Usually it is not possible to simply find a ‘drop-in replacement’ for a solvent due to the fact that the solvent can affect various parameters including:
- Rates of reaction;
- Chemo-, regio- and stereoselectivity;
- Outcome of reaction (i.e. the reaction may not work at all or may do something unexpected)
Biotransformations are particularly susceptible to solvent effects.
It is important to consider the process as a whole, and not focus solely on one aspect, as solvents are generally used throughout the process for example as reaction medium, and in work-up and purification. Awareness of where the solvent comes from and how it is made are also important for example in terms of energy consumption, hazards, raw materials and by-products.
Solvent replacement is not a straightforward process, and all replacements will have both advantages and disadvantages that should be taken into consideration when making a decision. A perhaps obvious solution is to avoid or minimise use of solvents in the first place. Other options include: using less toxic solvents; using solvents that are renewable (i.e. not derived from petrochemicals); and avoiding the use of VOCs (volatile organic compounds). However, replacing volatile solvents with ones with low vapour pressure/high boiling points can potentially lead to other complications. By their inherent nature volatile solvents are easy to distil and therefore are relatively easy to purify, whereas recycling of solvents with low vapour pressure/high boiling points can be difficult (and also energy intensive).