Potential Advantages of ‘On Glycerol’ Reactions Compared to ‘On Water’ Reactions

  "On Water' reactions refer to reactions between hydrophobic reaction partners that are conducted on the surface of a large bulk of water that itself is immiscible with both the reactants and the desired product.

‘On water’ reactions have a number of advantages:

• The water phase will buffer the shock of any sudden exothermicity
• The bulk water can occupy the minimum stirrable volume allowing for a smaller quantity of neat reactants in the hydrophobic phase
• The bulk water is easily separated from any immiscible organic solvent used in the workup
• The bulk water can remove water-soluble co-products, including Bronsted acids or bases, from the water-insoluble product
• The bulk water forces hydrophobic reaction partners to come together more intimately

Because glycerol/glycerin, like water, will no dissolve a wide range of organic solutes,  ‘on glycerin’ reactions can be contemplated where inexpensive glycerin fills up a substantial part of a reactor’s minimum stirrable volume. These reactions might be expected to display some of the same advantages as ‘on water’ reactions; perhaps to an even greater extent.

Reactions ‘on water’ seem inappropriate when water partially or completely interferes with the desired transformation. If water interferes by protonation, neither water nor glycerol can realistically be considered since both are protic; but if water only interferes by participating in byproduct formation, such as acting as a competing nucleophile, glycerol may not present the same problem. For example, even if glycerol creates a small amount of related byproduct by forming an adduct, that material will most likely be easily removed from the reaction mixture. Since it still retains a vic-diol functionality it will most likely be easily separated by passing the entire crude product mixture through a plug of solid adsorbent. A vic-diol substructure endows most parent structures with distinguishably different physical properties. Thus ‘on glycerol’ reactions can have an advantage over the water analog.