Oxidation of primary alcohols to aldehydes

by James

Description: Treatment of primary 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 known as the Collins reagent.

Oxidation of the aldehyde to the carboxylic acid will not happen as long as water is excluded.

Examples: 

Notes: 

Mechanism:  Not generally considered important for the purposes of Org 1/ Org 2, but provided here for completeness.

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 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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{ 2 comments… read them below or add one }

Alex Goodnough

The title of this page is “Oxidation of primary alcohols to aldehydes,” But every example shows primary alcohols being oxidised to carboxylic acids. What’s the deal?

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james

Fixed. Thanks for catching that.

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