2 - Synthesis of halogenolkanes

Question 1

What is the mechanism called for the synthesis of halogenoalkanes?

Answer 1

Free radical substitution

Question 2

What are the 3 steps of free radical substitution?

Answer 2

1) Initiation
2) Propagation
3) Termination

Question 3

What happens in step 1 initiation?

Answer 3

• UV light supplies energy to break the Cl-Cl or Br-Br bond
• Bonds are broken via homolytic fission
• Produces 2 free radicals

Question 4

Write the equation showing initiation in chlorine

Answer 4

Cl₂ → 2Cl^.

Question 5

Write the equation showing initiation in bromine

Answer 5

Br₂ → 2Br^.

Question 6

What is the essential condition for alkanes to react with Cl / Br?

Answer 6

UV light

Question 7

Why is UV light needed to start free radical substitution?

Answer 7

It supplies energy to break the Cl-Cl or Br-Br bond

Question 8

What happens in step 2 propagation?

Answer 8

• A Cl / Br free radical will react with a methane molecule
• This removes a H atom from the methane to form a methyl free radical
• The methyl free radical reacts to with a Cl₂ / Br₂ molecule to produce chloro / bromomethane and a Cl / Br free radical
• This cycle repeats until termination occurs

Question 9

Write the 2 equations for step 2 propagation involving chlorine and methane

Answer 9

CH₄ + Cl^. → HCl + ^.CH₃

^.CH₃ + Cl₂ → CH₃Cl + Cl^.

Question 10

Write the 2 equations for step 2 propagation involving bromine and methane

Answer 10

CH₄ + Br^. → HBr + ^.CH₃

^.CH₃ + Br₂ → CH₃Br + Br^.

Question 11

What happens in step 3 termination?

Answer 11

• Two free radicals react together which terminates the chain reaction
• This forms an unreactive molecule

Question 12

Write 3 possible equations for step 3 termination involving chlorine and methane

Answer 12

Cl^. + Cl^. → Cl₂
^.CH₃ + ^.CH₃ → CH₃CH₃
^.CH₃ + Cl^. → CH₃Cl

Question 13

Write 3 possible equations for step 3 termination involving bromine and methane

Answer 13

Br^. + Br^. → Br₂
^.CH₃ + ^.CH₃ → CH₃CH₃
^.CH₃ + Br^. → CH₃Br