Dichloromethane And Its Uses

By Adrian Dingle

Introduction

Dichloromethane, also commonly known as methylene dichloride, is a clear, colorless liquid at room temperature. It is volatile, and when it evaporates gives off a distinct odor that is sometimes characterized as being sweet or ether-like.

Dichloromethane is categorized as a haloalkane, meaning that it is an alkane (a hydrocarbon with only single bonds), with halogen (group 17 element) substitution of some hydrogen atoms. It is specifically based upon the simplest of all the alkanes, methane, or CH₄, where two of the hydrogen atoms in that molecule are replaced by chlorine atoms.

Methane, CH4	Dichloromethane, CH2Cl2

Dichloromethane as a solvent

By far the most popular application of dichloromethane is as a general solvent. It is used extensively in industry for this purpose, where in each case it dissolves certain compounds and then, as the dichloromethane is removed, it takes the unwanted substances with it.

Properties that make dichloromethane a good solvent choice

Attributes that make DCM a good solvent include:

• polarity (it has positive and negative parts that allow it to be attracted to some molecules),
• relative inertness (it does not tend to react chemically with other substances that it comes into contact with)
• miscibility with a very wide range of substances

Another important property of dichloromethane related to its role as a solvent is its density. The chlorine atoms present in such compounds tend to create molecules that have higher densities than water, and dichloromethane is no exception. It has a density of around 1.33 g/mL, with water being around 1.00 g/mL (dependent upon temperature). This allows dichloromethane to form a separate, lower layer by sinking when it comes into contact with aqueous solutions. In such extractions the dichloromethane lower layer can easily be removed by gravity, leaving the aqueous layer behind, and affecting the extraction and the separation.

The fact that dichloromethane is non-flammable is also a huge asset. This is especially true since so many of the alternative solvents are indeed flammable, and as such can create a hazard.

Dichloromethane’s specific uses as a solvent

So, knowing the general mechanism of a solvent, and knowing some of the properties of dichloromethane that make it good in that role, what about some of the more specific uses?

Caffeine extraction – one of dichloromethane’s major uses as a solvent is to extract caffeine from coffee to make what we know as decaffeinated coffee. Dichloromethane can dissolve the caffeine from the moistened coffee beans, and thereby remove it from the beans. The beans retain their flavor and other properties but are ‘decaffeinated’ in the process. The same extraction process can be applied to tea-leaves in order to produce decaffeinated tea.

Paint and varnish stripper – in its role as a solvent, dichloromethane can also be used to strip paint and varnish. The dichloromethane will dissolve some of the compounds that make paint and varnishes, and they will be removed with the removal of dichloromethane.

Degreaser – because dichloromethane has the ability to dissolve so many different compounds, it is used as a degreaser and cleaner in industrial situations. Again, the dichloromethane will dissolve the grease and dirt and remove it. One extra twist here is that this can be done in the gaseous phase in a process called vapor degreasing. Using the solvent as a gas that is produced by evaporating the liquid will result in a more thorough cleaning since the gaseous solvent will condenses on the equipment, getting into smaller areas that might otherwise be missed by the liquid. In a similar manner dichloromethane is used in the electronics industry where cleaning surfaces before applying conducting materials is vital to success.

Chemical manufacturing – dichloromethane is used in the manufacture of insecticides and herbicides, and in the pharmaceutical industry in the manufacture of steroids, antibiotics, and vitamins. It is also used in the coating of tablets.

Aerosol solvent and propellent – dichloromethane can be used as an aerosol propellent and solvent for products such as spray paints, products used in the automotive industry, for adhesives, and even for hairsprays.

Grades

Like most chemicals that have a wide variety of uses, dichloromethane comes in several different grades and types depending on the situation where it is used. The purity can vary from relatively low all the way up to food grade. In many cases the dichloromethane is mixed with stabilizers of various types. The stabilizers are present in order to stop the dichloromethane degrading and/or reacting with oxygen in the air, thus preserving its shelf-life.

Safety concerns

Despite all of the properties that make dichloromethane such an effective and widely used solvent, its use is not without significant concern. Both the U.S. Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) classify dichloromethane as a potential human carcinogen.

Those concerns are chiefly connected with long-term, chronic exposure to dichloromethane but there are also potential short terms effects that can come with the direct inhalation of dichloromethane. Several ailments are possible, including breathing difficulties, nervous system issues, and lung damage.

The knowledge that dichloromethane is such a potentially hazardous substance makes it all the more alarming to learn that dichloromethane once had some limited use as an anesthetic. Alarming yes, but not entirely surprising when the structure of the more popular anesthetic chloroform is known. Chloroform’s use in anesthesia is far more well-known and was far more widespread, and the difference between its chemical structure and that of dichloromethane is slight. Chloroform (with the formula CHCl₃) has just one more hydrogen atom of the base methane molecule replaced with a chlorine atom.

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