The oxidation of alcohols is a useful set of reactions that makes ketones, aldehydes, and carboxylic acids. These reactions have important uses in synthesis and other practical applications.
Topics Covered in Other Articles
- Redox Reactions
- Oxidation States
- Alcohol Functional Group
- Carboxylic Acids
- Cannizzaro reaction
Overview of Oxidation and Alcohols
Alcohols consist of an oxygen bonded to a hydrogen on one side and a carbon on the other. This functional group is very common, but sometimes chemists want to change it into other oxygen-containing functional groups. Alcohols oxidize into three main classes of compounds; aldehydes, ketones, and carboxylic acids. These products depend on the initial alcohol structure and the reagents used.
Oxidation of Primary Alcohols
Primary alcohols consist of an alcohol group attached to a carbon joined to only one carbon. This carbon attached to the alcohol generally has an oxidation state of -1. However, primary alcohols oxidize into aldehydes (oxidation state of +1) or later carboxylic acids (oxidation state of +3).
Common inorganic oxidizing agents can oxidize alcohols. Often, Potassium Permanganate (KMnO4) or Potassium Dichromate (K2Cr2O7) oxidize alcohols in the presence of acid. If other alcohols are present, however, the must react with a protecting group such as a methoxymethyl ether (OMOM) or a tertiary butyl ether group (OtBu) to ensure they don’t form an aldehyde or acid. Milder reagents selectively oxidize a primary alcohol into an aldehyde without making the acid. These are usually milder organic reagents such as TPAP to TEMPO.
When oxidizing a primary alcohol, however, there is no good way to ensure that the aldehyde or the carboxylic acid form the major product using standard reagents. To make an aldehyde, one must distill the reaction mixture at the boiling point of the aldehyde to separate it out and prevent it from reacting further. Refluxing the reaction mixture reintroduces the aldehyde to the oxidizer, so it reacts again to make almost exclusively the acid.
Oxidation of Secondary Alcohols
Secondary alcohols usually have an oxidation state of +0, and oxidize in to a ketone which has an oxidation state of +2. This is achievable using the same reagents as above. However, a secondary alcohol are not oxidizable any further, as the carbonyl carbon already has a full valence shell. Thus, no refluxing is necessary, and the ketone is either distilled out as it is made or left in situ.
Tertiary alcohols cannot oxidize because the carbon already has a full valence shell. Another electronegative substituent must bond to the carbon to oxidize the tertiary alcohol. However, the carbon has no room in its valence shell.
Uses for the Oxidation of Alcohols
The oxidation of alcohols is a very predictable and simple reaction, and hence it has many uses. The oxidation of alcohols is a very important reaction in natural product synthesis, and many natural organic molecules have aldehydes and acids easily accessible from alcohols. Fragrances often contain esters, a class of chemicals prepared by oxidizing an alcohol into an acid, and esterifying that with another alcohol. Breathalyzers analyze the content of ethanol (a primary alcohol) in the breath of a person suspected of drinking alcohol. The ethanol in a persons breath oxidizes from potassium dichromate (orange) in the apparatus into ethanoic acid an the chromate ion (blue green). The color change is a measure of the the alcohol in the person’s breath.
Practice Problem 3: Suggest a method to turn ethanol into ethyl ethanoate with reference to specific reagents using our article on Fisher Esterification.