Alkanes are exceptionally boring and unreactive molecules chemically speaking but we can persuade them to react with halogens in the presence of high energy / high frequency UV light.
CH4(g) + Cl2(g) ⇾ CH3Cl(g) + HCl(g)
The radiation causes homolytic fission of the halogen-halogen bond, forming radical halogen atoms and starting a chain reaction which ends in a haloalkane (halogenoalkane) such as choromethane.
You need to be able to explain the terminology and draw out the mechanism for the radical substitution reaction.
A mechanism shows the movement of electrons in a reaction, breaking it down into simple steps.
This is the first of many that you will meet and is important for understanding the role of CFCs in the depletion of ozone in the stratosphere.
- Initiation
Homolytic fission of a covalent bond is when the bond breaks evenly and each fragment takes one electron from the bond. The resulting fragments each have an unpaired electron and are called radicals. Radicals are very reactive species.
- Propagation
There are two propagation steps. Each step produces a radical leading to a chain reaction.
The key to understanding this mechanism is to appreciate that of the two propagation steps, the first always makes the by-product and the haloalkane is made in the second propagation step. Both steps are exothermic as the bonds that are made in each stage are stronger than those broken.
- Termination
In the termination step, two radicals react to form a non-radical. Since the concentration of radicals in the reaction mixture is actually very low, chain termination is rare.
Practice questions
- The majority of chloromethane and bromomethane molecules in the troposphere are short lived because they react with hydroxyl radicals.
(a) What is a radical?
(b) Draw a dot and cross diagram for a hydroxyl radical, •OH.
- Ozone, O3, found in the troposphere breaks down to form an oxygen molecule, O2, and an oxygen radical, O•.
(a) What type of bond breaking occurs in this reaction?
(b) Bond breaking requires energy. Name the type and source of radiation needed to break a covalent bond.
(c) An oxygen radical reacts with a water molecule to produce hydroxyl radicals. Write an equation for this reaction.
- Two of the steps in the chain reaction between hydroxyl radicals and chloromethane are shown below.
CH3Cl+ OH ⇾ CH2Cl + H2O equation 1
CH2Cl + OH ⇾ CH2ClOH equation 2
(a) Draw the reaction mechanism using curly arrows to show the reaction happening in equation 1.
(b) Classify, with reason, the steps shown in equations 1 and 2 as being initiation, propagation or termination.
- Ethane, C2H6, reacts with bromine, Br2, in the presence of sunlight via a radical chain reaction. Draw out a possible mechanism for the reaction labelling each step as initiation, propagation or termination.
Answers
- (a) a particle with an unpaired or lone electrn
(b)
- (a) homolytic fission
(b) high energy uv radiation from the sun
(c) H2O + O• ⇾ 2 •OH
(showing the unpaired electron is optional on radicals when writing equations but if you do, make sure it is on the correct atom i.e. in the hydroxyl radical the unpaired electron is on the oxygen not the hydrogen)
- (a)
(b) Equation 1 is a propagation step as both reactants and products include a radical. Equation 2 is termination because two radicals react to form a non-radical product.
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