NECTAR - WGs

Synthesis and purification of Ionic Liquids

Purification and Challenges

Purification
Starting compounds should be purified to prevent the formation of byproducts during the metathesis reaction.

It is recommended to carry out the quaternization step right after purifying the starting compounds, as prolonged exposure to light or moisture can generate new impurities. Alternatively, starting compounds like amines can be stored under dark and inert conditions until needed.

Dry solvents and inert conditions must also be used during the generation of the quaternized salt. Precautions may vary depending on the alkyl halide used. For example, the synthesis of [C₂mim]I requires the reaction to be carried out in the dark to avoid photo-oxidation of the iodide, compared to [C2mim]Br synthesis. In general, it is recommended to purify the quaternized salt before using it in the metathesis reaction.

When a solid is generated in the quaternization reaction, recrystallization from dry ethyl acetate can produce white crystals.

If a liquid is formed, then the IL can be washed with a nonpolar solvent to remove impurities.

To find a suitable purification solvent, you can check the solubility and miscibility of the starting components and the final product in different solvents. Choose a solvent in which the initial components dissolve easily, but the main product does not. This process will help you find the most appropriate solvent for purification.

After purification, the IL is then dried under vacuum with moderate heating for several days to remove traces of water and other organic solvents. While ILs themselves cannot generally be distilled because of their negligible vapor pressure, many liquid impurities such as solvents and starting compounds can be evaporated from the IL by stirring under vacuum under mildly elevated temperatures.

Note, however, that Henry’s Law applies, that is, the volatility of the component decreases as its concentration falls – in other words, it becomes harder to remove the volatile impurities as their concentration decreases. Where vacuum treatment is not entirely satisfactory, sparging with nitrogen (i.e. bubbling nitrogen through the IL) may be more successful.

Example:

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