
Water is one of those chemicals, much like carbon dioxide, that is often referenced when chemistry and the environment find themselves in the same paragraph. It must be the near- deity like status that we tend to place on good old H2O.
It is the image of a pristine mountain stream, clear and cool, and its inevitable destruction that people think of when they think of industrial chemistry. How wonderful would it be if society (industry) could use that clear, clean, natural water as a solvent for industrial chemistry? Could we reduce our dependance on oil as a source of nasty chemicals like toluene, benzene, hexane and….gasp….MTBE? (ok, I don’t know a lot about MTBE, but I know that it is public enemy number 1 when it comes to evil solvents….)? If we, as chemists, design processes that use water, we could pipe it from the cool stream, to our reactor and run our process and then just put it back. It is only water, right? It is natural and clean and golly… green!
Unless you have been in a coma for the past couple of years, you have probably read at least one article that touts its chemistry as being “an environmentally benign method of….(insert chemical transformation) due to the use of water as the solvent”. I get miffed, nay offended, when researchers spout off in their Chem Comm, JOC or Org Lett papers about how great water is, how it is the greatest and greenest solvent in the world. Here is the truth: it is really difficult to take a process and make it “green” by running it in water. I am not even considering the fact that the chemistry may not even work. I am only condsidering the following two things:
- Water has a specific heat of 4.186 J /g K (toluene’s specific heat is 1.67 J /gK), which is higher than most common substances.
- The ecosystem is really sensitivve to things like acetone, boron, chromium… ie. waste that needs to be removed from the aqueous waste stream.
Regarding 1. It takes almost three time the energy to bring a kilo of water to reflux than it does a kilo of toluene. Where does that energy come from? Your solar farm? Not yet buddy. It comes from coal, oil or the plant’s own burning of organic waste. No matter how, there is probably a lot of CO2 getting generated. Not green to me sir.
Regarding 2. as was suggested here, if the bugs cannot chew it up, it needs to be hauled away and treated. Forget distillation, that is expensive (see #1).
So yes, using water can be great. But it can also be a false hope. In any event, stop using it to sell your research. If you really want to do research in to green chemistry, do it right.
And really, running something in water (even a BOC deprotection!), beit room temperature or super critical, does not an environmentally acceptable process make.
I hate it when people try to sell their research by stretching things too much… makes me nuts.
I once heard an executive type say: “Can we greenify that product by making it in water?” That was a fun day.
milo Science BullSh*t, Chemistry, Green, non-scientist goons, Water
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