Organic Chemistry: Aldehydes, Ketones, and Carboxylic Acids - SS3 Chemistry Past Questions and Answers - page 2

Aldehydes and ketones are important classes of organic compounds containing the carbonyl functional group (C=O). They differ based on the location of the carbonyl group within the molecule. In aldehydes, the carbonyl group is located at the end of the carbon chain, while in ketones, it is found in the middle.

Properties:

1.    Structural Formula: Both aldehydes and ketones have a carbonyl group, which imparts polarity to the molecules.

2.    Boiling Points: Aldehydes and ketones have higher boiling points than alkanes of similar molecular weight due to the presence of dipole-dipole interactions between the carbonyl groups.

3.    Solubility: Small aldehydes and ketones are soluble in water due to hydrogen bonding with water molecules.

4.    Odour: Many aldehydes and ketones possess strong and distinctive odours.

Reactions:

1.    Nucleophilic Addition: Aldehydes and ketones undergo nucleophilic addition reactions with nucleophiles such as water, alcohol, and amines, leading to the formation of hydrates, hemiacetals, and imines, respectively.

Example: Acetal formation with alcohols:

RCHO + 2ROH ⇌ RCH(OR)2 + H2O

2.    Oxidation: Aldehydes can be oxidised to carboxylic acids using mild oxidising agents like Tollens' reagent or Fehling's solution.

Example: Oxidation of ethanol to acetic acid:

CH3CHO + [O] → CH3COOH

3.    Reduction: Aldehydes and ketones can be reduced to primary and secondary alcohols, respectively, using reducing agents like NaBH4 or LiAlH4.

Example: Reduction of propanone to propan-2-ol:

CH3COCH3 + 2[H] → CH3CHOHCH3

4.    Aldol Condensation: Aldehydes and ketones can undergo aldol condensation to form β-hydroxy carbonyl compounds in the presence of a base.

Example: Aldol condensation of propanal:

CH3CH2CHO + CH3CH2CHO ⇌ CH3CH2CH(OH)CH2CHO

Applications:

1.    Flavours and Fragrances: Aldehydes and ketones contribute to the flavours and fragrances of various natural products and synthetic perfumes.

2.    Pharmaceuticals: Many aldehydes and ketones serve as essential intermediates in the synthesis of pharmaceutical drugs.

3.    Polymers: Ketones like acetone are important solvents in the production of plastics and synthetic fibres.

4.    Testing and Identification: Tollens' reagent and Fehling's solution are used for testing the presence of aldehydes in organic compounds.

In conclusion, aldehydes and ketones are versatile functional groups with significant applications in various fields. Their reactivity and unique properties make them valuable building blocks in organic synthesis, pharmaceuticals, flavours, and fragrances. The wide range of reactions and applications of aldehydes and ketones highlight their importance in the field of organic chemistry.