Oxidation of Primary Alcohols to Aldehydes using PCC
Description: Treatment of alcohols with PCC leads to formation of the aldehyde.
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Notes: The same transformation can also be carried out by CrO3 and pyridine, a combination sometimes known as the Collins reagent. Oxidation of the aldehyde to the carboxylic acid will not happen so long as water is excluded.
Examples:
Notes:
Mechanism: Not generally considered “important” for the purposes of Org 1/ Org 2.
Oxygen from the alcohol attacks chromium, in a [1,2]-addition (Step 1, arrows A and B) and then a proton is transferred to the oxygen on chromium (Step 2, arrows C and D). After 1,2-elimination of chloride ion (Step 3, arrows E and F), a proton is removed from C–2 in an elimination reaction (Step 4, arrows G, H, I and J) to give the aldehyde.
Notes: Note here that all we’re basically doing is putting a good leaving group on oxygen (the chromium) and then doing an elimination to form the C–O π bond.
There are many other reasonable ways of depicting the proton transfer step. Furthermore there are several species present which could alternatively be used as the base in Step 4.
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{ 14 comments… read them below or add one }
how to decompose the tar obtained after using the PCC as an oxidising agent in the reaction
Best approach is to prevent tar formation by adding molecular sieves or Celite to the reaction; the tar will stick to that instead of the flask.
Thanks for the tip. Would you happen to know the best way to get the chromium out of the sample afterwards? I was going to filter it through Celite, but if that’s already in the reaction vessel, would it even be necessary?
Generally I’d suggest filtering through a pad of fresh Celite to get rid of the chromate ester byproducts and other crap; typically rinse with CH2Cl2. You’ll find that the brown color will be very difficult to remove, however, without purification through a plug of silica or by distillation.
Also, recently found this tip:
” Quench with 1M citric acid: it complexes all the brown Chromium junk. Much better than the usual “Celite filtration” work up.
God bless E J Corey :-)
Nice tip james, but normally DMC is used as the solvent for PCC oxidation, how did you use mix 1M citric acid water solution with it to complex the brown junk and how to get rid of residual citric acid? Looking forward to your answer. Thank you.
Hi there,
Why is it that oxidation of ALDEHYDE will not happen under PCC? What is exactly the reason for that? We have a hydrogen in aldehyde, how come that does not get oxidized?
and why is that you say “as long as” Water is excluded? does that imply we can deliberately introduce water do the system to further oxidize the aldehyde if we want to?
Good question!
Water is key.
If water is not added, the aldehyde will not oxidize.
However, if water is added, it will add to the aldehyde to form a hydrate (carbon with two OH groups attached). And this will undergo oxidation, (through a mechanism similar to that of a primary alcohol) to give a carboxylic acid.
if we add water then how we stop reaction at aldehyde stage while converting primary alcohol to aldehyde
Generally you don’t want to add water in this reaction, except in the workup stage when trying to isolate the product.
check the last step of the mechanism, you accidentally made the hexane ring into a benzene ring
and what might be a solvent then
Solvent for this reaction is usually CH2Cl2 or DMF.
DMC? Do you mean DMF?