Hydrocarbons Flashcards
(21 cards)
What type of reaction can alkanes undergo?
Alkanes can undergo free-radical substitution.
What is substituted in the free-radical substitution of alkanes?
A hydrogen atom gets substituted by a halogen (chlorine/bromine).
What is needed for the free-radical substitution reaction to occur?
Ultraviolet light (sunlight) is needed for this substitution reaction to occur.
What is the first step in the free radical substitution of methane?
Initiation
What is the second step in the free radical substitution of methane?
Propagation
What is the third step in the free radical substitution of methane?
Termination
What initiates the initiation step in the reaction?
The covalent Cl-Cl bond is broken by energy from the UV light.
What is produced when the Cl-Cl bond breaks?
This produces two radicals in a homolytic fission reaction.
What is the equation representing the initiation step?
Cl–Cl → 2Cl•
This reaction occurs with UV light.
What do halogen free radicals do in the propagation step?
They are very reactive and will attack the unreactive alkanes.
What happens when a methane C-H bond breaks homolytically?
It produces an alkyl radical.
What is the equation for the reaction between methane and a chlorine radical?
CH4 + Cl• → •CH3 + HCl
What does the alkyl radical do in the propagation step?
It can attack another chlorine molecule to form a halogenoalkane and regenerate the chlorine free radical.
What is the equation for the reaction between an alkyl radical and chlorine?
•CH3 + Cl2 → CH3Cl + Cl•
What happens if there is enough chlorine or bromine present?
All the hydrogens in the alkane will eventually get substituted.
What is the outcome of the termination step?
Multiple products are possible, dependent on the radicals involved.
What is an example of a single substitution of methane with chlorine?
•CH3 + Cl• → CH3Cl
This shows the formation of chloromethane.
What happens when two alkyl radicals combine?
•CH3 + •CH3 → CH3CH3
What is the reaction between two chlorine radicals?
Cl• + Cl• → Cl2
What should you avoid giving in the termination step of a free radical reaction?
You should not give the equation reforming the original halogen.
This is often marked as ‘ignore’ on mark schemes.
