Oxidation of Alcohols A Peer-Assisted Learning Demonstration

Discussion The most important reaction of alcohols is their oxidation to carbonyl compounds. Primary alcohols yield either aldehydes or carboxylic acids, depending on the conditions used. Secondary alcohols yield ketones - and tertiary alcohols do not react under the usual oxidizing conditions. 1-propanol is oxidized rapidly, 2-propanol more slowly. Under the given reaction conditions tert- butanol does not react. Primary and secondary alcohol reduce yellow Cr 2 O 7 2- ions (Cr 6+ ) to blue Cr 3+ ions.

Mechanism for Oxidation of Alcohols

1 o Alcohol Oxidations Primary alcohols  aldehydes PCC/CH 2 Cl 2 (pyridinium chlorochromate, C 5 H 5 NH + ClCrO 3 - )\ Primary alcohols  carboxylic acids Cr VI in one form or another (H 2 CrO 4 or K 2 Cr 2 O 7 ) Mn VII (KMnO 4 /NaOH/H 2 O/heat) Color of reagents can be useful. Cr VI is yellow; Cr III is blue

2 o Alcohol Oxidations Secondary alcohols  ketones Cr VI in one form or another CrO 3 /aqueous acetone = “Jones Reagent” – good oxidizer, doesn’t affect alkenes where KMnO 4 would.

3 o Alcohol Oxidations Tertiary alcohols cannot be oxidized under normal conditions. Heat them too much in the presence of strong oxidizers; start cleaving C-C bonds.

Why? When an alcohol is oxidized, a hydrogen is removed from the carbon. If that hydrogen is not present, no oxidation can occur.

Examples