Glycol cleavage

Glycol cleavage is a specific type of organic chemistry oxidation. The C-C bond between two OH groups is cleaved and replaced with C=O bonds. The ketone is formed on the more highly substituted carbon next to the hydroxyl group being attacked. The aldehyde forms on the less substituted side.

Glycol cleavage is an important reaction in the laboratory because it is useful for determining the structures of sugars. After cleavage takes place the keto-aldehyde fragments can be inspected and the location of the former hydroxyl groups ascertained.

Reagents
Periodic acid (HIO4) is one of the most convenient ways to cleave a glycol. It is thought to involve a cyclic periodate intermediate which then stabilizes into a keto-aldehyde. Periodic cleavage of a glycol yields the same result as ozonolysis reduction of an alkene (followed by dimethyl sulfate).

Warm and concentrated potassium permanganate (KMnO4) will react with an alkene to form a glycol and will then immediately cleave the glycol to give a stable ketone and an oxidizable aldehyde. The aldehyde reacts to become a carboxylic acid. Controlling the temperature and concentration of the reagent can keep the reaction from continuing past the formation of the glycol.

Nomenclature
The term glycol means a 1,2-diol, also called a vicinal diol. This indicates a molecular structure consisting of two hydroxyl (OH) groups on adjacent (hence 1,2) carbon atoms. Common glycols are ethylene glycol (antifreeze) and propylene glycol (used in medicine and foods).

Synthesis of Glycols
Glycols can be synthesized by the hydroxylation of alkenes. Hydroxyl groups are added across the double bond yielding cis stereochemistry. This oxidation reaction can be accomplished with osmium tetroxide (OsO4) followed by hydrogen peroxide (H2O2). Cold, dilute potassium permanganate (KMnO4) followed by a strong base (HO-) will yield the same result with reduced yields.