The Formation of Halogenoalkanes
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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?
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
Free radical substitution takes place in a chain reaction, what are the three stages involved?
1.Initiation 2.Propagation 3.Termination
What is the general overall equation for a free radical substitution reaction?
Alkane + Halogen → Halogenoalkane + Hydrogen halide
Write the balanced equation for the reaction of methane with chlorine?
CH4 + Cl2 ⇒ CH3Cl + HCl
What happens in the initiation step of free radical substitution?
The halogen bond (Cl-Cl) breaks to form two halogen radicals (Cl*)
Why is the UV light enough to cause the halogen bond (Cl-Cl) to break?
The energy fo one quantum of UV light is greater than the halogen bond energy
What type of bond fission occurs when the halogen bond (Cl-Cl) breaks?
Homolytic fission - one electron goes to each Cl atom
What is produced when the halogen bond (Cl-Cl) undergoes homolytic fission?
Two halogen (chlorine) free radicals (Cl*)
Why doesn’t the C-H bond in the alkane break during initiation?
It requires more energy than is provided by a quantum of UV light
What happens in the first propagation stage of free radical substitution Example methane reacting with chlorine
A halogen free radical (Cl) removes a hydrogen atom from the alkane (CH3), forming a hydrogen halide (HCl) and an alkyl radical (CH₃)
Write the equation for the first propagation step (methane and chlorine)
Cl* + CH₄ → HCl + *CH₃
What happens in the second propagation step? (methane and chlorine)
The alkyl radical (CH₃) reacts with another halogen molecule (Cl₂), forming a halogenoalkane (CH₃Cl) and a new halogen radical (Cl)
Write the equation for the second propagation step (methane and chlorine)
CH₃ + Cl₂ → CH₃Cl + Cl
What are the three overall products of the two propagation steps?
• A hydrogen halide (HCl) • A halogenoalkane (CH3Cl) • A side product depending on the radicals present (Cl*)
What happens during the termination step of free radical substitution? (methane and chlorine)
Free radicals (Cl* + CH3*)are removed by reacting together to form stable molecules
Write a termination reaction between two chlorine atoms
Cl* + Cl* → Cl₂
Write a termination reaction between two methyl radicals.
*CH₃ + *CH₃ → C₂H₆ (ethane)
Write a termination reaction between a chlorine radical and a methyl radical.
Cl* + *CH₃ → CH₃Cl
Why is termination less frequent than propagation?
Free radicals are low in concentration, despite being highly reactive, so mostly react 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
Why is the ozone layer important?
It protects the Earth from harmful exposure to, too many ultraviolet rays