Halogenoalkanes Flashcards
What is required for an alkane to react with a halogen?
Ultraviolet (UV) light is required to initiate the reaction
Why don’t alkanes react with halogens in the dark at room temperature?
Alkanes are unreactive because the they have strong C-H and C-C bonds, so energy (UV light) is needed to break the halogen bond and start the reaction
What type of reaction occurs between an alkane and a halogen under UV light?
Free radical substitution, where hydrogen atoms in the alkane are replaced by halogen atoms
It follows a chain reaction mechanism with three stages: initiation, propagation and termination
Write the balanced equation for the reaction of hexane with bromine?
C6H14 + Br2 ⇒ C6H13Br + HBr
What is a chain reaction?
A reaction in which the products of one step continue the reaction in further steps until stopped
What are the three steps in the free radical substitution mechanism?
Initiation, propagation and termination
What happens in the initiation step of free radical substitution?
Homolytic fission occurs, where the halogen molecule absorbs UV light and splits into two free radicals
What happens in the first propagation step of free radical substitution?
Example (hexane and bromine)
A halogen free radical removes a hydrogen atom from the alkane, forming a hydrogen halide and an alkyl free radical
C6H14 + Br* ⇒ *C6H13 + HBr
What happens in the second propagation step of free radical substitution?
The alkyl radical reacts with another halogen molecule, forming a halogenoalkane and a new halogen radical
C6H13* + Br2 ⇒ C6H13Br + Br*
Why is the propagation stage considered a chain reaction?
This is because the reaction keeps generating new free radicals, which continue reacting until stopped
What happens in the termination step of free radical substitution?
Two free radicals combine to form a stable molecule, stopping the chain reaction
Why can multiple substitution products form in these reactions?
If excess halogen is present, more hydrogen atoms in the alkane can be replaced forming compounds that have di- or tri-halogenated compounds
Why doesn’t UV light break the C-H bond in the alkane?
The C-H bond requires more energy to break than a single quantum of UV radiation provides
Why are free radicals highly reactive?
They have an unpaired electron, making them eager to react with other molecules to achieve stability
What are the main products of the propagation stage in methane chlorination?
Chloromethane and hydrogen chloride
Why does the termination step stop the reaction?
No new free radicals are produced, so the chain reaction cannot continue
Why is termination less frequent than propagation?
Free radicals are highly reactive, but they are low in concentration. so most reactions involve radicals reacting with non - radical molecules instead
Can free radical substitution occur without UV light?
Yes, it can occur at high temperatures, even without light
