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Redox Reactions

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  1. 33 Redox Reactions 33.1 Organic Synthesis 33.2 Redox Reactions 33.3 Oxidation of Alkylbenzenes 33.4 Oxidation of Alcohols 33.5 Redox Reactions of Aldehydes and Ketones 33.6 Redox Reactions of Carboxylic Acids 33.7 Redox Reactions of Alkenes 33.8 Autooxidation of Fats and Oils

  2. 33.1 Organic Synthesis

  3. Precursors 33.1 Organic Synthesis (SB p.51) Organic Synthesis • In planning syntheses, •  we need to think backwards •  think backwards from the desired product to simpler molecules (precursors) Target molecule

  4. 1st Precursor 2nd Precursor Starting material 33.1 Organic Synthesis (SB p.51) Organic Synthesis • A synthesis usually involves more than one step Target molecule

  5. 2nd Precursor a 1st Precursor A 2nd Precursor b 2nd Precursor c 1st Precursor B 2nd Precursor d 2nd Precursor e 1st Precursor C 2nd Precursor f 33.1 Organic Synthesis (SB p.51) Organic Synthesis • Usually more than one way to carry out a synthesis Target molecule

  6. 33.1 Organic Synthesis (SB p.52) Number of Steps Involved in the Synthesis • Most organic reactions are • reversible reactions •  seldom proceed to completion • impossible to have a 100% yield of the product from each step of the synthetic route

  7. 60 % conversion 60 % conversion 60 % conversion 60 % conversion A  B  C  D  E 33.1 Organic Synthesis (SB p.52) Number of Steps Involved in the Synthesis • Consider the following synthetic route: •  each step has a yield of 60 % What is the yield of the desired product?

  8. 60 % conversion 60 % conversion 60 % conversion 60 % conversion A  B  C  D  E 33.1 Organic Synthesis (SB p.52) Number of Steps Involved in the Synthesis Yield of the desired product = 60 %  60 %  60 %  60 % = 12.96 %

  9. 33.1 Organic Synthesis (SB p.52) Number of Steps Involved in the Synthesis • An efficient route of synthesis should consist of a minimal number of steps • Limit the total number of reaction steps in a synthesis to not more than four

  10. Let's Think 1 33.1 Organic Synthesis (SB p.52) Availability of Starting Materials and Reagents • Only a restricted number of simple, relatively cheap starting materials is available • Include: •  simple haloalkanes and alcohols of not more than four carbon atoms •  simple aromatic compounds (e.g. benzene and methylbenzene)

  11. Check Point 33-1 33.1 Organic Synthesis (SB p.52) Duration of the Synthetic Process • Many organic reactions proceed at a relatively low rate • e.g. the acid-catalyzed esterification requires refluxing the reaction mixture of alcohols and carboxylic acids for a whole day • Inclusion of these slow reactions in a synthetic route is impractical

  12. 33.2 Redox Reactions

  13. 33.2 Redox Reactions (SB p.53) Redox Reactions • Redox reactions are reactions that involve a change of oxygen or hydrogen content in organic compounds

  14. 33.2 Redox Reactions (SB p.53) Oxidation • Oxidation of an organic compound usually corresponds to: •  an increase in oxygen content •  a decrease in hydrogen content

  15. 33.2 Redox Reactions (SB p.53) Oxidation • e.g. • The change of ethanol to ethanoic acid is an oxidation •  the oxygen content of ethanoic acid is higher than that of ethanol

  16. 33.2 Redox Reactions (SB p.53) Oxidation • e.g. • Converting ethanol to ethanal is also an oxidation process •  the hydrogen content of ethanal is lower than that of ethanol

  17. 33.2 Redox Reactions (SB p.53) Oxidation • Common oxidizing agents used in organic reactions include: • Acidified potassium manganate(VII) (KMnO4/H+) • Alkaline potassium manganate(VII) (KMnO4/OH–) • Acidified potassium dichromate(VI) (K2Cr2O7/H+) • Ozone (O3/CH3CCl3, Zn/H2O)

  18. 33.2 Redox Reactions (SB p.54) Reduction • Reduction of an organic compound usually corresponds to: •  an increase in hydrogen content •  a decrease in oxygen content

  19. 33.2 Redox Reactions (SB p.54) Reduction • e.g. • Converting ethanoic acid to ethanal is a reduction •  the oxygen content of ethanal is lower than that of ethanoic acid

  20. 33.2 Redox Reactions (SB p.54) Reduction • e.g. • Converting ethanal to ethanol is also a reduction process •  the hydrogen content of ethanol is higher than that of ethanal

  21. Check Point 33-2 33.2 Redox Reactions (SB p.54) Reduction • Common reducing agents used in organic reactions include: • Lithium tetrahydridoaluminate in dry ether (LiAlH4/ether, H3O+) • Sodium tetrahydridoborate (NaBH4/H2O) • Hydrogen with palladium (H2/Pd)

  22. 33.3 Oxidation of Alkylbenzenes

  23. 33.3 Oxidation of Alkylbenzenes (SB p.55) Alkylbenzenes • A group of aromatic hydrocarbons in which an alkyl group is bonded directly to a benzene ring • Sometimes called arenes

  24. 33.3 Oxidation of Alkylbenzenes (SB p.55) Alkylbenzenes • Examples of alkylbenzenes:

  25. 33.3 Oxidation of Alkylbenzenes (SB p.55) Oxidation of Alkylbenzenes • Oxidation of alkylbenzenes •  carried out by the action of hot alkaline potassium manganate(VII) solution • In the oxidation process, a benzoate is formed

  26. 33.3 Oxidation of Alkylbenzenes (SB p.55) Oxidation of Alkylbenzenes • Benzoic acid can be recovered •  by adding a mineral acid such as dilute H2SO4 to the benzoate • This method gives benzoic acid in almost quantitative yield

  27. 33.3 Oxidation of Alkylbenzenes (SB p.55) Oxidation of Alkylbenzenes

  28. 33.3 Oxidation of Alkylbenzenes (SB p.55) Oxidation of Alkylbenzenes

  29. 33.3 Oxidation of Alkylbenzenes (SB p.56) Oxidation of Alkylbenzenes • All alkylbenzenes are oxidized to benzoic acid • except the alkylbenzenes with a tertiary alkyl group •  they do not have a benzylic hydrogen atom

  30. 33.3 Oxidation of Alkylbenzenes (SB p.56) Oxidation of Alkylbenzenes • In the above oxidation processes, •  the alkyl groups of alkylbenzenes are oxidized, rather than the benzene ring • In the first step, the oxidizing agent abstracts a benzylic hydrogen atom • The oxidizing agent oxidizes the side chain to a carboxyl group

  31. 33.3 Oxidation of Alkylbenzenes (SB p.56) Oxidation of Alkylbenzenes • Side-chain oxidation by KMnO4 is not restricted to alkyl groups • C = C bonds and C = O groups in the side chain are also oxidized by hot alkaline KMnO4

  32. 33.3 Oxidation of Alkylbenzenes (SB p.56) Oxidation of Alkylbenzenes • e.g.

  33. 33.3 Oxidation of Alkylbenzenes (SB p.56) Let's Think 2 Check Point 33-3

  34. 33.4 Oxidation of Alcohols

  35. 33.4 Oxidation of Alcohols (SB p.56) Alcohols • A group of compounds with one or more hydroxyl groups (OH) attached to an alkyl group • For alcohols having only one hydroxyl group, •  their general formula is CnH2n+1OH

  36. 33.4 Oxidation of Alcohols (SB p.56) Alcohols • Examples of alcohols:

  37. 33.4 Oxidation of Alcohols (SB p.57) Alcohols • Depending on the number of alkyl groups attached to the carbon to which the hydroxyl group is linked, •  alcohols can be classified as primary, secondary and tertiary alcohols

  38. 33.4 Oxidation of Alcohols (SB p.57) Alcohols • Differentiating an alcohol as a 1o alcohol, a 2o alcohol or a 3o alcohol is extremely important •  when oxidized, these alcohols give different products

  39. 33.4 Oxidation of Alcohols (SB p.57) Alcohols

  40. 33.4 Oxidation of Alcohols (SB p.57) Oxidation of Primary Alcohols • Primary alcohols are firstly oxidized to aldehydes and subsequently to carboxylic acids • Using oxidizing agents like acidified KMnO4 and acidified K2Cr2O7

  41. 33.4 Oxidation of Alcohols (SB p.57) 1. Oxidation of Primary Alcohols to Aldehydes • The oxidation of alcohols is difficult to stop at the aldehyde stage •  aldehydes are a reducing agent • One way of solving this problem •  remove the aldehyde as soon as it is formed •  by distilling off the aldehydes from the reaction mixture

  42. 33.4 Oxidation of Alcohols (SB p.57) 1. Oxidation of Primary Alcohols to Aldehydes • e.g. • Ethanal can be synthesized from ethanol using acidified K2Cr2O7 •  ethanal is removed by distillation

  43. 33.4 Oxidation of Alcohols (SB p.58) 1. Oxidation of Primary Alcohols to Aldehydes A typical laboratory set-up for the oxidation of ethanol to ethanal

  44. 33.4 Oxidation of Alcohols (SB p.58) 2. Oxidation of Primary Alcohols to Carboxylic Acids • Primary alcohols can be oxidized to carboxylic acids by acidified KMnO4 • Acidified KMnO4 is a powerful oxidizing agent •  the oxidation of the alcohols does not stop at the aldehydes •  but directly to the carboxylic acids

  45. 33.4 Oxidation of Alcohols (SB p.58) Ethanol Ethanoic acid 2. Oxidation of Primary Alcohols to Carboxylic Acids • e.g. • Ethanol can be oxidized to ethanoic acid by acidified KMnO4

  46. 33.4 Oxidation of Alcohols (SB p.59) 2. Oxidation of Primary Alcohols to Carboxylic Acids A reflux apparatus used for the oxidation of ethanol to ethanoic acid

  47. 33.4 Oxidation of Alcohols (SB p.59) 2. Oxidation of Primary Alcohols to Carboxylic Acids A distillation apparatus used for the separation of ethanoic acid from the reaction mixture

  48. 33.4 Oxidation of Alcohols (SB p.59) 2. Oxidation of Primary Alcohols to Carboxylic Acids • The oxidation of ethanol by acidified K2Cr2O7 •  the basis of the breathalyser used by the police •  to rapidly estimate the ethanol content of the breath of suspected drunken drivers

  49. 33.4 Oxidation of Alcohols (SB p.59) 2. Oxidation of Primary Alcohols to Carboxylic Acids • When the drunken driver blows into the bag •  the ethanol molecules reduce the orange Cr2O72- ions to green Cr3+ ions • If more than a certain amount of the orange crystal changes colour, •  the driver is likely to be “over the limit”

  50. 33.4 Oxidation of Alcohols (SB p.59) 2. Oxidation of Primary Alcohols to Carboxylic Acids Demonstration of the use of the breathalyser