Science Class 12 - Aldehydes Notes

Aldehydes

Welcome to the chapter on Aldehydes for Class 12. In this chapter, you will learn about the structure, nomenclature, preparation, properties, and uses of aldehydes. By the end of this chapter, you will understand the chemical behavior of aldehydes and their importance in organic chemistry.

Introduction

Aldehydes are organic compounds containing the functional group -CHO. They are important intermediates in many chemical reactions and are widely used in industry and laboratories.

Structure and Nomenclature

• General formula: R-CHO, where R is a hydrogen atom or an alkyl/aryl group.
• The carbonyl group (C=O) is always at the end of the carbon chain.
• IUPAC naming: Replace the -e of the parent alkane with -al.
  Example: Methanal (formaldehyde), Ethanal (acetaldehyde)

Preparation of Aldehydes

• Oxidation of primary alcohols (using PCC or mild oxidizing agents).
• Ozonolysis of alkenes.
• Rosenmund reduction (hydrogenation of acid chlorides).
• Hydroformylation of alkenes (industrial method).

Physical Properties

• Lower aldehydes are colorless, volatile liquids with pungent odors.
• Soluble in water (lower members); solubility decreases with increasing molecular mass.
• Boiling points are higher than alkanes but lower than alcohols of similar mass.

Chemical Properties

• Nucleophilic addition reactions: Addition of HCN, NaHSO₃, alcohols, ammonia derivatives.
• Oxidation: Aldehydes are easily oxidized to carboxylic acids (Tollen's and Fehling's tests).
• Reduction: Reduced to primary alcohols using LiAlH₄ or NaBH₄.
• Aldol condensation: Aldehydes with α-hydrogen undergo self-condensation in the presence of base.
• Cannizzaro reaction: Aldehydes without α-hydrogen undergo disproportionation in the presence of base.

Tests for Aldehydes

• Tollen's test: Silver mirror forms with aldehydes.
• Fehling's test: Red precipitate forms with aliphatic aldehydes.
• Schiff's test: Pink color develops with aldehydes.

Uses of Aldehydes

• Formaldehyde is used in making plastics, resins, and disinfectants.
• Acetaldehyde is used in the manufacture of acetic acid, perfumes, and dyes.
• Aldehydes are used as intermediates in organic synthesis.

Practice Questions

1. Write the IUPAC name and structure of formaldehyde and acetaldehyde.
2. How can you prepare an aldehyde from a primary alcohol?
3. What is the difference between Tollen's and Fehling's tests?
4. Explain aldol condensation with an example.
5. List two uses of aldehydes in daily life or industry.

Challenge Yourself

• Describe the Cannizzaro reaction and give an example.
• Explain why aldehydes are more reactive than ketones towards nucleophilic addition.

Did You Know?

• Formaldehyde is the simplest aldehyde and is used to preserve biological specimens.
• Benzaldehyde gives the smell of almonds and is used in perfumes and flavorings.

Glossary

• Aldehyde: An organic compound with the -CHO group.
• Carbonyl group: The C=O group present in aldehydes and ketones.
• Aldol condensation: A reaction where two aldehyde molecules combine to form a larger molecule.
• Cannizzaro reaction: A reaction of aldehydes without α-hydrogen with base, giving alcohol and acid.

Answers to Practice Questions

1. Formaldehyde: Methanal, HCHO; Acetaldehyde: Ethanal, CH₃CHO
2. By oxidation of a primary alcohol using mild oxidizing agents like PCC.
3. Tollen's test gives a silver mirror with aldehydes; Fehling's test gives a red precipitate with aliphatic aldehydes.
4. Aldol condensation: 2 CH₃CHO (acetaldehyde) → CH₃CH(OH)CH₂CHO (aldol) in presence of base.
5. Formaldehyde is used in making plastics; acetaldehyde is used in making acetic acid.

---

Understanding aldehydes helps you master organic chemistry and prepares you for higher studies and practical applications!