What this chapter is about
Amines are organic compounds derived from ammonia (NH₃) by replacing one, two or all three hydrogen atoms with alkyl or aryl groups. They are nitrogen-containing compounds that play a crucial role in biochemistry—proteins, alkaloids, vitamins and many pharmaceuticals contain amine groups. Understanding amines helps explain how drugs like antihistamines, local anaesthetics and sulfa drugs work.
This chapter builds on your knowledge of organic reactions from Class 11 and earlier chapters of Class 12. You will learn how to name amines using IUPAC rules, classify them, understand their structure and basicity, study methods of preparation, and examine their chemical reactions. The chapter also covers diazonium salts, which are extremely useful intermediates for synthesising azo dyes and various aromatic compounds.
After studying this chapter, you should be able to write IUPAC names of amines, predict their relative basicity, describe their preparation from nitro compounds, halides and amides, write mechanisms of important reactions, and explain the synthetic importance of diazonium salts.
Key ideas
- Classification: Amines are primary (1°), secondary (2°) or tertiary (3°) depending on whether one, two or three hydrogen atoms of NH₃ are replaced. A quaternary ammonium salt has four groups attached to nitrogen with a positive charge, such as (CH₃)₄N⁺Cl⁻.
- Structure: The nitrogen in amines has a lone pair and is sp³ hybridised, giving a pyramidal shape. This lone pair is responsible for the basic character and nucleophilic behaviour of amines.
- Basicity order: Aliphatic amines are more basic than ammonia, which is more basic than aromatic amines. In aqueous solution, the order is often: secondary > primary > tertiary for aliphatic amines, due to a balance between inductive effect and solvation.
- Aromatic amines are weaker bases: In aniline, the lone pair on nitrogen delocalises into the benzene ring (resonance), making it less available for accepting a proton.
- Preparation methods: Amines can be prepared by reduction of nitro compounds, ammonolysis of alkyl halides, Gabriel phthalimide synthesis (gives pure primary amines), Hoffmann bromamide reaction (converts amide to amine with one fewer carbon).
- Reactions: Amines undergo alkylation, acylation, reaction with nitrous acid (HNO₂), carbylamine reaction (isocyanide test for 1° amines), and electrophilic substitution in aromatic amines.
- Diazonium salts: Formed by treating aromatic primary amines with NaNO₂ and HCl at 0–5 °C. They are unstable but extremely versatile, undergoing Sandmeyer reaction, coupling reactions to form azo dyes, and replacement reactions.
Formulas and facts to remember
- General formula of amines: Primary amine R–NH₂, secondary amine R₂NH, tertiary amine R₃N.
- IUPAC naming: Replace the ending -e of the parent alkane with -amine. Example: CH₃–NH₂ is methanamine; C₆H₅–NH₂ is aniline (common) or benzenamine (IUPAC).
- Basicity order in gas phase: (CH₃)₃N > (CH₃)₂NH > CH₃NH₂ > NH₃ (inductive effect dominates).
- Basicity in aqueous solution: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ (solvation effect also matters).
- Reduction of nitro compounds: R–NO₂ + 3H₂ (Ni catalyst, or Sn/HCl, or Fe/HCl) → R–NH₂ + 2H₂O.
- Gabriel synthesis: Potassium phthalimide + R–X → N-alkylphthalimide; then hydrolysis with aqueous NaOH or hydrazine gives R–NH₂.
- Hoffmann bromamide degradation: R–CO–NH₂ + Br₂ + 4NaOH → R–NH₂ + Na₂CO₃ + 2NaBr + 2H₂O (amine has one carbon less).
- Diazotisation: C₆H₅–NH₂ + NaNO₂ + 2HCl (0–5 °C) → C₆H₅–N₂⁺Cl⁻ + NaCl + 2H₂O.
Worked examples
Example 1: Naming an amine
Problem: Write the IUPAC name of CH₃–CH₂–CH(NH₂)–CH₃.
Solution: Step 1: Identify the longest carbon chain containing the amino group – four carbons, so parent is butane. Step 2: Number from the end nearest to –NH₂. The amino group is on carbon 2. Step 3: Replace -e with -amine and add locant: butan-2-amine.
Answer: Butan-2-amine.
Example 2: Comparing basicity
Problem: Arrange the following in increasing order of basic strength: aniline (C₆H₅NH₂), methylamine (CH₃NH₂), ammonia (NH₃).
Solution: Step 1: Methylamine has an electron-donating methyl group attached to nitrogen, increasing electron density on N; it is more basic than ammonia. Step 2: In aniline, the lone pair on nitrogen is delocalised into the benzene ring by resonance, reducing its availability for proton acceptance; it is less basic than ammonia. Step 3: Order of increasing basicity: aniline < ammonia < methylamine.
Answer: C₆H₅NH₂ < NH₃ < CH₃NH₂.
Example 3: Identifying an amine using the carbylamine reaction
Problem: Compound A (C₃H₉N) when heated with chloroform and alcoholic KOH gives a foul-smelling product. Identify the type of amine and suggest a possible structure.
Solution: Step 1: The carbylamine (isocyanide) reaction is given only by primary amines: R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O. Step 2: The foul smell indicates isocyanide formation, confirming compound A is a primary amine. Step 3: Molecular formula C₃H₉N fits propan-1-amine (CH₃–CH₂–CH₂–NH₂) or propan-2-amine (CH₃–CH(NH₂)–CH₃). Both are primary amines and would give this test.
Answer: A is a primary amine; possible structures are propan-1-amine or propan-2-amine.
Common mistakes
- Thinking tertiary amines are most basic in water because they have three alkyl groups → in aqueous solution, solvation of the conjugate acid matters; secondary amines are often most basic.
- Forgetting to keep temperature at 0–5 °C during diazotisation → diazonium salts decompose at higher temperatures; always maintain ice-cold conditions.
- Confusing primary, secondary and tertiary amines with primary, secondary and tertiary carbons → amine classification is based on the number of carbon groups attached to nitrogen, not the carbon type.
- Applying Gabriel synthesis to prepare aromatic amines → aryl halides do not undergo nucleophilic substitution easily; Gabriel synthesis works only for primary aliphatic amines.
- Writing the wrong carbon count in Hoffmann bromamide reaction → the product amine has one carbon atom fewer than the starting amide.
Quick revision
- Amines are derivatives of NH₃; classified as 1°, 2° or 3° based on the number of C groups on N.
- Aliphatic amines are stronger bases than aniline because resonance in aniline delocalises the lone pair.
- Reduction of nitro compounds and Hoffmann bromamide degradation are key preparation methods.
- Carbylamine test (foul smell of isocyanide) identifies primary amines only.
- Diazonium salts are prepared below 5 °C and are starting points for Sandmeyer reaction and azo dye synthesis.
- In aqueous solution, basicity order of aliphatic amines: secondary > primary > tertiary > ammonia.