Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
33 33.1 33.2 33.3 33.4 33.5 33.6 33.7 33.8 1 Redox Reactions Organic Synthesis Redox Reactions Oxidation of Alkylbenzenes Oxidation of Alcohols Redox Reactions of Aldehydes and Ketones Redox Reactions of Carboxylic Acids Redox Reactions of Alkenes Autooxidation of Fats and Oils New Way Chemistry for Hong Kong A-Level 3B 33.1 2 Organic Synthesis New Way Chemistry for Hong Kong A-Level 3B 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 3 Precursors New Way Chemistry for Hong Kong A-Level 3B 33.1 Organic Synthesis (SB p.51) Organic Synthesis • A synthesis usually involves more than one step Target molecule 1st Precursor 2nd Precursor Starting material 4 New Way Chemistry for Hong Kong A-Level 3B 33.1 Organic Synthesis (SB p.51) Organic Synthesis • Usually more than one way to carry out a synthesis 2nd Precursor a 1st Precursor A Target molecule 2nd Precursor b 2nd Precursor c 1st Precursor B 2nd Precursor d 1st Precursor C 2nd Precursor e 2nd Precursor f 5 New Way Chemistry for Hong Kong A-Level 3B 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 6 New Way Chemistry for Hong Kong A-Level 3B 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 % 60 % 60 % 60 % 60 % conversion conversion conversion conversion A B C D E 7 What is the yield of the desired product? New Way Chemistry for Hong Kong A-Level 3B 33.1 Organic Synthesis (SB p.52) Number of Steps Involved in the Synthesis 60 % 60 % 60 % 60 % conversion conversion conversion conversion A B C D E Yield of the desired product = 60 % 60 % 60 % 60 % = 12.96 % 8 New Way Chemistry for Hong Kong A-Level 3B 33.1 Organic Synthesis (SB p.52) Number of Steps Involved in the Synthesis 9 • 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 New Way Chemistry for Hong Kong A-Level 3B 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) 10 New Way Chemistry for Hong Kong A-Level 3B 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 Check Point 33-1 11 New Way Chemistry for Hong Kong A-Level 3B 33.2 12 Redox Reactions New Way Chemistry for Hong Kong A-Level 3B 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 13 New Way Chemistry for Hong Kong A-Level 3B 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 14 New Way Chemistry for Hong Kong A-Level 3B 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 15 New Way Chemistry for Hong Kong A-Level 3B 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 16 New Way Chemistry for Hong Kong A-Level 3B 33.2 Redox Reactions (SB p.53) Oxidation Common oxidizing agents used in organic reactions include: 17 • Acidified potassium manganate(VII) (KMnO4/H+) • Alkaline potassium manganate(VII) (KMnO4/OH–) • Acidified potassium dichromate(VI) (K2Cr2O7/H+) • Ozone (O3/CH3CCl3, Zn/H2O) New Way Chemistry for Hong Kong A-Level 3B 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 18 New Way Chemistry for Hong Kong A-Level 3B 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 19 New Way Chemistry for Hong Kong A-Level 3B 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 20 New Way Chemistry for Hong Kong A-Level 3B 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) Check Point 33-2 21 New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes 22 New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.55) Alkylbenzenes 23 • A group of aromatic hydrocarbons in which an alkyl group is bonded directly to a benzene ring • Sometimes called arenes New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.55) Alkylbenzenes • 24 Examples of alkylbenzenes: New Way Chemistry for Hong Kong A-Level 3B 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 • 25 In the oxidation process, a benzoate is formed New Way Chemistry for Hong Kong A-Level 3B 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 • 26 This method gives benzoic acid in almost quantitative yield New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.55) Oxidation of Alkylbenzenes 27 New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.55) Oxidation of Alkylbenzenes 28 New Way Chemistry for Hong Kong A-Level 3B 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 29 New Way Chemistry for Hong Kong A-Level 3B 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 30 New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.56) Oxidation of Alkylbenzenes 31 • 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 New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.56) Oxidation of Alkylbenzenes • 32 e.g. New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.56) Check Point 33-3 33 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols 34 New Way Chemistry for Hong Kong A-Level 3B 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 35 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.56) Alcohols • Examples of alcohols: 36 New Way Chemistry for Hong Kong A-Level 3B 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 37 New Way Chemistry for Hong Kong A-Level 3B 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 38 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.57) Alcohols Primary alcohols • Can be oxidized to aldehydes • Further oxidized to carboxylic acids 39 Secondary alcohols • Can be oxidized to ketones • Cannot be further oxidized to carboxylic acids Tertiary alcohols • Generally resistant to oxidation New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.57) Oxidation of Primary Alcohols 40 • Primary alcohols are firstly oxidized to aldehydes and subsequently to carboxylic acids • Using oxidizing agents like acidified KMnO4 and acidified K2Cr2O7 New Way Chemistry for Hong Kong A-Level 3B 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 41 New Way Chemistry for Hong Kong A-Level 3B 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 42 New Way Chemistry for Hong Kong A-Level 3B 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 43 New Way Chemistry for Hong Kong A-Level 3B 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 44 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.58) 2. Oxidation of Primary Alcohols to Carboxylic Acids • e.g. Ethanol can be oxidized to ethanoic acid by acidified KMnO4 Ethanol 45 Ethanoic acid New Way Chemistry for Hong Kong A-Level 3B 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 46 New Way Chemistry for Hong Kong A-Level 3B 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 47 New Way Chemistry for Hong Kong A-Level 3B 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 48 New Way Chemistry for Hong Kong A-Level 3B 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” 49 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.59) 2. Oxidation of Primary Alcohols to Carboxylic Acids Demonstration of the use of the breathalyser 50 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.59) Oxidation of Secondary Alcohols • Secondary alcohols can be oxidized to ketones by either acidified K2Cr2O7 or acidified KMnO4 51 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.59) Oxidation of Secondary Alcohols • The reaction usually stops at the ketone stage further oxidation requires the breaking of a carbon-carbon bond difficult to proceed 52 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.60) Oxidation of Secondary Alcohols • e.g. Propan-2-ol can be oxidized to form propanone 53 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.60) Oxidation of Tertiary Alcohols • Tertiary alcohols are generally resistant to oxidation unless they are subjected to severe oxidation conditions any oxidation would immediately involve the cleavage of the strong C C bonds in the alcohol molecule 54 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.60) Oxidation of Tertiary Alcohols • Tertiary alcohols can be oxidized by acidified KMnO4 give a mixture of ketones and carboxylic acids both with fewer carbon atoms than the original alcohol 55 New Way Chemistry for Hong Kong A-Level 3B 33.4 Oxidation of Alcohols (SB p.60) Oxidation of Tertiary Alcohols • e.g. heat 2-Methylbutan-2-ol Propanone Ethanoic acid Check Point 33-4 56 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones 57 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.62) Aldehydes and Ketones • 58 Carbonyl compounds that contain the carbonyl group New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.62) Oxidation of Carbonyl Compounds • Aldehydes are readily oxidized by acidified KMnO4 or K2Cr2O7 to form carboxylic acids 59 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.62) Oxidation of Carbonyl Compounds • Ketones do not undergo oxidations readily their oxidation involves the cleavage of the strong CC bond more severe conditions are required to bring about the oxidation 60 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.62) Oxidation of Carbonyl Compounds • With the action of hot acidified KMnO4, the CC bonds in ketones would be broken a mixture of carboxylic acids would be formed 61 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Compounds • Both aldehydes and ketones undergo reduction reactions readily forming 1o and 2o alcohols respectively • Reducing agents: lithium tetrahydridoaluminate (LiAlH4) sodium tetrahydridoborate (NaBH4) 62 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Compounds • LiAlH4 is a powerful reducing agent it reacts violently with water • Those reduction reactions using LiAlH4 must be carried out in anhydrous solutions usually in dry ether 63 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Compounds 64 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Compounds • The reduction of aldehydes and ketones to alcohols is most often carried out by NaBH4 • NaBH4 is a less powerful reducing agent it does not react with water the reduction reactions using NaBH4 can be carried out in water or alcohols 65 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) Reduction of Carbonyl Compounds Check Point 33-5 66 New Way Chemistry for Hong Kong A-Level 3B 33.6 Redox Reactions of Carboxylic Acids 67 New Way Chemistry for Hong Kong A-Level 3B 33.6 Redox Reactions of Carboxylic Acids (SB p.64) Carboxylic Acids 68 • A group of organic compounds containing the carboxyl group • Examples: New Way Chemistry for Hong Kong A-Level 3B 33.6 Redox Reactions of Carboxylic Acids (SB p.64) Reduction of Carboxylic Acids • Reductions of carboxylic acids are difficult to carry out • Can be achieved with the use of very powerful reducing agents (e.g. LiAlH4) • LiAlH4 reduces carboxylic acids to 1o alcohols in excellent yields Check Point 33-6 69 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes 70 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.65) Alkenes • Alkenes are unsaturated hydrocarbons containing C = C bonds • The C = C bonds are readily oxidized alkenes are able to undergo oxidation reactions 71 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.65) Alkenes • Alkenes can accept hydrogen to form alkanes alkenes are also able to undergo reduction reactions 72 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.65) Oxidation of Alkenes by Potassium Manganate(VII) • Alkenes react with alkaline KMnO4 form 1,2-diols called glycols 73 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.65) Oxidation of Alkenes by Potassium Manganate(VII) 74 • Ethene is oxidized to ethane-1,2-diol • The manganate(VII) ions are reduced to manganese(IV) oxide New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.65) Oxidation of Alkenes by Potassium Manganate(VII) • A change from the purple colour of manganate(VII) ions to the brown precipitate of manganese(IV) oxide a chemical test to distinguish between alkenes and alkanes 75 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.66) Oxidation of Alkenes by Ozone • Alkenes react rapidly and quantitatively with ozone form an unstable compound, known as ozonide • Ozonides are very unstable they are not usually isolated treated directly with a reducing agent (Zn/H3O+) 76 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.66) Oxidation of Alkenes by Ozone • The reduction produces carbonyl compounds can be safely isolated and identified 77 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.66) Oxidation of Alkenes by Ozone • The net result of this reaction is the breaking of the C = C bond to form two carbonyl groups • This process is called ozonolysis can be used to locate the position of the C = C bond in an alkene 78 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.66) Oxidation of Alkenes by Ozone • e.g. Ozonolysis of but-1-ene gives two different aldehydes 79 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.66) Oxidation of Alkenes by Ozone • e.g. Ozonolysis of but-2-ene gives one aldehyde Example 33-7 80 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.68) Reduction of Alkenes (Hydrogenation of Alkenes) • Alkenes react with hydrogen in the presence of metal catalysts (Ni, Pd and Pt) form alkanes 81 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.68) Reduction of Alkenes (Hydrogenation of Alkenes) • The atoms of the hydrogen molecule add to each carbon atom of the C = C bond of the alkene the alkene is converted to an alkane 82 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.68) Reduction of Alkenes (Hydrogenation of Alkenes) • Useful in analyzing unsaturated hydrocarbons (alkenes or alkynes) • By measuring the number of moles of hydrogen reacted with one mole of an unsaturated hydrocarbon the number of double or triple bonds 83 present in an unsaturated hydrocarbon molecule can be deduced Check Point 33-7 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils 84 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.69) Oxidation of Fats and Oils • Fats and oils are esters of propane-1,2,3triol and carboxylic acids of fairly long carbon chains • Some of these acids may contain one or more C = C bonds in them known as unsaturated carboxylic acids 85 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.69) Oxidation of Fats and Oils • When fats and oils are exposed to air, the C = C bonds will be oxidized the fats and oils will develop an “off ” odour and unpleasant flavour 86 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.70) Oxidation of Fats and Oils • Fats and oils having a high degree of unsaturation are more susceptible to oxidation • The oxidation follows a free radical mechanism accelerated by trace metals, light and free radical initiators 87 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.70) Oxidation of Fats and Oils • The hydroperoxides produced are flavourless and odourless decompose readily to form highly reactive hydroperoxide free radicals 88 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.70) Oxidation of Fats and Oils • These radicals set up a chain reaction produce volatile, flavoured compounds of aldehydes, ketones and carboxylic acids responsible for their rancid flavour • 89 This process is called autooxidation New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.70) Oxidation of Fats and Oils • Autooxidation can be controlled but cannot be stopped • The addition of antioxidants (e.g. BHA and BHT) can slow down the oxidative spoilage of fats and oils • Many vegetable oils contain natural antioxidants (e.g. vitamin C) can withstand autooxidation for a longer time 90 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.70) Principle of BHA/BHT as Antioxidants • BHA and BHT are common antioxidants used in food retard the development of oxidative rancidity in unsaturated fats and oils 91 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.70) Principle of BHA/BHT as Antioxidants • BHA and BHT work by: donating the hydrogen atom of the OH group to the free hydroperoxide radical (ROO •) involved in the autooxidation of fats and oils stopping the chain reactions in oxidative spoilage: AH + ROO • ROOH + A • 92 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.70) Principle of BHA/BHT as Antioxidants AH + ROO • ROOH + A • where AH represents the antioxidant, and A • is a radical derived from the antioxidant e.g. 93 New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.71) Principle of BHA/BHT as Antioxidants Foods containing BHA and BHT 94 Check Point 33-8 New Way Chemistry for Hong Kong A-Level 3B The END 95 New Way Chemistry for Hong Kong A-Level 3B 33.1 Organic Synthesis (SB p.52) Why are simple alcohols and simple aromatic compounds relatively cheap starting materials for organic syntheses? Answer They can be made from alkanes and benzene which can be obtained directly from fractional distillation of petroleum. Back 96 New Way Chemistry for Hong Kong A-Level 3B 33.1 Organic Synthesis (SB p.53) (a) What are the main reasons for carrying out an organic synthesis? Answer (a) To make new substances such as medicines, dyes, plastics or pesticides To make new organic compounds for studying reaction mechanisms or metabolic pathways 97 New Way Chemistry for Hong Kong A-Level 3B 33.1 Organic Synthesis (SB p.53) (b) What are the factors that determine the feasibility of an organic synthesis? Answer (b) Number of steps involved in the synthesis Availability of starting materials and reagents Duration of the synthetic process Back 98 New Way Chemistry for Hong Kong A-Level 3B 33.2 Redox Reactions (SB p.54) (a) State two common oxidizing agents used in organic reactions. Answer (a) Acidified potassium manganate(VII) (KMnO4/H+) Alkaline potassium manganate(VII) (KMnO4/OH–) Acidified potassium dichromate(VI) (K2Cr2O7/H+) Ozone (O3/CH3Cl3, Zn/H2O) (Any two) 99 New Way Chemistry for Hong Kong A-Level 3B 33.2 Redox Reactions (SB p.54) (b) State two common reducing agents used in organic reactions. Answer (b) Lithium tetrahydridoaluminate in dry ether (LiAlH4/ether, H3O+) Sodium tetrahydridoborate (NaBH4/H2O) Hydrogen with palladium (H2/Pd) (Any two) 100 New Way Chemistry for Hong Kong A-Level 3B 33.2 Redox Reactions (SB p.54) Back (c) State whether each of the following reactions is an oxidation or a reduction. (i) Conversion of ethanol to ethanal (ii) Conversion of ethene to ethanol (iii) Conversion of nitrobenzene to phenylamine (iv) Conversion of propene to propane (v) Conversion of propan-2-ol to propanone (c) (i) Oxidation (iii) Reduction (v) Oxidation 101 (ii) Oxidation (iv) Reduction New Way Chemistry for Hong Kong A-Level 3B Answer 33.3 Oxidation of Alkylbenzenes (SB p.56) Why is tert-butylbenzene resistant to side-chain oxidation? Answer tert-Butylbenzene does not have a benzylic hydrogen atom. Back 102 New Way Chemistry for Hong Kong A-Level 3B 33.3 Oxidation of Alkylbenzenes (SB p.56) State the conditions under which ethylbenzene can be converted to benzoic acid in the laboratory. Reagents: 1. potassium manganate(VII), sodium hydroxide 2. dilute sulphuric acid Condition: heating under reflux Back 103 New Way Chemistry for Hong Kong A-Level 3B Answer 33.4 Oxidation of Alcohols (SB p.60) Back Draw the structural formulae for the major organic products in the following reactions: K2Cr2O7/H+ (a) Propan-1-ol reflux K2Cr2O7/H+ (b) Propan-2-ol reflux (a) 104 (b) New Way Chemistry for Hong Kong A-Level 3B Answer 33.5 Redox Reactions of Aldehydes and Ketones (SB p.63) What is the species responsible for the reducing property of LiAlH4 and NaBH4? Answer Hydride ion, H– Back 105 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.64) Give the structural formulae for the major organic products of the following reactions: (a) (b) (c) (d) Answer 106 New Way Chemistry for Hong Kong A-Level 3B 33.5 Redox Reactions of Aldehydes and Ketones (SB p.64) (a) (b) (c) CH3CH2OH (d) Back 107 New Way Chemistry for Hong Kong A-Level 3B 33.6 Redox Reactions of Carboxylic Acids (SB p.65) Give the structural formulae for the major organic products, if any, in the following reactions: (a) (b) (a) (b) (c) No reaction (c) 108 New Way Chemistry for Hong Kong A-Level 3B Answer 33.6 Redox Reactions of Carboxylic Acids (SB p.65) Back Give the structural formulae for the major organic products, if any, in the following reactions: (d) (d) (e) (e) (f) (f) 109 New Way Chemistry for Hong Kong A-Level 3B Answer 33.7 Redox Reactions of Alkenes (SB p.67) Predict the structures of the following hydrocarbons A, B and C using the information given below: 110 Hydrocarbon Molecular formula A C3H6 B C6H10 C C10H16 Products after ozonolysis New Way Chemistry for Hong Kong A-Level 3B Answer 33.7 Redox Reactions of Alkenes (SB p.67) A: As C3H6 can be expressed as CnH2n, the hydrocarbon is a compound with one C = C double bond. When A undergoes ozonolysis, ∴ 111 and are formed. The possible structure of A is CH3CH = CH2. New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.67) B: As C6H10 can be expressed as CnH2n–2 and only one dicarbonyl compound is formed on ozonolysis, the hydrocarbon is an alicyclic compound with one C = C double bond. ∴ The possible structure of B is 112 . New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.67) C: C10H16 can be expressed as CnH2n–4. Two products with totally five carbon atoms are formed on ozonolysis. So the original hydrocarbon is an acyclic compound with three C = C double bonds. ozonolysis ∴ The possible structure of C is CH3CH = CHCH2CH = CHCH2CH = CHCH3. Back 113 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.68) Give the structural formulae for the major organic products, if any, in the following reactions: (a) (b) (c) Answer 114 New Way Chemistry for Hong Kong A-Level 3B 33.7 Redox Reactions of Alkenes (SB p.68) (a) (b) (c) 115 Back New Way Chemistry for Hong Kong A-Level 3B 33.8 Autooxidation of Fats and Oils (SB p.71) (a) What causes fats and oils to go rancid? (b) Explain how BHA and BHT can slow down the oxidative spoilage of fats and oils. Answer (a) Carbon-carbon double bonds in fats and oils as well as oxygen in air (b) BHA and BHT donate the hydrogen atoms of their hydroxyl group to the free hydroperoxide radical involved in the autooxidation of fats and oils. Back 116 New Way Chemistry for Hong Kong A-Level 3B