All solutions have a solvent

For some time now papers have been being submitted to Green Chemistry where reactions have been variously described as ‘solventless’ or ‘solvent-free’ in spite of the fact that the reaction mixture is clearly a homogeneous solution. This appears to be particularly the case where one of the neoteric solvents, such as an ionic liquid or supercritical fluid, is being used and a claim to greenness is being made.

Although there are many environmentally damaging volatile organic liquids that can, and do, get used as solvents, this is not the definition of what a solvent is. Therefore, eliminating their use by using an environmentally benign alternative does not lead to solvent-free conditions.

Leaving to one side the issue of solid solutions; a solution is a liquid mixture in which the solute(s) is(are) uniformly distributed within the solvent. Whenever there is a solution present there must be a solvent present. The solvent is the major component of the solution and it is usually a liquid under the conditions described when pure. Of course, in more complex systems, it is possible to have a mixture of liquids acting as the solvent.

One source of confusion seems to be when the solvent has more than one function. For instance, it is often the case that one of the starting materials for a reaction is a liquid that can dissolve all of the other components. In these cases it is often unnecessary to add another liquid to act as the solvent. However, this is not a solvent-free process; it is a process where one of the components is acting both as a reagent and a solvent.

In my own subject area of ionic liquids many examples are coming to the fore where the ionic liquid is acting as a catalyst for a reaction. Again, some care is needed here. If the ionic liquid is a minor component of the reaction solution, it is a catalyst that happens to be a liquid and the major component (whatever that is) is the solvent; if it is the major component it is acting as both the catalyst and the solvent. However, in this circumstance, further care is needed. Whenever changing solvent leads to a faster reaction the new solvent can be viewed as being a catalyst. After all, the reaction has been accelerated and the solvent has not been permanently changed by the process. However, this is usually regarded as a solvent effect rather than catalysis. In this case a good rule of thumb is that if a specific chemical interaction can be identified between the solvent and the solute that is giving rise to the acceleration of the reaction then it can be described as catalysis, whereas if it is a generalised polarity effect then the term should not be used.

So when can a reaction be described as solvent-free? Obviously, a dry solid phase reaction is solvent-free, also a reaction where there is a liquid present, but it is not acting as a solvent (i.e. nothing is dissolved in it) is also solvent-free, but that is the extent of the list. If there is a solution present there is a solvent present.

Tom Welton, Imperial College of Science, Technology and Medicine, London

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