Chapter 9: Amines

Structure of Amines

Nitrogen atom in amines is sp³ hybridized. The geometry is pyramidal due to the presence of one unshared electron pair (lone pair). The C-N-C or C-N-H bond angle is slightly less than 109.5° (e.g., 108° in trimethylamine) due to lone pair-bond pair repulsion.

Classification

• Primary (1°) Amine: Replace one H of NH₃ with an alkyl/aryl group (R-NH₂).
• Secondary (2°) Amine: Replace two H atoms (R₂NH).
• Tertiary (3°) Amine: Replace all three H atoms (R₃N).

Nomenclature Rules

Aliphatic amines are named by adding suffix 'amine' to the parent alkane (Alkanamine). For 2° and 3° amines, the largest group is chosen as the parent chain, and other groups are indicated with prefix 'N-alkyl'.

Reduction of Nitro, Nitriles & Amides

• Nitro compounds: Reduced by H₂/Pd or Sn/HCl or Fe/HCl (Fe/HCl is preferred because FeCl₂ formed gets hydrolyzed to release HCl during the reaction).
  R-NO₂ + 6[H] (Sn/HCl) → R-NH₂ + 2H₂O
• Nitriles: Reduced using LiAlH₄ or H₂/Ni or Na(Hg)/C₂H₅OH (Mendius Reduction).
  R-C≡N + 4[H] → R-CH₂-NH₂ (Steps up the carbon chain).
• Amides: Reduced by LiAlH₄ to yield amines with same number of carbon atoms.
  R-CO-NH₂ + LiAlH₄ → R-CH₂-NH₂

Gabriel Phthalimide Synthesis (For 1° Aliphatic Amines Only)

Phthalimide reacts with ethanolic KOH to form potassium salt. Heating this salt with alkyl halide followed by alkaline hydrolysis produces Primary Aliphatic Amines.

Note: Aromatic primary amines (Aniline) cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.

Hoffmann Bromamide Degradation (Step-Down Reaction)

Primary amides react with bromine (Br₂) in aqueous or ethanolic solution of NaOH to form primary amines. The amine formed contains one carbon less than the starting amide.

Basic Nature

Amines act as Lewis bases due to the lone pair of electrons on the nitrogen atom. They react with acids to form salts. Higher the basicity, lower is the pKb value.

Aliphatic Amines vs Ammonia

Aliphatic amines are stronger bases than NH₃ due to the +I (electron donating) effect of alkyl groups, which increases electron density on the nitrogen atom.

Basicity Order in Gas Phase

In gas phase, only +I effect works. Order: 3° > 2° > 1° > NH₃.

Basicity Order in Aqueous Phase (Most Asked)

In aqueous phase, basicity depends on three factors: +I effect, Solvation effect (hydrogen bonding with water), and Steric hindrance. The order is anomalous:

• For Methyl Group (-CH₃): 2° > 1° > 3° > NH₃
  (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃
• For Ethyl Group (-C₂H₅): 2° > 3° > 1° > NH₃
  (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃

Aromatic Amines (Aniline) vs Ammonia

Aniline is a weaker base than ammonia. The lone pair of electrons on nitrogen is involved in resonance with the benzene ring, making it less available for protonation.

Carbylamine Reaction (Isocyanide Test)

Aliphatic and aromatic Primary (1°) Amines on heating with chloroform and ethanolic potassium hydroxide form foul-smelling isocyanides (carbylamines). Secondary and tertiary amines do NOT show this reaction.

Reaction with Nitrous Acid (HNO₂)

• 1° Aliphatic Amines: Form aliphatic diazonium salts which are unstable and instantly decompose to yield alcohols and nitrogen gas (effervescence).
• 1° Aromatic Amines (Aniline): React with HNO₂ (NaNO₂ + HCl) at low temperature (0-5°C) to form stable Arenediazonium salts.

Hinsberg's Test (Distinction of 1°, 2°, 3° Amines)

Reagent: Benzenesulfonyl chloride (C₆H₅SO₂Cl).

• 1° Amine: Forms N-alkylbenzenesulfonamide. The H atom attached to nitrogen is highly acidic. It is soluble in alkali.
• 2° Amine: Forms N,N-dialkylbenzenesulfonamide. Does not have an acidic H atom attached to N. It is insoluble in alkali.
• 3° Amine: Does not react with Hinsberg's reagent at all.

Electrophilic Substitution of Aniline

• Bromination: Aniline reacts with Bromine water at room temp to give a white precipitate of 2,4,6-tribromoaniline. (To get mono-substituted product, NH₂ group is first protected by acetylation with acetic anhydride).
• Nitration: Direct nitration gives a significant amount of meta-nitroaniline (47%) along with para and ortho, because in acidic medium aniline gets protonated to anilinium ion which is meta-directing.

Structure & Stability

General formula: ArN₂⁺X⁻. Arene diazonium salts are stable for a short time at low temperatures (0-5°C) due to resonance stabilization.

Replacement Reactions

• Sandmeyer Reaction: Replacement of diazo group by Cl, Br, or CN using Cu(I) salts (Cu₂Cl₂, Cu₂Br₂, CuCN).
• Gattermann Reaction: Replacement of diazo group by Cl or Br using Copper powder in presence of corresponding halogen acid (Cu/HCl or Cu/HBr).
• Balz-Schiemann Reaction (Fluorination): Diazonium salt is treated with fluoroboric acid (HBF₄) to form a precipitate, which on heating decomposes to yield fluorobenzene.
• Replacement by H (Reduction): Treated with mild reducing agents like hypophosphorous acid (H₃PO₂) or ethanol to give Benzene.

Coupling Reactions (Azo Dye Test)

Diazonium salts react with highly reactive aromatic compounds (like phenol, aniline) to form highly colored azo compounds (-N=N- bond).

• With Phenol (in weakly basic medium pH 9-10) → p-Hydroxyazobenzene (Orange Dye).
• With Aniline (in weakly acidic medium pH 4-5) → p-Aminoazobenzene (Yellow Dye).