An Overview of Ncert Books Class 12 Chemistry Chapter 13 Free Download
FAQs on Ncert Books Class 12 Chemistry Chapter 13 Free Download
1. What are the most important types of questions to prepare from the Amines chapter for the CBSE Class 12 Chemistry board exam 2025-26?
For the Class 12 board exam, the most frequently asked questions from the Amines chapter typically fall into these categories:
- Reasoning Questions (2 Marks): Usually based on the basicity of amines, boiling points, and solubility.
- Named Reactions (2-3 Marks): Questions on reactions like Hoffmann bromamide degradation, Gabriel phthalimide synthesis, Carbylamine test, Sandmeyer, and Gattermann reactions are very common.
- Distinction Tests (2 Marks): Chemical tests to distinguish between primary, secondary, and tertiary amines.
- Conversions (3-5 Marks): Multi-step conversions involving the preparation or reaction of amines and diazonium salts.
- Complete the Reaction (1-2 Marks): Questions involving reactants and reagents where you must predict the final product.
2. Which named reactions from Chapter 13, Amines, are considered high-yield for 3-mark questions?
Based on previous board exam trends, the following named reactions are extremely important and often carry a weightage of 2 to 3 marks:
- Gabriel Phthalimide Synthesis: Valued for its ability to produce pure primary amines.
- Hoffmann Bromamide Degradation Reaction: An important step-down reaction for preparing primary amines with one less carbon atom.
- Carbylamine Reaction (Isocyanide Test): A definitive test for primary amines.
- Sandmeyer Reaction: Crucial for converting diazonium salts into aryl halides.
- Coupling Reactions: Essential for the synthesis of azo dyes from diazonium salts.
Students should focus on the reactants, reagents, conditions, and final products for these reactions.
3. How do you answer a 2-mark question asking to distinguish between primary, secondary, and tertiary amines?
A classic and expected question. The best way to answer is by using Hinsberg's Test. You should state the reagent (benzenesulphonyl chloride) and describe the observation for each type of amine:
- Primary Amine (1°): Reacts with Hinsberg's reagent to form a precipitate (N-alkyl benzenesulphonamide) which is soluble in alkali (KOH).
- Secondary Amine (2°): Reacts to form a precipitate (N,N-dialkyl benzenesulphonamide) that is insoluble in alkali.
- Tertiary Amine (3°): Does not react with Hinsberg's reagent at all.
Mentioning the solubility in alkali is the key to securing full marks.
4. Why is aniline a weaker base than cyclohexylamine? This is a frequently asked HOTS question.
This is a fundamental reasoning question. Aniline is a weaker base than cyclohexylamine due to resonance. In aniline, the lone pair of electrons on the nitrogen atom is delocalised into the benzene ring. This makes the electron pair less available for donation to a proton. In contrast, cyclohexylamine has an alkyl group (cyclohexyl) attached to the -NH₂ group. This alkyl group exerts a +I (positive inductive) effect, which increases the electron density on the nitrogen atom, making it a stronger base.
5. What is the expected answer for why primary amines have a higher boiling point than tertiary amines of comparable mass?
The key difference lies in intermolecular hydrogen bonding. Primary amines (R-NH₂) have two hydrogen atoms directly bonded to the nitrogen, allowing them to form strong, extensive hydrogen bonds with other molecules. Tertiary amines (R₃N) have no hydrogen atoms on the nitrogen and therefore cannot form intermolecular hydrogen bonds among themselves. The stronger intermolecular forces in primary amines require more energy to break, resulting in a significantly higher boiling point.
6. Why can't aromatic primary amines like aniline be prepared using Gabriel phthalimide synthesis?
This is a common conceptual trap. Gabriel phthalimide synthesis involves the nucleophilic substitution of an alkyl halide by the phthalimide anion. Aromatic primary amines cannot be prepared because aryl halides (like chlorobenzene or bromobenzene) do not undergo this nucleophilic substitution reaction easily. The reason is the partial double bond character of the carbon-halogen bond in aryl halides due to resonance with the benzene ring, making it difficult to break.
7. What is the importance of diazonium salts, and what are two important conversion reactions from this topic for the board exam?
Benzene diazonium chloride is a highly versatile intermediate in organic chemistry, used to synthesise a wide range of aromatic compounds. For the CBSE 2025-26 exam, two crucial conversions are:
- Conversion to Phenol: Achieved by warming the aqueous solution of diazonium salt. This is an important method for introducing an -OH group onto the benzene ring.
- Conversion to Fluorobenzene (Balz-Schiemann Reaction): Reacting the diazonium salt with fluoroboric acid (HBF₄) to form an intermediate which on heating gives fluorobenzene.
8. What are some of the most important 3-mark conversion questions that can be expected from the Amines chapter?
For a 3-mark question, you can expect multi-step conversions. Some important examples include:
- Converting Nitrobenzene to Phenol (via aniline and diazonium salt).
- Converting Ethanoic acid to Methanamine (involving Hoffmann bromamide reaction).
- Converting Aniline to Iodobenzene (using diazotisation followed by reaction with KI).
- Converting Propanenitrile to Ethanamine (involving reduction followed by Hoffmann degradation).
Practising these conversions helps in understanding the application of various reactions.
