Class 12 Chemistry Notes: Aldehydes, Ketones and Carboxylic Acids | Complete Guide

Aldehydes, Ketones, and Carboxylic Acids form an essential part of organic chemistry and the Class 12 CBSE Chemistry curriculum. These organic compounds contain the carbonyl functional group (C=O) and are highly significant due to their wide applications in industries, pharmaceuticals, and biological systems.

• Aldehydes have the –CHO group.

• Ketones have the C=O group bonded to two carbon atoms.

• Carboxylic Acids have the –COOH group.

In this article, we will cover nomenclature, structure, physical and chemical properties, reactions, preparation methods, and uses of these compounds in detail for Class 12 board exams and competitive exams like JEE and NEET.

Class 12 Chemistry Notes: Aldehydes, Ketones and Carboxylic Acids | Complete Guide

Aldehydes

Structure and Functional Group

• Aldehydes contain the formyl group (–CHO) attached to a carbon chain.

• The carbon atom of the group is sp² hybridized, leading to a planar structure.

General formula: R–CHO

Nomenclature of Aldehydes

1. Common system: Derived from corresponding carboxylic acids by replacing –ic acid with –aldehyde.

   • Example: HCHO → Formaldehyde

2. IUPAC system: Add the suffix –al to the parent hydrocarbon chain.

   • Example: CH₃CHO → Ethanal

Methods of Preparation of Aldehydes

1. Oxidation of primary alcohols:
R–CH2OH→R–CHOR–CH₂OH → R–CHOR–CH2​OH→R–CHO
Using PCC or Cu at 573 K.

2. Rosenmund reduction:
Acid chlorides are hydrogenated in the presence of Pd/BaSO₄ catalyst.

3. Stephen reaction:
Nitriles reduced with stannous chloride (SnCl₂) and HCl to imines, followed by hydrolysis.

Physical Properties of Aldehydes

• State: Lower aldehydes are gases; higher ones are liquids/solids.

• Boiling point: Higher than hydrocarbons but lower than alcohols due to weak hydrogen bonding.

• Solubility: Lower aldehydes soluble in water due to H-bonding; solubility decreases with chain length.

Chemical Reactions of Aldehydes

1. Nucleophilic Addition Reactions:

• HCN addition → Cyanohydrins

• NaHSO₃ addition → Bisulfite compounds

2. Reduction Reactions:

• Aldehydes → Primary alcohols (LiAlH₄, NaBH₄)

3. Oxidation Reactions:

• Aldehydes → Carboxylic acids (KMnO₄, K₂Cr₂O₇)

4. Aldol Condensation:

• Aldehydes with α-H atom react with base → β-hydroxy aldehyde.

5. Cannizzaro Reaction:

• Aldehydes without α-H atom react with base → one molecule oxidized, one reduced.

Ketones

Structure and Functional Group

• Ketones have the carbonyl group (C=O) bonded to two alkyl/aryl groups.

• General formula: R–CO–R

Nomenclature of Ketones

1. Common system: Name alkyl groups + ketone. Example: CH₃–CO–CH₃ → Dimethyl ketone.

2. IUPAC system: Add suffix –one to parent hydrocarbon. Example: CH₃–CO–CH₃ → Propanone.

Methods of Preparation of Ketones

1. Oxidation of secondary alcohols:
   R2CHOH→R2C=OR₂CHOH → R₂C=OR2​CHOH→R2​C=O

2. Friedel–Crafts Acylation:
   Benzene + acid chloride → aromatic ketones using AlCl₃.

3. Hydration of Alkynes:
   Alkyne + H₂O in presence of H₂SO₄/HgSO₄ → Ketone.

Physical Properties of Ketones

• Similar to aldehydes in solubility and boiling points.

• Generally liquids with pleasant odor.

Chemical Reactions of Ketones

• Nucleophilic addition reactions similar to aldehydes but less reactive.

• Reduction to secondary alcohols.

• Aldol condensation if α-H present.

Carboxylic Acids

Structure and Functional Group

• Carboxylic acids have –COOH group attached to carbon chain.

• Structure: sp² hybridized, planar geometry.

Nomenclature of Carboxylic Acids

1. Common system: Replace –ic acid with parent name: HCOOH → Formic acid.

2. IUPAC system: Replace –e of alkane with –oic acid: CH₃COOH → Ethanoic acid.

Methods of Preparation

1. Oxidation of aldehydes/primary alcohols:
   R–CH2OH→R–COOHR–CH₂OH → R–COOHR–CH2​OH→R–COOH

2. Hydrolysis of nitriles:
   R–C≡N + H₂O → R–COOH

3. Grignard reagents with CO₂:
   R–MgX + CO₂ → R–COOH

Physical Properties

• Higher boiling points due to strong hydrogen bonding.

• Soluble in water up to four carbon atoms.

• Form dimers due to hydrogen bonding.

Chemical Reactions of Carboxylic Acids

1. Acidity:

• Weak acids; form salts with alkalis.

2. Reduction:

• R–COOH → Primary alcohols using LiAlH₄.

3. Substitution reactions:

• α-H replaced by halogens (Hell–Volhard–Zelinsky reaction).

4. Esterification:

• R–COOH + R’–OH → Ester + H₂O (H₂SO₄ as catalyst).

Uses and Applications

• Aldehydes: Formaldehyde in plastics, acetaldehyde in acetic acid production.

• Ketones: Acetone as a solvent, methyl ethyl ketone in paints.

• Carboxylic acids: Acetic acid in vinegar, fatty acids in soaps and detergents.

Objective Questions-Aldehydes, Ketones and Carboxylic Acids

Q1. Which functional group is present in aldehydes?

a) –COOH
b) –CHO
c) –OH
d) –C=O

Answer: b) –CHO

Q2. What is the IUPAC name of CH₃–CHO?

a) Methanal
b) Ethanal
c) Propanal
d) Formaldehyde

Answer: b) Ethanal

Q3. Which reagent is used in Rosenmund reduction?

a) Pd/BaSO₄
b) LiAlH₄
c) KMnO₄
d) Zn/HCl

Answer: a) Pd/BaSO₄

Q4. Cannizzaro reaction is given by which aldehydes?

a) With α-H atom
b) Without α-H atom
c) Aromatic aldehydes only
d) None

Answer: b) Without α-H atom

Q5. Which of the following gives iodoform test?

a) Methanal
b) Ethanal
c) Propanone
d) Both b and c

Answer: d) Both b and c

Q6. What is the product of oxidation of aldehydes?

a) Alcohol
b) Ketone
c) Carboxylic acid
d) Ester

Answer: c) Carboxylic acid

Q7. Which compound is known as formaldehyde?

a) HCHO
b) CH₃CHO
c) CH₃COCH₃
d) CH₃COOH

Answer: a) HCHO

Q8. Friedel–Crafts acylation gives what product?

a) Aldehydes
b) Alcohols
c) Aromatic ketones
d) Alkanes

Answer: c) Aromatic ketones

Q9. The general formula of carboxylic acids is:

a) R–CHO
b) R–COOH
c) R–CO–R’
d) R–OH

Answer: b) R–COOH

Q10. Which acid is present in vinegar?

a) Formic acid
b) Acetic acid
c) Oxalic acid
d) Benzoic acid

Answer: b) Acetic acid

Q11. Which aldehyde is used in plastics?

a) Acetaldehyde
b) Formaldehyde
c) Benzaldehyde
d) Propionaldehyde

Answer: b) Formaldehyde

Q12. What is the functional group of ketones?

a) –OH
b) –CHO
c) C=O between two carbon atoms
d) –COOH

Answer: c) C=O between two carbon atoms

Q13. Which reagent is used for oxidation of aldehydes?

a) KMnO₄
b) NaBH₄
c) Zn/HCl
d) Pd/BaSO₄

Answer: a) KMnO₄

Q14. Hell–Volhard–Zelinsky reaction halogenates:

a) Alcohols
b) Carboxylic acids
c) Ketones
d) Aldehydes

Answer: b) Carboxylic acids

Q15. Which ketone is commonly used as a solvent?

a) Benzophenone
b) Acetone
c) Propanone
d) Ethanal

Answer: b) Acetone

Q16. Oxidation of primary alcohol gives:

a) Ketones
b) Aldehydes
c) Esters
d) Alkanes

Answer: b) Aldehydes

Q17. The reaction of aldehydes with HCN gives:

a) Alcohols
b) Cyanohydrins
c) Carboxylic acids
d) Alkanes

Answer: b) Cyanohydrins

Q18. Which reaction converts acid chlorides to aldehydes?

a) Aldol reaction
b) Rosenmund reaction
c) Cannizzaro reaction
d) HVZ reaction

Answer: b) Rosenmund reaction

Q19. Esters are formed by the reaction between:

a) Alcohols and aldehydes
b) Aldehydes and ketones
c) Alcohols and carboxylic acids
d) Alcohols and ketones

Answer: c) Alcohols and carboxylic acids

Q20. Acetic acid is also known as:

a) Methanoic acid
b) Ethanoic acid
c) Propanoic acid
d) Benzoic acid

Answer: b) Ethanoic acid

Short Answer Questions-Aldehydes, Ketones and Carboxylic Acids

Q1. What is the difference between aldehydes and ketones?

Answer:
Aldehydes have the –CHO group attached to at least one hydrogen atom, while ketones have the C=O group attached to two carbon atoms. Aldehydes are more reactive due to the presence of the hydrogen atom, whereas ketones are less reactive towards nucleophilic addition reactions.

Q2. Write two methods of preparation of aldehydes.

Answer:
Aldehydes can be prepared by:

1. Oxidation of primary alcohols using PCC or Cu at 573 K.

2. Rosenmund reduction of acid chlorides in the presence of Pd/BaSO₄ catalyst.

Q3. Explain aldol condensation reaction.

Answer:
Aldehydes and ketones with α-hydrogen atoms react in the presence of a dilute base like NaOH to form β-hydroxy aldehydes or ketones, called aldol. On heating, these undergo dehydration to form α,β-unsaturated aldehydes or ketones.

Q4. What is the acidity of carboxylic acids due to?

Answer:
Carboxylic acids are acidic because the carboxylate ion formed after losing a proton is stabilized by resonance, making the proton loss easier. This resonance stabilization increases acidity compared to alcohols.

Q5. Write two uses of acetone.

Answer:

1. Used as an industrial solvent for plastics and paints.

2. Used in nail polish removers and cleaning purposes.

Long Answer Questions-Aldehydes, Ketones and Carboxylic Acids

Q1. Describe the physical properties of aldehydes, ketones, and carboxylic acids.

Answer:

• Aldehydes and Ketones: Lower members are liquids or gases with sharp odors. They have higher boiling points than hydrocarbons but lower than alcohols due to lack of hydrogen bonding between molecules.

• Carboxylic Acids: They have the highest boiling points due to strong hydrogen bonding and form dimers in both liquid and vapor phases. They are soluble in water up to four carbon atoms due to hydrogen bonding with water molecules.

Q2. Explain the chemical reactions of carboxylic acids with examples.

Answer:
Carboxylic acids undergo:

1. Neutralization: Form salts with alkalis.

2. Esterification: React with alcohols in the presence of conc. H₂SO₄ to form esters.

3. Reduction: Give primary alcohols with LiAlH₄.

4. HVZ Reaction: Halogenation at the α-position using halogen and red phosphorus.

Q3. Write the methods of preparation of ketones with equations.

Answer:

1. Oxidation of secondary alcohols:
   R2CHOH→R2C=OR₂CHOH → R₂C=OR2​CHOH→R2​C=O using PCC or KMnO₄.

2. Friedel–Crafts Acylation: Benzene reacts with acid chlorides in the presence of AlCl₃ to form aromatic ketones.

3. Hydration of alkynes: Alkynes react with H₂O in the presence of H₂SO₄/HgSO₄ to form ketones.

Q4. Explain Cannizzaro reaction with mechanism.

Answer:
Cannizzaro reaction occurs with aldehydes having no α-hydrogen atoms. In the presence of concentrated NaOH, one aldehyde molecule undergoes oxidation to carboxylic acid, and the other gets reduced to alcohol. This is a disproportionation reaction.

Q5. Give the differences between aldehydes, ketones, and carboxylic acids with examples.

Answer:

Conclusion

Aldehydes, Ketones, and Carboxylic Acids are fundamental organic compounds with diverse preparation methods, properties, and applications. Understanding their reactions, structures, and mechanisms is crucial for excelling in Class 12 board exams and entrance tests like JEE/NEET.

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