What are Amines?
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
1. What are amines and how are they classified?
Amines are organic compounds derived from ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups. They are classified based on the number of hydrogen atoms replaced:
- Primary (1°) amines: One hydrogen atom is replaced (e.g., R-NH₂).
- Secondary (2°) amines: Two hydrogen atoms are replaced (e.g., R₂-NH).
- Tertiary (3°) amines: All three hydrogen atoms are replaced (e.g., R₃-N).
2. How does the basic character of amines compare in the gaseous phase versus in an aqueous solution?
The basicity of amines changes based on the medium due to different stabilising factors.
- In the gaseous phase, basicity depends solely on the +I (inductive) effect of alkyl groups, which increases electron density on the nitrogen atom. The order is generally: Tertiary > Secondary > Primary.
- In an aqueous solution, basicity is a combined result of the +I effect, solvation effect (hydration), and steric hindrance. The hydrogen bonding with water stabilises the conjugate acid, making the overall order more complex, often with secondary amines being the strongest bases.
3. What is the carbylamine reaction and what is its primary use in chemistry?
The carbylamine reaction, also known as the isocyanide test, is a chemical test for the detection of primary amines. In this reaction, a primary amine is heated with chloroform (CHCl₃) and an alcoholic solution of potassium hydroxide (KOH). This results in the formation of a carbylamine or isocyanide, which has a highly unpleasant odour. Its primary use is to distinguish primary amines from secondary and tertiary amines, as only primary amines give a positive result for this test.
4. How does Hinsberg's reagent help distinguish between primary, secondary, and tertiary amines?
Hinsberg's reagent (benzenesulphonyl chloride, C₆H₅SO₂Cl) reacts differently with the three classes of amines, allowing for their distinction based on the product's solubility in alkali:
- A primary amine reacts to form N-alkylbenzenesulphonamide, which has an acidic hydrogen and is therefore soluble in alkali (like KOH).
- A secondary amine reacts to form N,N-dialkylbenzenesulphonamide, which lacks an acidic hydrogen and is insoluble in alkali.
- A tertiary amine does not have a replaceable hydrogen on the nitrogen atom and therefore does not react with Hinsberg's reagent.
5. What is a coupling reaction, and why is it important in the chemical industry?
A coupling reaction is an electrophilic substitution reaction where a diazonium salt reacts with an electron-rich aromatic compound, such as phenol or aniline, to form a brightly coloured azo compound (containing the -N=N- group). This reaction is extremely important in the chemical industry for the synthesis of a wide variety of azo dyes, which are used extensively in colouring textiles, paper, and food products.
6. Explain the process of acylation of amines.
Acylation is the process of introducing an acyl group (R-C=O) into a molecule. Primary and secondary amines undergo acylation by reacting with acid chlorides, anhydrides, or esters in a nucleophilic substitution reaction. The lone pair of electrons on the nitrogen atom of the amine attacks the carbonyl carbon of the acylating agent. A hydrogen atom from the amine is eliminated, forming a substituted amide. This reaction is often carried out in the presence of a stronger base like pyridine to neutralise the acid by-product and drive the reaction forward.
7. Why does aniline not undergo the Friedel-Crafts reaction?
Aniline does not undergo the Friedel-Crafts reaction (both alkylation and acylation) because of a side reaction with the Lewis acid catalyst. The Lewis acid (e.g., AlCl₃), required for the reaction, is acidic. Aniline, being a Lewis base due to the lone pair of electrons on its nitrogen atom, reacts with AlCl₃ to form a stable salt. This deactivates the benzene ring by creating a positive charge on the nitrogen, which strongly pulls electron density away from the ring, thus preventing the necessary electrophilic substitution.
8. What is diazotisation and how is it used to prepare other benzene derivatives?
Diazotisation is the reaction of a primary aromatic amine, like aniline, with nitrous acid (HNO₂, prepared in-situ from NaNO₂ and a mineral acid like HCl) at a low temperature (0-5°C) to form a benzenediazonium salt. This salt is a highly versatile intermediate because the diazonium group (-N₂⁺) is an excellent leaving group. It can be easily replaced by various nucleophiles (e.g., Cl⁻, Br⁻, CN⁻, OH⁻) in reactions like the Sandmeyer and Gattermann reactions to synthesise a wide range of substituted benzene compounds that are otherwise difficult to prepare.
9. Why are aryl diazonium salts more stable than alkyl diazonium salts?
Aryl diazonium salts are significantly more stable than alkyl diazonium salts due to resonance. The positive charge on the diazonium group in an aryl diazonium salt is delocalised across the entire benzene ring, spreading the charge and increasing the molecule's stability. In contrast, alkyl diazonium salts lack this resonance stabilisation. The positive charge is localised on the nitrogen atom, making them highly unstable and prone to immediate decomposition to release nitrogen gas.
10. How does the basicity of aniline compare to that of aliphatic amines like ethylamine?
Aniline is significantly less basic than aliphatic amines like ethylamine. This is because the lone pair of electrons on the nitrogen atom in aniline is involved in resonance with the benzene ring. This delocalisation makes the lone pair less available for donation to a proton. In contrast, the lone pair on the nitrogen in ethylamine is localised, and its availability is enhanced by the electron-donating inductive effect (+I) of the ethyl group, making it a much stronger base.
