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Overview of Amines

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Estimated time: 37 minutes
CBSE: Class 12

Definition: Amines

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

CBSE: Class 12

Definition: Primary amine

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

R–NH₂

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Definition: Secondary amine

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

R–NH–R′

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Definition: Tertiary amine

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

R–N(R′)–R″

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Definition: Quaternary ammonium salt

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

R₄N⁺X⁻

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Definition: Ammonolysis

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

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Definition: Acylation

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

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Definition: Gabriel phthalimide synthesis

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

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Definition: Hofmann bromamide degradation reaction

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

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Definition: Diazotization reaction

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

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Ammonolysis

R X + NH3R NH2+ HX

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Gabriel Synthesis

Salt + R – X → R – NH2

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Hofmann Bromamide Reaction

R CONH2 + Br2+ 4NaOH R NH2+ Na2CO3 + 2NaBr + 2H2O

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Acylation

R NH2+ RCOCl R NHCO R+ HCl

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Reduction of Nitro Compounds

\[R–NO_2\xrightarrow{H_2/Pd}R–NH_2\]

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Reduction of Nitriles

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

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Diazotization (Aromatic Amines)

Ar NH2 + NaNO2 + HCl Ar N2+Cl

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Key Points: Basic Character of Amines

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

CBSE: Class 12

Key Points: Gabriel Phthalimide Synthesis Principle

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.

CBSE: Class 12

Key Points: Hofmann Bromamide Degradation Principle

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.

CBSE: Class 12

Key Points: Acylation Principle

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.

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Definition: Carbylamine reaction

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 + 3H2O

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Definition: Diazotization reaction

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

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Definition: Hinsberg reaction

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

Primary amine:

R NH2 + C6H5SO2Cl Sulphonamide

Secondary amine:

R2NH + C6H5SO2Cl N,N-dialkyl sulphonamide

Tertiary amine → No reaction

CBSE: Class 12

Definition: Sandmeyer reaction

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.

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

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

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

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Definition: Coupling 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

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