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Simon Muller
Simon Muller

There is quite an analogy between cooking and chemistry. In both cases you have to mix the right amount of your ingredients together in a pan and boil it for a specific time to then obtain  the desired result.
If your  ingredients or time of cooking were chosen poorly, you get way too salty pasta that you can hardly bite into anymore (happens even to organic chemists pursuing a PhD…especially when they are trying to write articles for Yourformula at the same time!). In the early days of chemistry the analogy even went  so far that chemists actually tasted their products.

Doesn’t look like this: Simons pasta

However, there is a parameter in cooking that  you hardly ever think about but which becomes crucial in a chemistry laboratory: your medium of reaction. In cooking, there are two possible choices

1.) water

2.) oil.

You boil your pasta in water, but you fry your fries or chips (depending on where you live) in boiling oil. A chemist would say you usually choose water as your solvent for your reactions. In a real chemistry lab however, water is hardly ever the solvent of choice for a chemical reaction.

The reason for this is that water has very unique physical and chemical properties: very high heat capacity, strong hydrogen bonding, high surface tension, high dielectric constant and so on.  These properties make water a great solvent for salts but a terrible one  for complex organic molecules, which we need for most chemical reactions. Furthermore, water interacts strongly with the molecules through hydrogen bonding and other interactions, leading to side reactions and byproducts. So we chemists use other solvents such as diethylether, acetone, methanol, dichloromethane, hexane and ethyl acetate to dissolve our components and to perform the reactions.

As you might already guess, the above mentioned solvents have their own disadvantage: they are not really sustainable. Diethylether, ethyl acetate and acetone are flammable; methanol makes you go blind; dichloromethane is carcinogenic; n-hexane is poisonous for your nervous system and dangerous to the environment. A  nice group of compounds, isn’t it? When I look at this list, I am quite happy that chemists don’t taste their chemicals anymore.

            

Flammable (left), toxic (m), dangerous for the environment (r): the properties of common solvents

So how can we solve this problem?

One very fascinating approach is the use of subcritical – or superheated water – as solvent in organic synthesis. You surely remember from your chemistry class that water has a boiling point of 100°C. However, if you apply pressure to  your hot water, the boiling point will be shifted to higher temperatures, as the water needs more energy to leave its liquid state. This can be done until the critical point where liquid and gas become one and the same phase, so called supercritical fluids (pretty cool effect, check out the video!).

 

Superheating  water is a method we actually use in cooking too. You have most certainly already encountered pressure cookers.

 

Superheated water in the kitchen

The physical properties of superheated water actually differ quite dramatically from  those of normal water. Most importantly, the dielectric constant of water drops significantly at higher temperatures, the thermal motion of the molecules disrupting  the hydrogen bonding between water molecules. This makes water a better solvent for more apolar molecules. In fact, just beneath its critical conditions (373.95°C and 217.76 bar) water is as a solvent comparable with acetone, dissolving highly complex organic molecules easily.

Subcritical or superheated water is not only an impressively versatile solvent for organic reactions, but it can also be used for extractions of substances from plant material. Organic materials dissolve better in solvents with higher temperatures, but they don’t do it at the same rate. Hence, subcritical water can be used to extract essential oils from plants, as these oils are extracted at  a much higher rate than hydrocarbons and other molecules in the plants.

Even as a catalyst in organic reactions, subcritical water finds application. The dissociation constant of superheated water (maybe you remember from high school that water can dissociate 2 H2O à H3O+ + OH-) is 100 times higher than of normal water, making it a stronger acid and a stronger base and a valuable catalyst. Furthermore, superheated water can be used to oxidize hazardous materials in a very sustainable way.

In conclusion, subcritical water is a real all-rounder and its byproducts are only water.

What could be cleaner than this?

So heat it up! And now I have to check my pasta again…