redox reactions in organic chemistry
Summary
In organic chemistry, oxidation of a carbon means gaining bonds to oxygen (or another more electronegative atom) or losing bonds to H; reduction is the reverse. Most testable: alcohol oxidation, where PCC stops at the aldehyde and stronger oxidants give the carboxylic acid.
Detail
Meaning: Oxidation of a carbon increases its bonds to O (or other more electronegative atoms) or removes its bonds to H. Reduction does the opposite: more C–H bonds, fewer C–O bonds. The two always occur together, so an oxidizing agent is itself reduced. Oxidation level rises in this order: alkane → alcohol → aldehyde or ketone → carboxylic acid → CO₂.
On the MCAT: You are given a substrate and a reagent and asked for the product. PCC oxidizes a primary alcohol to an aldehyde and a secondary alcohol to a ketone. Stronger oxidants (CrO₃ with acid, Na₂Cr₂O₇, KMnO₄) take a primary alcohol on to the carboxylic acid. A tertiary alcohol has no H on its carbinol carbon, so it does not oxidize under these conditions. Reducing agents: LiAlH₄ reduces aldehydes, ketones, carboxylic acids and esters to alcohols; milder NaBH₄ reduces aldehydes and ketones. H₂ with Pd, Pt or Ni reduces C=C bonds. The biochemical parallel is NAD⁺ being reduced to NADH as an alcohol is oxidized.
Common confusion: Students look for electron movement that is not drawn and lose track of direction. Count bonds instead: gaining H on carbon is reduction, and gaining O is oxidation. Also, do not assume every hydride reagent is equally strong: NaBH₄ leaves carboxylic acids and esters unchanged, while LiAlH₄ reduces them.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (Clayden)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.