There are several ways by which to convert a secondary pseudo-alcohol (fig 1) to a carbonyl group. Some methods are more effective than others experimentally. The first method involves the use of potassium permanganate (KMnO4) in acidic media such as a simple carboxylic acid like acetic acid. This is done at an elevated temperature (28C) and done so in a controlled environment.
This resulting carbonyl group is much more manageable than the preceding alcohol group and thus can be either converted to another functional group or eliminated entirely. Here, we will focus on eliminating this group completely.
The second method of producing the carbonyl is by use of sodium hypochlorite (NaOCl), or bleach, in basic media. These are both oxidative reactions in which the alcohol is oxidized to the carbonyl. For a follow up on how they work specifically, see KMnO4 and NaOCl.
From this point on, the carbonyl can be removed by a few reactions. The first is the reduction of the carbonyl by use of Zinc metal shavings in conjunction with Mercury in concentrated hydrochloric acid. The reductive ability of this solution acts quickly but leaves a possibly toxic environment due to the presence of Mercury. Therefore, this method is not preferred. The alternative method is by using hydrazine, N2H4.
Hydrazine is best synthesized by the Atofina Pcuk Cycle, which uses acetone and concentrated ammonia in solution with the addition of a small heat source to remove water. The reaction occurs where the oxygen of the carbonyl is replaced with nitrogen to give a double-bonded carbon-nitrogen group. Upon oxidation with hydrogen peroxide, H2O2, a carbon-nitrogen-oxygen ring is formed, whereupon more ammonia reacts to lose water from the molecule. The resulting molecule has an amine group bonded to the aforementioned double-bonded carbon-nitrogen compound. Reacting this molecule with more acetone under heat results in the desired hydrazine compound in water.
Therefore, using hydrazine in base, such as potassium hydroxide, KOH or NaOEt, and glycol reduces the carbonyl directly to the alkyl chain. This is the most reliable reaction as it is well documented and gives relatively high yield.
A less effective reaction uses the reduction of the alcohol directly, however may have side effects with surrounding functional groups. Using the original secondary pseudo-alcohol, one places it in solution with concentrated sulfuric acid while heating. The heating is to remove excess water that is produced from this reaction instead of hydrating it. This is known as a dehydration reaction. This reaction forms the adjacent alkene on the alkyl chain in question. The resulting chain can then be hydrogenised to give the racemic mixture of alkane chains. This is not desired as the presence of nearby methyl groups will change isomerism in the overall molecule. Thus, the aforementioned reactions are far more preferential as they give the correct isomers.
