Important reactions of amines such as alkylation acylation carbylamine and diazotization
Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl groups. They constitute an important class of organic compounds. They occur widely throughout both plants and animals. They are found among proteins, vitamins, alkaloids, hormones, etc. Synthetic examples include polymers, drugs, dyestuffs, etc.
Classification of Amines
The amines are classified as primary, secondary, or tertiary according to one, two or three hydrogen atoms of ammonia molecule are replaced by alkyl or aryl groups in ammonia molecule. If one hydrogen atom of ammonia is replaced by alkyl or aryl group, we get RNH2 or ArNH2, a primary amine. If two hydrogen atoms of ammonia are replaced by an alkyl or aryl group, we get secondary amine. If three hydrogen atoms of ammonia are replaced by an alkyl or aryl group, we get tertiary amine.
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Reaction of Amines
Reaction with Acids - Amines being basic react with acids to form salts.
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The salts of amines are ionic compounds and are soluble in water. On treatment with aqueous hydroxide, amines are regenerated.
Reaction of Amines With Metal Ions - Like ammonia, amines combine with metal ions such as Ag and Cu ions to form complex compounds. The lone pair of electrons in ammonia is used to form a co-ordinate bond of the amine with metal ions.
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Alkylation - Primary and secondary amines react with alkyl halides to form tertiary amines. The primary or secondary amine acts as a nucleophile and performs nucleophilic substitution at an alkyl halide. On removal of HX, secondary or a tertiary amine is regenerated respectively. The secondary amine is a more powerful nucleophile and again reacts similarly with another alkyl halide forming tertiary amine. At each stage of the reaction, an equivalent amount of a strong acid is formed. This can protonate the amine making lone pair on nitrogen not available for nucleophilic attack and therefore, stop the reaction before completion. Therefore, for the neutralisation of the acid and for liberating the nucleophile, a base such as carbonate is added. Finally, the tertiary amine reacts with alkyl halides to form quaternary ammonium salts.
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Acylation - Aliphatic and aromatic primary and secondary amines (which contain replaceable hydrogen atoms) react with an acid chloride, acid anhydride and esters to form substituted amides. The process of introducing an acyl group into a molecule is called acylation.
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Carbonyl Amine Reaction - This reaction is also known as “Schiff’s base formation reaction”. In this reaction, amines react with aldehydes and ketones. Both aliphatic and aromatic primary amines react with aldehydes and ketones to form imines also called Schiff’s bases or anils.
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Carbylamine Reaction - Aliphatic and aromatic primary amines when warmed with chloroform and an alcoholic solution of potassium hydroxide, form isocyanide or carbylamine which have a very unpleasant or foul smell. This reaction cannot be called a Carbon Amine Reaction.
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Secondary and tertiary amines do not give this test. Therefore, this test can be used to distinguish between primary amines from secondary and tertiary amines.
Reaction with Aryl Sulphonyl Chloride - Benzene sulphonyl chloride which is also known as “Hinsberg’s reagent” reacts with primary and secondary amines to form sulphonamides.
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Since different amines react differently with benzene sulphonyl chloride, this reaction can be used for the distinction between primary, secondary, and tertiary amines and also for the separation of their mixtures.
Coupling Reaction in Amines - Aromatic amines react with diazonium salts to form azo compounds in an acidic medium called dyes. The reaction is known as coupling or diazo reaction.
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Amine Reaction with Water - Nitrogen of amine group donates its lone pair to proton and form positive ion on reacting with water.
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Did You Know?
Aliphatic amines of low molecular weight are used as solvents.
Amines are used as intermediates in drug manufacture and as reagents in organic synthesis.
Aromatic amines are used for the manufacture of polymers, dyes and as intermediates for additives in the rubber industry.
FAQs on Reactions of Amines Explained with Mechanisms and Examples
1. What are the common reactions of amines?
The common reactions of amines include reactions with acids, alkyl halides, acyl chlorides, nitrous acid, and oxidation reactions.
- With acids: Amines form ammonium salts (e.g., RNH2 + HCl → RNH3+Cl-).
- Alkylation: Reaction with alkyl halides to give higher amines.
- Acylation: Formation of amides using acyl chlorides.
- With nitrous acid (HNO2): Primary amines form diazonium salts or alcohols.
- Oxidation: Especially important for aromatic amines.
2. How do amines react with acids?
Amines react with acids to form ammonium salts by protonation of the nitrogen atom.
- The lone pair on nitrogen accepts a proton (H+).
- Example reaction: CH3NH2(aq) + HCl(aq) → CH3NH3+Cl-(aq)
- This reaction shows that amines behave as Brønsted–Lowry bases.
3. What happens when primary amines react with nitrous acid?
Primary amines react with nitrous acid (HNO2) to form diazonium salts (aromatic) or alcohols (aliphatic).
- Aliphatic primary amines: Form unstable diazonium salts that decompose to alcohols.
- Aromatic primary amines: Form stable diazonium salts at 0–5°C.
- Example (aniline): C6H5NH2 + NaNO2 + 2HCl → C6H5N2+Cl- + NaCl + 2H2O
4. How do amines undergo acylation reactions?
Amines undergo acylation when they react with acyl chlorides or acid anhydrides to form amides.
- The hydrogen of the –NH2 group is replaced by an acyl group.
- Example: RNH2 + CH3COCl → CH3CONHR + HCl
- Only primary and secondary amines undergo acylation.
5. What is the reaction of amines with alkyl halides?
Amines react with alkyl halides in a nucleophilic substitution reaction called alkylation to form higher amines.
- The nitrogen lone pair attacks the carbon bonded to the halogen.
- Example: NH3 + CH3Br → CH3NH2 + HBr
- Further alkylation can produce secondary, tertiary amines, and quaternary ammonium salts.
6. Why are amines basic in nature?
Amines are basic because the nitrogen atom contains a lone pair of electrons that can accept a proton (H+).
- They act as Lewis bases (electron pair donors).
- They also act as Brønsted–Lowry bases (proton acceptors).
- Alkyl groups increase basicity due to the +I (electron-donating) effect.
7. What is the carbylamine reaction of amines?
The carbylamine reaction is a test in which primary amines react with chloroform and alcoholic KOH to form isocyanides with a foul smell.
- Only primary amines give this test.
- Example: RNH2 + CHCl3 + 3KOH → RNC + 3KCl + 3H2O
- The product is an isocyanide (carbylamine).
8. How do secondary and tertiary amines react with nitrous acid?
Secondary amines form N-nitrosamines, while tertiary amines form soluble ammonium salts with nitrous acid.
- Secondary amines: R2NH + HNO2 → R2N–NO + H2O
- Tertiary amines: Form trialkylammonium nitrite salts without nitrogen gas evolution.
9. Do amines undergo oxidation reactions?
Yes, amines undergo oxidation reactions, especially aromatic amines which can form azo compounds or nitro derivatives.
- Primary aliphatic amines can oxidize to alcohols or aldehydes under strong conditions.
- Aromatic amines may oxidize to colored products.
- The reaction conditions determine the oxidation product.
10. What is the difference between alkylation and acylation of amines?
The key difference is that alkylation adds an alkyl group to nitrogen, while acylation adds an acyl group to form an amide.
- Alkylation: RNH2 + R′X → RNH–R′ + HX (forms higher amines).
- Acylation: RNH2 + R′COCl → R′CONHR + HCl (forms amides).
- Alkylation increases substitution; acylation reduces basicity.
