Halogenated hydrocarbons

General characteristics

Halogenated hydrocarbons are obtained as a result of attaching halogen atoms to the molecule of an organic compound. The elements from group 17 of the periodic table that are involved in such reactions are chlorine, bromine, iodine or fluorine. The resulting compounds are often referred to as halocarbons. Depending on the number of halogen atoms in a molecule, mono-, di-, tri- and polyhalogenated compounds are distinguished. It is important to remember that chemical compounds consisting only of a carbon chain and halogens attached to it are also halocarbons. Depending on the ‘base’ hydrocarbon, we can distinguish:

• Halogenated saturated hydrocarbons – these compounds are derived from the corresponding hydrocarbons, by replacing one or more hydrogen atoms with elements from group 17 of the periodic table.
• Halogenated unsaturated hydrocarbons – in the case of unsaturated hydrocarbons, i.e. those with a double or triple bond, the halogen molecule undergoes addition. An unsaturated bond is broken, and halogen atoms or hydrogen and halogen atoms are attached to the carbon atoms.
• Halogenated aromatic hydrocarbons – these compounds are formed by the reaction of a benzene ring and the corresponding halogens. In benzene, one to six carbon atoms are substituted, with the formation of carbon-halogen bonds.

In addition, most halogenated hydrocarbons exhibit isomerism resulting from different positions of the halogen atom in the molecule. The longer the carbon chain, the greater the number of possible combinations. Moreover, halogenated hydrocarbons are associated with the concept of the order of carbon atoms. According to this criterion, we can distinguish primary, secondary and tertiary carbon atoms. This classification is important in organic chemistry as it allows the prediction of the properties and reactivity of chemical compounds.

Preparation and properties

Preparation

Halogenated hydrocarbons can be obtained in several different ways. The choice of preparation method depends on the substrates and the efficiency of the overall process. The basic ways of obtaining these compounds are listed below:

• Radical substitution reaction – occurs with light or heating. It is one of the basic methods for preparing hydrocarbon halides and is primarily used to obtain methane derivatives. Radical substitution mainly occurs in the presence of chlorine.
• Reaction of halides with unsaturated hydrocarbons – during this process, an addition of a halide or a hydride of the corresponding element from group 17 of the periodic table to an unsaturated bond in the hydrocarbon takes place.
• Electrophilic substitution of aromatic hydrocarbons – used to produce halogenated hydrocarbons containing a benzene ring in their molecule. The reaction is carried out in the presence of a catalyst.
• Reaction of halides with alcohols – a halogen hydride molecule is attached to an alcohol molecule. The most commonly used halogen hydrides are HCl or HBr. As a result of this reaction, the hydroxyl group is replaced with a chlorine or bromine atom, respectively (it is a substitution reaction). The by-product is a water molecule.

Halogenated methane derivatives are among the most popular derivatives of saturated hydrocarbons, particularly those containing chlorine molecules. The attachment of this element is an example of a radical chain substitution reaction initiated by sunlight. Methane and chlorine do not react with each other in the dark. A chlorine atom bonds with one of the hydrogen atoms from a methane molecule to form chloromethane and hydrogen chloride. The reaction does not end at this stage and another chlorine molecule reacts with chloromethane. The replacement of another hydrogen atom by chlorine results in dichloromethane, which in the next step changes to trichloromethane and then tetrachloromethane. The latter, also known as carbon tetrachloride, is the final product of the radical chlorination of methane. Further reaction with chlorine is not possible. In reality, during this process, the final chemical mixture contains all four chloromethanes. Their quantitative ratio depends on the conditions under which the reaction is performed (their separation is carried out using distillation).

Properties

The specific properties exhibited by individual halogenated hydrocarbons are directly dependent on the type of organic compound (carbon chain length, presence of unsaturated bonds, etc.) and the number of substituted halogen atoms. As a general rule, the more halogens there are in a molecule, the higher the density and boiling point of such a compound.  Furthermore, in the case of isomers, the more branched a molecule is, the lower its boiling point and the higher its volatility. The least active are derivatives containing a chlorine atom, more active are those with bromine, and the most active are compounds having iodine atoms. All of them decompose at very high temperatures. Halogenated hydrocarbons are very chemically active due to the presence of a highly electronegative element in their molecule, such as fluorine. They undergo reactions such as electrophilic substitution of a halogen.

The most important examples of halogenated hydrocarbons

Vinyl chloride

Vinyl chloride (the common name), formally named chloroethene or chloroethylene, is the product of the addition reaction between hydrogen chloride and ethyne, in the presence of a catalyst, at high temperature and pressure. Vinyl chloride is a gas that, like other ethene derivatives, is susceptible to polymerisation, the result of which is a macromolecular substance, a polymer – polyvinyl chloride, the popular PVC. It is a durable and low-cost raw material with a wide range of industrial and business applications. The most popular are window and door components made of polyvinyl chloride. In addition, it is used, among other things, as a material for electrical, plumbing and ventilation systems, as an additive in the manufacture of toys, wall coverings, insulating curtains, panels, advertising boards, prostheses in medical applications and much more.

Freons

For decades, freons (or CFCs) have been used in aerosols and refrigeration systems for fridges or freezers. Their use is now banned worldwide. Chemically, they are chlorinated and fluorinated derivatives of saturated hydrocarbons, mainly methane or ethane. They are highly volatile liquids, providing a lot of heat energy as they change their state of matter. Freons are insoluble in water and exhibit no chemical activity with other compounds. The most well-known CFCs are dichlorodifluoromethane and 1,1,1,2-tetrafluoroethane. After years of intensive use of CFCs in household chemicals, they have been demonstrated to destroy the ozone layer, the loss of which increases the harmful effects of solar radiation. These compounds are responsible for 14% of the greenhouse effect. Unfortunately, CFC molecules are persistent and can remain in the atmosphere for up to 130 years.

Chlorobenzene

Among the halogenated aromatic hydrocarbons of greatest industrial importance is chlorobenzene. It is obtained by chlorination of the benzene ring with chlorine, in the presence of iron, which catalyses the process. Chlorobenzene is a colourless, highly volatile, highly water-soluble liquid with a wide range of applications in the chemical industry.  This compound is a key substrate in the production of agrochemicals, including (but not limited to) herbicides, fungicides and other plant protection products. The chemical industry readily uses chlorobenzene in the plastics industry, where it serves as an antioxidant additive for rubbers or an additive in the production of polyaniline. Other uses of chlorobenzene include the production of dyes, active pharmaceutical ingredients (so-called APIs), solvents and chemical compounds.

Tetrachloromethane

Tetrachloromethane (carbon tetrachloride) is a fully chlorinated derivative of methane. It belongs to the halogenoalkane group. At room temperature, it is a colourless liquid with a sweet odour. It is non-flammable. It is slightly soluble in water, but it is very well soluble in organic solvents. Until recently, it has been widely used as a solvent, particularly in the chemical industry, but it is highly toxic and dangerous to the environment. This is why for several years, the goal has been to significantly reduce the use of carbon tetrachloride. Today, it can be found in products such as solvents, cleaning and washing agents or fire extinguisher fluids (especially dedicated to extinguishing burning petroleum products).