10-29-08 - 318M Krische, Lecture 28: W - 10/29/08, Alcohols...

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Our main topic today is oxidation of alcohols. After discussing the various oxidations states possible for carbon, we notice that an oxidation (loss of electrons) typically involves an increase in the number of carbon-oxygen bonds, and/or a decrease in the number of oxygen-hydrogen bonds. In living organisms, the oxidation of an alcohol (for instance, ethanol) is catalyzed by enzymes. In the laboratory, chromium based oxidants are typically used. Potassium dichromate (K 2 Cr 2 O 7 ) in aqueous sulfuric acid is commonly used to oxidize alcohols. The actual reactive species is chromic acid (H 2 CrO 4 , know the structure), which forms from the reaction of K 2 Cr 2 O 7 with the acid. Secondary alcohols are oxidized to ketones. The reaction mechanism involves a chromate ester intermediate, which undergoes an E2-type elimination upon reaction with H 2 O to give the product. Chromium is reduced from oxidation state +6 to oxidation state +4 during the course of the reaction.
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