Aldehydes Ketones And Carboxylic Acids Notes

Introduction

Aldehydes, ketones, and carboxylic acids are fundamental classes of organic compounds that play crucial roles in both chemistry and biology. They are characterized by the presence of the carbonyl group ($C=O$), but each has distinct structural features and chemical properties. This study note will cover their nomenclature, structure, preparation, properties, and reactions, essential for the JEE Main Chemistry syllabus.

Aldehydes

Nomenclature

Aldehydes are named by replacing the terminal "-e" of the parent alkane with "-al." For example, methane becomes methanal (commonly known as formaldehyde).

Common Names vs. IUPAC Names

• Methanal: Formaldehyde
• Ethanal: Acetaldehyde
• Propanal: Propionaldehyde

Structure

Aldehydes have the general formula $RCHO$, where $R$ can be a hydrogen atom or an alkyl/aryl group. The carbonyl carbon is sp² hybridized, forming a planar structure.

Preparation

1. Oxidation of Primary Alcohols: $$ RCH_2OH + [O] \rightarrow RCHO + H_2O $$
2. Ozonolysis of Alkenes: $$ R_2C=CR_2 + O_3 \rightarrow R_2C=O + R_2C=O $$

Properties

• Physical Properties: Lower aldehydes are typically gases or liquids with a pungent odor.
• Chemical Properties:
  • Nucleophilic Addition Reactions: $$ RCHO + HCN \rightarrow RCH(OH)CN $$
  • Oxidation: $$ RCHO + [O] \rightarrow RCOOH $$
  • Reduction: $$ RCHO + H_2 \rightarrow RCH_2OH $$

Ketones

Nomenclature

Ketones are named by replacing the terminal "-e" of the parent alkane with "-one." The position of the carbonyl group is indicated by a number.

Examples

• Propanone: Acetone
• Butanone: Methyl ethyl ketone

Structure

Ketones have the general formula $RCOR'$, where $R$ and $R'$ are alkyl or aryl groups. The carbonyl carbon is sp² hybridized.

Preparation

1. Oxidation of Secondary Alcohols: $$ R_2CHOH + [O] \rightarrow R_2CO + H_2O $$
2. Ozonolysis of Alkenes: $$ R_2C=CR_2 + O_3 \rightarrow 2R_2CO $$

Properties

• Physical Properties: Ketones are generally liquids with pleasant odors.
• Chemical Properties:
  • Nucleophilic Addition Reactions: $$ RCOR' + HCN \rightarrow RCR'(OH)CN $$
  • Reduction: $$ RCOR' + H_2 \rightarrow RCH(OH)R' $$

Carboxylic Acids

Nomenclature

Carboxylic acids are named by replacing the terminal "-e" of the parent alkane with "-oic acid."

Examples

• Methanoic Acid: Formic Acid
• Ethanoic Acid: Acetic Acid

Structure

Carboxylic acids have the general formula $RCOOH$. The carboxyl group ($-COOH$) consists of a carbonyl group ($C=O$) and a hydroxyl group ($-OH$).

Preparation

1. Oxidation of Primary Alcohols: $$ RCH_2OH + 2[O] \rightarrow RCOOH + H_2O $$
2. Hydrolysis of Nitriles: $$ RCN + 2H_2O \rightarrow RCOOH + NH_3 $$

Properties

• Physical Properties: Carboxylic acids have higher boiling points due to hydrogen bonding.
• Chemical Properties:
  • Acidity: $$ RCOOH \rightarrow RCOO^- + H^+ $$
  • Esterification: $$ RCOOH + R'OH \rightarrow RCOOR' + H_2O $$
  • Reduction: $$ RCOOH + 4[H] \rightarrow RCH_2OH + H_2O $$

Important Reactions

Aldol Condensation

Occurs between aldehydes or ketones with at least one α-hydrogen: $$ 2RCHO \rightarrow RCH(OH)CH_2CHO \rightarrow RCH=CHCHO + H_2O $$

Cannizzaro Reaction

Occurs in aldehydes without α-hydrogens: $$ 2RCHO + NaOH \rightarrow RCH_2OH + RCOONa $$

Decarboxylation

Removal of $CO_2$ from carboxylic acids: $$ RCOOH \rightarrow RH + CO_2 $$

Summary

Aldehydes, ketones, and carboxylic acids are essential organic compounds with distinct properties and reactions. Understanding their nomenclature, structure, preparation, and chemical behavior is crucial for mastering organic chemistry in the JEE Main syllabus.

Conclusion

Mastering the chemistry of aldehydes, ketones, and carboxylic acids involves understanding their structures, properties, and reactions. This knowledge is fundamental for solving problems in organic chemistry and is critical for success in JEE Main Chemistry.