Amines are one of the most important classes of organic compounds in chemistry. They are derivatives of ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups. Amines play a significant role in the chemical industry, pharmaceuticals, dyes, and biological systems as they form the backbone of many neurotransmitters, hormones, and alkaloids. Understanding amines involves learning their classification, nomenclature, preparation, properties, and reactions, which are crucial for both theoretical and practical aspects of chemistry.
Amines Class 12 Chemistry Notes | Complete Guide with Examples, MCQs & Q&A
1. Classification of Amines
Amines are classified based on the number of alkyl or aryl groups attached to the nitrogen atom:
1.1 Primary Amines (RNH₂)
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Contains one alkyl or aryl group attached to nitrogen.
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General formula: R–NH₂
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Example: Methylamine (CH₃NH₂), Ethylamine (C₂H₅NH₂)
1.2 Secondary Amines (R₂NH)
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Contains two alkyl or aryl groups attached to nitrogen.
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General formula: R₂NH
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Example: Dimethylamine ((CH₃)₂NH), Diethylamine ((C₂H₅)₂NH)
1.3 Tertiary Amines (R₃N)
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Contains three alkyl or aryl groups attached to nitrogen.
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General formula: R₃N
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Example: Trimethylamine ((CH₃)₃N), Triethylamine ((C₂H₅)₃N)
1.4 Aromatic Amines
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Amines in which the nitrogen atom is directly attached to an aromatic ring.
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Example: Aniline (C₆H₅NH₂)
2. Nomenclature of Amines
2.1 IUPAC Nomenclature
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Identify the longest carbon chain attached to the nitrogen atom.
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Use the suffix -amine.
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For secondary and tertiary amines, use the prefix N- to indicate substituents on nitrogen.
Examples:
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CH₃NH₂ → Methylamine (Primary)
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(CH₃)₂NH → N-Methylmethanamine (Secondary)
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(C₂H₅)₃N → N,N-Diethylethanamine (Tertiary)
2.2 Common Nomenclature
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Based on the source of amine.
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Example: CH₃NH₂ is called Methylamine; C₂H₅NH₂ is called Ethylamine
3. Structure of Amines
3.1 Geometry
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The nitrogen atom in amines is sp³ hybridized.
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Bond angle ≈ 107° (slightly less than tetrahedral due to lone pair repulsion).
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Lone pair on nitrogen makes amines nucleophilic.
3.2 Resonance in Aromatic Amines
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In aniline, the lone pair on nitrogen delocalizes into the benzene ring, reducing its basicity.
4. Methods of Preparation of Amines
Amines can be prepared by several methods:
4.1 From Alkyl Halides (Gabriel Phthalimide Synthesis)
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Primary amines are prepared using phthalimide and alkyl halides.
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Example:
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Phthalimide + CH₃I → N-Methylphthalimide
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Hydrolysis → CH₃NH₂
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4.2 Reduction of Nitro Compounds
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Nitro compounds (R–NO₂) are reduced to amines using:
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H₂ / Pd-C (Catalytic Hydrogenation)
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Sn / HCl (Chemical Reduction)
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Example: Nitrobenzene → Aniline
4.3 Reduction of Amides (Hofmann’s Method)
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Primary amides react with Br₂ and NaOH to give primary amines with one less carbon atom.
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Example: CH₃CONH₂ + Br₂ + NaOH → CH₃NH₂
4.4 Reductive Amination
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Aldehydes or ketones react with ammonia or primary amines in the presence of H₂ and a catalyst to form amines.
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Example: CH₃CHO + NH₃ → CH₃CH₂NH₂
4.5 From Cyanides (Nitrile Reduction)
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R–CN + 2[H] → R–CH₂NH₂
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Reducing agents: LiAlH₄, H₂ / Ni
5. Physical Properties of Amines
5.1 Boiling Point
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Amines have higher boiling points than hydrocarbons but lower than alcohols.
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Hydrogen bonding in primary and secondary amines increases boiling points.
5.2 Solubility
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Amines are soluble in water due to hydrogen bonding.
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Solubility decreases with increasing alkyl chain length.
5.3 Odor
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Low molecular weight amines have a fishy smell.
6. Chemical Properties of Amines
6.1 Basic Nature
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Amines are basic due to the lone pair of electrons on nitrogen.
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They form salts with acids:
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CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻
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6.2 Acylation
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Reaction with acyl chlorides forms amides.
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Example: CH₃NH₂ + CH₃COCl → CH₃NHCOCH₃ + HCl
6.3 Alkylation
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Amines react with alkyl halides to form higher amines.
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Over-alkylation can occur, giving secondary and tertiary amines.
6.4 Reaction with Nitrous Acid (HNO₂)
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Primary aliphatic amines → Alcohols (via diazonium ion)
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Primary aromatic amines → Diazonium salts
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Example: C₆H₅NH₂ + HNO₂ → C₆H₅N₂⁺Cl⁻
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6.5 Hoffmann’s Bromamide Reaction
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Converts primary amides to primary amines with one less carbon.
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Example: CH₃CONH₂ → CH₃NH₂
6.6 Carbylamine Reaction
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Primary amines react with chloroform and KOH to form isocyanides (R–NC), which have a foul odor.
7. Aromatic Amines
7.1 Structure and Resonance
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Lone pair on nitrogen delocalizes into the benzene ring.
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Reduces basicity compared to aliphatic amines.
7.2 Reactions
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Electrophilic Substitution: Amino group is an activating group for ortho- and para- positions.
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Diazotization: Aromatic amines react with nitrous acid at 0–5°C to form diazonium salts.
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C₆H₅NH₂ + HNO₂ → C₆H₅N₂⁺Cl⁻
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7.3 Coupling Reactions
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Diazonium salts react with phenols or aromatic compounds to form azo dyes.
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Example: C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅–N=N–C₆H₄OH
8. Identification Tests for Amines
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Hinsberg Test – Differentiates primary, secondary, and tertiary amines.
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Carbylamine Test – Primary amines form isocyanides with a foul smell.
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Reaction with Nitro Compounds – Used to detect aromatic amines.
9. Importance of Amines
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Amines are used in the synthesis of dyes, drugs, and polymers.
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Examples:
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Pharmaceuticals: Ephedrine, Morphine
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Dyes: Aniline dyes
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Polymers: Nylon (hexamethylenediamine)
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10. Summary Table of Amines
| Property/Reaction | Primary Amines | Secondary Amines | Tertiary Amines |
|---|---|---|---|
| General Formula | RNH₂ | R₂NH | R₃N |
| Boiling Point | Higher than hydrocarbons | Moderate | Lower than primary/secondary |
| Solubility | High in water | Moderate | Low |
| Reaction with HNO₂ | Alcohols | Do not react | Do not react |
| Carbylamine Test | Positive | Negative | Negative |
Objective Questions on Amines
1. Which of the following is a primary amine?
Options:
A) CH₃NH₂
B) (CH₃)₂NH
C) (CH₃)₃N
D) C₆H₅NHCH₃
Answer:
The primary amine has one alkyl group attached to the nitrogen atom. Here, CH₃NH₂ has only one methyl group bonded to nitrogen, making it a primary amine. The others are secondary, tertiary, or substituted aromatic amines.
2. What is the product when aniline reacts with nitrous acid at 0–5°C?
Options:
A) Alcohol
B) Diazonium salt
C) Ketone
D) Amide
Answer:
Aromatic primary amines react with nitrous acid at 0–5°C to form diazonium salts. Therefore, aniline (C₆H₅NH₂) reacts to produce C₆H₅N₂⁺Cl⁻, which is used in azo dye synthesis.
3. Which reaction is used to distinguish primary, secondary, and tertiary amines?
Options:
A) Carbylamine test
B) Hinsberg test
C) Reduction
D) Diazotization
Answer:
The Hinsberg test differentiates amines: primary amines form soluble sulfonamides, secondary amines form insoluble sulfonamides, and tertiary amines do not react.
4. What is the general formula of secondary amines?
Options:
A) RNH₂
B) R₂NH
C) R₃N
D) R–NH–R’
Answer:
Secondary amines have two alkyl or aryl groups attached to nitrogen. Their general formula is R₂NH.
5. Which method converts a primary amide into a primary amine with one less carbon atom?
Options:
A) Gabriel synthesis
B) Hofmann bromamide reaction
C) Reductive amination
D) Nitrile reduction
Answer:
The Hofmann bromamide reaction converts primary amides to primary amines by losing one carbon atom. Example: CH₃CONH₂ → CH₃NH₂.
6. Which reagent is used in the Carbylamine test?
Options:
A) HCl and NaNO₂
B) Br₂ and KOH
C) Acyl chloride
D) LiAlH₄
Answer:
The Carbylamine test uses chloroform (CHCl₃) and alcoholic KOH. Primary amines form isocyanides with a foul odor.
7. Which of the following amines has the highest basicity?
Options:
A) Aniline
B) Methylamine
C) Dimethylamine
D) Trimethylamine
Answer:
Aliphatic amines are more basic than aromatic amines due to the availability of the nitrogen lone pair. Dimethylamine ((CH₃)₂NH) is more basic than primary or tertiary amines due to the electron-donating effect of alkyl groups enhancing nucleophilicity.
8. Which product is obtained when CH₃NH₂ reacts with HCl?
Options:
A) CH₃NH₃⁺Cl⁻
B) CH₃NH₂Cl
C) CH₃Cl
D) CH₃N₂⁺Cl⁻
Answer:
Primary amines react with acids to form ammonium salts. Thus, methylamine reacts with HCl to form CH₃NH₃⁺Cl⁻, a water-soluble salt.
9. Which is an aromatic amine?
Options:
A) CH₃NH₂
B) C₂H₅NH₂
C) C₆H₅NH₂
D) (CH₃)₂NH
Answer:
Aromatic amines have nitrogen directly attached to a benzene ring. C₆H₅NH₂ (Aniline) is an aromatic amine, whereas the others are aliphatic amines.
10. What happens when a primary aliphatic amine reacts with nitrous acid?
Options:
A) Forms alcohol
B) Forms ketone
C) Forms aldehyde
D) Forms amide
Answer:
Primary aliphatic amines react with nitrous acid to form alcohols via the diazonium intermediate, which decomposes to give the corresponding alcohol.
11. Which method is used for the preparation of primary amines from alkyl halides?
Options:
A) Gabriel phthalimide synthesis
B) Hofmann reaction
C) Reductive amination
D) Nitrile reduction
Answer:
Gabriel phthalimide synthesis is used to prepare primary amines from alkyl halides. It prevents over-alkylation and yields a pure primary amine.
12. Which amine cannot be detected by the Carbylamine test?
Options:
A) Primary aliphatic amine
B) Secondary amine
C) Aromatic primary amine
D) Methylamine
Answer:
Secondary and tertiary amines do not react in the Carbylamine test. Only primary amines give isocyanides. Therefore, a secondary amine cannot be detected.
13. What type of hybridization does nitrogen in amines exhibit?
Options:
A) sp²
B) sp³
C) sp
D) None
Answer:
Nitrogen in amines is sp³ hybridized, forming a pyramidal shape with a lone pair of electrons, making it nucleophilic and basic.
14. Which amine gives a yellow precipitate with Hinsberg’s reagent?
Options:
A) Primary amine
B) Secondary amine
C) Tertiary amine
D) Aromatic amine
Answer:
Secondary amines react with Hinsberg’s reagent to form insoluble sulfonamides, often observed as a yellow precipitate. Primary amines give soluble sulfonamides, and tertiary amines do not react.
15. Which of the following is an example of a tertiary amine?
Options:
A) CH₃NH₂
B) (CH₃)₂NH
C) (CH₃)₃N
D) C₂H₅NH₂
Answer:
(CH₃)₃N (Trimethylamine) has three alkyl groups attached to nitrogen, making it a tertiary amine.
16. Which reaction produces amines using LiAlH₄?
Options:
A) Reduction of nitro compounds
B) Reduction of nitriles
C) Both A and B
D) Diazotization
Answer:
LiAlH₄ reduces both nitriles and nitro compounds to form amines. For example, R–CN + 4[H] → R–CH₂NH₂.
17. Which amine is used in dye synthesis?
Options:
A) Aniline
B) Methylamine
C) Ethylamine
D) Dimethylamine
Answer:
Aniline (C₆H₅NH₂) is widely used in the synthesis of azo dyes and other aromatic dyes due to its ability to form diazonium salts.
18. Which of the following is the product of acylation of methylamine?
Options:
A) CH₃CONHCH₃
B) CH₃CH₂NH₂
C) CH₃NH₂
D) CH₃N₃
Answer:
Acylation of methylamine (CH₃NH₂) with an acyl chloride produces an amide: CH₃NHCOCH₃ (N-methylacetamide).
19. Which is the main reason for the lower basicity of aniline compared to aliphatic amines?
Options:
A) Lone pair delocalization
B) Steric hindrance
C) High boiling point
D) Solubility
Answer:
In aniline, the lone pair on nitrogen delocalizes into the benzene ring, making it less available for protonation. This reduces basicity compared to aliphatic amines.
20. Which reaction forms isocyanides?
Options:
A) Carbylamine reaction
B) Hofmann bromamide reaction
C) Reductive amination
D) Diazotization
Answer:
The Carbylamine reaction converts primary amines into isocyanides (R–NC) using chloroform and KOH. These compounds have a foul odor and are characteristic for identification.
Short-Answer Questions on Amines
1. Give an example of a tertiary amine.
Answer:
Trimethylamine ((CH₃)₃N) is a tertiary amine. It contains three methyl groups attached to nitrogen and has no hydrogen atoms bonded to nitrogen. Tertiary amines are less reactive in hydrogen bonding compared to primary and secondary amines.
2. What is formed when aniline reacts with bromine water?
Answer:
Aniline reacts with bromine water to give 2,4,6-tribromoaniline as a white precipitate. The amino group activates the ortho and para positions on the benzene ring, allowing rapid substitution of bromine atoms.
3. State the general formula of primary, secondary, and tertiary amines.
Answer:
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Primary amine: RNH₂
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Secondary amine: R₂NH
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Tertiary amine: R₃N
These formulas indicate the number of alkyl/aryl groups attached to the nitrogen atom.
4. What is the product of the Hofmann bromamide reaction?
Answer:
The Hofmann bromamide reaction converts a primary amide into a primary amine with one less carbon atom. Example: CH₃CONH₂ + Br₂ + NaOH → CH₃NH₂ + Na₂CO₃. This reaction is useful for reducing amides to amines.
5. Why are aliphatic amines more basic than aromatic amines?
Answer:
Aliphatic amines are more basic because the nitrogen’s lone pair is fully available to accept a proton. In aromatic amines, such as aniline, the lone pair delocalizes into the benzene ring, reducing electron density on nitrogen and lowering its basicity.
Long-Answer Questions on Amines
1. Explain the classification of amines with examples.
Answer:
Amines are classified based on the number of alkyl or aryl groups attached to the nitrogen atom.
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Primary Amines (RNH₂): Nitrogen is attached to one alkyl or aryl group and two hydrogen atoms. Example: Methylamine (CH₃NH₂).
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Secondary Amines (R₂NH): Nitrogen is attached to two alkyl or aryl groups and one hydrogen atom. Example: Dimethylamine ((CH₃)₂NH).
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Tertiary Amines (R₃N): Nitrogen is attached to three alkyl or aryl groups and no hydrogen atom. Example: Trimethylamine ((CH₃)₃N).
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Aromatic Amines: Amines where nitrogen is directly attached to an aromatic ring. Example: Aniline (C₆H₅NH₂).
This classification helps in predicting chemical reactivity, basicity, and methods of preparation.
2. Describe the Gabriel phthalimide synthesis for preparing primary amines.
Answer:
The Gabriel phthalimide synthesis is a method to prepare primary amines without over-alkylation. It involves the following steps:
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Formation of potassium phthalimide: Phthalimide reacts with KOH to produce potassium phthalimide.
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Alkylation: Potassium phthalimide reacts with an alkyl halide (R–X) to give N-alkylphthalimide.
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Hydrolysis: The N-alkylphthalimide is hydrolyzed under acidic or basic conditions to yield the primary amine (R–NH₂).
Example:
C₆H₄(CO)₂NH + KOH → C₆H₄(CO)₂NK → C₆H₄(CO)₂NR → Hydrolysis → R–NH₂
This method is widely used because it prevents the formation of secondary and tertiary amines.
3. Explain the reactions of amines with nitrous acid.
Answer:
Amines react differently with nitrous acid (HNO₂) based on their type:
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Primary aliphatic amines: React to form alcohols. Example: CH₃NH₂ + HNO₂ → CH₃OH + N₂ + H₂O.
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Primary aromatic amines: React to form diazonium salts at 0–5°C. Example: C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O.
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Secondary amines: React to form N-nitrosoamines, which are often carcinogenic.
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Tertiary amines: Generally do not react with nitrous acid.
These reactions are crucial in organic synthesis, especially for preparing alcohols and azo dyes.
4. Discuss the basicity of amines. Compare aliphatic and aromatic amines.
Answer:
Amines are basic due to the lone pair of electrons on nitrogen, which can accept a proton.
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Aliphatic amines: Electron-donating alkyl groups increase the electron density on nitrogen, making them more basic. Example: CH₃NH₂.
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Aromatic amines: In aniline (C₆H₅NH₂), the lone pair on nitrogen delocalizes into the benzene ring, reducing availability for protonation. Hence, aromatic amines are less basic than aliphatic amines.
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Secondary amines: Often more basic than primary amines due to increased electron density but steric hindrance may lower basicity in tertiary amines.
Understanding basicity is important for predicting reactions like salt formation and electrophilic substitution.
5. Explain the Carbylamine reaction and its significance.
Answer:
The Carbylamine reaction is a test for primary amines. It involves the reaction of a primary amine with chloroform (CHCl₃) and alcoholic KOH to form an isocyanide (R–NC), which has a foul odor.
Example:
CH₃NH₂ + CHCl₃ + 3KOH → CH₃NC + 3KCl + 3H₂O
Significance:
-
It is a qualitative test for detecting primary amines.
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It helps differentiate primary amines from secondary and tertiary amines, which do not give this reaction.
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The isocyanides produced are also intermediates in organic synthesis.
Conclusion
Amines are a versatile and important class of compounds in organic chemistry. Understanding their classification, preparation, physical and chemical properties, and reactions is crucial for advanced studies in chemistry, pharmaceuticals, and industrial applications. Mastery of amines also lays the foundation for learning more complex organic compounds like amino acids, proteins, and alkaloids.
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