become anilinium ion.
In contrast, ammonia has no such resonance — its lone pair is fully available.
Q4. Write the reactions of aniline with (a) HCl, (b) CHCl₃ + KOH, (c) NaNO₂ + HCl (0-5°C).
C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻ (anilinium chloride)
Demonstrates basic nature of aniline.
C₆H₅NH₂ + CHCl₃ + 3KOH → C₆H₅-NC + 3KCl + 3H₂O
Product: phenyl isocyanide (carbylamine) — has foul odor.
Carbylamine test for 1° amines.
C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O
Product: benzenediazonium chloride (Ph-N₂⁺Cl⁻) — a versatile intermediate.
Temperature must be low (0-5°C) to prevent decomposition.
Q5. How is aniline prepared from nitrobenzene? How is it converted to: (a) phenol, (b) chlorobenzene, (c) p-hydroxyazobenzene?
C₆H₅NO₂ + 3H₂ (H₂/Ni or Sn/HCl) → C₆H₅NH₂ + 2H₂O
Nitrobenzene is reduced to aniline.
Step 1: C₆H₅NH₂ + NaNO₂ + HCl (0-5°C) → C₆H₅N₂⁺Cl⁻ (diazonium salt)
Step 2: C₆H₅N₂⁺Cl⁻ + H₂O (warm) → C₆H₅OH + N₂ + HCl
Product: phenol.
Step 1: C₆H₅NH₂ + NaNO₂ + HCl (0-5°C) → C₆H₅N₂⁺Cl⁻
Step 2: C₆H₅N₂⁺Cl⁻ + Cu₂Cl₂/HCl → C₆H₅Cl + N₂
This is the Sandmeyer reaction.
Step 1: Diazotization of aniline → C₆H₅N₂⁺Cl⁻
Step 2: Coupling with phenol (in alkaline medium) — the diazonium ion attacks the para position of phenoxide ion:
C₆H₅N₂⁺Cl⁻ + C₆H₅OH (NaOH) → p-HO-C₆H₄-N=N-C₆H₅ (p-hydroxyazobenzene, orange dye)
This is a coupling reaction — forms an azo dye.
Q6. Discuss the Hofmann bromamide degradation with mechanism.
R-CONH₂ + Br₂ + 4KOH → R-NH₂ + K₂CO₃ + 2KBr + 2H₂O
Product: primary amine with one carbon LESS than the amide.
• Chain shortening by one carbon
• Gives pure primary amine (no 2°/3° contamination)
• Useful for converting carboxylic acid → amine (via amide)
Br₂ + 2KOH → KBr + KOBr + H₂O (or Br₂ + 2OH⁻ → Br⁻ + OBr⁻ + H₂O)
R-CONH₂ + OBr⁻ → R-CONHBr + OH⁻ (N-bromoamide)
R-CONHBr + OH⁻ → R-CONBr⁻ + H₂O
Then rearrangement (alkyl group migrates from C to N with loss of Br⁻) → R-N=C=O (isocyanate).
This is the key step — it's an intramolecular rearrangement.
R-N=C=O + 2OH⁻ → R-NH₂ + CO₃²⁻ (via carbamate intermediate)
Q7. Why is Gabriel phthalimide synthesis preferred for primary amines? What is its limitation?
Step 1: Phthalimide + KOH → potassium phthalimide
Step 2: K-phthalimide + R-X → N-alkylphthalimide (SN2 mechanism)
Step 3: N-alkylphthalimide + NaOH (or NH₂NH₂) → R-NH₂ + phthalic acid (or phthalhydrazide)
Other methods (e.g., reduction of nitriles, alkylation of NH₃) give MIXTURES of 1°, 2°, 3° amines — hard to separate.
Gabriel uses a nitrogen that's "protected" in phthalimide form. Only ONE alkylation occurs (K-phthalimide + R-X), then the amine is liberated. No secondary/tertiary amines can form.
Result: pure primary amine — no contamination.
• Only works for aliphatic primary amines (from primary alkyl halides).
• Does NOT work for aromatic primary amines — aryl halides (Ph-X) don't undergo SN2 with phthalimide ion (the ring carbon is sp² and doesn't accept nucleophilic attack easily).
• Secondary/tertiary alkyl halides give elimination (E2) as the major product instead of substitution.
• Phthalimide is not a suitable nucleophile for tertiary halides (steric hindrance).
Selected exercise questions from NCERT Class 12 Chemistry Chapter 9. Click each to reveal the solution.
(a) CH₃CH₂NH₂: Ethanamine (ethylamine)
(b) (CH₃)₂CHNH₂: Propan-2-amine (isopropylamine)
(c) CH₃NHCH₃: N-Methylmethanamine (dimethylamine)
(d) (CH₃)₃N: N,N-Dimethylmethanamine (trimethylamine)
(e) C₆H₅NHCH₃: N-Methylaniline
(a) 2-Aminotoluene: Toluene with NH₂ at ortho position → 2-methylaniline, i.e., o-toluidine
Structure: o-CH₃-C₆H₄-NH₂
(b) N-Methylbenzylamine: Benzylamine with methyl on nitrogen
Structure: Ph-CH₂-NH-CH₃
(c) N,N-Dimethylaniline: Aniline with two methyl groups on nitrogen
Structure: Ph-N(CH₃)₂
(a) Aromatic amines: p-Nitroaniline < Aniline < p-Toluidine
p-NO₂ is strongly electron-withdrawing (-M, -I) → decreases lone pair availability → weakest base.
CH₃ is electron-donating (+I, +M) → increases basicity → p-toluidine is strongest.
(b) Aliphatic amines: (C₂H₅)₃N < NH₃... wait, in water: NH₃ < (C₂H₅)₃N < C₂H₅NH₂ < (C₂H₅)₂NH
Explanation: In water, the conjugate acids must be solvated. More N-H bonds = more H-bonding with water = more stable conjugate acid = more basic. So 2° > 1° > 3° > NH₃.
Note: In GAS PHASE, 3° > 2° > 1° > NH₃ (alkyl +I effect).
Hinsberg's test: Reaction with benzenesulfonyl chloride (PhSO₂Cl):
Primary amine: R-NH₂ + PhSO₂Cl → R-NH-SO₂Ph + HCl. Product: N-alkylbenzenesulfonamide. Soluble in base (forms sodium salt due to acidic N-H).
Secondary amine: R₂NH + PhSO₂Cl → R₂N-SO₂Ph + HCl. Product: N,N-dialkylbenzenesulfonamide. No acidic N-H → NOT soluble in base.
Tertiary amine: R₃N + PhSO₂Cl → no reaction (no N-H to react).
Carbylamine test: Only 1° amines react with CHCl₃/KOH → isocyanide (foul smell). 2° and 3° don't react.
(a) Hoffmann bromamide degradation:
R-CONH₂ + Br₂ + 4KOH → R-NH₂ + K₂CO₃ + 2KBr + 2H₂O
Product: primary amine with one less carbon than the amide.
Example: CH₃CH₂CONH₂ → CH₃CH₂NH₂
(b) Gabriel phthalimide synthesis:
Step 1: Phthalimide + KOH → K-phthalimide
Step 2: K-phthalimide + R-X (SN2) → N-alkylphthalimide
Step 3: N-alkylphthalimide + NaOH (or NH₂NH₂) → R-NH₂ + phthalic acid (or phthalhydrazide)
Result: pure primary amine.
(c) Carbylamine reaction:
R-NH₂ + CHCl₃ + 3KOH (ethanolic) → R-NC + 3KCl + 3H₂O
Only 1° amines give this reaction. Product has foul odor (isocyanide).
Used as a test for primary amines.
Answer: Gabriel phthalimide synthesis requires the reaction of K-phthalimide (a nucleophile) with an alkyl halide (R-X) via SN2 mechanism. In the SN2 reaction, the nucleophile attacks the carbon attached to the halogen from the backside.
Aryl halides (Ar-X) do NOT undergo SN2 reactions because:
• The C-X bond in aryl halides has partial double bond character (resonance) → hard to break
• The sp² carbon is more compact → harder for nucleophile to approach
• The electron-rich aromatic ring repels nucleophiles
So K-phthalimide cannot attack an aryl halide to form the N-arylphthalimide. Therefore, aromatic primary amines cannot be made by this method. Instead, aryl amines are made by reduction of nitroarenes (nitrobenzene → aniline).
Primary amine (aliphatic):
R-NH₂ + HNO₂ → R-OH + N₂ + H₂O
Product: alcohol + nitrogen gas. Quantitative N₂ evolution used for structure determination.
Primary amine (aromatic):
Ar-NH₂ + HNO₂/HCl (0-5°C) → Ar-N₂⁺Cl⁻ + 2H₂O
Product: diazonium salt. Requires low temperature.
Secondary amine:
R₂NH + HNO₂ → R₂N-N=O + H₂O
Product: N-nitrosamine (yellow oily liquid). No N₂ evolved.
Tertiary amine (aliphatic):
R₃N + HNO₂ → no reaction (forms unstable nitrite which hydrolyzes back).
Tertiary amine (aromatic):
Ar-NR₂ + HNO₂ → p-NO-C₆H₄-NR₂ (ring nitrosation)
Product: p-nitroso-N,N-dialkylaniline.
(a) Aniline → phenol:
Step 1: C₆H₅NH₂ + NaNO₂ + HCl (0-5°C) → C₆H₅N₂⁺Cl⁻
Step 2: C₆H₅N₂⁺Cl⁻ + H₂O (warm) → C₆H₅OH + N₂ + HCl
(b) Aniline → chlorobenzene:
Step 1: Diazotization → C₆H₅N₂⁺Cl⁻
Step 2: + Cu₂Cl₂/HCl (Sandmeyer) → C₆H₅Cl + N₂
(c) Aniline → benzoic acid:
Step 1: Diazotization → C₆H₅N₂⁺Cl⁻
Step 2: + CuCN/KCN (Sandmeyer) → C₆H₅CN (benzonitrile)
Step 3: C₆H₅CN + H₂O/H⁺ (hydrolysis) → C₆H₅COOH (benzoic acid)
(a) Sandmeyer reaction: Replacement of diazonium group by halide or cyanide using copper(I) salts:
Ar-N₂⁺Cl⁻ + Cu₂Cl₂/HCl → Ar-Cl + N₂ (chlorobenzene)
Ar-N₂⁺Cl⁻ + Cu₂Br₂/HBr → Ar-Br + N₂ (bromobenzene)
Ar-N₂⁺Cl⁻ + CuCN/KCN → Ar-CN + N₂ (benzonitrile)
(b) Balz-Schiemann reaction: Replacement of diazonium group by fluoride:
Ar-N₂⁺Cl⁻ + NaBF₄ → Ar-N₂⁺BF₄⁻
Ar-N₂⁺BF₄⁻ (heat) → Ar-F + N₂ + BF₃
Product: fluorobenzene. This is the main way to make aryl fluorides (since F is hard to introduce by other methods).
(c) Coupling reaction: Diazonium salt + activated arene (phenol or aniline):
C₆H₅N₂⁺Cl⁻ + C₆H₅OH/NaOH → p-HO-C₆H₄-N=N-C₆H₅ (p-hydroxyazobenzene, orange dye)
C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ → p-H₂N-C₆H₄-N=N-C₆H₅ (p-aminoazobenzene, yellow dye)
Forms azo compounds (Ar-N=N-Ar') — used as dyes and indicators.
(a) Aniline weaker than NH₃: In aniline, the lone pair on nitrogen participates in resonance with the aromatic ring → lone pair less available for protonation → weaker base. NH₃ has no such resonance; its lone pair is fully available.
(b) p-Nitroaniline is weaker than aniline:
The -NO₂ group is strongly electron-withdrawing (via -M and -I effects). It further delocalizes the lone pair of nitrogen into the ring (extending the resonance to the nitro group), making it even less available for protonation.
Result: p-nitroaniline (pKb ≈ 13) is much weaker base than aniline (pKb ≈ 9.4). The higher pKb means weaker base.
p-Toluidine (with -CH₃, an EDG) has pKb ≈ 8.9 — stronger than aniline.
(a) Acetamide + Br₂/KOH: CH₃CONH₂ → CH₃NH₂ (methylamine) — Hofmann bromamide degradation
(b) Aniline + CHCl₃/KOH: C₆H₅NH₂ → C₆H₅NC (phenyl isocyanide) — carbylamine reaction
(c) Aniline + NaNO₂/HCl (0-5°C): C₆H₅NH₂ → C₆H₅N₂⁺Cl⁻ (benzenediazonium chloride)
(d) N-Methylaniline + HNO₂: Ph-NH-CH₃ → Ph-N(NO)-CH₃ (N-methyl-N-nitrosoaniline, yellow oil)
(e) Benzenediazonium chloride + phenol (NaOH): C₆H₅N₂⁺Cl⁻ + C₆H₅OH → p-HO-C₆H₄-N=N-C₆H₅ (p-hydroxyazobenzene, orange dye) — coupling reaction
| Diethylamine (2°) | pKa ≈ 10.9 | Most basic (in water) |
| Ethanamine (1°) | pKa ≈ 10.7 | Strong base |
| Trimethylamine (3°) | pKa ≈ 9.8 | Lower than 2° (solvation) |
| Ammonia (NH₃) | pKa ≈ 9.25 | Reference |
| p-Toluidine | pKa ≈ 5.1 | EDG increases basicity |
| Aniline (Ph-NH₂) | pKa ≈ 4.6 | Weak base (resonance) |
| N-Methylaniline | pKa ≈ 4.8 | Slightly more than aniline |
| N,N-Dimethylaniline | pKa ≈ 5.1 | +I but solvation issues |
| p-Nitroaniline | pKa ≈ 1.0 | Very weak (EWG) |
| 2,4-Dinitroaniline | pKa ≈ −4.0 | Extremely weak (two EWG) |
| R-NH₂ | Primary amine |
| R₂NH | Secondary amine |
| R₃N | Tertiary amine |
| R₄N⁺X⁻ | Quaternary ammonium salt |
| Ar-NH₂ | Aromatic amine (aniline derivative) |
| Ar-N₂⁺Cl⁻ | Diazonium chloride |
| pKa | Negative log of acid dissociation constant |
| pKb | Negative log of base dissociation constant |
| Hinsberg reagent | Benzenesulfonyl chloride (PhSO₂Cl) |
| HNO₂ | Nitrous acid (NaNO₂ + HCl) |
| Hofmann | Bromamide degradation |
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
All key reactions & facts at one place — perfect for quick revision before exams.