12.7 Oxidation with Chromic Acid and PCC
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Oxidation of Alcohols
The general trend you'll notice is that when an alcohol is oxidized it will end up with more bonds to oxygen and fewer bonds to hydrogen. The carbon bonded to the hydroxyl group of an alcohol is referred to as the alpha carbon, and this is specifically where the oxidation takes place.
Oxidation of Primary Alcohols
The mechanism of oxidation involves an alpha hydrogen (hydrogen attached to the alpha carbon). As a primary alcohol has two alpha hydrogen atoms two steps of oxidation are possible. The first step of oxidation produces an aldehyde whereas the second step of oxidation produces a carboxylic acid. Note that each step of oxidation leads to the loss of an alpha hydrogen and an increase in the number of bonds to oxygen.
Oxidation of Secondary Alcohols
As a secondary alcohol has has only one alpha hydrogen only one step of oxidation is possible. Oxidation of a secondary alcohol produces a ketone. Note once again that oxidation is accompanied by the loss of an alpha hydrogen and an increase in the number of bonds to oxygen.
Tertiary Alcohols are not Oxidized
As a tertiary alcohol does not have any alpha hydrogens normal oxidation is not possible.
Chromic Acid and PCC
The two commonly used oxidizing agents used for alcohols are Chromic Acid and PCC. Chromic Acid (H2CrO4) is most commonly made from a combination of sodium or potassium dichromate and sulfuric acid (Na2Cr2O7 / H2SO4) or a combination of chromium trioxide and sulfuric acid (CrO3 / H2SO4). Chromic Acid is commonly represented by any of these three in an undergraduate organic chemistry course.
PCC stands for pyridinium chlorochromate but its acronym is much more commonly used. PCC is prepared from a combination of chromium trioxide, pyridine, and concentrated hydrochloric acid.
|Na2Cr2O7 / H2SO4|
|CrO3 / H2SO4|
Alcohol Oxidation Reactions with Chromic Acid
Chromic Acid is the stronger of the two oxidizing agents. It will carry out both steps of oxidation of a primary alcohol producing a carboxylic acid as well as oxidizing secondary alcohols to ketones. It will also oxidize an aldehyde to a carboxylic acid.
Alcohol Oxidation Reactions with PCC
PCC is a milder oxidizing agent than Chromic Acid. It will only oxidize a primary alcohol a single step to produce an aldehyde. This is the major difference in the alcohol oxidation reactions of PCC and Chromic Acid. And just like Chromic Acid, PCC will oxidize a secondary alcohol to a ketone.