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Water Buoys Organic Synthesis

May 23, 2016
System improves separation, is more environmentally friendly and less costly.

Researchers at The University of Texas at Arlington (UTA), Arlington, Texas, say a water-based system for making organic compounds offers significant benefits over conventional synthesis. The system, which can produce compounds typically used in pharmaceuticals, agrochemicals, plastics, textiles and household chemicals, boasts considerably higher yields of product than pure organic solvents, improves separation, is more environmentally friendly and less costly, they note.

Organic Synthesis

Figure 1. Two-phase aqueous system easily separates final product. Source: UTA/Green Chemistry.

“Using water as a solvent is ideal as it is benign, plentiful, cheap and not harmful to the environment,” says Morteza Khaledi, dean of UTA’s College of Science and co-investigator of the project.

The new medium, 80–90% water with hexafluoroisopropanol (HFIP), forms two separate phases during the reaction, allowing final products to be separated and extracted. Typically, additional organic solvents are needed to facilitate this.

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Khaledi and co-investigator Nathaniel Weisner of North Carolina State University, Raleigh, report that the water-based system benefits from the properties of HFIP when carrying out reactions such as Friedel-Crafts and Diels Alder ones; they have performed Diels Alder reactions and derivatization of primary amines into amides.

“HFIP and other fluorinated alcohols have already been shown … to be beneficial solvents for different classes of reactions such as oxidations, several types of C-C bond formation reactions as well as cycloadditions. This is due to a combination of several specific properties, which are strong H-bond donation, low nucleophilicity, [and] high ionizing power. While we have not fully investigated these reactions in our system, we anticipate observing similar results,” says Khaledi.

Trifluoroethanol (TFE) is another fluorinated alcohol compound with similar phase separation, “although it does not form a two-phase system as easily as HFIP due to lower hydrophobicity or fewer fluorinated groups,” Khaledi notes.

“The Friedel-Crafts reactions were shown to work in similar two-phase systems formed using TFE instead of HFIP. The results were not as good for all reactants, particularly the less nucleophilic ones, which is why HFIP was chosen as the focus. This may not be the case for all types of reactions though and should be investigated,” he adds.

The two-phase system also reduces consumption of HFIP by 80%, leading to a significant decrease in cost as well as toxicity of the reaction media, they report in an article in Green Chemistry. “HFIP is also more expensive than many traditional solvents. Thus, the significant reduction of the HFIP in our system would facilitate the broader use of this solvent,” believes Khaledi.

In addition, due to its low boiling point, it’s feasible to recycle HFIP by evaporation after separation from the aqueous phase, say the researchers. The aqueous phase also could absorb the heat given off by any exothermic reactions carried out in these systems. “This is a role which water should perform better than many traditional solvents due to its high heat capacity,” claims Khaledi.

Currently, the team has no plans to try the method on a larger laboratory scale, but “it would be interesting to try,” Khaledi muses.

Finding the proper equipment to use the two-phase approach on a commercial scale poses a challenge.

“The equipment would need to be designed to facilitate the separation of the HFIP-organic phase from the aqueous phase after the reaction is complete in order to more easily isolate the product. Luckily, due to the high density of HFIP, the HFIP-organic phase settles to the bottom fairly quickly when there is no agitation applied,” Khaledi explains. “HFIP is a very strong solvent and could cause a problem by dissolving certain materials, particularly plastics, which would need to be taken into account when deciding on what kind of equipment to use.”

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