Updated: Aug 6
The following is intended to be the first part in a loose series of brief articles on various aspects of sustainability and green technology. So, if you've read and heard quite enough about la maladie venue de chine (aka Covid19) why not take a gander into the world of chemistry in general and solvents in particular.
No, this is not an exhaustive, balanced treatment of all the alternatives to traditional solvents in existence, not even a comprehensive list of such alternatives.
This is merely a teaser to pique your interest in case you haven't given the subject matter any thought as yet.
For the sake of convenience (ours, not yours) the following text will treat both conventional and alternative solvents and their respective properties in a very broad way.
If you would really like to know more about the specific properties of solvent XYZ you should consult the relevant MSDS (Material Safety Data Sheet), a list of which will be given at the end.
1. Let's talk about solvents!
As many people will no doubt be aware, solvents have a bad name.
Surely, those little orange warning signs on their bottles aren't there for no reason?
Why use solvents in the first place?
Couldn't it be done without?
Solvents are of immense importance in chemistry. They serve a multitude of tasks in glues, paints, coatings etc. but here we'd like to concentrate on their role in synthetic reactions. In reactions, solvents serve a number of purposes, three of which will be briefly introduced here.
(a) Solvents facilitate transport.
It is much easier to pump a liquid through a tube from A to B than to shovel a powder from A to B.
(b) Solvents make every reagent molecule individually accessible to other reagent molecules.
Consider a reaction taking place in the solid state.
Here, only the molecules on the surface of the particles which are to react with each other, would get the chance to do so.
If the material is dissolved completely, every molecule is available for the reaction.
(c) Solvents serve as a 'heat sink'.
Quite often, the reaction between two molecules liberates energy, usually in the form of heat.
If the reaction partners are present in the solid state, this heat has got nowhere to go and results in local 'hot spots'.
This is problematic as the temperature in such hot spots can be significantly higher than in the rest of the mixture and even exceed the thermal stability of the reactants and/or products, leading to side reactions and degradation, hence lowering the yield.
A solvent helps to dissipate the reaction heat generated and thus helps to minimize side reactions and to improve the purity of the product.
The purer the product, the less effort has to be made in order to purify it.
It can be seen that chemistry without solvents is very difficult to imagine.
Solvent-less reactions exist (most prominently in cases where the reactants are already liquid in the first place) but they are the exception.
2. Traditional solvents – The cons and the pros
From an environmental point of view traditional solvents such as toluene, chloroform, acetonitrile and the like are dubious.
They are generally petroleum-based.
Even if they are (sometimes) side-products of the petroleum industry they still contribute to a line of business which is coming under immense societal and consequently political pressure, which in turn might negatively affect their availability.
The are highly volatile, making them difficult to contain in order to prevent pollution issues.
They are flammable thus special precautions in terms of equipment are necessary to use or just to transport them in order to minimise risks to staff and the general public.
Many organic solvents are carcinogenic and/or toxic and thus present a very real danger to plant, animal, and human life.
On the upside, they are well understood, cheap, readily available, and of low viscosity.
3. The alternatives
There are quite a few greener alternatives aimed at reducing the impact of solvents on our environment and they differ vastly in characteristics and properties.
(i) Fluorous solvents
These are essentially organic solvents such as alkanes in which many if not all of the hydrogen atoms have been replaced with fluorine atoms.
The main use of such solvents is in fluoro-chemistry, where organic reagents are 'tagged' with perfluoro-alkyl chains.
Post reaction, these tagged reagents can then be extracted from the reaction mixture by using a mixture of fluorous and conventional solvents.
So, what's the use of tagging a reagent in order to having to extract it with a very special solvent?
The advantage lies the extremely high affinity of the tagged reagent for the fluorous phase, reducing losses of the valuable reagents into the waste stream.
Economically, this methodology is obviously only viable in cases of very expensive products such as pharmaceuticals – and not for anything else.
(ii) Ionic Liquids
Ionic liquids (ILs) are, quite as their name suggests, liquids composed entirely of ions i.e. particles carrying positive and negative charges.
Considering the number of possible cations and anions to be used in an IL it is immediately obvious that there exists an enormous array of possible combinations making it possible to design an ionic liquid to
each and every application.
If, as stated above, on of the main problems associated with conventional solvents is their high volatility then Ionic Liquids (ILs) are the solution to that problem – they possess no vapour pressure.
This is due their very nature, namely the positive and negative charges carried by the respective ions and it is the coulombic interaction between these charges that reduces their vapour pressure to practically zero. This is an advantage.
However, the same characteristic also explains one of their biggest disadvantages: The coulombic attraction between the ions also imparts high viscosity on these liquids making it energy consuming to pump them from A to B and rendering them completely useless in fields such as lab-on-a-chip.
(iii) Supercritical fluids
If a gas is compressed its density increases.
If a gas is heated its density decreases.
In some cases it is possible to compress and heat a gas to such an extent that it collapses into what appears to be a liquid. Which it isn't.
It is a completely new phase not found anywhere else in nature and it is called a supercritical fluid.
Multiple industrial application for such phases have emerged, the decaffeination of coffee is perhaps the most well known.
The gas most often used as a supercritical solvent is CO2, in its supercritical state abbreviated as scCO2.
CO2 has the advantage of being cheap, readily available, and non-flammable.
It does, however, carry the popular stigma of being a greenhouse gas.
Another, more substantial, draw-back is the fact that due to its non-polar nature scCO2 will not dissolve polar solutes.
This obstacle can to some extent be overcome by the addition of additives.
To sum it up, scCO2 is a very clean solvent and applicable to many situations, however, it does have the disadvantage of requiring special high-pressure equipment.
(iv) Molecular alternatives
Nowadays, even major chemicals providers like Sigma-Aldrich advertise a selection of alternative solvents on their webpage.
Broadly speaking it is split into two categories:
Traditional solvents such as ethanol produced from waste feedstock, thus not relying on non-renewable resources, and
These are organic molecules not traditionally seen as solvents, such as cyclopentyl methyl ether, 2-methyl THF, dihydrolevoglocosenone (Cyrene™), or dimethyl sulfone.
Obviously, no general recommendation in favour of such alternatives can be given due to the specific criteria of the reaction to be carried out in the solvent.
However, advantages such as an increased liquidous range compared to more familiar solvents have been reported.
But whether the particular solvent characteristics suit the application in question or not, the defining qualities that make any alternative greener is its origin (petrochemical, from waste, or from bio-feedstocks) and its biodegradability.
As said above, if you haven't looked into alternative solvents for your processes this little article was meant to increase your awareness.
Links to MSDSs:
Cyclopentyl methyl ether: