Definitions [3]
Primary amines (1°) are the ammonia derivatives in which one H-atom has been replaced by one alkyl or aryl group.
Tertiary amines (3°) are the ammonia derivatives in which three hydrogen atoms have been replaced by three alkyl or aryl group.
Secondary amines (2°) are the ammonia derivatives in which two hydrogen atoms have been replaced by two alkyl or aryl groups.
Key Points
- 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.
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.
- Boiling point vs non-polar compounds: Due to intermolecular H-bonding, amines boil at a higher temperature than non-polar compounds such as hydrocarbons.
- Boiling point vs alcohols/acids: The boiling points of amines are less than those of alcohols and carboxylic acids.
- Solubility of lower amines: Lower members of aliphatic amines are soluble in water due to intermolecular H-bonding with water.
- Effect of alkyl group size: Solubility in water decreases as the size of the alkyl group increases.
- Order of boiling points & aromatic amines: Propane < ethylamine < diethylamine < n-butylamine < ethyl alcohol < propanoic acid; aromatic amines are insoluble in water due to the bulky size of the phenyl group.
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:
- 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:
| S. No. | Test | 1° amine | 2° amine | 3° amine |
|---|---|---|---|---|
| 1. | Reaction with nitrous acid | Give alcohol with effervescence of N₂ gas | Gives oily nitroso-amine which gives Liebermann’s nitroso amine test | In the cold, it forms nitrite, which is soluble in water and, upon heating, yields nitrosoamine |
| 2. | Hinsberg’s reagent | Gives N-alkyl benzene sulphonamide soluble in alkali | Gives N, N-dialkyl benzene sulphonamide insoluble in alkali | No reaction |
| 3. | Carbylamine test | Forms isocyanide with pungent odour | No reaction | No reaction |
| 4. | Hofmann’s mustard oil reaction | Forms isothio cyanate having smell of mustard oil | No reaction | No reaction |
- 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.
- 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.
Concepts [20]
- Introduction of Organic Compounds Containing Nitrogen
- Concept of Amines
- Classification of Amines
- Structure of Amines
- Nomenclature of Animes
- Physical Properties of Amines
- Preparation of Amines
- Chemical Reactions of Amines - Basic Character of Amines
- Chemical Reactions of Amines - Alkylation and Acylation
- Chemical Reactions of Amines - Carbylamine Reaction
- Chemical Reactions of Amines - Reaction with Nitrous Acid
- Chemical Reactions of Amines - Reaction with Arylsulphonyl Chloride
- Chemical Reactions of Amines - Electrophilic Substitution
- Identification of Primary, Secondary and Tertiary Amines
- Uses of Amines
- Diazonium Salts
- Method of Preparation of Diazonium Salts
- Chemical Reaction of Diazonium Salts - Reactions Involving Displacement of Nitrogen
- Chemical Reaction of Diazonium Salts - Reactions Involving Retention of Diazo Group
- Importance of Diazonium Salts in Synthesis of Aromatic Compounds
