Alkanes: Chlorination of Alkanes Flashcards

1
Q

What are halogenoalkanes?

A
  • 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
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2
Q

What are chloroalkanes?

A
  • Chloroalkanes are alkanes with one or more hydrogen atoms substituted by a chlorine atom
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3
Q

How are halogenoalkanes formed from alkanes?

A

• Photochemical reaction
- Reactions started by UV light

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

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4
Q

What other products are formed with a halogenoalkane?

A
  • The products of this reaction are always a halogenoalkane and a hydrogen halide
  • You DO NOT produce hydrogen
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5
Q

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

A
  • Initiation
  • Propagation
  • Termination
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6
Q

What happens during the initiation stage?

A
  • In initiation step, free radicals are produced
  • UV light provides enough energy to break some of the Cl-Cl bonds
  • Cl2 → 2Cl•
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7
Q

What happens during the first propagation stage?

A
  • 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
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8
Q

What happens during the second propagation stage?

A
  • The new methyl free radical, •CH3, collides with another Cl2 molecule
  • This forms chloromethane and more chlorine free radicals
  • •CH3 + Cl2 → CH3Cl + Cl•
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9
Q

What expression can be used to describe a propagation reaction?

A
  • Radical + stable molecule → Radical + another stable molecule
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10
Q

What happens during the termination stage?

A
  • 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
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11
Q

How can you deduce the overall equation?

A
  • 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
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12
Q

Describe any further substitutions that can occur

A
  • 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
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