Organic Chemistry II: Aldehydes and Ketones

Organic Chemistry II: Reactions of Aldehydes and Ketones

Overview of Aldehydes and Ketones
  • Aldehydes and ketones are significant functional groups in organic chemistry.
  • Reactions involve oxidations, reductions, and various transformations.
Key Reactions Involving Aldehydes and Ketones

  • Formation of Aldehydes and Ketones:

    • KMnO4 under warm, acidic conditions:
    • Converts secondary alcohols to ketones.
    • Converts aldehydes to carboxylic acids.

    ext{Secondary Alcohol} + ext{KMnO}4 ightarrow ext{Ketone} ext{Aldehyde} + ext{KMnO}4
    ightarrow ext{Carboxylic Acid}

Synthesis of Benzaldehyde

  • Reaction using CO and HCl followed by AlCl3 and water:

    • This is an acylation reaction; however, it does not occur with meta-directing substituents on a benzene ring.

    ext{Benzene} + ext{CO}, ext{HCl}
    ightarrow ext{Benzaldehyde}

Reactivity of Aldehydes vs. Ketones
  • Aldehydes react faster than ketones under both acidic and basic conditions.
  • Under acidic conditions:
    • Formation of alcohol.
Oxidation of Aldehydes

  • Common oxidizing agents:

    • Na2Cr2O7, H2SO4:
    • Converts aldehydes to carboxylic acids.

    ext{Aldehyde} + ext{Oxidizing Agent}
    ightarrow ext{Carboxylic Acid}

Cyanohydrin Formation

  • Conversion of aldehydes/ketones into cyanohydrins using NaCN or HCN:

    • Subsequent reactions can yield amines or carboxylic acids.
    • Either reaction transforms the CN into:
    • Carboxylic acid when treated with acid and water.
    • Primary amine when treated with LiAlH4 then water.

    ext{Aldehyde/Ketone} + ext{CN}
    ightarrow ext{Cyanohydrin}

Formation of Imines and Enamines

  • Imine Formation:

    • Reaction with amines in the presence of acid.
    • Can lead to stable imine products.

  • Enamine Formation:

    • Forms via reaction between aldehydes/ketones and secondary amines under acidic conditions.
Hemiacetal and Acetal Formation
  • Hemiacetals formed when aldehydes/ketones react with alcohol under acidic conditions.
  • Full acetals form upon further reaction with excess alcohol.
Protecting Groups
  • Use of protecting groups allows for selective reactions in complex organic synthesis.
Reduction of Aldehydes and Ketones

  • Common Reductants:

    • LiAlH4 and NaBH4:
      • Reduce aldehydes to primary alcohols.
      • Reduce ketones to secondary alcohols.

    ext{Aldehyde/Ketone} + ext{Reducing Agent}
    ightarrow ext{Alcohol}

Notable Reactions

  • Wolff-Kishner Reaction:

    • Used to convert carbonyl compounds to hydrocarbons.

  • Cannizzaro Reaction:

    • Aldehyde undergoes disproportionation in the presence of a strong base, yielding alcohol and carboxylic acid.
Summary of Reduction and Oxidation Mechanisms
  • Aldehydes can reduce to alcohols or oxidize to carboxylic acids depending on reagents used.
  • Ketones do not undergo oxidation as easily as aldehydes.