The Formation of 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 in the dark because there is not enough energy provided in the dark to break their strong C-C and C-H bonds
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 are free radicals highly reactive?
They have an unpaired electron, making them eager to react with other molecules to achieve stability
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
What is a free radical?
What is an electrophile?
An atom which has an unpaired electron
An atom that is attracted to an electron rich region in another molecule
Write two equations that show how chlorine atoms catalyse the decomposition of ozone into oxygen
O3 + •Cl -> O2 + ClO•
ClO• + O3 -> 2O2 + Cl•
