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.
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.