What You May Not Know About Methylene Chloride



As an organic chemist, Emily James has had a lot of exposure to organic solvents, especially the popular dichloromethane, also known as methylene chloride. Dichloromethane is a volatile, colorless liquid, with a mildly sweet, not unpleasant odor. It’s immiscible with water but can dissolve a wide range of organic compounds. These properties make it the perfect solvent for use in the lab, and indeed that how I used it - to separate and extract organic products.


But it's not just useful in the lab; coffee, a very popular drink amongst researchers, was once decaffeinated using DCM. Unroasted beans would be steamed and then repeatedly rinsed in DCM, which would extract the caffeine. The solvent would then be drained away, leaving behind coffee beans packed full of flavor, but without the buzz. An alternative method was to essentially make a giant pot of very strong coffee, then extract the caffeine using DCM. When a new batch of beans was added to the brew, the higher concentration of caffeine in the beans would leach out into the water, decaffeinating the beans without removing any of the compounds essential to the flavor of the coffee.


Although DCM has a toxic dark side, it’s found a light-hearted use in a Chinese toy – the drinking bird. The bird is made from a glass body containing dichloromethane - with two bulbs, representing the head and body, connected by a tube. Drinking birds are heat engines that use a temperature difference between the head and body to convert heat energy to mechanical work. This work takes the form of the bird tipping back and forth, just like it’s drinking from a cup of water.


Dichloromethane plays an important part in the mechanism; its low boiling point means the drinking bird can function at room temperature. The bird can move even without a water source, like in the Simpson’s episode, – as long as the body is heated to a higher temperature than the head. Some people believe that the toy is a perpetual motion machine; however this is unfortunately not true, as it uses temperature gradients as an energy source.


So the next time you're enjoying a cup of decaf, taking a bath or thinking about leaving a drinking bird in charge of your work whilst you sneak off on holiday, spare a thought for dichloromethane – the organic chemist’s favorite smelling solvent.