Alkyl halides or Haloalkanes as the name suggests are chemical compounds with a combination of halogen and alkanes.
Did You Know?
Methyl iodide does not deplete the ozone layer, unlike most other alkyl halides. Hence, has been announced as the non-ozone layer depleting substance by EPA of United States.
Alkanes are compounds of carbon and hydrogen. Alkanes have the general formula as CnH(2n+2). Halogen family comprises five elements namely fluorine, chlorine, iodine, bromine, and astatine. Haloalkanes are derived from saturated hydrocarbons when one or more hydrogen atom is replaced by a halogen atom.
Although haloalkanes are derived from alkanes, there is a large difference between their structural and physical properties. If alkanes and haloalkanes containing the same number of carbons are compared, it can be observed that the boiling point of alkanes are lower than those of haloalkanes. Alkanes are non-aromatic while some of the alkyl halides have an associated smell.
Nomenclature of Alkyl Halides
Since alkyl halides are compounds formed from alkanes and halogen, they are named after their parent constituents. Consider ethane reacting with chlorine. The resultant haloalkane is called chloroethane. However, this is for a single chlorine molecule. If the number of chlorine molecules is four, then the resultant will be called tetrachloroethane wherein tetra depicts four molecules of chlorine.
Also Known As: Haloalkanes, halogenoalkanes, halogenated hydrocarbons
General Formula: R-X
Where R represents hydrocarbon (alkane) and X represents halogen. As mentioned earlier, the general formula of an alkane is CnH(2n+2). One or more hydrogen is replaced by a halogen atom.
Types of Alkyl Halides
Alkyl halides can be classified depending on the atom of carbon to which the halogen atom is bonded.
Primary (1o): The carbon atom bonded to the halogen atom is bonded to one other alkyl group.
Secondary (2o): The carbon atom bonded to the halogen atom is bonded to two other alkyl groups.
Tertiary (3o): The carbon atom bonded to the halogen atom is bonded to three other alkyl groups.
Two examples of alkyl halides are presented below.
Consider the chlorination of methane (CH4) as represented in the image below.
When two atoms of chlorine are added to methane, a bond between central carbon atom and hydrogen is broken whereas central carbon and a chlorine bond is formed. The freed hydrogen atom combines with a chlorine atom to form hydrochloric acid. The resultants are chloromethane and hydrochloric acid (HCL) whose structures have been presented.
Chloromethane is toxic, colorless, pleasantly smelling, and highly flammable. It is used as weed killer, as a solvent in chemical reactions, anesthetic in medicine field, manufacture of silicon polymers, and petroleum refining process.
Consider the chlorination of chloromethane (CH3Cl) as represented in the image below.
When two atoms of chlorine are added to chloromethane, a bond between central carbon atom and hydrogen is broken whereas central carbon and a chlorine bond is formed. The freed hydrogen atom combines with a chlorine atom to form hydrochloric acid. The resultants of this reaction are dichloromethane and hydrochloric acid (HCL) whose structures have been presented.
Dichloromethane is a volatile, colorless, pleasantly smelling liquid which is insoluble with water. It is used as a degreaser, solvent for chemical processes, aerosol propellant, and heat engine (in drinking bird).
In the end, process of distillation is used to separate these products. Note that both the above reactions can take place only in presence of UV light or high temperatures.
Properties of Alkyl Halides
◈ Halogen imparts reactivity to alkyl halides.
◈ Alkanes impart odorlessness and colorlessness to alkyl halides.
◈ Some alkyl halides are less toxic and have high heat of vaporization.
◈ Alkyl halides are water-phobic, i.e. they repel water. Only a few of these compounds are sparingly soluble in water.
◈ Halogenated hydrocarbons are soluble in organic solvents.
◈ Some of the haloalkanes do not conduct electricity.
◈ Alkyl halides have higher boiling and melting point unlike alkanes.
◈ Haloalkanes are less flammable as compared to its component alkanes.
◈ R-F are the most stable alkyl halides. The stability in their increasing order is as expressed below. R-F << R-Cl < R-Br < R-I. This order indicates the strength of the carbon-hydrogen bond.
◈ Alkyl halides have stronger intermolecular forces (dipole-dipole interaction.)
◈ Some halogenoalkanes leave no residue post usage.
Uses of Alkyl Halides
Alkyl halides find a variety of uses and applications in our everyday lives which have been mentioned below:
◈ Alkyl halides are used in labs as synthetic intermediate compounds.
◈ They are used as cleansers for cleaning.
◈ Commercial uses of haloalkanes include its use in fire extinguishers.
◈ Carbon tetrachloride is used to detect neutrinos.
◈ Ethyl chloride can be used as an agent for bringing about cooling of the skin in tropical regions.
◈ The water-insoluble alkyl halides are used as aqua-phobic solvents in chemical processes.
◈ Alkyl halides are commonly used as varnish or paint removers.
◈ Haloalkanes are used as propellants.
◈ Naturally occurring haloalkanes found in oceans, sometimes act as guards against ocean-attackers.
◈ CFC’s or chlorofluorocarbons are used as refrigerants in refrigerators. However, they deplete the ozone layer and are thus harmful. They can also be used for manufacturing expanded polystyrene.
◈ Dichloromethane is used in plastic welding, and as a blowing agent for foamed plastics.
Alkyl halides find wide applications in the field of chemistry. However, the fact that these compounds have serious implications on health and environment cannot be ignored. Carbon tetrachloride used as a fabric cleaner is known to cause damage to liver. Similarly, chloroform a popular anesthetic has been proven to be cancerous. Therefore, use and circulation of alkyl halides has been restricted to some extent.