Oxidation of Alcohols
Oxidation of Alcohols to Aldehydes or Ketones: Biological Importance
The oxidation of alcohol groups to carbonyl groups represents an important step in the degradation of fats during the human metabolism (e.g. L- malate to oxaloacetate). Such oxidations are also part of the citric acid cycle. The body is using coenzyme NAD+ (nicotinamide adenine nucleotide) as reducing agent. Oxidation of the alcohol takes place when NAD+ acts in conjunction with a dehydrogenase. The reactive part in NAD+ is the nicotinamide ring. This pyridine derivative is bound through a nitrogen atom to the rest of the coenzyme. The positively charged, tetra-coordinated nitrogen in NAD+ makes this compound a strong Lewis acid which is transformed to NADH by hydrid transfer from an alcohol.
This mechanism is also responsible for the degradation of alcohol in the body according to the following overall formula:
Using a different dehydrogenase, the generated acetaldehyde is further oxidized with NAD+ to the safe acetate. If this second enzyme is missing, acetaldehyde accumulates in the blood and causes unpleasant side effects such as headaches and reddening of the face (hangover).
The reverse reaction in which NADH acts as biological hydrid donor and reducing agent is also possible.
The hydrid transfer from the alcohol to NAD+ can be compared to the Cannizaro reaction.