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Primary amines (1°) are the ammonia derivatives in which one H-atom has been replaced by one alkyl or aryl group.

Tertiary amines (3°) are the ammonia derivatives in which three hydrogen atoms have been replaced by three alkyl or aryl group.

Secondary amines (2°) are the ammonia derivatives in which two hydrogen atoms have been replaced by two alkyl or aryl groups.

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 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.

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.

• 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.

Replace 'e' of alkane by amine'.

For example, \[ \underset{\text{Ethanamine}}{\mathrm{CH}_3\mathrm{CH}_2\mathrm{NH}_2} \quad , \quad \underset{\text{Propan-1-amine}}{\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{NH}_2} \]

Secondary amines are named as N-alkylaminoalkanes, and tertiary amines are named as N, N-dialkylaminoalkanes

For example,

\[ \underset{\text{N-methylethanamine}}{\mathrm{CH}_3 - \underset{\underset{\displaystyle \mathrm{H}}{|}}{\mathrm{N}} - \mathrm{CH}_2 - \mathrm{CH}_3} \quad , \quad \underset{\text{N-methylmethanamine}}{\mathrm{CH}_3 - \underset{\underset{\displaystyle \mathrm{H}}{|}}{\mathrm{N}} - \mathrm{CH}_3} \]

Aromatic amine,

It is named as aniline or benzenamine.

• Common system: Aliphatic amine is named by adding 'amine' to the alkyl group (e.g., CH₃NH₂ – methylamine).
• Same groups in 2°/3° amines: Use prefix di or tri before the alkyl group name (e.g., trimethylamine).
• IUPAC – Primary amines: Named as alkanamines, replacing 'e' of alkane with 'amine' (e.g., methanamine).
• IUPAC – 2° & 3° amines: Named as N-alkylalkanamine or N,N-dialkylalkanamine (e.g., N,N-dimethylmethanamine).
• Multiple —NH₂ groups: Positions are numbered, terminal 'e' retained (e.g., ethane-1,2-diamine); aromatic amines named as aniline derivatives (e.g., N, N-dimethylaniline).

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.

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

• Amines are basic due to the lone pair on nitrogen → can donate to a proton (Bronsted base) or Lewis acids.
• In gas/non-aqueous phase: Basicity ∝ +I effect → 3° > 2° > 1° > NH₃
• In aqueous phase: Solvation (hydration) also plays a role.

Aqueous basicity order (aliphatic):

\[ \underset{(2^\circ)}{(\mathrm{CH}_3)_2\mathrm{NH}} > \underset{(1^\circ)}{\mathrm{CH}_3\mathrm{NH}_2} > \underset{(3^\circ)}{(\mathrm{CH}_3)_3\mathrm{N}} > \mathrm{NH}_3 \]

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:

• Diazonium salts: Compounds with the general formula ArN₂⁺X⁻ (e.g., C₆H₅N₂⁺Cl⁻ = Benzenediazonium chloride).
• X⁻ = Cl⁻, Br⁻, HSO₄⁻, NO₃⁻, BF₄⁻, etc.
• Benzenediazonium chloride: Colourless crystalline solid; readily soluble in water; stable in cold but decomposes on warming; decomposes easily in dry state.
• Benzenediazonium fluoroborate: Water insoluble, stable at room temperature.
• The stability of arenediazonium ions is due to resonance — the positive charge is delocalised into the benzene ring.

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​