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AS Organic CHEM > Formation of halogenoalkanes > Flashcards

Formation of halogenoalkanes Flashcards

1
Q

What type of mechanism forms halogenoalkanes?

A
  • Free radical substitution.
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2
Q

What is a “free radical?”

A
  • A free radical is a particle with an unpaired electron.
  • These particles are very reactive.
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3
Q

How are free radicals formed?

A
  • Forms when a covalent bond is split equally, leaving two particles - each having an unpaired electron.
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4
Q

What is required to break a covalent bond, forming two free radicals?

A
  • UV light.
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5
Q

What are the 3 stages of forming halogenoalkanes?

A

1.) Initiation
2.) Propagation
3.) Termination.

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6
Q

What happens in the stage of initiation?

A
  • 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.
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7
Q

What is the process called when sunlight/ UV light breaks a covalent bond to form 2 free radicals?

A
  • Photodissociation.
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8
Q

Why is free radical substitution also known as “free radical chain reactions?”

A
  • 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.
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9
Q

What is needed in order to form di/ tri/ tetr haloalkanes in free radical chain reactions?

A
  • Need an excess of halogen, if not you will only get one substitution rather than multiple substitutions.
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10
Q

What happens in propogation stage?

A
  • 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.
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11
Q

What is the termination stage? Give examples of termination equations for formation of chloromethane from methane?

A
  • 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.)
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12
Q

How should we write all molecules in propoagation/ termination equations?

A
  • All needs to be STRUCTURAL FORMULA (so that we can see what atom the free radical is exactly!!)

() - extra info for clarification.

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13
Q

What does termination do to the reaction?

A
  • Termination ends the free radical chain reaction (because all the free radicals react together in this stage.)

() - extra info for clarification.

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14
Q

Write the OVERALL equation for reaction of methane with chlorine to form chloromethane.

A
  • CH₄ + Cl₂ –> CH₃Cl + HCl (with UV written above the arrow!)
  • (not that there are no free radicals involved in the overall equation.)
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15
Q

Write the propogation steps for formation of chloromethane from methane.

A

1.) Clο + CH₄ –> CH₃ (with dot on top of C) + HCl
2.) CH₃ (with dot on top of C) + Cl₂ –> CH₃Cl + Clο

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16
Q

What is the STRUCTURE of every propogation equation?

A
  • 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!!
17
Q

How do you write the overall equation for the formation of halogenoalkane from an alkane?

A
  • Alkane + halogen –> halogenoalkane + hydrogen halide.

(no free radicals involved in overall equation!)

18
Q

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.

A
  • 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!
19
Q

How is the unpaired electron of a free radical represented?

A
  • Represented as a dot.

AS Organic CHEM (6 decks)

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