Oxidation of Methylidene Groups to Carbonyl Groups
Oxidation of Methylidene Groups to Carbonyl Groups: Experimental Procedure
The oxidation to carbonyl compounds most easily succeeds with palladium as catalyst. In the Wacker process, 1-alkenes are oxidized mainly to methyl ketones while aldehydes are formed only in yields of 1 - 3%. This result can be explained by the fact that the intermediate delocalized π-complex preferrably adds water according to the Markovnikov rule.
The portion of aldehyde generated in the Wacker oxidation can be raised to a maximum of 25% by increasing the concentration of acid and chloride ions as well as by using higher reaction temperatures. The disadvantage of these concentration increases is a sharp decrease in reaction rates. Therefore, the preparation of higher aldehydes using this methodology is of little preparative value. An exception is the oxidation of styrene to phenyl acetaldehyde and acetophenone formed in the ratio of 4:1.
2,2-Disustituted 1-alkenes form aldehydes. However, the reaction rates decrease with increasing volume of the substituents. The reactions require higher temperatures and generate more side products, for example in the oxidation of 2-methylprop-1-ene and acrolein (2-propenal).
An electron-withdrawing group at the olefinic carbon directs the carbonyl group to the unsubstituted carbon atom.
Increasingly, higher alkenes yield more side products caused by rearrangements and chlorinations. Shifting of the double bond or chlorination can be largely avoided by using aqueous polar organic solvents, such as dimethylformamide, or by substituting copper(II) chloride with ferric sulfate as the oxdizing agent. In industry, parts of copper(II) chloride are substituted with copper(II) acetate to counteract the negative effects on product ratios resulting from high concentration of chloride ions and acid generated during the process. The reaction with unsymmetrical alkenes yields a mixture of ketones. However, changes in reaction conditions can bring about the formation of a single product.
In the laboratory, it is more convenient in most cases to synthesize the carbonyl compounds in multiple steps. For example, alcohols are synthesized first and subsequently converted to carbonyl compounds by dehydrogenation. The alcohols can be prepared by adding either water or sulfuric acid or acetic acid to the reaction mixture. In the latter case, the generated esters are hydrolyzed to alcohols. .