Chlorination of Alkanes Questions
1. Methane and chlorine can react to make chloromethane.
a) State the condition required for methane and chlorine to react.
b) Write the overall equation for the reaction.
c) State the type of bond breaking that occurs in the first step of the mechanism.
Homolytic Fission – The bond breaks evenly and one electron goes to each atom.
d) The steps of the mechanism can be split into three stages: initiation, propagation, and termination.
i) State the features of an initiation step.
You start with one stable molecule and end up with two free radicals (species with an unpaired electron).
ii) State the features of a propagation step.
You start with one radical and one stable molecule, and end up with a different radical and a different stable molecule. (This maintains the chain reaction).
iii) State the features of a termination step.
You start with two radicals and end up with one stable non-radical molecule.
iv) Write an equation for the initiation step.
v) Write two equations for two possible propagation steps.
vi) Write an equation for the termination step that forms chloromethane.
vii) Write an equation for the termination step that forms a different product.
Two methyl radicals colliding to form ethane:
2. Chloroethane and chlorine can react in much the same way to produce 1,2-dichloroethane.
a) Write an equation for the termination step that produces 1,2-dichloroethane.
•CH2CH2Cl + Cl•
→
CH2ClCH2Cl
b) Explain why this method only produces a low yield of the desired product.
Free radical substitution is uncontrollable. You cannot control which carbon the chlorine is added to, so you may end up with the isomer 1,1-dichloroethane. Additionally, you cannot control how many chlorine atoms are added (multi-substitution), so you could end up with trichloroethane, tetrachloroethane, etc.
3. CHF3 can react with bromine under certain conditions.
a) Write an equation for the initiation step.
b) Write an equation for two propagation steps.
c) Write an equation for the termination step that results in the formation of bromotrifluoromethane.
d) Write an equation for the termination step that results in the formation of hexafluoroethane.
e) Write the overall equation for the reaction between CHF3 and Br2 that produces CBrF3.
