Oxidation of Primary Alcohols to Aldehydes using PCC
Description: Treatment of alcohols with PCC leads to formation of the aldehyde.
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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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.