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Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters. The reaction mainly applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids. [1] A variety of oxidants can be used.
Notably, in contrast to the general oxidation of tertiary alcohols, the secondary alcohol case only works with aromatic substrates (Ar-: an aryl group). This, along with the strongly acidic conditions due to the stoichiometric amount of periodic acid, suggest that the initially formed chromate ester isomerises through a carbocationic route ...
Tertiary alcohols (R 1 R 2 R 3 C−OH) are resistant to oxidation. The direct oxidation of primary alcohols to carboxylic acids normally proceeds via the corresponding aldehyde, which is transformed via an aldehyde hydrate (R−CH(OH) 2) by reaction with water before it can be further oxidized to the carboxylic acid.
Like conventional esters, the formation of this chromate ester is accelerated by the acid. These esters can be isolated when the alcohol is tertiary because these lack the α hydrogen that would be lost to form the carbonyl. For example, using tert-butyl alcohol, one can isolate tert-butyl chromate ((CH 3) 3 CO) 2 CrO 2), which is itself a good ...
Enones can be synthesized from tertiary allylic alcohols through the action of a variety of chromium(VI)-amine reagents, in a reaction known as the Babler oxidation. The reaction is driven by the formation of a more substituted double bond. (E)-Enones form in greater amounts than (Z) isomers because of chromium-mediated geometric isomerization.
Steric hindrance of the hydrogen alpha to the alcohol is a major determination of the rate of oxidation as it affects the rate of association. [5] Tertiary alcohols lacking an alpha hydrogen are selected against and generally do not oxidize in the presence of Fétizon's reagent. [7]
The Babler-Dauben oxidation of cyclic tertiary allylic alcohols to cyclic enones using PCC. This type of oxidative transposition reaction has been synthetically utilized, e.g. for the synthesis of morphine. [10] Using other common oxidants in the place of PCC usually leads to dehydration, because such alcohols cannot be oxidized directly.
The reaction mechanism has similarities with the Baeyer-Villiger oxidation where the intermediate hydroxyperacid is called a Criegee intermediate. The per-acid forms a per-ester with the alcohol group. One alkyl substituent migrates from carbon to the adjacent oxygen atom, replacing the carboxylic acid leaving behind a carbocation.