Introduction and Oxidations on the Alcohol Level
It is not the aim of this section to catalogue all means of oxidation for every compound imaginable, as this would be a mammoth task for both reader and writer (!), and furthermore quite unnecessary.
The oxidations included here are the more commonly encountered reagents and functional groups, but should provide a general understanding of the processes involved in oxidation. To work from the ‘bottom’ up in terms of levels, it is worth mentioning that the hydrocarbon level (0) is actually quite hard to oxidise in a controlled manner – they can be burned in air to take them straight to CO2 (level 4), but to break the C-H bonds and form ‘C-X‘ bonds in a controlled fashion takes unusual conditions best achieved with radicals. Therefore we start at;
Oxidation on the Alcohol Level to Aldehyde/Ketone
|i. with Chromic acid|
|Points of note:
i. Further reaction of H2CrO4 and aldehyde/ketone can occur to give carboxylic acid.
ii. Hindered 20 alcohols react more quickly than less hindered 10/20 ones because the RDS involves greater steric relief.
|ii. Swern Reaction
(for primary alcohols) The active species in the reaction is prepared by reaction of DMSO with (COCl)2:
|iii. with Magnganese Dioxide
This will only oxidise 10/20 alcohols that are activated by adjacent unsaturated groups (to stabilise the radicalformed in the second step) e.g. aromatic or alkenes – this can be a useful selectivity.
Note the use of single-hooked arrows to represent the movements of single electrons, as in radical reactions.