Revision: Organic Compounds Containing Nitrogen >> Amines Chemistry HSC Science (General) 12th Standard Board Exam Maharashtra State Board

Primary amines (1°) are the ammonia derivatives in which one H-atom has been replaced by one alkyl or aryl group.

Tertiary amines (3°) are the ammonia derivatives in which three hydrogen atoms have been replaced by three alkyl or aryl group.

Secondary amines (2°) are the ammonia derivatives in which two hydrogen atoms have been replaced by two alkyl or aryl groups.

• Amines are derivatives of ammonia (NH₃) where one, two, or all three hydrogen atoms are replaced by alkyl or aryl groups.
• They contain a nitrogen atom with a lone pair of electrons → this lone pair makes amines basic in nature.
• Compounds of nitrogen connected to a carbonyl group are called amides (R–CO–NR'R'') — different from amines.

• 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.

In amines, N atom is sp³ hybridised. Among four sp³ hybridised orbital one contains lone pair of electrons and other three contain bond pairs of electrons.

Due to a lone pair of electrons, it is pyramidal in shape.

• Lower aliphatic amines: Used as solvents.
• Aromatic amines: Used in the manufacture of polymers, dyes (azo dyes), and in the rubber industry.
• Quaternary ammonium salts: Used as detergents (e.g., cetyltrimethylammonium bromide).
• Aniline: Starting material for dyes (e.g., aniline dyes), drugs (sulphonamide drugs), and polymers.

Only 1° amines (aliphatic and aromatic) react with CHCl₃ and alc. KOH to give isocyanides (carbylamines) with very bad smell.

• Used as a test for 1° amines (Carbylamine test / Isocyanide test).
• 2° and 3° amines do NOT give this reaction.

\[ \underset{1^\circ \text{ amine}}{\mathrm{RNH}_2} + \mathrm{CHCl}_3 + \text{Alc. } 3\mathrm{KOH} \longrightarrow \underset{\text{Alkyl isocyanide}}{\mathrm{RNC}} + 3\mathrm{KCl} + 3\mathrm{H}_2\mathrm{O} \]

HNO₂ is generated in situ: NaNO₂ + HCl → HNO₂

Primary Aliphatic Amines:

Primary Aromatic Amines (Diazotisation):

'Aromatic diazonium salts are stable due to delocalisation of the +ve charge in the benzene ring.

Secondary Amines (both aliphatic and aromatic):

\[ \underset{\text{Dimethylamine}}{(\mathrm{CH}_3)_2 \ddot{\mathrm{N}}\mathrm{H}} + \mathrm{HCl} + \mathrm{NaNO}_2 \xrightarrow{\mathrm{H}_2\mathrm{O}} [(\mathrm{HONO})] \underset{\substack{\text{N-Nitrosodimethylamine} \\ \text{(a yellow oil)}}}{(\mathrm{CH}_3)_2 \ddot{\mathrm{N}} - \ddot{\mathrm{N}} = \mathrm{O}} \]

Tertiary Aliphatic Amines:

\[ \underset{\text{Tertiary aliphatic amine}}{2\mathrm{R}_3\mathrm{N}} + \mathrm{HX} + \mathrm{NaNO}_2 \rightleftharpoons \underset{\text{Amine salt}}{\mathrm{R}_3\overset{+}{\mathrm{N}}\mathrm{H} \text{ } \mathrm{X}^-} + \underset{\text{N-Nitrosoammonium compound}}{\mathrm{R}_3\overset{+}{\mathrm{N}}-\ddot{\mathrm{N}}=\mathrm{O}\mathrm{X}^-} \]

Tertiary Aromatic Amines:

Electrophilic substitution reaction of arylamines:

Reduction of Alkyl Nitriles (Nitrile Reduction):

\[ \mathrm{R} - \mathrm{C} \equiv \mathrm{N} \xrightarrow[\mathrm{LiAlH}_4 \text{ or } \mathrm{Na} / \mathrm{C}_2\mathrm{H}_5\mathrm{OH}]{\text{Raney Ni} / \mathrm{H}_2} \mathrm{R} - \mathrm{CH}_2\mathrm{NH}_2 \]

Reduction of Amides:

\[ \mathrm{R} - \mathrm{CONH}_2 \xrightarrow{\mathrm{LiAlH}_4 / \text{ether}} \mathrm{R} - \mathrm{CH}_2\mathrm{NH}_2 \]

Reduction of Oximes:

\[ \mathrm{R} - \mathrm{CH} = \mathrm{NOH} + 4[\mathrm{H}] \xrightarrow[\mathrm{LiAlH}_4]{\mathrm{Na} / \mathrm{C}_2\mathrm{H}_5\mathrm{OH}} \mathrm{R} - \mathrm{CH}_2\mathrm{NH}_2 + \mathrm{H}_2\mathrm{O} \]

Reductive Amination (from Aldehydes/Ketones):

\[ \mathrm{R} - \mathrm{CH} = \mathrm{O} \xrightarrow{\mathrm{NH}_2\mathrm{OH}} \mathrm{R} - \mathrm{CH} = \mathrm{N} - \mathrm{OH} \xrightarrow{\mathrm{LiAlH}_4} \mathrm{R} - \mathrm{CH}_2 - \mathrm{NH}_2 \]

Gabriel Phthalimide Reaction (for 1° amines only):

Hoffmann Bromamide Degradation (gives 1° amine with one less carbon):

\[ \underset{\text{Amide}}{\mathrm{RCONH}_2} + \mathrm{Br}_2 + 4\mathrm{KOH} \longrightarrow \underset{1^\circ \text{ amine}}{\mathrm{RNH}_2} + 2\mathrm{KBr} + 2\mathrm{H}_2\mathrm{O} + \mathrm{K}_2\mathrm{CO}_3 \]

Mechanism:

 

Schmidt Reaction:

Monocarboxylic acid react with hydrazoic acid give primary amine.

\[ \underset{\text{Acid}}{\mathrm{RCOOH}} + \underset{\text{Hydrazoic acid}}{\mathrm{N}_3\mathrm{H}} \xrightarrow{\mathrm{H}_2\mathrm{SO}_4} \underset{1^\circ\text{amine}}{\mathrm{RNH}_2} + \mathrm{CO}_2 + \mathrm{N}_2 \]

In this reaction

 

Mendius Reaction:

Alkyl cyanide on reduction with Na⁺ C₂H₅OH gives a primary amine. This reaction is known as Mendius reaction.

\[ \underset{\text{Alkylcyanide}}{\mathrm{RCN}} + 4[\mathrm{H}] \xrightarrow[\text{or } \mathrm{Na}/\mathrm{C}_2\mathrm{H}_5\mathrm{OH} \text{ or } \mathrm{LiAlH}_4]{\mathrm{H}_2/\mathrm{Ni}} \underset{1^\circ\text{amine}}{\mathrm{RCH}_2\mathrm{NH}_2} \]