OZ3: Radical Chain Reactions
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What is bond fission?
The breaking of bonds
What is heterolytic fission?
• Both of the shared electrons in the covalent bond go to just one of the atoms when the bond breaks • So one atom becomes a negatively charged anion (more electrons than protons) and the other becomes a positively charged cation (more protons than electrons)
Where is heterolytic fission common?
In polar bonds with a large difference in electronegativity
What is homolytic fission?
• One of the two shared electrons goes to each atom in the covalent bond • Two electrically uncharged “radicals” are formed
What is a radical?
A particle with an unpaired electron
Why do radicals have no overall charge?
They have the electronic structure they had before they shared their electrons
Why are radicals very reactive?
• Due to their unpaired electron - Gives them a strong tendency to pair up again with another electron from another substance
What does the amount of energy needed to photodissociate depend on?
The bond enthalpy of the bond
What is initiation?
• The first step of the radical chain reaction - Free radicals are produced
What is propagation?
• A sequence of reactions • Free radicals react with molecules to form new radicals • These new radicals go on to react with more molecules, producing new radicals • Keeps the reaction going
What is termination?
• The final step of the radical chain reaction - Two radicals react together to form a stable molecule
Is termination exothermic or endothermic and why?
Exothermic – energy is released as bonds are formed
Give the overall formula for the chlorine and hydrogen radical chain reaction.
H2 + Cl2 ==UV==> 2HCl
Give the initiation reaction equation for the chlorine and hydrogen radical chain reaction.
Cl2 + hv ===> Cl. + Cl.
How does the Cl2 molecule undergo photodissociation?
Sunlight provides enough energy to break the Cl-Cl bond
What type of fission is photodissociation?
Homolytic fission
Give the propagation reaction equations for the chlorine and hydrogen radical chain reaction.
• Cl. + H2 ====> HCl + H. - H. + Cl2 ====> HCl + Cl.
Give the termination reaction equations for the chlorine and hydrogen radical chain reaction.
• H. + H. ====> H2 • Cl. + Cl. ====> Cl2 • H. + Cl. ====> HCl
Give the overall formula for the methane and chlorine radical chain reaction.
CH4 + Cl2 ==UV==> CH3Cl + HCl
Give the initiation reaction equation for the methane and chlorine radical chain reaction.
Cl2 + hv ====> Cl. + Cl.
Give the propagation reaction equations for the methane and chlorine radical chain reaction.
• CH4 + Cl. ====> CH3. (methyl radical) + HCl - CH3. + Cl2 ===> CH3Cl + Cl.
Give the termination reaction equations for the methane and chlorine radical chain reaction.
• Cl. + Cl. ====> Cl2 • CH3. + Cl. ====> CH3Cl • CH3. + CH3. ====> C2H6
Give the products of the methane and chlorine radical chain reaction.
• Hydrogen chloride • Ethane • Chloromethane
Give 2 possible further side products of the methane and chlorine radical chain reaction.
• Dichloromethane - Trichloromethane