Chemoselectivity - Oxidation¶
| Reagent | → aldehyde | 2\(^\circ\) alcohol → ketone | Functions |
|---|---|---|---|
| Jones Conditions (\(\ce{Na2Cr2O7, H2SO4, acetone}\)) | if anhydrous | yes | common for non-acid sensitive |
| PCC (Pyridinium chlorochromate) | if anhydrous | yes | acid sensitive alcohols |
| PDC (Pyridinium dichromate) | if anhydrous | yes | |
| TPAP (terapropylammonium perruthenate) NMO (optional) will regenerate the TPAP catalytically |
if anhydrous | yes | can be regenerated catalytically |
| Dess-Martin periodinate | if anhydrous | yes | IBX precursor is insoluble and slow (good for control) |
| Swern | yes | yes | |
| \(\ce{KMnO4}\) | no | yes | used if you want to over-oxidise to an acid |
Solvent: Use DCM unless water is required¶
Aldehydes → Carboxylic Acid (overoxidation/complet oxidation)¶
While most reagents in an aqueous solution will manage to completely oxidise an aldehyde due to their equilibrium with their hydrated form, possibly the most simple method is to use an aqueous solution of \(\ce{Cr^{VI}}\) or an acidic/basic solution of \(\ce{KMnO4}\).
Secondary Alcohols → ketones¶
For secondary alcohols, pretty much all oxidising agents will work to convert them to a ketone.
Jones Oxidation¶
Jones oxidation is often used for alcohols that aren’t acid sensitive, as it’s difficult to over oxidise
A good alternative if the alcohol is acid sensitive is to use PCC
Mechanism
Primary Alcohols → Aldehyde¶
Primary alcohols can be oxidised to either an aldehyde (partial oxidation) or a carboxylic acid (complete oxidation).
In aqueous situations, the carboxylic acid will be in equilibrium with its hydrated form, which is readily oxidised to a carboxylic acid. This means that primary alcohols cannot be oxidised to aldehydes in an aqueous environment
Mechanism
Other Oxidising Agents¶
Some other oxidising agents to use could be Pyridinium chlorochromate PCC, pyridinium dichromate (PDC) or tetrapropylammonium perruthenate (TPAP)
Tetrapropylammonium Perruthenate (TPAP)¶
TPAP is a mild oxidising agent that can be regenerated in situ to use it catalytically
Dess-Martin Periodinane¶
Is another mild oxidising agent that has the ability to oxidise very sensitive alcohols to carbonyl compounds. Dess-Martin periodinane is made from IBX, which is a useful oxdising agent in itself, as it’s insoluble in many organic solvents, meaning that it can be reacted slowly and filtered off, to control the level of oxidation.
Dess-Martin periodinane is acetylated which makes it much more soluble in organic solvents and is in turn more reactive.
Swern Oxidation¶
Is another mild oxidising agent that will convert a primary alchol to an aldehyde. One of the products is \(\ce{Me2S}\) which smells really bad, so the reaction needs to be carried out in a fume cupboard and all the glassware involved needs to be bleached ASAP.
