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Free Radical Substitution

Free Radical Substitution. Homolytic Fission. Substitution Rxn (free radical substitution). Is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms. Mechanism of reaction.

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Free Radical Substitution

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  1. Free Radical Substitution Homolytic Fission

  2. Substitution Rxn(free radical substitution) • Is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms

  3. Mechanism of reaction • Is the detailed step by step description of how the overall reaction occurs

  4. H H Cl Cl Cl Cl Methane Chloromethane = + + Hydrogen Chloride Chlorine

  5. Simple mechanism H Cl Cl Chloromethane Methane Hydrogen and Chlorine have swapped places Substitution Hydrogen Chloride Chlorine

  6. Stage 1 Initiation Getting Started

  7. Ultra violet light breaks the bond Chlorine molecule Cl2 2 Chlorine radicals each with an unpaired electron Both species are the same Called Homolytic Fission

  8. Stage 2 Propagation Keeping it going

  9. H Cl Lets put in the 2 electrons in this bond Chlorine radical Methane The chlorine radical pulls the hydrogen and one electron across to it. Hydrogen chloride Methyl radical The methyl radical is now free to react with a chlorine molecule

  10. Cl Cl Chlorine radical Methyl radical Chlorine Chloromethane Chlorine radical can now go and react with a methane molecule

  11. Stage 3 Termination Grinding to a halt

  12. Three different ways this can happen

  13. Cl Cl Chlorine molecule Reaction stops No free radicals to keep it going Chlorine radical Chlorine radical

  14. Cl Chlorine radical Methyl radical Chloromethane forms Reaction stops Because there are no free radicals to keep it going

  15. Methyl radical Methyl radical Ethane Reaction stops because no free radicals produced to keep it going The formation of ethane proves that this is the mechanism Reaction speeded up by sources of free radicals such as tetramethyl lead.

  16. Proof of mechanism Small amount of ethane detected Not initiated (start) in dark needs UV light Tetra methyl lead decomposes to form methyl free radicals, if Tetra methyl lead added it increases rate of reaction Pb (CH3)4 = Pb + 4 CH3º THERFORE Methyl radicals are used in reaction Halogenated alkanes are used as flame retardants

  17. Tetramethyl lead is added to speed up the rxn • It supplies the solution with methl free radicals. • Evidence for free radicals comes from small amounts of ethane being found in the solution • Halogenation of alknaes makes them more flame resistant

  18. Addition Reaction (pg. 367) • When two substances react together to form a single substance

  19. Addition Reaction Mechanism and evidence for Heterolytic Fission

  20. Step 1 Polarising of the bond in bromine

  21. Concentration of negative charge Because 4 electrons in this area δ+ δ- Moves in this direction At this point the negative charge of the double bond in ethene forces the electrons to the right Br and it becomes δ− and other one becomes δ+ Ethene Bromine Br2

  22. Step 2 Heterolytic Fission Occurs

  23. The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+ δ+ δ- The Br2 has been split into 2 different species i.e. Br+ and Br- this is called Heterolytic Fission At this point the negative charge of the double bond in ethene forces the electrons to the right Br and it becomes δ− and other one becomes δ+ Br+ Br- Br-

  24. Step 3 Formation of the Carbonium Ion

  25. The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+ Br Br+ Let us put in the two electrons of this bond The Br2 has been split into 2 different species i.e. Br+ and Br- this is called Heterolytic Fission Br- • At this point a lot of things happen at the same time • The two electrons are pulled to the Br+ • A bond is formed • one of the bonds between the two carbons disappears • The lower carbon becomes +ve because it has lost an electron • The two hydrogens on the upper carbon move to make way for the Br Br- + Carbonium ion Cyclic bromium ion

  26. Step 4 Attack on carbonium ion by Br-

  27. Br Br- Br- Br Br- Br- The negative bromide ion is attracted by the positive carbonium ion The two electrons of the bromide ion are used to form the bond The two hydrogen atoms move round to allow the Br in The negative and positive cancel each other out + Carbonium ion 1,2 dibromoethane Called Ionic Addition because the species are ions when they add on

  28. Step 5 Proof of mechanism

  29. Br Br- Br- Br- Br- Cl Br- Cl- Br- Br- + Proof of the mechanism is that if there are Cl-in the environment then some 1-bromo, 2-chloroethane will be formed. This can be identified by its different Relative Molecular Mass

  30. Hydrogenation • Adding of hydrogen's into a molecule (addition) • Occurs in manufacture of margarine • Add hydrogen into double bonds causes oils to become solid • Unsaturated fats are better for you that saturated fats

  31. Evidence for the carbonium ion • When bromine and chlorine ions present • Ethene forms 1-bromo-2-chloroethane as well as 1, 2 dibromoethane

  32. Polymerisation rxns • Molecules that contain double bonds undergo addition to become less unsaturated (addition polymers such as polythene and polypropene)

  33. Polymerisation Reactions • Example of an addition reaction • Ethene molecules add together • Polymers are long chain molecules made by joining together many small molecules + =

  34. Polymers • Commonly reffered to as plastics • Polyethene used for plastic bags, bowls, lunch boxes, bottles etc • Polypropene is used in toys, jugs, chairs etc • Crude oil is raw material for their manufacture

  35. Elimination reactions

  36. Elimination reactions • When a small molecule is removed from a larger molecule to leave a double bond in the larger molecule

  37. Elimination rxns • a compound breaks down into 2 or more simpler substances • Double bond created • only one reactant AB  A + B

  38. Elimination reactions • Ethene is made from ethanol from removing water using AlO as catalyst • Elimination reaction is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecules • Dehydration reaction • Only need to know dehydration of alcohol

  39. Elimination reaction • Dehydration of an alcohol is an example of an elimination reaction • In this reaction, a larger alcohol molecule reacts to form a smaller alkene molecule and an even smaller water molecule • The change in structure is from tetrahedral to planar

  40. Dehydration of ethanol • Ethanol is dehydrated to ethene • This reaction is used in the preparation of ethene

  41. Dehydration of ethanol to ethene

  42. Reaction conditions • Heat • Aluminium oxide catalyst

  43. Preparation of ethene

  44. Elimination rxn • Is when a small molecule is removed from a larger molecule to leave a double bond in the larger molecule • Alcohol =water + alkene • Dehydration reaction since water is removed • Ethanol=ethene + water • 2 methanol +sulphuric acid =methoxymethane ether +water

  45. C. Decomposition 2 H2O(l)  2 H2(g) + O2(g)

  46. Redox reactions

  47. Redox reactions • These reactions involves oxidation and reduction reactions • The removal or addition of lectrons from the molecule

  48. Reduction Oxidation

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