Formation of halogenoalkanes Flashcards
What type of mechanism forms halogenoalkanes?
- Free radical substitution.
What is a “free radical?”
- A free radical is a particle with an unpaired electron.
- These particles are very reactive.
How are free radicals formed?
- Forms when a covalent bond is split equally, leaving two particles - each having an unpaired electron.
What is required to break a covalent bond, forming two free radicals?
- UV light.
What are the 3 stages of forming halogenoalkanes?
1.) Initiation
2.) Propagation
3.) Termination.
What happens in the stage of initiation?
- Production of free radicals
- Covalent bond breaks equally, producing 2 highly reactive free radicals.
- UV light provides energy required to break covalent bond - process is called photodissociation.
What is the process called when sunlight/ UV light breaks a covalent bond to form 2 free radicals?
- Photodissociation.
Why is free radical substitution also known as “free radical chain reactions?”
- Same free radical used in first propogation reaction is produced in second propogation reaction.
- The free radical produced by second propogation reaction can react with the alkane multiple times in a chain reaction - forming di, tri, tetr haloalkanes.
What is needed in order to form di/ tri/ tetr haloalkanes in free radical chain reactions?
- Need an excess of halogen, if not you will only get one substitution rather than multiple substitutions.
What happens in propogation stage?
- 1st equation: free radical will react with alkane - forming alkyl radical and a hydrogen halide (ie. HF, HCl, HBr.)
- 2nd equation: alkyl radical will react with halogen molecule - forming halogenalkane and free radical.
- Free radicals are used up and created in these chain reactions.
- These two different equations will repeat until either halogen molecule/ alkane runs out.
What is the termination stage? Give examples of termination equations for formation of chloromethane from methane?
- 2 free-radicals will react to form stable non-radical molecule.
- 2Cl ο –> Cl₂
- 2CH₃ –> C₂H₆ (with dot ON TOP of C in CH₃. )
- Clο + CH₃ –> CH₃Cl (with dot on top of C in CH₃.)
How should we write all molecules in propoagation/ termination equations?
- All needs to be STRUCTURAL FORMULA (so that we can see what atom the free radical is exactly!!)
() - extra info for clarification.
What does termination do to the reaction?
- Termination ends the free radical chain reaction (because all the free radicals react together in this stage.)
() - extra info for clarification.
Write the OVERALL equation for reaction of methane with chlorine to form chloromethane.
- CH₄ + Cl₂ –> CH₃Cl + HCl (with UV written above the arrow!)
- (not that there are no free radicals involved in the overall equation.)
Write the propogation steps for formation of chloromethane from methane.
1.) Clο + CH₄ –> CH₃ (with dot on top of C) + HCl
2.) CH₃ (with dot on top of C) + Cl₂ –> CH₃Cl + Clο
What is the STRUCTURE of every propogation equation?
- On both sides of equation, you will have one free radical and one non-free radical
- The free radical produced by 2nd equation must be the same as the free radical started off with in 1st equation!!
How do you write the overall equation for the formation of halogenoalkane from an alkane?
- Alkane + halogen –> halogenoalkane + hydrogen halide.
(no free radicals involved in overall equation!)
Write the overall balanced equation for formation of C₂HF₅ from C₂H₅F and F₂ using UV light.
Explain why the equation balances in the way it does.
- C₂H₅F + 4F₂ –> C₂HF₅ + 4HF (with UV written above the arrow.)
- You have 4 subsitutions because 4 hydrogens have been removed ad five fluorines have been added.
- 4 F₂ needed for 4 substitutions.
- No in front of hydrogen halide will ALWAYS be the same as no in front of halogen!
How is the unpaired electron of a free radical represented?
- Represented as a dot.
