Oxidation and Reduction of Aldehydes and Ketones

Oxidation–Reduction Spectrum Positioning

• Aldehydes occupy an intermediate oxidation state.
  • More oxidized than alcohols.
  • Less oxidized than carboxylic acids.
• Ketones represent the maximum oxidation level attainable for a secondary carbon.

Oxidation of Aldehydes → Carboxylic Acids

• Any oxidizing agent stronger than PCC will drive the conversion.
• Common reagents you are expected to recognize on the MCAT:
  • $\mathrm{KMnO_4}$ (potassium permanganate)
  • $\mathrm{CrO_3}$ (chromium trioxide)
  • $\mathrm{Ag_2O}$ (silver(I) oxide)
  • $\mathrm{H<em>2O</em>2}$ (hydrogen peroxide)
• Conceptual point: the reaction is a two-electron oxidation of the aldehydic carbon, generating a new C–O bond (O of the carboxyl OH).

Reduction of Aldehydes & Ketones → Alcohols

• Proceeds via nucleophilic hydride addition to the carbonyl carbon, followed by protonation.
• Two primary hydride donors:
  • $\mathrm{LiAlH_4}$ (lithium aluminum hydride)
  • Very strong; requires anhydrous conditions.
  • $\mathrm{NaBH_4}$ (sodium borohydride)
  • Weaker/milder; compatible with protic solvents.
• Product profile:
  • Aldehyde → primary alcohol.
  • Ketone → secondary alcohol.

Broader Context & MCAT Relevance

• Reactivity hinges on the electrophilic carbonyl carbon which readily participates in nucleophilic addition.
• Carbonyl chemistry is ubiquitous in biosynthetic pathways (e.g., glycolysis, fatty-acid metabolism), explaining its high test frequency.
• Mastery of oxidation/reduction patterns aids in predicting multi-step synthetic schemes and metabolic transformations.

Looking Ahead

• Next chapter extends into enolate chemistry: nucleophilic species derived from carbonyl compounds that participate in C–C bond-forming reactions.