Dichloromethane (DCM), also known as methylene chloride, is a volatile chemical with the formula CH2Cl2. This colourless liquid is an organic solvent that is widely used in many industries. Dichloromethane can be identified by its sweet aroma – but don’t let that fool you, because DCM also has a toxic dark side.
Dichloromethane was first discovered in 1839 when Henri Victor Regnault, a French chemist, isolated it from a mixture of chloromethane and chlorine which had been left in sunlight. When a mixture of methane and chlorine is exposed to ultraviolet light in this way, it produces chloromethane and hydrogen chloride, as shown below:
CH4 + Cl2 → CH3Cl + HCl
Here we can see that a hydrogen atom has been replaced with a chlorine atom. This is because, under UV light, the chlorine molecule has broken down into free radicals. This means that a chlorine radical can replace a hydrogen atom.
During the reaction, methane is used up and replaced with chloromethane. This increases the likelihood of the chlorine radical hitting the chloromethane molecule, and taking a hydrogen from it in the same way it did with methane. When this happens, a radical chloromethyl is formed:
CH3Cl + Cl• → •CH2Cl + HCl
This result creates a new propagation step in which the radical chloromethyl interacts with chlorine, rather than hydrogen. When chloromethyl takes a chlorine atom, dichloromethane is formed, and a radical chlorine is leftover to continue the chain reaction. Read more about how this reaction occurs here.
•CH2Cl + Cl2 → CH2Cl2 + Cl•
A non-polar substance is defined when two atoms in a molecule are equally electronegative, meaning that the electrons they attract are shared evenly. This bond is often found in H2 and Cl2 molecules. Polar substances, on the other hand, have a difference in electronegativity.
While dichloromethane is relatively polar, it is not as polar as water and teeters on the edge of being a non-polar substance. This is because the chlorines that form part of its structure are more electronegative than the hydrogens, thus forming a net dipole moment that balances itself out. This also means that DCM is immiscible in water.
Organic solvents like dichloromethane are immiscible in water because water is, by contrast, a very polar solvent. When non-polar substances enter an aqueous solution, they exhibit hydrophobic behaviour by aggregating together and excluding the water molecules.
Organic solvents, like ethyl acetate, are mostly less dense than water. However, DCM is actually denser than water, and leaves an organic layer below the aqueous layer rather than above like other solvents.
How is Dichloromethane Used?
While dichloromethane isn’t miscible with water, it is able to dissolve a wide range of organic compounds. These properties, combined with its volatility, makes DCM a highly effective solvent in many industrial processes.
Most commonly, DCM is used as a paint remover. However, it has a range of applications and is commonly used in aerosol products like spray paint, adhesives, insecticides, and bathtub refinishing. It was once even used to decaffeinate drinks!
The technique used to separate an organic compound, such as caffeine, from a mixture of compounds is called extraction. Solvent based extractions are when the caffeine is removed by a chemical solvent, such as dichloromethane or ethyl acetate.
Because caffeine is much more soluble in DCM (140mg/ml) than it is in water (22mg/ml), DCM became the preferred solvent to be used in the extraction of caffeine. In coffee, unroasted beans would be repeatedly rinsed in dichloromethane after being steamed. The DCM would extract the caffeine from them before being drained away. The result were flavoursome coffee beans without the kick.
However, because of the health risks associated with dichloromethane, it was soon replaced by non-toxic alternatives. White DCM has many advantages in industrial applications, it also has its risks.
Dangers of Dichloromethane
Organic solvents like DCM are dangerous because of their high volatility. This means that they can easily evaporate. Therefore, inhalation is the most common form of exposure to solvents.
The toxicity of DCM not only comes from its high volatility, but also from its great stability. This means that it doesn’t just have a tendency to easily vaporise, it’s also extremely persistent in the air.
Because of these characteristics, dichloromethane can quickly build up in small spaces. When it does this, DCM can cause respiratory depression and even narcosis. Symptoms of DCM exposure may include headaches, nausea, drowsiness, and confusion.
This makes it an inhalation risk because prolonged exposure to DCM could be fatal. In poorly ventilated areas, for example, high enough levels of DCM can even cause suffocation. Many bathtub refinishers have unfortunately passed away from concentrated exposure to dichloromethane.
Carbon Monoxide Poisoning
Once dichloromethane has been inhaled, and therefore absorbed into the bloodstream, it can give rise to carbon monoxide. This happens when the enzyme Cytochrome P-450 metabolises with DCM. When this happens, carbon monoxide is produced, and this can lead to carbon monoxide poisoning.
The biotransformation of DCM into carbon monoxide induces hypoxia, a state in which oxygen supply is insufficient in regions of the body. This has potential to threaten the brain in the form cerebral hypoxia. This can happen because of damaging effects dichloromethane can have on the central nervous system (CNS).
Because of its narcotic effects after inhalation, dichloromethane begins to behave as a CNS-depressant. This not only causes feelings of narcosis, it also decreases motor activity, reduces the rate of breathing, impairs the memory, and can lead to a loss of consciousness, or in extreme circumstances, death.
Dichloromethane is considered a carcinogen. However, while human studies have linked occupational exposure to DCM with increased risk for several specific cancers, these studies have been extremely limited (for obvious reasons). Because human data is insufficient in this regard, it means that DCM is regarded as a potential carcinogen. But it is still treated with extreme caution.
This is because while human studies have been inconclusive, animal studies have been extremely revealing. Studies have shown a clear increase in lung and liver cancer, brain cancer and tumours when animals were exposed to concentrated levels of dichloromethane. This is why it is largely considered and treated as a carcinogen.
It is important to note that these extreme effects only occur during chronic exposure. This is defined as being in a small, poorly ventilated space for between 4 – 8 hours whilst being exposed to high levels of DCM. Acute effects are very treatable, and often reverse themselves once the exposure has ceased. While this is a relief, the dangers dichloromethane presents are always worth keeping in mind when handling this sweet-smelling solvent.
At ReAgent, we supply businesses with several types of DCM. We sell dichloromethane that is suited for laboratory applications, general use, and some that has been mixed with acetic acid. No matter what application you require DCM for, ReAgent are sure to supply your business with the highest quality product. Contact us today with your query!
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