Chapter 44. Typical reactions of various functional groups Introducing organic reactions Typical reactions of alkanes

Transcription

1 Chapter 44 Typical reactions of various functional groups 44.1 Introducing organic reactions 44.2 Typical reactions of alkanes 44.3 Typical reactions of alkenes 44.4 Typical reactions of haloalkanes 44.5 Typical reactions of alcohols P. 1 / 112

2 44.6 Typical reactions of aldehydes and ketones 44.7 Typical reactions of carboxylic acids 44.8 Typical reactions of esters 44.9 Typical reactions of amides Key terms Progress check Summary Concept map P. 2 / 112

3 44.1 Introducing organic reactions Organic reactions: chemical reactions involving organic compounds Compounds of different homologous series possess different functional groups and participate in different types of reactions. P. 3 / 112

4 Basic reaction types: substitution reactions addition reactions redox reactions hydrolysis Figure 44.1 Production of many important organic substances depends on organic reactions Introducing organic reactions P. 4 / 112

5 44.2 Typical reactions of alkanes Alkanes: saturated hydrocarbons in which all atoms are linked by single covalent bonds. Alkanes have few reactions other than the combustion reaction. C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l) H c = 2220 kj mol 1 P. 5 / 112

6 Substitution with halogens When hexane and bromine (dissolved in an organic solvent) are mixed in the dark, no reaction occurs even after a long time. In sunlight, hexane reacts with bromine. Red-orange bromine solution is decolorized Typical reactions of alkanes P. 6 / 112

7 (a) sunlight the red-orange bromine solution is not decolorized in darkness hexane + bromine solution Figure 44.2 Hexane does not react with bromine (a) in the dark but reacts with it (b) in sunlight. (b) the redorange bromine solution is decolorized Some sort of energy (e.g. ultraviolet light or heat) must be supplied for the reaction to take place Typical reactions of alkanes P. 7 / 112

8 Energy breaks the Br Br bonds in bromine molecules to form very reactive bromine atoms, starting the reaction. In direct sunlight, when more energy is available, the reaction takes place very rapidly. Learning tip Hexane also reacts with bromine when the mixture is irradiated with ultraviolet light (UV) or when heated. Bromine atom replaces one of the hydrogen atoms in the alkane molecule Typical reactions of alkanes P. 8 / 112

9 As a result, bromoalkane and hydrogen bromide are produced. Steamy fumes of hydrogen bromide are observed and the red-orange bromine solution is decolorized. In general, alkanes react with halogens to form haloalkanes and hydrogen halides Typical reactions of alkanes P. 9 / 112

10 General equation of the reaction: R H + X 2 R X + HX alkane halogen UV light or heat haloalkane hydrogen halide where R is an alkyl group and X is a halogen (Cl or Br) Typical reactions of alkanes P. 10 / 112

11 Example: Reaction between methane and bromine in the presence of UV light UV light or heat methane bromomethane hydrogen bromide Learning tip Monosubstitution of methane with bromine involves three steps: initiation, propagation and termination. Refer to Book 2, Chapter 22, p Typical reactions of alkanes P. 11 / 112

12 CH 3 Br + Br 2 CH 2 Br 2 + Br 2 CHBr 3 + Br 2 CH 2 Br 2 + HBr UV light or heat dibromomethane CHBr 3 + HBr UV light or heat tribromomethane CBr 4 + HBr UV light or heat tetrabromomethane This is a chain reaction and a mixture of bromoalkanes is usually produced Typical reactions of alkanes P. 12 / 112

13 When methane is in excess, bromomethane is the major product. When bromine is in excess, tetrabromomethane is the major product. Learning tip The reactions of the alkane with chlorine and with bromine are known as chlorination and bromination respectively Typical reactions of alkanes P. 13 / 112

14 A similar but faster reaction takes place when chlorine is used. Reactivity of the halogens follows the order F 2 > Cl 2 > Br 2 > I 2. Key point A substitution reaction is a chemical change in which an atom (or a group of atoms) of a molecule is replaced by another atom (or a group of atoms). Class practice Typical reactions of alkanes P. 14 / 112

15 44.3 Typical reactions of alkenes All alkenes have similar chemical properties as they have the same functional group. Alkenes are unsaturated and much more reactive than alkanes. Addition reaction Addition reaction: chemical reaction in which two or more molecules react to give a single molecule. Typical reaction of unsaturated hydrocarbons. P. 15 / 112

16 General equation of the reaction: where X may or may not be the same as Y. Most addition reactions are exothermic. Addition reactions can be further classified depending on the type of reactant added to the alkene Typical reactions of alkenes P. 16 / 112

17 Reactant added Name of addition reaction H H Hydrogenation X X (where X is a halogen) Halogenation H X (where X is a halogen) Hydrohalogenation Table 44.1 Different types of addition reactions. Key point An addition reaction is a chemical change in which two or more molecules react to give a single molecule Typical reactions of alkenes P. 17 / 112

18 1. Hydrogenation (addition of hydrogen) Hydrogenation: two hydrogen atoms are added across a carbon-carbon double bond, converting an alkene to an alkane Finely divided platinum is often used as catalyst to speed up the reaction. General equation of the reaction: Pt alkene alkane 44.3 Typical reactions of alkenes P. 18 / 112

19 Example: Hydrogenation of propene Usually carried out under room conditions in the presence of finely divided platinum. Propane is produced. Pt propene propane 44.3 Typical reactions of alkenes P. 19 / 112

20 Hydrogenation has an important application in the food industry. Used to convert liquid animal oils and vegetable oils (unsaturated esters) into solid fats (containing fewer double bonds) for making margarine and peanut butter. During this process, trans fats are produced Typical reactions of alkenes P. 20 / 112

21 Figure 44.3 Hydrogenated vegetable oils have higher melting points than the original vegetable oils. They can be used to make margarine and peanut butter. By controlling the degree of hydrogenation, margarine can be made as soft or hard as required. Key point In hydrogenation, two hydrogen atoms are added across a carbon-carbon double bond, converting an alkene to an alkane Typical reactions of alkenes P. 21 / 112

22 2. Halogenation(addition of halogens) Halogenation: two halogen atoms are added across a carbon-carbon double bond, converting an alkene to a dihaloalkane Reaction takes place under room conditions readily, without the use of any catalyst. General equation: alkene (in organic solvent) where X is a halogen (Cl, Br or I). dihaloalkane 44.3 Typical reactions of alkenes P. 22 / 112

23 When hex-1-ene is mixed with bromine (dissolved in an organic solvent), the red-orange bromine solution is decolorized rapidly. bromine (dissolved in an organic solvent) hex-1-ene the redorange bromine solution is decolorized Figure 44.4 Hex-1-ene decolorizes bromine (dissolved in an organic solvent) rapidly under room conditions Typical reactions of alkenes P. 23 / 112

24 The reaction can be represented by the following equation: hex-1-ene (in organic solvent) 1,2-dibromohexane Alkenes undergo addition reactions readily with bromine and chlorine in non-aqueous solvents (e.g. organic solvents). However, the reaction of alkene with iodine is very slow Typical reactions of alkenes P. 24 / 112

25 Very often, we use bromine to test for the presence of the carbon-carbon double bond in a compound. Key point In halogenation, two halogen atoms are added across a carbon-carbon double bond, converting an alkene to a dihaloalkane. Think about Class practice Typical reactions of alkenes P. 25 / 112

26 3. Hydrohalogenation(addition of hydrogen halides) Hydrohalogenation: a hydrogen halide (HF, HCl, HBr or HI) molecule is added across a carboncarbon double bond, converting an alkene to a haloalkane. General equation of the reaction: alkene haloalkane where HX is a hydrogen halide (HF, HCl, HBr or HI) Typical reactions of alkenes P. 26 / 112

27 For example, ethene reacts with hydrogen bromide to give bromoethane: ethene bromoethane Key point In hydrohalogenation, a hydrogen halide molecule is added across a carbon-carbon double bond, converting an alkene to a haloalkane Typical reactions of alkenes P. 27 / 112

28 Markovnikov s rule Hydrohalogenation of some alkenes may give rise to two products. For example, propene reacts with hydrogen bromide to give 2-bromopropane and 1-bromopropane: or propene 2-bromopropane (major product) 1-bromopropane (minor product) 44.3 Typical reactions of alkenes P. 28 / 112

29 Markovnikov s rule: the hydrogen atom in HX is added to the carbon atom of the carbon-carbon double bond that already carries a larger number of hydrogen atoms The halogen atom is added to the carbon atom that carries fewer hydrogen atoms Typical reactions of alkenes P. 29 / 112

30 Example: Addition of HBr to propene Hydrogen atom is added to the terminal doublybonded carbon atom. Bromine atom is added to the second carbon atom in propene. 2-bromopropane is the major product Typical reactions of alkenes P. 30 / 112

31 Key point Markovnikov s rule states that when a molecule of HX is added to an alkene, the hydrogen atom is added to the carbon atom of the carbon-carbon double bond that already carries a larger number of hydrogen atoms. Example 44.1 Example 44.2 Class practice Typical reactions of alkenes P. 31 / 112

32 RCH 2 CH 3 alkane H 2, Pt addition polymer catalyst for polymerization cold, dilute MnO 4 (aq)/ OH (aq) RCH=CH 2 alkene X 2 (in organic solvent) HX dihaloalkane diol and RCH 2 CH 2 X (minor product) (major product) haloalkane Figure 44.5 A summary of different reactions of alkenes Typical reactions of alkenes P. 32 / 112

33 44.4 Typical reactions of haloalkanes Substitution with hydroxide ions Hydroxide ions in aqueous solution (e.g. sodium hydroxide solution, NaOH(aq)) react with a haloalkane. Halogen atoms are substituted by OH groups, converting the haloalkane to an alcohol. Heating is required to speed up the reaction. P. 33 / 112

34 General equation of the reaction: haloalkane heat alcohol where X is a halogen (e.g. F, Cl, Br or I). For example, substitution reaction between chloroethane and sodium hydroxide produces ethanol: chloroethane heat ethanol 44.4 Typical reactions of haloalkanes Class practice 44.4 P. 34 / 112

35 44.5 Typical reactions of alcohols Functional group of alcohols: hydroxyl group, OH Depending on the number of alkyl groups attached to the carbon atom bearing the OH group, alcohols can be divided into three classes: primary secondary tertiary P. 35 / 112

36 Primary (1 ) alcohols Alcohols with the structure RCH 2 OH One alkyl group (regardless of the number of carbon atoms in the alkyl group) attached to the carbon atom bearing the OH group Secondary (2 ) alcohols Alcohols with the structure Two alkyl groups attached to the carbon atom bearing the OH group 44.5 Typical reactions of alcohols P. 36 / 112

37 Tertiary (3 ) alcohols Alcohols with the structure Three alkyl groups attached to the carbon atom bearing the OH group. Learning tip R, R and R are alkyl groups Typical reactions of alcohols P. 37 / 112

38 butan-1-ol primary (1 ) alcohol butan-2-ol secondary (2 ) alcohol methylpropan-2-ol tertiary (3 ) alcohol Figure 44.6 Examples of primary, secondary and tertiary alcohols. Reactions of alcohols involve the breakage of the C O bond or O H bond. Primary, secondary and tertiary alcohols react differently in some reactions Typical reactions of alcohols P. 38 / 112 Think about

39 Substitution with halides In substitution reactions, the OH groups of alcohols are substituted by other atoms or groups of atoms. Alcohols are often used for the preparation of haloalkanes by substitution reactions with halides. Hydrogen halides (HX) and phosphorus trihalides (PX 3 ) are usually used as a source of halides Typical reactions of alcohols P. 39 / 112

40 1. Hydrogen halides When a hydrogen halide is mixed with an alcohol, substitution reaction occurs, converting the alcohol to a haloalkane. General equation for the reaction: alcohol hydrogen halide haloalkane water where X is a halogen (e.g. Cl, Br or I) Typical reactions of alcohols P. 40 / 112

41 When concentrated hydrochloric acid is mixed with propan-1-ol, an insoluble oily layer of 1-chloropropane is produced, which is above the aqueous layer. oily layer (1-chloropropane) aqueous layer Figure 44.7 The oily layer of 1-chloropropane and the aqueous layer of hydrochloric acid are immiscible Typical reactions of alcohols P. 41 / 112

42 The equation for the reaction is written as follows: propan-1-ol 1-chloropropane Propan-2-ol reacts with hydrogen bromide to give 2-bromopropane and water. propan-2-ol 2-bromopropane 44.5 Typical reactions of alcohols P. 42 / 112

43 2. Phosphorus trihalides Phosphorus trihalides can also convert an alcohol to a haloalkane. When liquid phosphorus trichloride is mixed with propan-1-ol, 1-chloropropane and phosphorous acid are produced. The equation is written as follows: 3CH 3 CH 2 CH 2 OH + PCl 3 3CH 3 CH 2 CH 2 Cl + H 3 PO 3 propan-1-ol phosphorus trichloride 1-chloropropane phosphorous acid 44.5 Typical reactions of alcohols P. 43 / 112

44 Propan-2-ol reacts with phosphorus tribromide to give 2-bromopropane and phosphorous acid. propan-2-ol phosphorus tribromide 2-bromopropane phosphorous acid Experiment 44.1 Experiment Typical reactions of alcohols P. 44 / 112

45 Dehydration When a water molecule is eliminated from an alcohol, the alcohol is converted to an alkene. This type of reaction is known as dehydration. Dehydration is a chemical change in which hydrogen and oxygen are eliminated in a ratio of 2 : 1 from a compound Typical reactions of alcohols P. 45 / 112

46 Dehydration of alcohols using concentrated sulphuric acid Reaction is carried out at an elevated temperature in the presence of a dehydrating agent (e.g. concentrated sulphuric acid). General equation for the reaction: conc. H 2 SO 4 heat alcohol alkene 44.5 Typical reactions of alcohols P. 46 / 112

47 When a primary alcohol is dehydrated, only one alkene is produced. For example, when propan-1-ol is treated with concentrated sulphuric acid at 180 C, propene is obtained. conc. H 2 SO 4 propan-1-ol heat propene 44.5 Typical reactions of alcohols P. 47 / 112

48 Dehydration of some secondary and tertiary alcohols may give rise to two products. Example: Dehydration of butan-2-ol Dehydration of butan-2-ol gives but-2-ene and but-1-ene. conc. H 2 SO 4 butan-2-ol heat but-2-ene (major product) but-1-ene (minor product) 44.5 Typical reactions of alcohols P. 48 / 112

49 Water molecule can be eliminated from the butan-2-ol molecule in two ways: conc. H 2 SO 4 heat butan-2-ol but-2-ene (major product) or, conc. H 2 SO 4 heat butan-2-ol but-1-ene (minor product) 44.5 Typical reactions of alcohols P. 49 / 112

50 But-2-ene is the major product. major product of dehydration of an alcohol is normally the alkene with the largest number of alkyl groups attached to the carbon-carbon double bond. But-2-ene has two alkyl groups (methyl) attached to the carbon-carbon double bond while but-1-ene has one (ethyl). Think about Experiment 44.2 Experiment Typical reactions of alcohols P. 50 / 112

51 Example: Dehydration of 2-methylbutan-2-ol Dehydration of 2-methylbutan-2-ol (a 3 alcohol) gives 2-methylbut-2-ene as the major product. conc. H 2 SO 4 heat 2-methylbutan-2-ol 2-methylbut-2-ene (major product) 2-methylbut-1-ene (minor product) 44.5 Typical reactions of alcohols P. 51 / 112

52 Dehydration of alcohols using aluminium oxide Another way of preparing an alkene from an alcohol is by catalytic dehydration of alcohol. For example, propan-1-ol can be dehydrated by passing its vapour over a heated catalyst of aluminium oxide or pumice stones at a temperature of 350 C. Propene can be collected over water Typical reactions of alcohols P. 52 / 112

53 aluminium oxide ceramic wool soaked with propan-1-ol aluminium oxide ceramic wool soaked with propan-1-ol heat propene water propene Figure 44.8 Catalytic dehydration of propan-1-ol. water propan-1-ol Al 2 O 3 heat propene Example Typical reactions of alcohols P. 53 / 112

54 Oxidation reactions An oxidation reaction occurs when a molecule gains oxygen atom(s) or loses hydrogen atom(s). Under appropriate conditions, alcohols can be oxidized to aldehydes, ketones or carboxylic acids. Common oxidizing agents such as acidified potassium dichromate solution can be used Typical reactions of alcohols P. 54 / 112

55 Oxidation of primary (1 ) alcohols to aldehydesand carboxylic acids Example: Ethanol Ethanol is mixed with potassium dichromate solution (acidified by dilute sulphuric acid) and gently heated under reflux, a redox reaction occurs. Dichromate ions (orange) are reduced to chromium(iii) ions (green). Ethanol is first oxidized to ethanal and is then further oxidized to ethanoic acid Typical reactions of alcohols P. 55 / 112

56 reflux condenser water in pear-shaped flask anti-bumping granule water bath heat water out hot vapour condenses on the cold inner wall of the condenser condensed liquid returns to the flask ethanol + acidified potassium dichromate solution reflux condenser pearshaped flask anti-bumping granule P. 56 / 112 water out Figure 44.9 Oxidizing ethanol to ethanoic acid by acidified potassium dichromate solution (heating under reflux) Typical reactions of alcohols water in water bath ethanol + acidified potassium dichromate solution

57 loses 2 hydrogen ethanol atoms oxygen atom from an oxidizing agent; in this case, the oxygen atom comes from acidified potassium dichromate solution ethanal ethanoic acid gains 1 oxygen atom The equation for the reaction is written as follows: Cr 2 O 7 2 (aq)/h + (aq) Cr 2 O 7 2 (aq)/h + (aq) ethanol heat ethanal heat ethanoic acid 44.5 Typical reactions of alcohols P. 57 / 112

58 Ethanol is mixed with acidified potassium dichromate solution. Reaction mixture is warmed to a temperature that is above the boiling point of ethanal but below that of ethanol. Ethanal is distilled out as soon as it forms. This prevents ethanal from being oxidized to ethanoic acid Typical reactions of alcohols P. 58 / 112

59 ethanol + potassium dichromate solution added dropwise slowly anti-bumping granule heat thermometer water out dilute sulphuric acid water bath water in ethanol + potassium dichromate solution added dropwise slowly thermometer dilute sulphuric acid water out water in ice-water bath ethanal water bath P. 59 / 112 ethanal Figure Oxidizing ethanol to ethanal by acidified potassium dichromate solution and collecting ethanal produced using distillation Typical reactions of alcohols

60 The equation for the reaction can be written as follows: Cr 2 O 7 2 (aq)/h + (aq) ethanol distil out the ethanal ethanal Acidified potassium permanganate solution is a very strong oxidizing agent. It oxidizes primary alcohols to carboxylic acids directly. Activity Typical reactions of alcohols P. 60 / 112

61 Oxidation of secondary (2 ) alcohols to ketones Example: Propan-2-ol Oxidation of propan-2-ol by acidified potassium dichromate solution gives propanone. propan-2-ol loses 2 hydrogen atoms propanone no reaction 44.5 Typical reactions of alcohols P. 61 / 112

62 The equation for the reaction is written as follows: Cr 2 O 7 2 (aq)/h + (aq) propan-2-ol heat propanone Unlike aldehydes, ketones are resistant to further oxidation. Left as the end product of oxidation of secondary alcohols Typical reactions of alcohols P. 62 / 112

63 Oxidation of tertiary (3 ) alcohols Tertiary alcohols do not react with oxidizing agents under the same conditions. no reaction 3 alcohol Key point Upon oxidation, primary alcohols are oxidized to aldehydes and carboxylic acids whereas secondary alcohols are oxidized to ketones Typical reactions of alcohols P. 63 / 112

64 Alcohols Product(s) of oxidation Methanol methanal, methanoic acid 1 alcohols aldehydes, carboxylic acids 2 alcohols ketones 3 alcohols no reaction Table 44.2 Products of oxidation reactions of alcohols Typical reactions of alcohols P. 64 / 112

65 RX haloalkane HX or PX 3 alkene conc. H 2 SO 4, heat or Al 2 O 3, heat ROH Cr 2 O 2 7 (aq)/h + (aq) alcohol heat Cr 2 O 2 7 (aq)/h + (aq), heat Cr 2 O 72 (aq) /H + (aq), heat carboxylic acid (from 1º alcohol) aldehyde (from 2º alcohol) ketone Figure A summary of different reactions of alcohols Typical reactions of alcohols Example 44.4 Class practice 44.5 P. 65 / 112

66 44.6 Typical reactions of aldehydes and ketones Aldehydes can be further oxidized to carboxylic acids. Both aldehydes and ketones can be reduced to their parent alcohols under appropriate conditions. 1 alcohols [O] aldehydes [O] carboxylic acids [R] [R] [O] 2 alcohols ketones [R] [O] Oxidation [R] Reduction Figure Redox pathway for aldehydes and ketones. P. 66 / 112

67 Oxidation using Cr 2 O 7 2 (aq) Aldehydes are readily oxidized to carboxylic acids. Oxidation of aldehydes occurs so readily that it happens with atmospheric oxygen. Ketones are resistant to oxidation Typical reactions of aldehydes and ketones P. 67 / 112

68 Learning tip If a carbon compound changes the colour of a dichromate solution from orange to green, it is likely to be a 1 alcohol, a 2 alcohol or an aldehyde. When propanal is mixed with acidified potassium dichromate solution, the orange dichromate ions are reduced to green chromium(iii) ions. Propanal is oxidized to propanoic acid Typical reactions of aldehydes and ketones P. 68 / 112

69 Propanone does not react with acidified potassium dichromate solution. propanal acidified potassium dichromate solution the solution turns green (a) propanone acidified potassium dichromate solution Figure Attempted oxidation of (a) propanal and (b) propanone by using acidified potassium dichromate solution. (b) the solution remains orange 44.6 Typical reactions of aldehydes and ketones P. 69 / 112

70 Equation of the oxidation of propanal with acidified potassium dichromate solution: Cr 2 O 7 2 (aq)/h + (aq) propanal Note that: heat propanoic acid propanone Cr 2 O 7 2 (aq)/h + (aq) heat no reaction 44.6 Typical reactions of aldehydes and ketones P. 70 / 112

71 Reduction using LiAlH 4 or NaBH 4 Aldehydes and ketones can be reduced to alcohols by adding hydrogen atoms to the carbonyl group. Common reducing agents: lithium aluminium hydride (LiAlH 4 ) and sodium borohydride (NaBH 4 ) Possess hydrogen atoms which are readily available for addition to the carbonyl groups Typical reactions of aldehydes and ketones P. 71 / 112

72 Reduction of aldehydes gives primary (1 ) alcohols, reduction of ketones gives secondary (2 ) alcohols. [R] [R] aldehyde 1º alcohol ketone 2º alcohol 44.6 Typical reactions of aldehydes and ketones P. 72 / 112

73 Lithium aluminium hydride (LiAlH 4 ) is a powerful reducing agent. Figure Formula of LiAlH 4. Since lithium aluminium hydride reacts violently with water, it must be used in a dry solvent e.g. dry ether Typical reactions of aldehydes and ketones P. 73 / 112

74 Reduction of propanal can be carried out by mixing it with lithium aluminium hydride in dry ether. The reaction mixture is treated with a dilute acid (such as dilute sulphuric acid or dilute hydrochloric acid). Propan-1-ol is produced. It can be separated from the mixture by fractional distillation Typical reactions of aldehydes and ketones P. 74 / 112

75 propanal 1. LiAlH 4, dry ether 2. H + (aq) propan-1-ol (1 alcohol) If propanone reacts with the same reagents, it forms propan-2-ol. 1. LiAlH 4, dry ether 2. H + (aq) propanone propan-2-ol (2 alcohol) 44.6 Typical reactions of aldehydes and ketones P. 75 / 112

76 Sodium borohydride (NaBH 4 ) is a milder reducing agent than lithium aluminium hydride. It can be used in water. NaBH 4 propanal H 2 O propan-1-ol When propanone is mixed with sodium borohydride in water, propan-2-ol forms. propanone NaBH 4 H 2 O propan-2-ol 44.6 Typical reactions of aldehydes and ketones P. 76 / 112

77 Key point Aldehydes can be oxidized to carboxylic acids by acidified potassium dichromate solution but ketones cannot. Aldehydes and ketones can be reduced by lithium aluminium hydride or sodium borohydride to primary and secondary alcohols respectively. Example 44.5 Example 44.6 Class practice Typical reactions of aldehydes and ketones P. 77 / 112

78 44.7 Typical reactions of carboxylic acids Carboxylic acids possess acidic properties. They react with reactive metals, metal oxides and hydroxides, carbonates and hydrogencarbonates. 2CH 3 COOH(aq) + Mg(s) (CH 3 COO) 2 Mg(aq) + H 2 (g) CH 3 COOH(aq) + NaOH(aq) CH 3 COONa(aq) + H 2 O(l) 2CH 3 COOH(aq) + Na 2 CO 3 (aq) 2CH 3 COONa(aq) + CO 2 (g) + H 2 O(l) CH 3 COOH(aq) + NaHCO 3 (aq) CH 3 COONa(aq) + CO 2 (g) + H 2 O(l) P. 78 / 112

79 Esterification Esterification (or ester formation): when a mixture of a carboxylic acid and an alcohol is heated in the presence of concentrated sulphuric acid as catalyst, an ester is produced. General equation: conc. H 2 SO 4 carboxylic acid alcohol heat ester 44.7 Typical reactions of carboxylic acids P. 79 / 112

80 For example, ethanoic acid reacts with ethanol to give ethyl ethanoate (an ester). conc. H 2 SO 4 heat ethanoic acid ethanol ethyl ethanoate water 44.7 Typical reactions of carboxylic acids P. 80 / 112

81 a mixture of ethanoic acid, ethanol and a few drops of conc. sulphuric acid hot water The mixture is poured into cold water Thin, immiscible layer floats on water; sweet, fruity smell detected Figure Reaction between ethanoic acid and ethanol (in the presence of a little concentrated sulphuric acid) to form the ester, ethyl ethanoate. The ethyl ethanoate formed is a colourless volatile liquid Typical reactions of carboxylic acids P. 81 / 112

82 It has a characteristic sweet fruity smell. It is immiscible with water. Learning tip Concentrated sulphuric acid removes water produced from the reaction, thus shifting the equilibrium position of esterification to the right. As a result, a larger amount of ester is produced. Many esters have a sweet and pleasant smell Typical reactions of carboxylic acids P. 82 / 112

83 Esterification is a reversible reaction and thus it does not go to completion. The reaction takes place very slowly under room conditions. To speed up the reaction, concentrated sulphuric acid is added as a catalyst and the reaction mixture is heated. Key point Esterification is the reversible reaction of a carboxylic acid with an alcohol to form an ester and water Typical reactions of carboxylic acids P. 83 / 112

84 Reduction using LiAlH 4 When a carboxylic acid is mixed with lithium aluminium hydride in dry ether and then with dilute acid, it is reduced to a primary alcohol very quickly. Learning tip Carboxylic acid cannot be reduced by sodium borohydride, NaBH Typical reactions of carboxylic acids P. 84 / 112

85 Example Propanoic acid is reduced to propan-1-ol. 1. LiAlH 4, dry ether 2. H + (aq) propanoic acid propan-1-ol 44.7 Typical reactions of carboxylic acids P. 85 / 112

86 Amide formation In order to prepare an amide, a carboxylic acid is firstly converted to an acid chloride by using phosphorus trichloride. The acid chloride can then react with ammonia to give an amide. PCl 3 NH 3 carboxylic acid acid chloride unsubstituted amide 44.7 Typical reactions of carboxylic acids P. 86 / 112

87 Example When ethanoic acid reacts with phosphorus trichloride and then with ammonia, ethanamide is obtained. PCl 3 NH 3 ethanoic acid ethanamide Class practice Typical reactions of carboxylic acids P. 87 / 112

88 44.8 Typical reactions of esters Reverse of esterification is the hydrolysis of an ester. Ester reacts with water and is broken down to form an alcohol and a carboxylic acid (or a salt of the carboxylic acid). Key point Hydrolysis is a chemical change in which a compound is broken down by reaction with water. Strong acids or alkalis are usually used to catalyse the hydrolysis. P. 88 / 112

89 Acid hydrolysis The ester is heated under reflux with a dilute acid like dilute hydrochloric acid or dilute sulphuric acid. General equation: H + (aq) ester heat carboxylic acid alcohol 44.8 Typical reactions of esters P. 89 / 112

90 Example: Acid hydrolysis of methyl ethanoate Ethanoic acid and methanol are produced. H + (aq) methyl ethanoate heat ethanoic acid methanol Acid hydrolysis of esters is a reversible reaction that does not go to completion Typical reactions of esters P. 90 / 112

91 Alkaline hydrolysis Esters are often hydrolysed in alkaline medium in the laboratory. Ester is heated under reflux with a dilute alkali like sodium hydroxide solution. A salt of carboxylic acid (carboxylate) and an alcohol are produced. Soaps are made by the alkaline hydrolysis of fat or oil (also an ester) Typical reactions of esters P. 91 / 112

92 Unlike acid hydrolysis, alkaline hydrolysis of esters is irreversible. General equation: ester heat carboxylate ion alcohol In order to obtain a carboxylic acid, a dilute mineral acid (e.g. hydrochloric acid) is added to the product mixture: H + (aq) carboxylate ion carboxylic acid 44.8 Typical reactions of esters P. 92 / 112

93 Example Methyl propanoate is heated under reflux with sodium hydroxide solution. Sodium propanoate and methanol are obtained. 1. OH (aq), heat methyl propanoate 2. H + (aq) propanoic acid methanol Class practice Typical reactions of esters P. 93 / 112

94 44.9 Typical reactions of amides The hydrolysis of amides can be carried out in an acidic or alkaline medium. Both acid hydrolysis and alkaline hydrolysis of amides are irreversible reactions and can go to completion. P. 94 / 112

95 Acid hydrolysis If an amide is heated under reflux with a dilute acid, it is hydrolysed to give a carboxylic acid and ammonium ion. General equation: amide H + (aq) heat carboxylic acid ammonium ion 44.9 Typical reactions of amides P. 95 / 112

96 Example: Acid hydrolysis of propanamide propanamide H + (aq) heat propanoic acid ammonium ion 44.9 Typical reactions of amides P. 96 / 112

97 Alkaline hydrolysis If an amide is heated under reflux with dilute sodium hydroxide solution, it is hydrolysed to give a carboxylate ion and ammonia. General equation: heat amide carboxylate ion Ammonia is given out in this reaction. ammonia This can be used as a test for amides Typical reactions of amides P. 97 / 112

98 Example If propanamide is heated under reflux with sodium hydroxide solution, sodium propanoate and ammonia are produced. 1. OH (aq), heat propanamide 2. H + (aq) propanoic acid ammonium ion Experiment 44.3 Example 44.7 Class practice 44.9 Experiment Typical reactions of amides P. 98 / 112

99 Key terms 1. acid hydrolysis 加酸水解 2. addition reaction 加成反應 3. alkaline hydrolysis 加鹼水解 4. catalytic dehydration 催化脫水作用 5. dehydration 脫水作用 6. esterification 酯化作用 7. halogenation 鹵化作用 8. hydrogenation 加氫作用 9. hydrohalogenation 鹵氫化作用 10. hydrolysis 水解 P. 99 / 112

100 11. lithium aluminium hydride 氫化鋁鋰 12. Markovnikov s rule 馬科尼科夫規則 13. organic reaction 有機化學反應 14. primary (1 ) alcohol 一級醇 15. secondary (2 ) alcohol 二級醇 16. sodium borohydride 硼氫化鈉 17. substitution reaction 取代反應 18. tertiary (3 ) alcohol 三級醇 Key terms P. 100 / 112

101 Progress check 1. What is a substitution reaction? 2. What is an addition reaction? 3. What are the products of hydrogenation, halogenation and hydrohalogenation of alkenes respectively? 4. What does Markovnikov s rule state? 5. What are the products formed from the reaction between a haloalkane and sodium hydroxide solution? 6. What reagents can be used to convert an alcohol to a haloalkane? P. 101 / 112

102 7. What reagents and conditions are necessary for the dehydration of alcohols? 8. What reagents and conditions are necessary for the oxidation of alcohols? 9. What are the products formed from the oxidation of methanol, 1 alcohols and 2 alcohols respectively? 10.What reagents and conditions are necessary for the oxidation of aldehydes? 11.What reagents and conditions are necessary for the reduction of aldehydes and ketones? 12.What is an esterification? Progress check P. 102 / 112

103 13.What reagents and conditions are necessary for the reduction of carboxylic acids? 14.What reagents and conditions are necessary for converting carboxylic acids to amides? 15.What are the products formed from the acid hydrolysis and alkaline hydrolysis of esters respectively? 16.What are the products formed from the acid hydrolysis and alkaline hydrolysis of amides respectively? Progress check P. 103 / 112

104 Summary 44.1 Introducing organic reactions 1. Organic reactions are chemical reactions involving organic compounds. Some basic reaction types are substitution reactions, addition reactions, redox reactions and hydrolysis Typical reactions of alkanes 2. A substitution reaction is a chemical change in which an atom (or a group of atoms) of a molecule is replaced by another atom (or a group of atoms). P. 104 / 112

105 3. Alkanes undergo substitution reaction with halogen in UV light or heat to give haloalkanes Typical reactions of alkenes 4. Addition reaction is a typical reaction of unsaturated hydrocarbons. It is a chemical reaction in which two or more molecules react to give a single molecule. 5. In hydrogenation, two atoms of hydrogen are added across a carbon-carbon double bond, converting alkenes to alkanes. Summary P. 105 / 112

106 6. In halogenation, two halogen atoms are added across a carbon-carbon double bond, converting alkenes to dihaloalkanes. 7. In hydrohalogenation, a hydrogen halide molecule is added across a carbon-carbon double bond, converting alkenes to haloalkanes. 8. Markovnikov s rule states that when a molecule of HX is added to an alkene, the hydrogen atom is added to the carbon atom of the carbon-carbon double bond that already carries the larger number of hydrogen atoms. Summary P. 106 / 112

107 44.4 Typical reactions of haloalkanes 9. When mixed with sodium hydroxide solution, the halogen atoms of haloalkanes are substituted by the hydroxide ions, converting them to alcohols Typical reactions of alcohols 10. When a hydrogen halide or phosphorus trihalide reacts with an alcohol, substitution occurs, converting the alcohols to haloalkanes. Summary P. 107 / 112

108 11. Dehydration is the chemical change in which hydrogen and oxygen are eliminated in a ratio of 2 : 1 from a compound. 12. Different classes of alcohols behave differently in oxidation. The products of their oxidation reactions are summarized in Table 44.2 on p Typical reactions of aldehydes and ketones 13. Aldehydes can be oxidized to carboxylic acids by acidified potassium dichromate solution but ketones are resistant to oxidation. Summary P. 108 / 112

109 14. Aldehydes and ketones can be reduced to primary and secondary alcohols respectively by common reducing agents such as lithium aluminium hydride (LiAlH 4 ) and sodium borohydride (NaBH 4 ) Typical reactions of carboxylic acids 15. Esterification is a reversible reaction of a carboxylic acid with an alcohol to form an ester and water. Summary P. 109 / 112

110 16. When mixed with lithium aluminium hydride in dry ether, a carboxylic acid is reduced to an alcohol very quickly. 17. In order to prepare an amide, a carboxylic acid is converted to an acid chloride by phosphorus trichloride, which can then react with ammonia to form an amide Typical reactions of esters 18. Hydrolysis is a chemical change in which a compound is broken down by the reaction with water. Summary P. 110 / 112

111 19. Acid hydrolysis of an ester is a reversible reaction. It gives a carboxylic acid and an alcohol. 20. Alkaline hydrolysis of an ester is an irreversible reaction. It gives a carboxylate ion and an alcohol Typical reactions of amides 21. Acid hydrolysis of an amide gives a carboxylic acid and ammonium ion. The reaction is irreversible. 22. Alkaline hydrolysis of an amide gives a carboxylate ion and ammonia. The reaction is irreversible. Summary P. 111 / 112

112 Concept map acid hydrolysis Esters acid hydrolysis dehydration substitution with hydroxide ion Alcohols Alkenes Alkanes addition of hydrogen Haloalkanes (For 1º alcohols) oxidation reduction substitution with halogens Aldehydes reduction (For 2º alcohols) oxidation addition of halogens or hydrogen halides esterification oxidation Ketones Carboxylic acids phosphorus trichloride Acid chlorides acid hydrolysis ammonia unsubstituted amides P. 112 / 112