NCERT Solutions for Class 12 Chemistry Chapter 9 – Amines
Chapter 9 of Class 12 Chemistry explores amines — nitrogen-containing organic compounds derived by replacing hydrogen atoms in ammonia with alkyl or aryl groups. These seemingly simple substitutions lead to a rich and nuanced chemistry, especially when it comes to basicity comparisons, which are among the most conceptually challenging aspects of the entire Class 12 Chemistry syllabus. For Subject Wise NCERT Solutions for class 12 and Chapter-wise NCERT solutions for class 12 Chemistry, check out these pages.
Find the PDF of All the Exercises of NCERT Class 12 Chemistry Chapter 9 Solutions
Exercise solutions for Amines are available as a downloadable PDF covering all NCERT textbook questions, including problems on nomenclature, basicity, reactions, and identification. These PDFs are structured for efficient revision, with clear explanations and reaction diagrams that help students reinforce their understanding. Downloading and reviewing these PDFs alongside regular study is an effective strategy for comprehensive exam preparation.
Amines are biologically significant: amino acids (the building blocks of proteins), neurotransmitters like dopamine and adrenaline, and pharmaceutical compounds like antihistamines are all amine derivatives. For CBSE Class 12 boards, basicity order questions, preparation of amines, reactions with nitrous acid (diazotisation), and the distinction between primary, secondary, and tertiary amines are routinely tested. For JEE and NEET, the mechanistic understanding of why aniline is less basic than aliphatic amines and how diazonium salts participate in coupling reactions is essential.
These NCERT Solutions for Class 12 Chemistry offer logically structured answers for all exercises in the chapter. The NCERT Solutions Class 12 Chemistry Chapter 9 content provided here explains every answer with the chemical reasoning behind it, helping students understand basicity trends, reaction mechanisms, and the identification of amines through chemical tests with complete confidence.
Important Topics Covered in NCERT Class 12 Chemistry Chapter 9
Structure and Classification: Amines are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of hydrogen atoms in NH₃ replaced by organic groups. Aliphatic and aromatic amines have distinct properties.
Nomenclature: IUPAC and common names for simple and complex amines; naming of diamines and cyclic amines.
Preparation of Amines: Reduction of nitro compounds, ammonolysis of haloalkanes (Hofmann's method), reduction of nitriles and amides, Gabriel phthalimide synthesis, and Hofmann bromamide degradation.
Physical Properties: Hydrogen bonding in amines; boiling points; solubility in water; characteristic fishy smell of lower aliphatic amines.
Basic Nature and Basicity Order: The basicity of amines in aqueous solution is determined by the availability of the lone pair on nitrogen, the stability of the resulting ammonium ion, steric factors, and solvation effects. This leads to the somewhat counterintuitive basicity order for aliphatic amines.
Chemical Reactions: Reaction with acids, alkylation, acylation, carbylamine reaction (for primary amines), reaction with nitrous acid (diazotisation for aromatic amines), and electrophilic aromatic substitution of aniline.
Diazonium Salts: Preparation, stability, and reactions of benzene diazonium chloride including replacement reactions (Sandmeyer reaction, Gattermann reaction) and coupling reactions (azo dye formation).
Important Formulas and Key Points of Chapter 9
Formula / Concept | Explanation / Application |
|---|---|
R–NO₂ + 6[H] (Fe/HCl or Sn/HCl) → R–NH₂ + 2H₂O | Reduction of nitro compounds to primary amines; most common lab preparation |
R–X + NH₃ (excess) → R–NH₂ + HX (ammonolysis) | Ammonolysis of haloalkane; excess ammonia favours primary amine; can give mixture of 1°, 2°, 3° amines and quaternary ammonium salts |
Gabriel Phthalimide Synthesis: Phthalimide + KOH + R–X → N-alkyl phthalimide → (hydrolysis) → R–NH₂ | Used exclusively to prepare pure primary amines without secondary or tertiary contamination |
Hofmann Bromamide Degradation: RCONH₂ + Br₂ + 4NaOH → R–NH₂ + Na₂CO₃ + 2NaBr + 2H₂O | Converts amide to primary amine with one fewer carbon; product has fewer carbons than starting amide |
R–NC (Carbylamine test): R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O | Positive carbylamine test (foul-smelling isocyanide) confirms primary amine; 2° and 3° amines do not give this test |
Diazotisation: C₆H₅NH₂ + NaNO₂ + HCl (0–5°C) → C₆H₅N₂⁺Cl⁻ + 2H₂O | Formation of diazonium salt from primary aromatic amine; must be performed at 0–5°C to prevent decomposition |
Sandmeyer Reaction: C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂ | Replaces –N₂⁺ with –Cl using Cu₂Cl₂; similarly –Br (Cu₂Br₂) and –CN (CuCN) can be introduced |
Gattermann Reaction: C₆H₅N₂⁺Cl⁻ + Cu (powder) + HCl → C₆H₅Cl + N₂ | Similar to Sandmeyer but uses Cu metal + HCl instead of cuprous salt |
Coupling Reaction: C₆H₅N₂⁺Cl⁻ + C₆H₅OH (alkaline) → C₆H₅–N=N–C₆H₄OH (Orange azo dye) | Electrophilic aromatic substitution; diazonium ion attacks para position of activated arene; produces coloured azo dyes |
Basicity Order (aqueous): (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ | Secondary aliphatic amine is most basic in aqueous solution due to best balance of induction and solvation effects |
Aniline vs Aliphatic Amines Basicity: C₆H₅NH₂ << R–NH₂ | In aniline, lone pair on N is delocalised into benzene ring, making it less available for protonation; hence aniline is far less basic than aliphatic amines |
Acylation: R–NH₂ + CH₃COCl → R–NHCOCH₃ + HCl | Forms amide (acetamide derivative); used to protect the amine group during synthesis |
Hinsberg Test: 1° amine gives soluble product, 2° gives insoluble product, 3° does not react | Hinsberg's reagent (benzene sulfonyl chloride) used to distinguish between 1°, 2°, and 3° amines |
Electrophilic Substitution of Aniline: –NH₂ is ortho/para director | Aniline undergoes bromination with Br₂/water to give 2,4,6-tribromoaniline (no Lewis acid needed due to strong activation) |
Kb = [RNH₃⁺][OH⁻] / [RNH₂] | Base dissociation constant; higher Kb means stronger base; pKb = –log Kb |
Key Points, Exam Tips & Common Mistakes
The basicity of amines in aqueous solution depends on three competing factors: the inductive effect of alkyl groups (increases electron density on N), steric hindrance (reduces accessibility), and solvation of the resulting ammonium ion (stabilises the conjugate acid).
In aqueous solution, the order of basicity for aliphatic amines is: 2° > 1° > 3° > NH₃ — this is different from the gas phase order (3° > 2° > 1°) because solvation is absent in gas phase.
Aniline (C₆H₅NH₂) is much less basic than methylamine (CH₃NH₂) because the nitrogen lone pair in aniline is delocalised into the benzene ring through resonance, making it less available for protonation.
Gabriel phthalimide synthesis yields only pure primary amines — it is the go-to method when a contamination-free primary amine is needed.
Hofmann bromamide degradation is an example of a reaction that decreases the carbon chain by one — a frequently asked point in both boards and JEE.
Diazotisation must be performed at 0–5°C because diazonium salts are unstable at higher temperatures and decompose rapidly.
Common mistake: Students confuse the Sandmeyer reaction (uses CuX salt) with the Gattermann reaction (uses Cu powder + HX) — remember both by their reagents.
The carbylamine reaction is a confirmatory test for primary amines only — it does not work with secondary or tertiary amines.
In the coupling reaction of diazonium salt with phenol, the coupling always occurs at the para position preferentially; if para is blocked, coupling occurs at ortho.
Azo dyes produced by coupling reactions of diazonium salts with activated benzene rings are commercially important — this connects organic chemistry to industrial applications.
When aniline undergoes acylation to form acetanilide, the amino group is protected — this is widely used in synthesis to moderate the strong activating power of –NH₂ and direct to desired positions.
Exam tip: For questions on "arrange in increasing order of basicity," always check whether the amines are aliphatic or aromatic, and whether solvent effects are relevant.
Tertiary amines cannot undergo acylation or Hinsberg's test — their inability to form N–H bonds after reaction is the key distinguishing feature.
The fishy smell of amines is due to volatile, low molecular mass amines — this sensory property is sometimes the basis of short descriptive questions.
Electrophilic substitution on aniline: Without protection, aniline undergoes tribromination. Acetylation first protects the ring, allowing mono-substitution at the para position.
Detailed Explanation of NCERT Class 12 Chemistry Chapter 9
Amines are one of the most biologically important classes of organic compounds. Every protein in your body contains amino groups. Neurotransmitters that regulate your mood, alertness, and muscle contractions are amines. Medications from antihistamines to local anaesthetics are built around the amine functional group. Understanding amines at the level demanded by Class 12 Chemistry means understanding a fundamental piece of life's chemistry.
The centrepiece of this chapter — and the concept that trips up the most students — is the basicity order of amines. At first glance, one might expect tertiary amines to be the most basic since they have the greatest electron density on nitrogen from three alkyl groups. In solution, however, steric effects prevent the nitrogen from being easily protonated, and the resulting bulky ammonium ion is not well-solvated by water molecules. The result is that secondary amines win the basicity contest in aqueous conditions. This interplay of electronic, steric, and solvation effects is a beautiful example of how multiple factors compete in real chemical systems — and it is exactly the kind of nuanced reasoning that examiners reward.
Aniline represents an entirely different situation. Here, the lone pair on nitrogen is partially delocalised into the π system of the benzene ring through resonance. This makes the nitrogen less electron-rich, the compound less basic, and the ring itself more activated toward electrophilic substitution. The dual consequence — decreased basicity, increased ring reactivity — is a key insight that connects amine chemistry to aromatic chemistry.
Board Exam Tip: In questions asking you to compare basicities, always state the reason explicitly — do not just write the order. A well-reasoned answer explaining inductive effect, steric hindrance, and solvation will earn full marks even if one comparison is incorrect.
The section on diazonium salts is especially rich for examination purposes. These reactive intermediates act as electrophiles in coupling reactions to produce azo dyes — one of the most colourful applications of organic chemistry. The Sandmeyer and Gattermann reactions give access to aryl halides that cannot be prepared directly by electrophilic substitution (especially aryl fluorides via the Balz–Schiemann reaction). Understanding diazonium chemistry essentially gives you a versatile synthetic toolkit for modifying aromatic rings in controlled ways.
This chapter connects naturally to Chapter 8 (where amides are made from amines and acids) and to Chapter 10 (where amino acids and proteins are discussed). The broader theme is that nitrogen-containing groups are everywhere in biochemistry, and the reactions learned here — acylation, diazotisation, coupling — appear repeatedly in the synthesis of pharmaceuticals and dyes that society depends on.
FAQs – NCERT Solutions for Class 12 Chemistry Chapter 9: Amines
Amines are organic compounds derived from ammonia by replacing one or more hydrogen atoms with alkyl or aryl groups. They are classified as primary, secondary, or tertiary amines depending on the number of carbon groups attached to the nitrogen atom. They can also be classified as aliphatic or aromatic based on the nature of the attached group. Understanding this classification is important because it influences their basicity, reactivity, and physical properties. Students should also focus on nomenclature and structure, as these are commonly asked in board exams through identification and naming-based questions.
Amines are basic because the nitrogen atom in the amine group has a lone pair of electrons that can accept a proton. This ability to donate electrons makes them behave as Lewis bases. The basic strength of amines depends on factors such as electron-releasing or withdrawing groups, solvation effects, and steric hindrance. Aliphatic amines are generally more basic than ammonia due to the +I effect of alkyl groups, while aromatic amines are less basic because of resonance delocalisation. Students should understand these factors clearly to answer reasoning and comparison-based questions effectively.
Amines undergo several important reactions such as alkylation, acylation, reaction with nitrous acid, and diazotisation in the case of aromatic amines. Primary amines react with nitrous acid to form alcohols or diazonium salts depending on the type of amine. Amines also form amides when treated with acyl chlorides or acid anhydrides. These reactions are important for understanding conversions and synthesis pathways. Students should focus on reagents, reaction conditions, and products, as exam questions often involve stepwise transformations and mechanism-based reasoning.
Diazotisation is the process in which a primary aromatic amine reacts with nitrous acid at low temperature to form a diazonium salt. This reaction is highly important because diazonium salts are versatile intermediates used in the synthesis of many aromatic compounds. They can undergo substitution reactions to form halides, phenols, and other derivatives. Students should understand the conditions required for stability and the applications of diazonium compounds. This topic is frequently asked in board exams due to its importance in aromatic chemistry and reaction mechanisms.