Aldehydes, Ketones and Carboxylic Acids is one of the most important chapters of Class 12 Chemistry (Organic Chemistry Aldehydes Ketones and Carboxylic Acids Class12 Notes) for CBSE Board Exams, NEET, and other competitive examinations. This chapter deals with carbonyl compounds and carboxylic acids, their preparation, properties, reactions, mechanisms, and uses. Questions from this chapter are frequently repeated in board exams and entrance tests, so clear conceptual understanding is essential.
In these Aldehydes Ketones and Carboxylic Acids Class 12 Notes, all topics are explained step by step, with important formulas, reactions, mechanisms, and examples, written in simple English to help students score high marks.
Aldehydes Ketones and Carboxylic Acids Class12 Notes | Reactions & Important Questions
Carbonyl Compounds
Organic compounds containing the carbonyl group (>C=O) are called carbonyl compounds.
Classification of Carbonyl Compounds
- Aldehydes – Carbonyl carbon attached to at least one hydrogen atom
- General formula: R–CHO
- Example: Formaldehyde (HCHO), Acetaldehyde (CH3CHO)
- Ketones – Carbonyl carbon attached to two alkyl or aryl groups
- General formula: R–CO–R’
- Example: Acetone (CH3COCH3)
Nomenclature of Aldehydes and Ketones
IUPAC Naming
- Aldehydes: Suffix -al
- Methanal (HCHO), Ethanal (CH3CHO)
- Ketones: Suffix -one
- Propanone (CH3COCH3), Butanone (CH3COCH2CH3)
Preparation of Aldehydes and Ketones
1. From Alcohols (Oxidation)
- Primary alcohol → Aldehyde
- Secondary alcohol → Ketone
Reagents: PCC, CrO3, K2Cr2O7
Reaction: R–CH2OH → R–CHO R–CHOH–R’ → R–CO–R’
2. From Acyl Chlorides (Rosenmund Reduction)
- Aldehydes prepared using Pd/BaSO4 catalyst
3. From Nitriles
- Reduction using SnCl2/HCl or LiAlH4
Physical Properties of Aldehydes and Ketones
- Polar molecules due to C=O bond
- Higher boiling point than alkanes
- Lower boiling point than alcohols
- Lower members are soluble in water
Chemical Reactions of Aldehydes and Ketones
1. Nucleophilic Addition Reactions
The carbonyl carbon is electrophilic and undergoes nucleophilic addition.
(a) Addition of HCN
R–CHO + HCN → Cyanohydrin
(b) Addition of NaHSO3
Forms bisulphite addition compounds
(c) Addition of Grignard Reagent
R–CHO + RMgX → Alcohol
2. Oxidation Reactions
- Aldehydes oxidize easily to carboxylic acids
- Ketones resist oxidation
Tollens’ Test: Silver mirror test (only aldehydes) Fehling’s Test: Red ppt of Cu2O (aliphatic aldehydes)
3. Reduction Reactions
- Reduction to alcohols using NaBH4 or LiAlH4
Aldol Condensation
Aldehydes and ketones with α-hydrogen undergo aldol condensation.
Example: 2 CH3CHO → CH3CH(OH)CH2CHO
Cannizzaro Reaction
Aldehydes without α-hydrogen undergo disproportionation.
Example: 2 HCHO + NaOH → HCOONa + CH3OH
Carboxylic Acids
Organic compounds containing –COOH (carboxyl group) are called carboxylic acids.
General Formula
R–COOH
Examples
- Formic acid (HCOOH)
- Acetic acid (CH3COOH)
Nomenclature of Carboxylic Acids
- Suffix -oic acid
- Example: Ethanoic acid, Propanoic acid
Preparation of Carboxylic Acids
1. Oxidation of Aldehydes and Alcohols
R–CHO → R–COOH
2. Hydrolysis of Nitriles
R–CN → R–COOH
Physical Properties of Carboxylic Acids
- Strong hydrogen bonding
- Exist as dimers
- Higher boiling points
- Lower acids soluble in water
Chemical Reactions of Carboxylic Acids
1. Acidic Nature
R–COOH ⇌ R–COO⁻ + H⁺
2. Esterification
R–COOH + R’–OH → R–COOR’ + H2O
3. Reduction
Carboxylic acids reduce to alcohols using LiAlH4
Uses of Aldehydes, Ketones and Carboxylic Acids
- Formaldehyde: Preservative
- Acetone: Solvent
- Acetic acid: Vinegar, food industry
Important Exam Tips
- Learn all named reactions
- Practice reaction mechanisms
- Memorize tests and reagents
Important Repeated Questions (Board + NEET)
These are the most searched and repeatedly asked questions from Aldehydes, Ketones and Carboxylic Acids Class 12, based on CBSE board papers, sample papers, and competitive exams:
Very Short Answer Questions (1 Mark)
- What is the functional group present in aldehydes?
- Write the general formula of ketones.
- Which test is used to distinguish aldehydes from ketones?
- Name the reagent used in Tollens’ test.
- Why do carboxylic acids have higher boiling points than alcohols of similar molecular mass?
Short Answer Questions (2–3 Marks)
- Write the IUPAC name of CH3COCH3 and CH3CHO.
- Explain why aldehydes are more reactive than ketones towards nucleophilic addition reactions.
- Write the chemical reaction involved in Fehling’s test.
- What is aldol condensation? Write one example.
- Give reason: Carboxylic acids exist as dimers.
Long Answer Questions (4–5 Marks)
- Explain the preparation of aldehydes from alcohols with suitable reactions.
- Describe the nucleophilic addition reaction of aldehydes and ketones.
- Explain Cannizzaro reaction with mechanism.
- Write the preparation, properties, and uses of ethanoic acid.
- How can you distinguish between aldehydes and ketones using chemical tests?
Assertion–Reason Questions (Repeated)
- Assertion (A): Aldehydes are easily oxidized to carboxylic acids. Reason (R): Aldehydes contain at least one hydrogen atom attached to carbonyl carbon.
- Assertion (A): Ketones do not give Tollens’ test. Reason (R): Ketones resist oxidation.
Case-Based / Competency-Based Questions
- A carbonyl compound gives silver mirror test but does not reduce Fehling solution. (a) Identify the compound. (b) Give reason. (c) Write the chemical equation involved.
Numericals / Reaction-Based Questions
- Predict the product when ethanal reacts with NaBH4.
- Write the product formed when formaldehyde undergoes Cannizzaro reaction.
Conclusion
Aldehydes, Ketones and Carboxylic Acids is a high-weightage and frequently searched chapter in Class 12 Chemistry. By focusing on important reactions, named reactions, formulas, and repeated questions, students can score excellent marks in board exams and competitive tests. These notes are designed to be SEO-friendly, exam-oriented, and easy to revise, making them perfect for last-minute preparation.