Carbonyl Compounds

Carbohydrates
 Elmhurst College
Synthesis - Aldehydes/Acids Reduction  Synthesis - Acetals  Chemistry Department
Synthesis - Ketones Synthesis - Hemiacetals  Virtual ChemBook

GENERAL EQUATION: REDUCTION OF ALDEHYDES AND KETONES

aldehyde + H2 ---> primary alcohol
ketone + H22 ---> secondary alcohol

Reduction of Aldehydes and Ketones

Reduction:

The reduction process is merely the reverse of oxidation. If two hydrogens are removed during oxidation, then two hydrogens should be added during reduction. Carbonyl compounds act as the oxidizing agent and hydrogen gas in the presence of a metal catalyst is the reducing agent. Other reducing agents which give up hydrogen may also be used in this reaction

The definitions which are used in inorganic chemistry involving the gain or loss of electrons; or the increase or decrease in oxidation numbers are not very useful in organic chemistry. In organic chemistry, although changes in electrons are important, oxidation and reduction is more readily observed from changes in the number of hydrogen or oxygen atoms. The definitions are as follows:

OXIDATION: loss of hydrogen, gain in oxygen, or loss of electrons

REDUCTION: gain of hydrogen, loss of oxygen, or gain of electrons.

Reduction of an Aldehyde:

 

Example:

1) One hydrogen adds to the carbon with the double bond oxygen.
2) One hydrogen adds to the double bond oxygen.
3) The two electrons which were in the double bond are used in the bonding of both hydrogens.
4) The double bond is converted to a single bond.
5) The final product is a primary alcohol.

Anaerobic Synthesis of Lactic Acid:

The end product of glycolysis is pyruvic acid
If conditions are anaerobic, pyruvic acid is converted into lactic acid. The purpose of this reaction is to convert NADH from Step 5 back into NAD+. This occurs under strenuous exercise conditions in the muscles. Looking closely you can see that two hydrogen atoms or ions from NADH are involved as a reduction of of the ketone group on pyruvic acid. The product of the reaction is lactic acid with a secondary alcohol group on the second carbon of lactic acid. The reaction is shown on the left:

Fermentation to make Ethanol:

Yeast and several other microorganisms undergo glycolysis or fermentation to produce ethanol from sugars. This is the process to make wine, beer, and other alcoholic products from grains and fruits.
The reactions of glycolysis are identical to those described up to the production of pyruvic acid. The difference is in the fate of pyruvic acid. In fermentation, pyruvic acid is converted first to ethanal (acetaldehyde) and then to ethanol. In the second step, the NADH from Step 5 is converted back to NAD+ in this process. The NADH supplies the hydrogen for the reduction of the ethanal (an aldehyde) into ethanol (an alcohol).

The reaction sequence is shown on the left.