Also known as halocarbons, halogenoalkanes or alkyl halides; whichever name you choose this is a class of organic
compounds in which halogen
atoms are bonded to the carbon chain or ring. Among the simplest examples is chloromethane (CH3
Cl), in which one hydrogen
atom from methane
) has been replaced by chlorine
, a halogen.
Displayed formula of chloromethane:
H - C - Cl
Haloalkanes can be created readily by the reaction of alkene
s with halogens or hydrogen halides. The two atoms of the diatomic
) or the hydrogen halide (HX) bond across the carbon-carbon double bond
(C=C). Thus the double bond is saturated and the attacking molecule combines with the alkene - this is an addition reaction, nothing is removed.
For example, ethene reacts with bromine (Br2) to form dibromoethane (CH2Br-CH2Br). If it were hydrogen bromide (HBr), one of the atoms of the attacking molecule would be hydrogen, so the product would simply be bromoethane (CH2Br-CH3).
Creating haloalkanes from alkanes is more difficult. Alkanes are less reactive than alkenes as there is no double bond waiting to be broken. In the presence of ultraviolet radiation, a halogen molecule, e.g. chlorine (Cl2), will split into its constituent atoms. This is homolytic fission. These atoms will both have an uneven number of electrons in their outer shell, making them extremely reactive - they are free radicals, represented as Cl•. In the presence of an alkane, this splitting initiates a chain reaction.
The chlorine free radicals split the alkane - let's say methane (CH4) - into two more free radicals, in this case CH3• and H•. The Cl•, CH3• and H• free radicals combine in various ways to end the chain reaction with several products:
- Two Cl•s may combine to give us back molecular chlorine, Cl2.
- A CH3• may combine with an H• to give us back methane, CH4.
- An H• and a Cl• may combine to give us HCl, hydrogen chloride.
- Two CH3•s may combine to produce ethane, C2H6.
- Two H•s may combine to produce molecular hydrogen, H2.
- Finally, a Cl• may combine with a CH3• to give us the halogenalkane, chloromethane (CH3Cl).
Many common anaesthetic
s are haloalkanes. Halothane
, for example, is 1,1,1-trifluoro-2-bromo-2-chloroethane, CHBrCl-CF3
, and chloroform
is simply trichloromethane, CHCl3
s (chlorofluorocarbons) also belong to this class of compound.
The pesticide Lindane
, 1,2,3,4,5,6-hexachlorocyclohexane (C6
), is an example of a halogenated cycloalkane, and so is also a type of haloalkane.
Cl - C - H
Cl C H
\ / \ /
H - C C - Cl
Cl - C C - H
/ \ / \
H C Cl
Haloalkanes can also be used in the preparation of alcohol
s and alkene
s. Reaction of a haloalkane with potassium hydroxide in aqueous solution and in ethanol
will replace the halogen atoms with hydroxyl groups, -OH. The water acts to dissociate
the hydroxide, and the ethanol provides a solvent suitable for both reactants. For example, 2-chloropropane (CH3
) would react to form 2-propanol (CH3
Alternatively, reaction of a haloalkane with potassium hydroxide in a pure ethanolic solution will remove HCl from the haloalkane and produce an alkene. 2-chloropropane would be converted in this case to propene (CH3-CH=CH2).