Free Radical Substitution Flashcards
Define Bond Fission:
Breaking a covalent bond.
List the two types of bond fission:
Heterolytic and Homolytic.
Explain Heterolytic Fusion:
- Bonds break UNEVENLY, with one of the bonded atoms receiving BOTH electrons from the bonded pair.
- This forms a Cation and an Anion.
X ÷ Y = X+ and Y-
What do curly arrows show?
Movement of electron pairs.
Explain Homolytic Fusion:
The bond breaks EVENLY and each bonding atom receives one electron from the bond pair. Two electrically uncharged radicals are formed.
X ÷ Y = X• + Y•
What is a radical?
Particles with an unpaired electron. Represented by a big dot next to the molecular formula (showing unpaired electron). e.g. X•
This unpaired electron makes radicals very reactive.
What type of reactions do Halogens react with Alkanes in?
Photochemical reactions. These require light (often UV) to start. e.g CH₄ + Cl₂ = CH₃Cl + HCl
What are the three steps to a free radical substitution reaction?
1) Initiation Reaction.
2) Propagation Reaction.
3) Termination Reaction.
Explain what happens during the Initiation Reaction:
-Free radicals are produced.
UV light provides enough energy to break the bond between the halogens. e.g. Cl₂ = 2Cl•
This is Homolytic Fusion as the bonds are split evenly, so each receive one electron.
As such, each atom becomes a highly reactive free radical (due to having an unpaired electron).
Explain what happens during the Propagation Reaction:
- Free radicals are used up and created in a chain reaction.
1) The free halogen radical attacks a hydrocarbon molecule e.g. Cl• + CH₄ = •CH₃ + HCl
2) This new methyl free-radical (•CH₃) attacks another halogen (Cl₂) molecule. e.g. •CH₃ + Cl₂ = CH₃Cl + Cl•
3) This new free halogen radical can attack another hydrocarbon molecule (loops back to step 1), until all the halogen or alkane molecules run out.
Explain what happens during the Termination Reaction:
- Free radicals are used up.
If two radicals join, they make a stable molecule.
There can be many termination reactions e.g. Cl• + •CH₃ = CH₃Cl or •CH₃ + • CH₃ = C₂H₆
What is a Free-Radical Substitution reaction?
Where a hydrogen atom is substituted by a chlorine or bromine atom.
What is a downside to a Free-Radical Substitution reaction?
It has a low atom economy since a big mix of products is made. These products can be hard to separate.
FRS can also occur at any point on the carbon chain, so a mix of isomers can be formed.
