Magnesia Adsorbent for Removing Olefinic or Aromatic or Coloured Impurities from a Reaction Mixture

 Even a chemistry undergraduate is familiar with the use of carbon powders for decolourizing solutions of organic compounds during recrystallization from organic solvents. However, charcoaling for decolourizing has some serious shortcomings when contemplated for application at scale. [Neal G.Anderson, Practical Process Research & Development Academic Press 2000, pg. 17-18]. 

For a very long time, it has been known that magnesia strongly adsorbs unsaturated and aromatic compounds while having less affinity for oxygen-containing functional groups such as carbonyls and ethers [L.R. Snyder, Water Deactivated Magnesia as a Chromatographic Adsorbant,  J. Chromatog., 28 (1967) 300-316.] Indeed so strong is this binding if the magnesia is not deactivated with small amounts of water, that chemisorption of polyaromatics can occur and these materials cannot be liberated without destruction of the adsorbent. These chemisorbed substances can often be eluted with water-wet organic solvents.

 
 Coloured impurities very often are polyunsaturated or polyaromatic materials. In removing these coloured impurities from a less unsaturated material there is no concern whether the coloured impurities become strongly adsorbed or chemisorbed so long as they are retained sufficiently tightly to the solid so that passing a solution of the desired compound with its coloured impurities through a plug of magnesia traps the colour and the filtrate contains the desired material, now colourless.

One would think that magnesia adsorption could also be used to remove even colourless contaminants in a reaction mixture so long as they were significantly aromatic or olefinic and the desired substance was not.