Lesson 22: Aldehydes, Ketones, Alpha Carbon Chemistry Reactions

Lesson 22 explores the reactivity and reactions of aldehydes, ketones, and alpha carbon chemistry. These fundamental organic compounds participate in various reactions that are crucial for synthetic chemistry.

Aldehydes and Ketones:

Aldehydes (R-CHO) and ketones (R-CO-R') are carbonyl compounds with distinctive reactivity due to the polarized carbon-oxygen double bond. This polarization makes the carbonyl carbon electrophilic, susceptible to nucleophilic attack, and the oxygen nucleophilic, capable of forming hydrogen bonds. Both aldehydes and ketones undergo several key reactions:

1. Nucleophilic Addition:

   • Hydration: Aldehydes and ketones react with water to form geminal diols (hydrates).

   • Alcohol Addition: Reaction with alcohols forms hemiacetals and hemiketals, which can further react to form acetals and ketals.

2. Reduction:

   • Hydride Addition: Aldehydes are reduced to primary alcohols, and ketones to secondary alcohols, using reducing agents like NaBH4 or LiAlH4.

3. Oxidation:

   • Aldehydes: Easily oxidized to carboxylic acids by oxidizing agents like KMnO4 or CrO3.

   • Ketones: Resistant to oxidation but can be cleaved by strong oxidizing agents under harsh conditions.

4. Condensation Reactions:

   • Aldol Condensation: Involves the enolate ion of one carbonyl compound reacting with the carbonyl carbon of another to form β-hydroxy aldehyde or ketone, which can dehydrate to form an α,β-unsaturated carbonyl compound.

Alpha Carbon Chemistry:

The alpha carbon, adjacent to the carbonyl group, exhibits significant reactivity due to the acidity of its hydrogen atoms. This allows for the formation of enolates, which are crucial intermediates in many reactions:

1. Aldol Reaction:

   • The enolate ion formed from deprotonation of the alpha carbon attacks another carbonyl compound to form β-hydroxy carbonyl compounds, which can further dehydrate to α,β-unsaturated carbonyl compounds.

2. Michael Addition:

   • The enolate adds to the β-carbon of an α,β-unsaturated carbonyl compound, forming a 1,5-dicarbonyl compound.

3. Claisen Condensation:

   • An enolate ion reacts with an ester to form a β-keto ester, a reaction important for forming carbon-carbon bonds in organic synthesis.