In a paper published online in 2006, Glycerol as a Green Solvent for high product yields and selectivities, A.Wolfson, C. Dlugy, and Y. Shotland provide ineluctable evidence that increased utilization of glycerol should be forthcoming in organic chemistry processing. Their reasoning included environmental, economic, safety, handling, and product isolation considerations. The only difficulty using glycerol appears to be a viscosity much higher than with standard organic solvents.
Yet irrefragable as their teaching was there are still things unsaid that further boost this processing chemical! Glycerol can be used to conveniently perform all kinds of solvent switches which are otherwise a weakness with the standard reactors in multipurpose plants.
The best solvent for conducting a chemical reaction may not be the best solvent to purify the product thereof. Furthermore, when a first and a second reaction in a process scheme are telescoped (avoiding the isolation and purification of the product of that first reaction) the best reaction solvents for the first and second reactions are likely to be different. But, as I have explained in another blog article, switching solvents in the plant cannot be done in the simple fashion of evaporating a first solvent to dryness on a rotary evaporator and pouring in the second solvent. Because of the non-zero minimum stirrable volume problem, the complete removal of a first solvent becomes complex and time-consuming.
This problem can be solved if a volume of glycerol sufficient to completely occupy the minimum stirrable volume is placed into the reactor. Then, all of a first solvent (for example a first reaction solvent) can be removed without distilling any of the glycerol, which is so high-boiling, yet the reactor remains stirrable throughout because glycerol occupies the minimum stirrable volume and provides continuous mass and heat transport. All of the product, coproducts, byproducts, and other processing chemicals (everything that can’t distill with the first solvent) remain with the glycerol. Now, all that is necessary to complete a solvent change is to add a second solvent that is immiscible with glycerol but will solubilize and extract at least the desired product and potentially the entire non-solvent reaction contents.
Here, another benefit of glycerol becomes apparent. According to the (1974-1975) CRC Press, Handbook of Chemistry and Physics, (the old one I have) glycerol is immiscible with at least:
acetone, benzene, butyl acetate, carbon tetrachloride, chloroform, dibutyl ether, diethyl ether, ethyl acetate, isoamyl alcohol, methyl isobutyl ketone (MIBK), nitromethane, petroleum ether, tributyl phosphine.
Furthermore, by analogy, glycerol can be expected to be immiscible with the additional common solvents:
hexane, cyclohexane, methylcyclohexane, heptane, toluene, methylene chloride, trichloroethylene, tetrachloroethylene, isopropyl acetate, nitroethane, 2-nitropropane, and t-butyl methyl ether.
So a variety of extractions are possible.
In a further extension, glycerol can be used as a cosolvent with fluids with which it is miscible, such as the lower alcohols methanol, ethanol, isopropanol, etc. It can also form gas-expanded liquid phases with such amines as ammonia, methylamine, and ethylamine. A mixed solvent will generally have a more moderate viscosity than glycerol alone and so will at the same time make processing easier. All the above materials, immiscible or miscible with glycerol, can be completely expelled from the reactor so long as the amount of glycerol equals, at least, the minimum stirrable volume.
When a reaction is conducted wherein the desired product is itself usefully volatile, glycerol can act as an effective chaser in a distillation that purifies the desired product.
Glycerol is cheap, biodegradable, and has a bp of 182 ºC @ 10 mm Hg. It would be expected to remain behind in a standard distillation when combined with any of the common organic reaction solvents. Even DMSO (bp 189℃), DMF (bp153℃), and NMP (bp 81-82℃ 10 mm Hg) would be expected to be chased by glycerol.
The distilled first solvent contaminated with traces of glycerin upon simple treatment might be ready for reuse. Thus the first solvent is no longer waste and there are no mixed fractions of solvents to dispose of. The waste glycerin is a biodegradable material and the quantity used is no more than the minimum stirrable volume of the reactor.
Whether used just to drive the removal of a first solvent during reaction work-up or as cosolvent for the first reaction and then a chaser for the first solvent, glycerol it seems has the properties that enable simpler processing.
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