HALOGENOALKANES: Fluorohydrocarbons and Chlorofluorocarbons

After a serious accident, such as a plane crash in a remote area. Blood transfusions may have to be carried out under difficult conditions, but there may not be time to identify and match a victim’s blood group beforehand. Ideally, the rescue team needs a synthetic blood substitute that will suit any victim and do the vital job of carrying large mounts of oxygen efficiently around the body.

The blood substitute must also be inert and non-toxic. It needs to be chemically stable so that it has a long shelf-life. Some polyfluorohydrocarbons have these properties, and so the search is now on for the polyfluorocarbon compound that is the best synthetic blood substitute.

For over 60 years, it was because of these properties – of being inert, non-toxic and chemically stable –that the related chemicals chlorofluorocarbons (CFCs) were thought to be the best choice for industrial and domestic applications as refrigerants, electrical insulators and aerosol propellants.

Then, in the 1980s, the same properties were found to make CFCS an environmental hazard: scientists confirmed that CFCS released into the air break down the ozone in the stratosphere. They warned that, even if CFCS were banned, because they are so stable it would be a long time before the ozone could build up to its original level. Since then, CFCS are being replaced in their previous applications by alternative compounds.

^{Immiscible layers of colored water (top) and much denser perfluoroheptane (bottom) in a beaker; a goldfish and crab cannot penetrate the boundary; quarters rest at the bottom. Sbharris (Steven B. Harris), CC BY-SA 3.0}

HALOGENO-HYDROCARBONS

A halogeno-hydrocarbon (also called a halogenated hydrocarbon) is a compound that contains carbon, hydrogen and at least one of the halogens. This means that the molecule contains at least one polar covalent bond, namely, the carbon-halogen bond. A halogenoalkane or haloalkane is an alkane in which one or more of the hydrogen atoms have been substituted by halogen atoms.

USES OF HALOGENO-HYDROCARBONS

Halogenated hydrocarbons are synthesised as useful chemicals in their own right, as well as being intermediates in the synthesis of other chemicals.

As chemicals in their own right

Chloroalkanes have been used as solvents and anaesthetics for at leas a century, but they are poisonous and damage vital organs in the body, including the liver. Synthetic methods were then developed that successfully introduced several different halogen atoms into a carbon skeleton. This changed the situation immediately. The new chlorofluorocarbons were non-toxic, low-boiling liquids or gases at room temperature and chemically inert. They seemed just the answer for many applications, for example as the fluids for air conditioning systems, refrigerators, aerosols and blowing plastic foams.

Later, other halogen atoms were introduced and the halons were developed for fire extinguishers. Unfortunately, the very properties that make halogeno hydrocarbons so useful have caused a global environmental problem – ozone depletion in the upper atmosphere. Some chlorinated hydrocarbons have been synthesised as insecticides and pesticides. But again, their chemical stability leads to environmental problems. The sales of aerosol cans that contain CFCs have been phased out since the damage that CFCs do to the atmosphere became understood.

As synthetic intermediates

The manufacture of many pharmaceutical products and polymers requires halogenated hydrocarbons as intermediates in their organic synthesis. A chlorine or bromine atom in an organic molecule often increases its reactivity and allows a change of functional groups or an extension of the carbon skeleton. The polymer poly(chloroethene), or polyvinyl chloride (PVC), is manufactured from chloroethene.

NAMING HALOGENO-HYDROCARBONS

Although many halogeno-hydrocarbons have traditional names, such as chloroform (CHCI₃) or vinyl chloride (CH₂CHCI), it is important that you can systematically name halogeno-hydrocarbons. The names of all organic compounds are based on the carbon skeleton and the functional groups present. In halogeno-hydrocarbons the functional group is the carbon-halogen bond. The presence of a halogen is indicated by the use of a prefix, namely fluoro, chloro, bromo or iodo. The number and location of the halogen atoms must also be specified. The number of each halogen atom is specified by the use of mono, di, tri, tetra, penta, hexa, and so on, before the prefix. For example, the trichloromethane molecule contains three chlorine atoms, and the dibromo-dichloroethane molecule contains two chlorine atoms and two bromine atoms. Sometimes, all the hydrogen atoms in an alkane are replaced by halogen atoms. If the halogen atoms are all identical, the prefix ‘per’ is often used. So that, for example, C₂F₆ is often referred to as perfluoroethane rather than hexafluoroethane.

Isomers are substances that have the same molecular formula, but their atoms are arranged differently. They may have different structural or displayed formulae, or different arrangements about a double bond. This means that, for example, dibromo-dichloroethane is not sufficient to describe fully one particular compound. There are four different structural isomers that could each be called dichloro-dibromoethane. This means that the position of the halogen atom must also be specified in the name. I use the system of numbering each carbon atom e.g 2,2-dibromo-1,1-dichloroethane, 1,2-dibromo-1,1-dichloroethane. Notice that the order of the halogens in the name is given alphabetically.

^{Tetrafluoroethane (a haloalkane) is a colorless liquid that boils well below room temperature (as seen here) and can be extracted from common canned air canisters by simply inverting them during use. Deglr6328, CC BY-SA 3.}0

PHYSICAL PROPERTIES OF HALOGENOALKANES

The uses of halogenoalkanes generally depend on their physical properties. One notable property is their low boiling point, which accounts for many of their applications. The boiling point of a substance depends on the nature and strength of the intermolecular forces in the liquid.

INTERMOLECULAR FORCES IN HALOGENOALKANES

Halogenoalkanes are all covalently bonded and form simple molecules. The presence of simple molecules in the liquid phase makes halogenoalkanes solvents that are electrically non-conducting. Halogen atoms tend to withdraw electrons from a carbon-halogen bond. In a monohalogenoalkane, the halogen atom will be slightly negative (δ-) and the carbon atom it is bonded to will be slightly positive (δ+). We say that the molecule has a permanent dipole and that it possesses a dipole moment, a measurable degree of polarity.

The high electronegativity of chlorine gives both chloromethane and trichloromethane a dipole moment. Chloromethane and trichloromethane are both molecules with a dipole moment, so they have permanent dipoles. The positive part of one molecule can attract the negative part of another molecule. This intermolecular force is referred to as a permanent dipole-permanent dipole interaction. When a large alkyl group is present as well, part of the intermolecular attraction consists of induced dipole-induced dipole van der Waals attractions.

CHLOROFLUOROCARBONS AND HALONS

The most well-known and infamous of the halogenoalkanes are the chlorofluorocarbons, known as CFCs. As their name suggests, these are halogenoalkanes in which every hydrogen atom has been replaced by either a chlorine or a fluorine atom. When a hydrogen atom is included in a CFC, the compound is called HCFC (hydrogen-chlorofluorocarbon).

Another variety of fully halogenated alkanes is called the halons. These contain bromine atoms as well as fluorine and chlorine atoms. Halons are used extensively in fire extinguishers because they are chemically unreactive and non-toxic. Being much denser than air, they effectively blanket a fire, which keeps out the air and so extinguishes the fire. Typical halons include trifluoro-bromoethane (Halon 1301), and bromo-chloro-difluoromethane (Halon 1211). Unfortunately, these halons also cause the same environmental problems as CFCs.

BOILING POINTS OF CFCs

Boiling occurs when intermolecular forces are overcome, so the molecules can separate from each other and spread out. Thus, the stronger the intermolecular attraction, the higher the boiling point. The type of intermolecular forces in CFCs are similar to those in other halogenoalkanes, but the dipole moment of the molecules is likely to be much smaller since the individual bond polarities tend to cancel out. This means that the attraction due to permanent dipole-permanent dipole interactions is liable to be quite weak. As a result, even though the relative molecular masses of CFCs are quite large compared with, say, the monohalogenoalkanes, they will have lower boiling points.

As CFCs have weak intermolecular attraction, they are volatile liquid and vaporise easily at low temperatures. Many CFCs have two properties that make them efficient refrigerants: they absorb heat energy readily when vaporising, and they can be easily compressed back into liquids, releasing this energy. Provided a heat exchanger efficiently removes the heat released during liquefaction, net cooling of the refrigerant CFC will occur. If the intermolecular forces were larger, it would be difficult to vaporise the CFC and so little heat would be absorbed.

^{Halon 1211 Fire Extinguisher, USA, early 1990s. Firetech117, CC BY-SA 4.}0

PREPARATION OF HALOGENOALKANES BY FREE RADICAL SUBSTITUTION

Even before the wide-scale use of halons and CFCS, the preparation of halogenoalkanes had become an important synthetic reaction because halogenoalkanes are extremely useful in making other chemicals. The main methods of preparation of halogenoalkanes are described below. One route involves a direct substitution of a hydrogen atom by a halogen atom. It is called free radical substitution.

FREE RADICALS

Alkanes are very unreactive compounds, with the exception of their reaction with oxygen. Their lack of polar covalent bonds is the reason for this behaviour. Nucleophiles are particles that seek out electron-deficient centres and donate pairs of electrons to form a covalent bond, while electrophiles are particles that seek out electron-rich centres and accept pairs of electrons to form a covalent bond. It is difficult for alkanes to react with nucleophiles of electrophiles, because alkanes are non-polar molecules.

Alkanes require a different type of particle with which they can react, that is, one which is highly reactive and does not need to seek out a polar covalent bond before it can react. The particle in question is called a free radical. This is an atom or group of atoms that possesses at least one unpaired electron in its outer shell.

Most free radicals are extremely reactive because they pair up their unpaired electron with an electron removed from a covalent bond. Alternatively, two free radicals can react with each other, pairing up the unpaired electrons to form a covalent bond. A free radical is usually indicated by a dot on the right-hand side of the formula of the particle. For example, a chlorine atom Cl is a free radical, which is made clear by writing Cl•.

HOMOLYTIC FISSION

As they are so reactive, free radicals cannot be stored in reagent bottles. They must be made in the reaction vessel, ready to react immediately. To make a free radical, a covalent bond is broken in such a way that the electrons in the shared pair of the covalent bond become two single electrons, one electron per atom. This is known as homolytic fission of a covalent bond.

In the representation of the process, to make a chlorine free radical from a chlorine molecule. A curly half-headed arrow shows the movement of one electron. The homolytic fission of a covalent bond requires energy (it is an endothermic process). For example, the production of the chlorine free radical requires half of the bond dissociation energy of a chlorine molecule:

½ Cl₂(g) → Cl•

FREE-RADICAL SUBSTITUTION OF METHANE

Methane is a typical alkane, which reacts with chlorine in the presence of ultraviolet light to give a large variety of products. There are varieties of substitution products that result from the free-radical chlorination of methane. Substitution is the process by which a hydrogen atom is swapped for a chlorine atom. But there are also other methane products whose origin is less obvious. The only way to explain this large collection of products is to examine the mechanism of the reaction, which I will discuss in my next post so as not to make this post lengthy.