UNIT 12: Aldehydes, Ketones & Carboxylic Acids

Introduction

This unit covers the chemistry of aldehydes, ketones, and carboxylic acids, focusing on their structure, preparation, properties, and reactions. These compounds are central to organic chemistry due to their functional groups and reactivity.

Structure and Nomenclature

Aldehydes and Ketones

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

• Ketones have a carbonyl group (>C=O) bonded to two alkyl or aryl groups.

• Nomenclature follows IUPAC rules: Aldehydes use the suffix -al (e.g., ethanal), ketones use -one (e.g., propanone).

Carboxylic Acids

• Contain the carboxyl group (-COOH).

• IUPAC names end with -oic acid (e.g., ethanoic acid).

Preparation Methods

Preparation of Aldehydes

• Oxidation of primary alcohols:

• Ozonolysis of alkenes:

Preparation of Ketones

• Oxidation of secondary alcohols:

• Friedel-Crafts acylation of aromatic compounds:

Preparation of Carboxylic Acids

• Oxidation of aldehydes:

• Hydrolysis of nitriles:

Physical Properties

• Aldehydes and ketones are generally polar and have higher boiling points than alkanes of similar molecular mass.

• Carboxylic acids form strong hydrogen bonds, resulting in even higher boiling points.

• Lower members are soluble in water; solubility decreases with increasing chain length.

Chemical Properties and Reactions

Reactions of Aldehydes and Ketones

• Nucleophilic Addition Reactions: The carbonyl carbon is electrophilic and undergoes addition with nucleophiles.

• Addition of HCN: Formation of cyanohydrins.

• Addition of NaHSO3: Formation of bisulfite adducts.

• Addition of Grignard Reagents: Formation of alcohols.

• Reduction: Aldehydes and ketones can be reduced to alcohols using NaBH4 or LiAlH4.

• Oxidation: Aldehydes are easily oxidized to carboxylic acids; ketones are more resistant.

Distinguishing Tests

• Tollens' Test: Aldehydes reduce Ag+ to metallic silver (silver mirror); ketones do not.

• Fehling's Test: Aldehydes reduce Cu2+ to Cu2O (red precipitate); ketones do not.

• Iodoform Test: Methyl ketones and compounds with CH3CO- group give yellow precipitate of iodoform.

Reactions of Carboxylic Acids

• Acidic Nature: Carboxylic acids are weak acids; their acidity is due to resonance stabilization of the carboxylate ion.

• Reactions with Bases: Form salts and water.

• Decarboxylation: Loss of CO2 on heating with soda lime.

• Reduction: Carboxylic acids can be reduced to primary alcohols.

Important Named Reactions

• Aldol Condensation: Aldehydes and ketones with α-hydrogen undergo condensation in the presence of base to form β-hydroxy aldehydes/ketones.

• Cannizzaro Reaction: Aldehydes without α-hydrogen undergo disproportionation in the presence of base.

Comparison Table: Aldehydes vs. Ketones

Applications and Examples

• Formaldehyde is used in polymer production (e.g., Bakelite).

• Acetone is a common solvent.

• Acetic acid is used in food industry (vinegar).

Sample Reaction Sequences

• Conversion of esters to aldehydes using DIBAL-H:

• Oxidation of primary alcohols to acids:

Distinguishing Between Compounds

• Benzaldehyde vs. Acetaldehyde: Benzaldehyde does not give Fehling's test; acetaldehyde does.

• Propanal vs. Propanone: Propanal gives positive Tollens' and Fehling's tests; propanone does not.

Additional info:

• Some questions refer to CBSE board exam patterns and focus on reaction mechanisms, conversions, and distinguishing tests.

• Students should practice writing reaction mechanisms and identifying reagents for conversions.