Alkanes: Chlorination of Alkanes Flashcards
What are halogenoalkanes?
- A halogenoalkane is an alkane with at least one halogen atom in place of a hydrogen atom
- They are named as substituted alkanes with fluoro-, chloro-, bromo, or iodo used as a prefix
What are chloroalkanes?
- Chloroalkanes are alkanes with one or more hydrogen atoms substituted by a chlorine atom
How are halogenoalkanes formed from alkanes?
• Photochemical reaction
- Reactions started by UV light
• Free radical substitution reaction
- A hydrogen atom is substituted by a halogen free radical (e.g chlorine)
What other products are formed with a halogenoalkane?
- The products of this reaction are always a halogenoalkane and a hydrogen halide
- You DO NOT produce hydrogen
What are the stages of the reaction mechanism for the synthesis of chloromethane?
- Initiation
- Propagation
- Termination
What happens during the initiation stage?
- In initiation step, free radicals are produced
- UV light provides enough energy to break some of the Cl-Cl bonds
- Cl2 → 2Cl•
What happens during the first propagation stage?
- During propagation, free radicals are used up and created in a chain reaction
- First, Cl• collides with methane molecules
- CH4 + Cl• → •CH3 + HCl
- This causes the C―H bond to break
- This forms a stable H―Cl molecule and a highly unstable •CH3 methyl radicals
What happens during the second propagation stage?
- The new methyl free radical, •CH3, collides with another Cl2 molecule
- This forms chloromethane and more chlorine free radicals
- •CH3 + Cl2 → CH3Cl + Cl•
What expression can be used to describe a propagation reaction?
- Radical + stable molecule → Radical + another stable molecule
What happens during the termination stage?
- This occurs when any two free radicals collide
- The product formed is therefore a stable molecule
- These are 3 possible termination reactions from synthesis of chloromethane
- •CH3 + Cl• → CH3Cl
- Cl• + Cl• → Cl2
- •CH3 + •CH3 → CH3CH3
How can you deduce the overall equation?
- Combine the two propagation steps
- Eliminate any radicals or molecules that remain the same
- CH4 + Cl• → •CH3 + HCl
- •CH3 + Cl2 → CH3Cl + Cl•
- CH4 + Cl2 → CH3Cl + HCl
Describe any further substitutions that can occur
- If chlorine is in excess, the hydrogen atoms on methane will be replaced by chlorine atoms
- This will form dichloromethane, trichloromethane and tetrachloromethane
- CH4 + Cl2 → CH3Cl + HCl
- CH3Cl + Cl2 → CH2Cl2 + HCl
- CH2Cl2 + Cl2 → CHCl3 + HCl
- CHCl3 + Cl2 → CHCl4 + HCl
- If methane is in excess, the chlorine will be used up quickly and the product formed will mostly be chloromethane
- CH4 + Cl2 → CH3Cl + HCl