Chlorination of alkanes

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  • Created by: Hindleyc
  • Created on: 13-05-18 18:04
What mechanism is used in the chlorination of alkanes?
Free radical substitution
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How is the formation of a covalent bond shown? How is the breaking of a covalent bond shown?
Curly arrows from the lone pair or another covalent bond. Curly arrow starting from the bond
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What is the initiation step?
UV light absorbed by Cl-Cl bond providing energy to break bond. Homolytic fission occurs with one electron to each chlorine creating 2 free radicals
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Equation? What does the dot represent? What does the curly arrow represent?
Cl2=2Cl. Unpaired electron. Movement of electrons
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Whats Propagation 1?
Methyl free radical formation
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Steps?
Chlorine radical reacts with methane molecule removing a hydrogen to create a methyl radical and hydrogen chloride.
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Equation?
Cl.+CH4=HCl+.CH3
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What is propagation 2?
Formation of Haloalkane and Cl.
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Steps?
Methyl radical reacts with chlorine molecule creating another chlorine radical and chloromethane
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What remains the same during propagation?
Total number of free radicals
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Equation?
.CH3+Cl2=.Cl+CH3Cl
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What assumption is made here?
Cl radical will always bond to methane
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What can termination form?
Chloromethane, Chlorine, Ethane
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Eqn for Chloromethane
Cl.+.CH3=CH3Cl
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Eqn for Chlorine
Cl.+Cl.=Cl2
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Eqn for Ethane?
.CH3+.CH3=C2H6
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What are the main products?
Chloromethane and hydrogen chloride
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What other products are formed?
Ethane, Dichloromethane
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If an alkane is more than 2 carbons in length, what can happen?
Any Hydrogen may be substituted
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What does this make?
Mixture of different isomers/products that are difficult to separate
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What does this show?
Why chain reactions are not a good method of preparing halogenoalkanes
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What other impurities can be made?
Further substituted chloroalkanes
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How can you reduce this?
Decreasing Chlorine in the reaction mixture
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How/Where is the ozone layer formed?
Naturally is upper atmosphere
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Eqn? What is this reaction?
O+O3=2O2. reversible and in equilibrium
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What does the O3 produced by reversible reaction in ozone layer do?
Absorb harmful UV radiation that can cause skin cancer
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What happens if forward reaction increase?
Damages O3 ozone layer
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What affect this reaction?
Chloroflurocarbons
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How are the Cl atoms formed?
When UV radiation causes the C-Cl bond in the CFC's to break
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What provide evidence for legislation to ban use of CFC's as solvents and refrigerants?
Results of research in scientific community
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First 2 eqn?
Cl2=2Cl. O3+Cl.=ClO.+O2
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3rd eon?
ClO.+O3=2O2+Cl.
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What does the Chlorine free radical produced at end do and what do they do overall?
Go on to react further (chain reaction) Catalyse breakdown
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What happens even if termination occurs to form Cl2?
The excess UV light will make 2Cl.
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What do Cl atoms do and contribute to?
Catalyse decomposition of ozone and contribute to holes in ozone layer
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What does this lead to?
Chain reaction of ozone destruction as 1 Cl radical could destroy 1000s of ozone molecules
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What have chemists now done?
Develops alternative Chlorine free compounds- Hydroflurocarbons
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What are halogenoalkanes in comparison to alkanes?
Much more reactive
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What were they used in? (use restricted due to effect of CFC's on atmosphere)
Refrigerants as solvents and pharmaceuticals
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How is ozone O3 formed?
When a photon of UV light is absorbed by an Oxygen molecule causing it to split and form 2 Oxygen radicals
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Eqn
O2=(UV)2O. O.+O2=O3
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The overall reaction for the decomposition of ozone?
2O₃ → 3O₂
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Card 2

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How is the formation of a covalent bond shown? How is the breaking of a covalent bond shown?

Back

Curly arrows from the lone pair or another covalent bond. Curly arrow starting from the bond

Card 3

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What is the initiation step?

Back

Preview of the front of card 3

Card 4

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Equation? What does the dot represent? What does the curly arrow represent?

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Card 5

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Whats Propagation 1?

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