The oxidation and reduction of organic molecules forms a very important part of many organic reactions and syntheses. There is a massive variety of reagents available and a vast array of systems that can be oxidised and/or reduced. However, the simple ‘oxidation is losing’ ‘reduction is gaining’ (‘oilrig‘) way of identifying electron transfer redox processes in inorganic chemistry is difficult to use in terms of organic chemistry. So other methods are needed to identify the processes occurring.
Broadly, oxidation can be thought of as addition of oxygen or removal of hydrogen, and reduction vice versa.
However, a system of oxidation levels has been devised which is useful to consider. All organic molecules will be classified into a level between 0 and 4, where the level number is the number of bonds a given carbon makes to elements more electronegative than it. The levels as classifications are then useful because it becomes clear that transformations which involve changing levels are redox reactions, and those which stay on the same level are not. These levels are summarized here:
Level 0 – ‘hydrocarbon level’
Methane and all saturated hydrocarbons.
Level 1 – ‘alcohol level’
It is important to note that alkenes and benzenes have been included at this level – the reasoning is that, for example, an alkene can be transformed into an alcohol by use of acidified water (overall addition of H2O) – a non redox process.
Level 2 – ‘ketone level’
Note the inclusion of alkynes here – alkynes being obtained by the removal of H2 from alkenes – an oxidation.
Level 3 – ‘carboxylic acid level’
Level 4 – ‘CO2 level’
Once the levels have been classified it can be seen that moving up levels is oxidation and moving down is reduction.
One area of potential danger with this system is that multiple functional groups may either act separately (and have their own individual levels), or act together (with a combined level).
To read the chapter on OXIDATION, click here.
To read the chapter on REDUCTION, click here.