NEET Chemistry: Amines – Detailed Notes and Practice Questions

Chapter 13: Amines

1. Introduction to Amines

• Amines are organic compounds derived by replacing one or more hydrogen atoms of ammonia (NH3​) molecule by alkyl or aryl groups.
• They are basic in nature and act as Lewis bases due to the presence of a lone pair of electrons on the nitrogen atom.

2. Classification of Amines

Amines are classified based on the number of hydrogen atoms replaced in ammonia by alkyl or aryl groups:

1. Primary Amines (1° Amines): One hydrogen atom of NH3​ is replaced by an alkyl or aryl group.
   • General formula: R−NH2​ or Ar−NH2​
   • Examples: Methylamine (CH3​NH2​), Aniline (C6​H5​NH2​)
2. Secondary Amines (2° Amines): Two hydrogen atoms of NH3​ are replaced by alkyl or aryl groups.
   • General formula: R2​NH or Ar2​NH or R−NH−Ar
   • Examples: Dimethylamine (CH3​−NH−CH3​), N-Methylaniline (C6​H5​−NH−CH3​)
3. Tertiary Amines (3° Amines): All three hydrogen atoms of NH3​ are replaced by alkyl or aryl groups.
   • General formula: R3​N or Ar3​N or R2​N−Ar
   • Examples: Trimethylamine ((CH3​)3​N), N,N-Dimethylaniline (C6​H5​−N(CH3​)2​)

3. Nomenclature of Amines

A. Common System:

• Alkylamines are named by prefixing the alkyl group to “amine”.
• If two or three identical alkyl groups are present, prefixes di- or tri- are used.
• For mixed alkyl groups, they are named in alphabetical order.
• Examples: CH3​NH2​ (Methylamine), (CH3​)2​NH (Dimethylamine), CH3​−CH2​−NH−CH3​ (Ethylmethylamine)

B. IUPAC System:

• Amines are named as alkanamines. The ‘e’ of alkane is replaced by ‘amine’.
• The longest carbon chain containing the −NH2​ group is selected as the parent chain.
• Positions of substituents are indicated by numbers.
• For secondary and tertiary amines, the largest alkyl group is chosen as the parent amine, and other alkyl groups are designated as N-substituents.
• Examples: CH3​CH2​NH2​ (Ethanamine), CH3​CH(NH2​)CH3​ (Propan-2-amine), CH3​−NH−CH2​CH3​ (N-Methylethanamine), (CH3​)3​N (N,N-Dimethylmethanamine).
• Aromatic amines are generally named as derivatives of Aniline (C6​H5​NH2​).

4. Structure of Amines

• The nitrogen atom in amines is sp$^3$ hybridized.
• It has three bond pairs and one lone pair of electrons.
• Due to the presence of the lone pair, the geometry is pyramidal, not perfectly tetrahedral. The bond angle (C−N−H or C−N−C) is slightly less than 109.5∘, e.g., in trimethylamine, the C−N−C angle is 108∘.
• Amines undergo rapid interconversion (pyramidal inversion) at room temperature, making it difficult to isolate enantiomers for chiral amines unless specific conditions are met.

5. Preparation of Amines

A. Reduction of Nitro Compounds:

• Nitro compounds can be reduced to amines using various reducing agents.
  • By H2​/Pd, Pt, or Ni (catalytic hydrogenation): R−NO2​+3H2​Pd/Pt/Ni​R−NH2​+2H2​O
  • By Metals in Acidic Medium (e.g., Sn/HCl, Fe/HCl): R−NO2​+6[H]Sn/HCl or Fe/HCl​R−NH2​+2H2​O
  • Aromatic nitro compounds are best reduced by Fe/HCl because FeCl2​ formed is hydrolysed to produce HCl, so only a small amount of HCl is required initially.

B. Ammonolysis of Alkyl Halides:

• Alkyl halides react with alcoholic ammonia to give primary, secondary, tertiary amines, and finally quaternary ammonium salts. This is a nucleophilic substitution reaction. R−X+NH3​→R−NH2​+HX (Primary amine) R−NH2​+R−X→R2​NH+HX (Secondary amine) R2​NH+R−X→R3​N+HX (Tertiary amine) R3​N+R−X→R4​N+X− (Quaternary ammonium salt)
• Limitation: This method yields a mixture of amines, making it less suitable for preparing pure primary amines. Excess ammonia can maximize the yield of primary amine.

C. Reduction of Nitriles:

• Nitriles (cyanides) on reduction with LiAlH4​ (Lithium Aluminium Hydride) or catalytic hydrogenation (H2​/Ni, Pt, or Pd) produce primary amines. R−C≡N+4[H]LiAlH4​ or H2​/Ni​R−CH2​−NH2​
• This method is useful for synthesizing primary amines with one more carbon atom than the starting alkyl halide (if derived from alkyl halide converting to nitrile).

D. Reduction of Amides:

• Amides on reduction with LiAlH4​ yield primary amines. R−CO−NH2​+4[H]LiAlH4​​R−CH2​−NH2​+H2​O

E. Gabriel Phthalimide Synthesis:

• This method is used for the preparation of pure primary amines (aliphatic and aromatic with electron-donating groups).
• Phthalimide reacts with ethanolic KOH to form potassium phthalimide.
• Potassium phthalimide undergoes nucleophilic substitution with an alkyl halide (SN2 reaction) to give N-alkylphthalimide.
• N-alkylphthalimide on hydrolysis with dilute HCl or NaOH (or hydrazine, NH2​NH2​) forms a primary amine.
  • Limitation: Aromatic primary amines cannot be prepared by this method, because aryl halides do not undergo nucleophilic substitution reactions readily with potassium phthalimide.

F. Hoffmann Bromamide Degradation Reaction:

• This reaction is used for the preparation of primary amines by degrading an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. R−CO−NH2​+Br2​+4NaOH→R−NH2​+Na2​CO3​+2NaBr+2H2​O
• Key Feature: The amine formed contains one carbon atom less than the starting amide. This is a step-down reaction.

6. Physical Properties of Amines

• State: Lower aliphatic amines (up to C3​) are gases; C4​ to C11​ are liquids; higher amines are solids.
• Odour: Lower amines have a fishy ammoniacal smell. Aniline and other arylamines are generally colorless but get colored on storage due to atmospheric oxidation.
• Boiling Points:
  • Order: Primary amines > Secondary amines > Tertiary amines (for comparable molecular masses).
  • Reason: Primary and secondary amines can form intermolecular hydrogen bonds due to the presence of two and one hydrogen atoms on nitrogen, respectively. Tertiary amines do not have hydrogen directly bonded to nitrogen, so they do not form intermolecular hydrogen bonds.
  • Amines have lower boiling points than alcohols of comparable molecular masses (due to less electronegativity of N compared to O, making N-H bonds less polar than O-H bonds).
• Solubility:
  • Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
  • Solubility decreases with increase in molecular mass (due to increase in hydrophobic alkyl part).
  • Aromatic amines (like aniline) are largely insoluble in water due to the large hydrophobic aryl group. They are soluble in organic solvents like alcohol, ether, benzene.

7. Chemical Reactions of Amines (Basicity)

• Amines are basic due to the presence of a lone pair of electrons on the nitrogen atom, which can be donated to an acid. They act as Lewis bases and Brønsted bases.
• Order of Basicity:
  • In Gaseous Phase: Tertiary amines > Secondary amines > Primary amines > Ammonia.
    • Reason: The basicity in the gas phase is solely dependent on the +I effect (electron-donating effect) of alkyl groups, which increases electron density on nitrogen, making the lone pair more available. More alkyl groups mean stronger basicity. R3​N>R2​NH>RNH2​>NH3​
  • In Aqueous Solution: This depends on three factors:
    1. +I effect of alkyl groups: (Increases basicity as in gas phase).
    2. Steric hindrance: Alkyl groups hinder the approach of the proton.
    3. Solvation of ammonium ion: The stability of the conjugate acid (ammonium ion) formed by accepting a proton. Greater the solvation, greater the stability, greater the basicity. Solvation is achieved through hydrogen bonding with water.
       • R−NH3+​ can form 3 H-bonds.
       • R2​NH2+​ can form 2 H-bonds.
       • R3​NH+ can form 1 H-bond.
    • For Methyl Amines: (CH3​)2​NH>CH3​NH2​>(CH3​)3​N>NH3​ (Secondary > Primary > Tertiary > Ammonia)
    • For Ethyl Amines: (C2​H5​)2​NH>(C2​H5​)3​N>C2​H5​NH2​>NH3​ (Secondary > Tertiary > Primary > Ammonia)
    • Aromatic Amines vs. Aliphatic Amines: Aliphatic amines are generally much stronger bases than ammonia, while aromatic amines are weaker bases than ammonia.
      • Reason: In aromatic amines (like aniline), the lone pair on nitrogen is involved in resonance with the benzene ring, making it less available for protonation. This delocalization stabilizes the amine but destabilizes the anilinium ion (conjugate acid).
      • Electron-donating groups on the benzene ring increase basicity of aromatic amines.
      • Electron-withdrawing groups on the benzene ring decrease basicity of aromatic amines.

8. Alkylation of Amines

• Amines react with alkyl halides to form higher degree amines (similar to ammonolysis, but starting with amines). R−NH2​+R′−X→R−NHR′+HX R−NHR′+R′−X→R−NR2′​+HX R−NR2′​+R′−X→R−N+R3′​X− (Quaternary ammonium salt)

9. Acylation of Amines

• Primary and secondary amines react with acid chlorides, acid anhydrides, or esters to form amides. Tertiary amines do not undergo acylation due to the absence of a replaceable hydrogen atom on nitrogen. R−NH2​+R′−COClPyridine​R−NH−COR′+HCl (Pyridine acts as a base to remove HCl formed).
• Aromatic amines also undergo acylation. For example, aniline reacts with acetyl chloride to form acetanilide. This reaction is important for protecting the amino group during electrophilic substitution reactions.

10. Carbylamine Reaction (Isocyanide Test)

• This reaction is used to test for primary amines (aliphatic and aromatic).
• When a primary amine is heated with chloroform (CHCl3​) and alcoholic KOH, an offensive smelling isocyanide (carbylamine) is formed. R−NH2​+CHCl3​+3KOH(alc.)Heat​R−NC+3KCl+3H2​O
• Secondary and tertiary amines do not give this test.

11. Reaction with Nitrous Acid (HNO2​)

Nitrous acid (HNO2​) is prepared in situ by mixing NaNO2​ and dilute HCl. This reaction is used to distinguish between primary, secondary, and tertiary amines.

• Primary Aliphatic Amines: React with HNO2​ to form primary alcohols with the evolution of nitrogen gas and bubbles. R−NH2​+HNO2​NaNO2​/HCl​R−OH+N2​↑+H2​O (Often rearrangement and mixture of products occur)
• Primary Aromatic Amines (Diazotisation): React with HNO2​ at low temperature (0−5∘C) to form stable arenediazonium salts. Ar−NH2​+HNO2​+HCl273−278K​Ar−N2+​Cl−+2H2​O This reaction is called Diazotisation.
• Secondary Amines (Aliphatic and Aromatic): React with HNO2​ to form N-nitrosamines (yellow oily compounds). R2​NH+HNO2​→R2​N−N=O+H2​O (N-nitrosodimethylamine, etc.)
• Tertiary Aliphatic Amines: React with HNO2​ to form N-nitrosated ammonium salts.
• Tertiary Aromatic Amines: React with HNO2​ to undergo electrophilic substitution at para position to form p-nitroso derivative.

12. Reaction with Arylsulphonyl Chloride (Hinsberg’s Test)

• Used to distinguish primary, secondary, and tertiary amines using benzene sulphonyl chloride (C6​H5​SO2​Cl, also known as Hinsberg’s reagent).
• Primary Amine: Reacts with Hinsberg’s reagent to form N-alkylbenzenesulphonamide, which is soluble in KOH solution (due to acidic hydrogen on N). R−NH2​+C6​H5​SO2​Cl→C6​H5​SO2​NHR+HCl C6​H5​SO2​NHR+KOH→C6​H5​SO2​N−K+R+H2​O (Soluble salt)
• Secondary Amine: Reacts with Hinsberg’s reagent to form N,N-dialkylbenzenesulphonamide, which is insoluble in KOH solution (no acidic hydrogen on N). R2​NH+C6​H5​SO2​Cl→C6​H5​SO2​NR2​+HCl (Insoluble)
• Tertiary Amine: Does not react with Hinsberg’s reagent at all (no replaceable hydrogen on N). It remains insoluble in KOH.

13. Electrophilic Substitution in Aromatic Amines

• The −NH2​ group in aromatic amines (like aniline) is a strong activating group and ortho-para directing due to the resonance effect (+R effect).
• Bromination: Aniline reacts with bromine water at room temperature to give 2,4,6-tribromoaniline (a white precipitate) because the activating effect is so strong that substitution occurs at all available ortho and para positions.
  • To get monosubstituted product (e.g., p-bromoaniline), the amino group needs to be protected by acetylation (forming acetanilide), which reduces its activating power. The acetyl group can be removed by hydrolysis.
• Nitration: Direct nitration of aniline yields a mixture of ortho, meta, and para products, along with oxidation products. This is because in strongly acidic medium, aniline is protonated to form anilinium ion (C6​H5​NH3+​), which is meta-directing and deactivating.
  • To get p-nitroaniline, the amino group is protected by acetylation.
• Sulphonation: Aniline reacts with concentrated H2​SO4​ to form anilinium hydrogen sulphate, which on heating at 453−473K forms sulphanilic acid (p-aminobenzenesulphonic acid) as a zwitterion.

14. Diazonium Salts

• General formula: Ar−N2+​X− (where Ar is an aryl group, X− is Cl−, Br−, BF4−​, etc.).
• They are highly unstable and explosive in solid state, but stable in aqueous solution at 0−5∘C.
• Preparation (Diazotisation): By reacting primary aromatic amine (e.g., aniline) with nitrous acid (NaNO2​/HCl) at 0−5∘C. Ar−NH2​+NaNO2​+2HCl273−278K​Ar−N2+​Cl−+NaCl+2H2​O
• Reactions of Diazonium Salts:
  • Replacement by Halide or CN (Sandmeyer Reaction): Ar−N2+​Cl−Cu2​Cl2​/HCl or CuCl​Ar−Cl+N2​ Ar−N2+​Cl−Cu2​Br2​/HBr or CuBr​Ar−Br+N2​ Ar−N2+​Cl−Cu2​(CN)2​/KCN or CuCN​Ar−CN+N2​
  • Replacement by Halide (Gattermann Reaction): Uses copper powder and HX instead of cuprous halide. Ar−N2+​Cl−Cu/HCl​Ar−Cl+N2​+CuCl
  • Replacement by I: Ar−N2+​Cl−+KI→Ar−I+N2​+KCl
  • Replacement by F (Balz-Schiemann Reaction): Ar−N2+​Cl−+HBF4​→Ar−N2+​BF4−​Heat​Ar−F+BF3​+N2​
  • Replacement by H: Ar−N2+​Cl−+H3​PO2​+H2​O→Ar−H+N2​+H3​PO3​+HCl Ar−N2+​Cl−+C2​H5​OH→Ar−H+N2​+CH3​CHO+HCl
  • Replacement by -OH: Ar−N2+​Cl−+H2​OWarm​Ar−OH+N2​+HCl
  • Coupling Reactions: Diazonium salts react with phenols (in weakly alkaline medium) or aromatic amines (in weakly acidic medium) to form colored azo dyes (e.g., p-hydroxyazobenzene, p-aminoazobenzene). Ar−N2+​Cl−+C6​H5​OHH+​Ar−N=N−C6​H4​−OH(p)+HCl (Orange dye) Ar−N2+​Cl−+C6​H5​NH2​H+​Ar−N=N−C6​H4​−NH2​(p)+HCl (Yellow dye)

15. Uses of Amines and Diazonium Salts

• Amines are used as intermediates in the synthesis of drugs, dyes, polymers, and other organic compounds.
• Diazonium salts are versatile synthetic intermediates, particularly for preparing various substituted aromatic compounds (halides, cyanides, phenols) and azo dyes.