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Unit 9: Amines

9.1 Classification and Nomenclature

Amines constitute an important class of organic compounds derived by replacing one or more hydrogen atoms of ammonia molecule by alkyl/aryl group(s).

  • Primary (\(1^\circ\)) amines: One hydrogen replaced by R/Ar. (e.g., \(CH_3-NH_2\))
  • Secondary (\(2^\circ\)) amines: Two hydrogens replaced by R/Ar. (e.g., \(CH_3-NH-CH_3\))
  • Tertiary (\(3^\circ\)) amines: All three hydrogens replaced by R/Ar. (e.g., \((CH_3)_3N\))

Like ammonia, the nitrogen atom in amines is \(sp^3\) hybridized, and the geometry is pyramidal due to the presence of an unshared pair of electrons on the nitrogen atom.

9.2 Methods of Preparation

  1. Reduction of nitro compounds: \(-NO_2\) to \(-NH_2\) using \(H_2/Pd\) or \(Sn/HCl\) or \(Fe/HCl\).
  2. Ammonolysis of alkyl halides: Reaction of alkyl halides with alcoholic ammonia. Leads to a mixture of primary, secondary, tertiary amines and quaternary ammonium salts.
  3. Reduction of nitriles: \(R-C \equiv N\) to \(R-CH_2-NH_2\) using \(LiAlH_4\) or \(H_2/Ni\).
  4. Reduction of amides: \(R-CO-NH_2\) to \(R-CH_2-NH_2\) using \(LiAlH_4\).
  5. Gabriel phthalimide synthesis: Used for the preparation of pure primary aliphatic amines. (Phthalimide + KOH \(\rightarrow\) Potassium phthalimide \(\xrightarrow{RX}\) N-alkylphthalimide \(\xrightarrow{aq. NaOH}\) Primary amine).
  6. Hoffmann bromamide degradation reaction: Migration of an alkyl or aryl group from carbonyl carbon of the amide to the nitrogen atom to form a primary amine containing one carbon atom less than the original amide. \[ R-CO-NH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O \]

9.3 Physical and Chemical Properties

  • Boiling Points: Primary and secondary amines are engaged in intermolecular hydrogen bonding. Boiling points are \(1^\circ > 2^\circ > 3^\circ\) isomeric amines. Furthermore, they are lower than corresponding alcohols.
  • Basic Character: Amines are Lewis bases (electron pair donors). The basicity of amines is influenced by +I (inductive) effect, solvation (hydration) effect, and steric hindrance.
    • Due to +I effect, alkylamines are stronger bases than ammonia.
    • Arylamines (e.g., aniline) are much weaker bases than ammonia. The lone pair of electrons on nitrogen is delocalized over the benzene ring through resonance, making it less available for protonation.

Identification of Primary, Secondary, and Tertiary Amines (Hinsberg Test)

Hinsberg’s Reagent: Benzenesulphonyl chloride (\(C_6H_5SO_2Cl\)).

  1. Primary amine: Reacts to form N-alkylbenzenesulphonamide, which has an acidic hydrogen attached to nitrogen. It is soluble in alkali.
  2. Secondary amine: Reacts to form N,N-dialkylbenzenesulphonamide, which does not have any acidic hydrogen. It is insoluble in alkali.
  3. Tertiary amine: Does not react with Hinsberg’s reagent.

Other Important Chemical Reactions

  1. Carbylamine reaction (Isocyanide test): Only primary aliphatic and aromatic amines on heating with chloroform and ethanolic KOH form foul-smelling isocyanides or carbylamines.
  2. Reaction with Nitrous acid (\(HNO_2\)):
    • Primary aliphatic amines form highly unstable aliphatic diazonium salts, which decompose to give alcohols and nitrogen gas.
    • Primary aromatic amines (aniline) form relatively stable arenediazonium salts at low temperatures (273-278 K).
  3. Electrophilic substitution in Aniline: The \(-NH_2\) group is strongly activating and ortho, para directing.
    • Bromination with aqueous \(Br_2\) yields 2,4,6-tribromoaniline instantly.
    • To get a monosubstituted product, the \(-NH_2\) group must be protected by acetylation (with acetic anhydride) to form acetanilide before electrophilic substitution, and then hydrolyzed back.

9.4 Diazonium Salts

General formula: \(R-N_2^+ X^-\) Preparation (Diazotisation): \[ C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O \]

Chemical Reactions:

  • Sandmeyer’s Reaction: Treatment with CuCl/HCl, CuBr/HBr or CuCN/KCN yields chlorobenzene, bromobenzene, or cyanobenzene respectively.
  • Gattermann Reaction: Done with Cu powder / HCl or HBr.
  • Replacement by iodide ion: Warming with KI gives iodobenzene.
  • Replacement by fluoride ion (Balz-Schiemann Reaction): Reaction with \(HBF_4\) followed by heating gives fluorobenzene.
  • Coupling Reactions: Diazonium ions act as weak electrophiles and couple with activated aromatic rings such as phenols (giving orange dye) and anilines (giving yellow dye).

Competency-Based Questions (CBQs)

Q1. (CBSE 2022) Arrange the following compounds in decreasing order of their basic strength in aqueous solution: (a) \(C_6H_5NH_2\), \(C_2H_5NH_2\), \((C_2H_5)_2NH\), \(NH_3\) (b) Give the chemical equation for the Hinsberg test of a secondary amine.


Answer: (a) Decreasing order of basic strength in aqueous solution: \((C_2H_5)_2NH\) > \(C_2H_5NH_2\) > \(NH_3\) > \(C_6H_5NH_2\)

Reasoning:

  • Aliphatic amines are generally more basic than ammonia due to the electron-donating inductive (+I) effect of alkyl groups, which increases electron density on the nitrogen atom.
  • Among ethylamines in aqueous solution, the secondary amine is more basic than the primary amine because the +I effect of two ethyl groups is greater than one, while the steric hindrance is not huge for the ethyl group compared to the +I and hydration effects. (Note: For methylamines, it’s 2° > 1° > 3°; for ethylamines, it’s 2° > 3° > 1° but here only 1° and 2° are given).
  • Aniline (\(C_6H_5NH_2\)) is the weakest base because the lone pair of electrons on nitrogen is delocalized into the benzene ring via resonance.

(b) Hinsberg test for a secondary amine (e.g., diethylamine): \[ (C_2H_5)_2NH + C_6H_5SO_2Cl \rightarrow C_6H_5SO_2-N(C_2H_5)_2 + HCl \] (N,N-diethylbenzenesulphonamide precipitates out. Since there is no replaceable hydrogen on the nitrogen atom of the product, it is insoluble in aq. KOH/NaOH).

Q2. (CBSE 2021) How can you convert aniline to chlorobenzene via a diazonium salt? Provide the sequences of reactions.


Answer: This conversion is achieved in two steps using the Sandmeyer Reaction.

Step 1: Diazotisation of Aniline. Aniline is treated with nitrous acid (prepared in situ from sodium nitrite and hydrochloric acid) at a low temperature (0 - 5 °C or 273 - 278 K) to form benzene diazonium chloride. \[ C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O \]

Step 2: Sandmeyer’s Reaction. The freshly prepared benzene diazonium chloride solution is mixed with cuprous chloride (CuCl) dissolved in HCl. The diazonium group is replaced by chlorine. \[ C_6H_5N_2^+Cl^- + CuCl/HCl \xrightarrow{\Delta} C_6H_5-Cl , (\text{Chlorobenzene}) + N_2 \uparrow \]

Q3. (Sample Paper 2024) Explain why Gabriel phthalimide synthesis is preferred for synthesizing primary amines, and why it cannot be used to prepare primary aromatic amines (like aniline).


Answer:

  1. Preference for Primary Amines: Gabriel phthalimide synthesis produces purely primary aliphatic amines. It involves the \(S_N2\) attack of the phthalimide nucleophile on an alkyl halide. Because the nitrogen in phthalimide is bonded to two bulky carbonyl groups, it can only attack one molecule of alkyl halide. Over-alkylation (which commonly occurs and leads to a mixture of \(1^\circ\), \(2^\circ\), and \(3^\circ\) amines in ammonolysis) is prevented.
  2. Inability to prepare Primary Aromatic Amines: To prepare an aromatic amine like aniline using this method, the nucleophilic potassium phthalimide would have to undergo nucleophilic substitution with an unreactive aryl halide (like chlorobenzene). Since haloarenes do not undergo nucleophilic substitution easily (due to partial double bond character of the C-X bond and other factors), Gabriel synthesis fails for producing aromatic primary amines.