Revision: Amines Chemistry Science (English Medium) Class 12 CBSE

Primary amines (1°) are the ammonia derivatives in which one H-atom has been replaced by one alkyl or aryl group.

Secondary amines (2°) are the ammonia derivatives in which two hydrogen atoms have been replaced by two alkyl or aryl groups.

Tertiary amines (3°) are the ammonia derivatives in which three hydrogen atoms have been replaced by three alkyl or aryl group.

An amine in which three hydrogen atoms of ammonia are replaced by alkyl or aryl groups is called tertiary amine.

R–N(R′)–R″

The reaction in which amines react with acid chlorides or anhydrides to form amides is called acylation.

The method of preparation of primary amines using potassium phthalimide is called Gabriel phthalimide synthesis.

The degradation of amides to primary amines containing one carbon less is called Hofmann bromamide degradation reaction.

Organic compounds derived from ammonia by replacement of one or more hydrogen atoms by alkyl or aryl groups are called amines.

An amine in which two hydrogen atoms of ammonia are replaced by alkyl or aryl groups is called secondary amine.

R–NH–R′

The substitution reaction in which diazonium group is replaced by Cl, Br or CN in the presence of Cu(I) salts is called Sandmeyer reaction.

\[ArN_2^+Cl^-\xrightarrow{CuCl/HCl}ArCl+N_2\]

\[ArN_2^+Cl^-\xrightarrow{CuBr/HBr}ArBr+N_2\]

\[ArN_2^+Cl^-\xrightarrow{CuCN}ArCN+N_2\]

A positively charged nitrogen species formed by addition of four alkyl/aryl groups is called quaternary ammonium salt.

R₄N⁺X⁻

The reaction in which alkyl halides react with ammonia to form amines is called ammonolysis.

The reaction of amines with nitrous acid to form diazonium salts (in case of primary aromatic amines) is called diazotization reaction.

An amine in which one hydrogen atom of ammonia is replaced by an alkyl or aryl group is called primary amine.

R–NH₂

The reaction in which primary amines on heating with chloroform and alcoholic KOH form isocyanides is called Carbylamine reaction.

R – NH₂​ + CHCl₃​ + 3KOH → R – NC + 3KCl + 3H_2​O

The reaction of primary aromatic amines with nitrous acid at 273–278 K to form diazonium salts is called Diazotization reaction.

The reaction of primary and secondary amines with benzenesulphonyl chloride to form sulphonamides is called Hinsberg reaction.

Primary amine:

R – NH₂ ​+ C₆​H_5​SO_2​Cl → Sulphonamide

Secondary amine:

R_2​NH + C₆​H₅​SO₂​Cl → N,N-dialkyl sulphonamide

Tertiary amine → No reaction

The reaction in which diazonium salts couple with phenols or aromatic amines to form azo compounds is called Coupling reaction.

ArN₂⁺ ​Cl⁻ + Phenol → Azo compound

\[R–NO_2\xrightarrow{H_2/Pd}R–NH_2\]

\[R\neg CN\xrightarrow{LiAlH_4}R\neg CH_2NH_2\]

R – CONH_2​ + Br₂​ + 4NaOH → R – NH₂​ + Na_2​CO₃ ​+ 2NaBr + 2H_2​O

R – X + NH_3​ → R – NH₂​ + HX

Phthalimide + KOH → Potassium salt

Salt + R – X → R – NH₂​

R – NH₂​ + R′COCl → R – NHCO – R′ + HCl

Ar – NH_2​ + NaNO₂ ​+ HCl → Ar – N₂⁺​ Cl⁻

• Amines overview: Amines are derivatives of ammonia (NH₃) where one or more H-atoms are replaced by alkyl/aryl groups; they can be saturated or unsaturated.
• Classification by source: Aliphatic amines (e.g., CH₃–NH₂, methylamine) and Aromatic amines (e.g., C₆H₅–NH₂, aniline).
• Classification by H-substitution: Primary (1°, –NH₂, e.g., methylamine), Secondary (2°, –NH–, e.g., dimethylamine), and Tertiary (3°, –N–, e.g., trimethylamine).
• Sub-types of 2° and 3° amines: Simple/symmetrical (same groups, e.g., (C₂H₅)₂NH, (C₂H₅)₃N) and Mixed/unsymmetrical (different groups, e.g., C₂H₅–NH–CH₃, C₂H₅–N(CH₃)₂).
• Examples of each class: 1° – n-butylamine, sec-butylamine, tert-butylamine, aniline; 2° – dimethylamine, diethylamine, diphenylamine; 3° – trimethylamine, triethylamine, triphenylamine.

• Preparation: Amines are obtained by reduction of alkyl nitriles (LiAlH₄ / Na-ethanol – Mendius), nitroalkanes (LiAlH₄ or Sn/HCl), and amides (LiAlH₄).
• Hoffmann's ammonolysis: RX + excess alc. NH₃ → mixture of 1°, 2°, 3° amines & quaternary salt; reactivity RI > RBr > RCl.
• Gabriel phthalimide synthesis: Phthalimide + KOH → R–X → hydrolysis gives pure 1° amine (RNH₂).
• Hoffmann bromamide degradation: RCONH₂ + Br₂ + 4KOH → RNH₂ (amine with one less C).
• Reactions of ethanamine: exhaustive alkylation → (C₂H₅)₄N⁺Br⁻; CH₃COCl → N-ethylethanamide; CHCl₃/KOH → ethyl isocyanide (carbylamine test); HNO₂ → ethanol; C₆H₅SO₂Cl → Hinsberg's test product.

Primary amines are prepared by treating potassium phthalimide with alkyl halides followed by hydrolysis.

Key points:

• Gives only primary amines.

• Not suitable for aryl halides.

• Avoids formation of secondary and tertiary amines.

Primary and secondary amines react with acid chlorides or anhydrides to form amides via nucleophilic substitution.

Key Points:

• Used for identification of amines.

• Tertiary amines do not undergo acylation.

• Reaction carried out in presence of base.

Primary amides on treatment with bromine and sodium hydroxide give primary amines containing one carbon atom less than the parent amide.

Key Points:

• Carbonyl carbon is lost as CO₂.

• Rearrangement reaction.

• Useful for chain shortening.

Amines are basic in nature due to the presence of a lone pair of electrons on nitrogen which can accept a proton.

Factors affecting basicity:

• +I effect of alkyl groups increases basicity.
• Solvation effect in aqueous solution.
• Steric hindrance decreases basicity.
• Aromatic amines are less basic due to resonance.

Order in gaseous phase:

3^∘ > 2^∘ >1^∘ >NH₃​

Order in aqueous phase:

2^∘ > 1^∘ > 3^∘ > NH₃​