Oxidation of primary alcohols to aldehydes and esters.

Abstract

The initial aim of this research project was to oxidise Cr C4 alcohols to their respective aldehydes. However, isolation of these aldehydes was impossible due to their low boiling points. Another problem was the proximity of the boiling points of these aldehydes with the boiling points of solvents used in reactions. As a result, the alcohols investigated in this project were changed to C4-C6 alcohols. Different oxidising reagents are known that oxidise oxidise primary alcohols to aldehydes. Of these, chromium (VI) has been used the most. In this project, Chromium (VI) supported on different inert: inorganic supports was chosen as the oxidising reagent. The first task was therefore to synthesise these supported oxidants. The synthesis of some of these oxidants was lengthy. This is advantage the preparation of supported oxidants. Unsupported oxidants like PCC and chromium trioxide were only used as a basis of comparison to emphasise the usefulness of supported oxidants. Pyridinium chlorochromate was used to oxidise primary alcohols to aldehydes in moderate yields. The reaction was found to be moisture sensitive. The yield and the duration of the reaction was found to be better with the use of molecular sieves. Silica gel-supported pyridinium chromate was also used to oxidise alcohols to aldehydes in good yields. Poly[vinyl(pyridinium chlorochromate)] was used to oxidise I-hexanol and I-pentanol to their respective aldehydes also in good yields. The duration of these reactions ranged from 30 min. to 3 days. During the oxidation of alcohols using silica gel-supported chromium trioxide and diethyI ether/chromium trioxide it was found that the reaction conditions can be changed so that either aldehydes or symmetrical esters were the products. This was achieved by varying the duration of the reaction. Silica gel-supported chromium trioxide resulted in both the aldehydes and esters in good yields while the diethyl iii Aluminium-supported chromium trioxide, kieselguhr-supported chromic acid, alumium silicate-supported chromic acid and alumina-supported pyridinium chlorochromate were used to oxidise primary alcohols to symmetrical and unsymmetrical esters in good to excellent yields. The duration of these reactions varied from 1 hour to 48 hours depending on the conditions employed and the chain length of the reagents.