Alkanes: Chlorination of Alkanes

Question 1

What are halogenoalkanes?

Answer 1

• 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

Question 2

What are chloroalkanes?

Answer 2

• Chloroalkanes are alkanes with one or more hydrogen atoms substituted by a chlorine atom

Question 3

How are halogenoalkanes formed from alkanes?

Answer 3

• Photochemical reaction
- Reactions started by UV light

• Free radical substitution reaction
- A hydrogen atom is substituted by a halogen free radical (e.g chlorine)

Question 4

What other products are formed with a halogenoalkane?

Answer 4

• The products of this reaction are always a halogenoalkane and a hydrogen halide
• You DO NOT produce hydrogen

Question 5

What are the stages of the reaction mechanism for the synthesis of chloromethane?

Answer 5

• Initiation
• Propagation
• Termination

Question 6

What happens during the initiation stage?

Answer 6

• In initiation step, free radicals are produced
• UV light provides enough energy to break some of the Cl-Cl bonds
• Cl2 → 2Cl•

Question 7

What happens during the first propagation stage?

Answer 7

• 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

Question 8

What happens during the second propagation stage?

Answer 8

• The new methyl free radical, •CH3, collides with another Cl2 molecule
• This forms chloromethane and more chlorine free radicals
• •CH3 + Cl2 → CH3Cl + Cl•

Question 9

What expression can be used to describe a propagation reaction?

Answer 9

• Radical + stable molecule → Radical + another stable molecule

Question 10

What happens during the termination stage?

Answer 10

• 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

Question 11

How can you deduce the overall equation?

Answer 11

• 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

Question 12

Describe any further substitutions that can occur

Answer 12

• 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