NCERT Solutions for Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids
NCERT Solutions for Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids covers essential carbonyl chemistry including IUPAC naming, nucleophilic addition reaction mechanisms, important named reactions (Aldol condensation, Cannizzaro reaction, Clemmensen reduction, HVZ reaction), and acidity of carboxylic acids.
Overview of Topics & Subtopics (Click to Jump)
1. Nomenclature, Carbonyl Group Structure & Preparation
Short Note:
• Structure of Carbonyl Group (>C=O): The carbonyl carbon is sp2 hybridized and forms a trigonal planar structure with bond angles near 120°. Due to higher electronegativity of oxygen, the C=O bond is strongly polar (Cδ+=Oδ-).
• Methods of Preparation:
- Aldehydes & Ketones: Controlled oxidation of 1° alcohols to aldehydes (using PCC) and 2° alcohols to ketones (using CrO3 or KMnO4).
- Aldehydes Specifically: Rosenmund reduction (RCOCl + H2 / Pd-BaSO4 → RCHO), Stephen reaction (RCN + SnCl2 + HCl → RCHO), Etard reaction (Toluene + CrO2Cl2 → Benzaldehyde), and Gattermann-Koch reaction.
- Ketones Specifically: Friedel-Crafts acylation of benzene, and reaction of acyl chlorides with dialkylcadmium [R2Cd].
2. Chemical Reactions & Distinguish Tests for Carbonyls
Short Note:
• Nucleophilic Addition Reactions: Nucleophiles attack the electrophilic sp2 carbonyl carbon to form a tetrahedral intermediate. Aldehydes are generally more reactive than ketones due to lesser steric hindrance and lesser +I effect of alkyl groups.
• Named Reactions:
- Clemmensen Reduction: >C=O + Zn-Hg / conc. HCl → >CH2 + H2O (Alkane conversion).
- Wolff-Kishner Reduction: >C=O + NH2NH2 / KOH + ethylene glycol Δ → >CH2 + N2.
- Aldol Condensation: Aldehydes or ketones containing at least one α-hydrogen in dilute NaOH form β-hydroxy aldehydes/ketones, which on heating undergo dehydration to yield α,β-unsaturated carbonyls.
- Cannizzaro Reaction: Aldehydes without α-hydrogen (e.g., HCHO, C6H5CHO) in conc. KOH undergo self-redox (disproportionation) to yield an alcohol and a carboxylate salt.
• Chemical Distinction Tests:
- Tollens' Test (Silver Mirror Test): Aldehydes reduce Tollens' reagent [Ag(NH3)2]+ to metallic silver. Ketones do not respond.
- Fehling's Test: Aliphatic aldehydes reduce Fehling's solution to give a red precipitate of Cu2O. Ketones and aromatic aldehydes do not respond.
- Iodoform Test: Compounds with CH3C=O or CH3CH(OH) groups react with I2 + NaOH to form a yellow ppt of Iodoform (CHI3).
3. Carboxylic Acids: Preparation, Acidic Nature & Reactions
Short Note:
• Preparation of Carboxylic Acids: Primary alcohols/aldehydes oxidation with KMnO4, hydrolysis of nitriles/amides (RCN → RCOOH), or Grignard reagent + solid CO2 (dry ice) followed by hydrolysis.
• Acidic Nature: Carboxylic acids are stronger acids than alcohols and phenols because the negative charge in carboxylate ion (RCOO-) is resonance-delocalised over two electronegative oxygen atoms.
- Effect of Substituents: Electron-withdrawing groups (-NO2, -F, -Cl) increase acidity by stabilizing carboxylate anion. Electron-donating groups (-CH3, -OCH3) decrease acidity.
• Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids with α-hydrogens react with X2 (Cl2 or Br2) in the presence of red phosphorus to yield α-halocarboxylic acids.
Intext & Conceptual Questions
(i) CH3CH(CH3)CH2CHO
(ii) CH3-CH=CH-CHO
(iii) Ph-CH=CH-COOH
1. CH3CH(CH3)CH2CHO: 3-Methylbutanal
2. CH3-CH=CH-CHO: But-2-enal
3. Ph-CH=CH-COOH: 3-Phenylprop-2-enoic acid
| Feature | Aldol Condensation | Cannizzaro Reaction |
|---|---|---|
| Requirement | Aldehydes/Ketones containing at least one α-hydrogen. | Aldehydes having no α-hydrogen (e.g., HCHO, C6H5CHO). |
| Reagent | Dilute alkali (dil. NaOH or Ba(OH)2). | Concentrated alkali (50% conc. NaOH or KOH). |
| Reaction Type | Nucleophilic addition followed by dehydration. | Self-oxidation and reduction (Disproportionation). |
| Products | α,β-unsaturated aldehyde or ketone. | Equimolar mixture of primary alcohol and sodium carboxylate salt. |
Textbook Chapter End Exercise Questions
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to two primary reasons:
• Steric Reasons: In aldehydes, there is only one alkyl group attached to the carbonyl carbon, whereas in ketones, there are two bulky alkyl groups. These bulky groups sterically hinder the attack of the incoming nucleophile.
• Inductive Reasons: Alkyl groups are electron-donating (+I effect). Ketones have two alkyl groups which donate electrons to the carbonyl carbon, reducing its positive charge (electrophilicity) more effectively than the single alkyl group in aldehydes.
Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids having an α-hydrogen atom react with chlorine (Cl2) or bromine (Br2) in the presence of a small amount of red phosphorus to yield α-halocarboxylic acids.
Chemical Equation:
R-CH2-COOH + X2 → (Red P) → R-CH(X)-COOH + HX (where X = Cl, Br)
Example:
CH3COOH + Cl2 → (Red P) → Cl-CH2-COOH (Monochloroacetic acid) + HCl
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.