Oxidation Reactions of Sugars

Oxidation of Alcohol Groups

Alcohols are organic molecules with the C-OH functional group and sugars always have many of these groups. Oxidizing agents, such as chromium trioxide, convert the C-OH group of alcohols into the C=O group of an aldehyde or a ketone. In an aldehyde, the carbonyl carbon is bonded to one other carbon atom and one hydrogen atom. In ketones, the carbonyl carbon is bonded to two other carbon atoms.

Let's look at the possible changes in oxidation state in the reaction of one of these alcohols and chromium trioxide.

Only the carbon of the alcohol functional group and the chromium center change oxidation states.

We can write the oxidation reaction of ethanol to ethanal in half reactions. The carbon of the alcohol is oxidized by 2 electrons, from -I to I, and the chromium atom is reduced by 3 electrons, from Cr(VI) to Cr(III). Because the number of electrons transferred must be equal, we multiply the oxidation half reaction by 3 and the reduction half reaction by 2 before we add them to get the net oxidation-reduction reaction.

Other oxidizing agents (easily reduced compounds) can oxidize alcohols. This includes other chromium(VI) compounds, like K2Cr2O7 and Kr2CrO4.

Oxidation of Aldehyde Groups

Aldehydes are much easier to oxidize than ketones. The oxidizing agents Ag+, Cu+2, and K2CrO4 can transform an aldehyde to a carboxylic acid.

The carbonyl carbon is oxidized by 2 electrons from the I to the III oxidation state. Silver metal is produced when Ag+ combines with 1 electron.

The half reactions for the oxidation of ethanal to ethanoic acid by silver(I) are below.

Naming organic compounds

To find the standard name of any simple alcohol, aldehyde, ketone, or carboxylic acid it is necessary to find the name of the longest carbon chain that contains that functional group.

The standard ending for any saturate hydrocarbon is "ane". Replace the final e with "ol" for an alcohol, "al" for an aldehyde, or "oic acid" for a carboxylic acid. Number the carbon atoms (with the functional group at the lower number end of the chain) to indicate the position of the functional group for alcohols or for ketones. The aldehyde and carboxylic acid functional groups are always on C1 and don't need to be numbered.

Learn more about organic nomenclature by going to the links below:
  1. alkanes

  2. alcohols

  3. carbonyl compounds

Professor Patricia Shapley, University of Illinois, 2012