CfE Higher Chemistry. Unit 2: Natures Chemistry. Esters, Fats and Oils, Soaps

Transcription

1 CfE igher Chemistry Unit 2: Natures Chemistry Esters, Fats and ils, Soaps 27/02/2018

2 Lesson Starter: Name / Draw these molecules a) b) c) d) e) 2-methylpentan-1-ol f) Pentanoic acid 27/02/2018

3 Esters 27/02/2018 Learning utcomes : Recognise an ester molecule and the ester link functional group Name esters and draw the structural formulae for esters Describe some uses of esters Explain the formation and break down of esters by condensation and hydrolysis Name and draw the reactants and products of a condensation or hydrolysis reaction

4 What are Esters? An ester is a compound which is formed when a carboxylic acid reacts with an alcohol. They contain the Ester Link functional group (-C- or -C-) The name of the parent alcohol and carboxylic acid give the ester it s name. It is easy to spot an ester from it s name; Ester names always finish with oate Esters can be natural or synthetic and have many uses related to their properties. 27/02/2018

5 Esters Esters are formed when an alcohol reacts with a carboxylic acid, water is also produced during this reaction. Carboxylic acid + Alcohol Ester + water Ester link The functional group of an ester is the ester link 27/02/2018

6 Naming Esters The name of the ester is made up of two parts the alcohol ( yl) part and the carboxylic acid ( anoate) part. C C C C C ethyl propanoate made by reacting ethanol with propanoic acid 27/02/2018

7 Naming Esters C C C C Ester link methanol + methanoic acid methyl methanoate + water Studies have revealed that the oxygen atom in the main chain of the ester always comes from the alcohol. 27/02/2018

8 Practice Question Name the esters that the following alkanols and alkanoic acids would produce. 1. Propanol & Ethanoic acid 2. Butanol & Pentanioc acid 3. Propanoic acid & Methanol 4. Methanoic acid & Pentanol 5. Butanol & Butanoic acid 6. ctanol & ctanoic acid 7. Ethanoic acid & eptanol Propyl ethanoate Butyl pentanoate Methyl propanoate Pentyl methanoate Butyl butanoate ctyl octanoate eptyl ethanoate 27/02/2018

9 Practice Questions Now try the practice questions on the worksheet. 27/02/2018

10 Making Esters Esters can be prepared by reaction of an alcohol with a carboxylic acid: C R' C R' + 2 R + R alcohol carboxylic acid ester The reaction is slow at room temperature and the yield of ester is low. The rate can be increased by: 1) heating the reaction mixture 2) using concentrated sulphuric acid as a catalyst. The presence of the concentrated sulphuric acid also increases the yield of ester. 27/02/2018

11 Experiment: Making esters Collect an experiment work card from the front. Acid Alcohol Notes: Copy the above diagram and explain the purpose of the concentrated sulphuric acid; the wet paper condenser and the water bath. What two pieces of evidence would indicate that an ester had been formed? 27/02/2018

12 Notes: Properties of Esters 1. Esters are. so we never use a naked flame near them, we would use a heating mantle or a water bath as a source of heat. 2. Esters have a distinctive When using them we must ensure that we have 3. Esters are very (meaning that they turn into a very easily) 4. Esters are in water (they form a layer with water or other solutions). If you learn these properties the uses are easier to recall in an exam. 27/02/2018

13 Uses of esters: Flavours and Scents Many esters are used as flavourings and in perfumes. Natural fruit flavours contain subtle blends of some of the esters in the table below: Name Methyl Butanoate Shortened Structural Formula C 3 (C 2 ) 4 C 3 dour/flavour Banana Pineapple 3-Methylbutyl Butanoate C 3 (C 2 ) 2 (C 2 ) 2 C(C 3 ) 2 Apple C 3 CC 3 7 Pear Methyl-1-butyl ethanoate C 3 CC(C 3 )C 4 9 Banana 2-Methylpropyl Raspberry methanoate C 3 7 CC 5 11 Apricot, Strawberry Benzyl ethanoate C 3 CC 2 C 6 5 Peach, flowers Methyl 2-aminobenzoate C 6 4 (N 2 )CC 3 Grapes Benzyl butanoate C 3 7 CC 2 C 6 5 Cherry 27/02/2018

14 27/02/2018

15 Uses of esters: Solvents Esters are also used as non-polar industrial solvents. Some of the smaller esters are quite volatile and are used as solvents in adhesives, inks and paints pentyl ethanoate is used in nail varnish for example. 27/02/2018

16 Uses of esters: Decafination Ethyl ethanoate is one of a number of solvents used to extract caffeine from coffee and tea. De-caffeinated products produced with ethyl ethanoate are often described on the packaging as "naturally decaffeinated" because ethyl ethanoate is a chemical found naturally in many fruits. 27/02/2018

17 Experiment: Decafination Caffeine (C810N42) is an example of a class of compounds called alkaloids which are produced by plants. The name alkaloid means alkali-like, where alkali is a base and hence refers to these basic properties. Carry out the experiment to extract caffeine from tea. 27/02/2018

18 Uses of esters Caffeine is more soluble in the organic solvent ethyl ethanoate than in water, so we will extract caffeine into the organic solvent to separate it from glucose, tannins, and other water soluble compounds using a separating funnel.. The ethyl ethanoate portions can be combined and the ethyl ethanoate removed by evaporation to leave the caffeine 27/02/2018

19 ydrolysis of Esters Condensation Alcohol + Carboxylic Acid Ester + Water ydrolysis The ester is split up by the chemical action of water, hydrolysis. The hydrolysis and formation of an ester is a reversible reaction. R C + R R C + R Bonds broken Ester + Water Bonds formed Carboxylic Acid + Alcohol 27/02/2018

20 ydrolysis of an Ester To reverse the formation of ester (ie hydrolyse it) we need to react the ester with water. owever in practice the ester is heated with either dilute acid or dilute alkali. The ester is said to be heated under reflux. When sodium hydroxide solution is used the ester is split up into the alkanol and the sodium salt of the acid. The alkanol can be removed by distillation and the alkanoic acid can be regenerated by reacting the sodium alkanoate with dilute hydrochloric acid. 27/02/2018

21 Starter Questions Name the products formed by hydrolysis of each of the following compounds: 1) ethyl ethanoate 2) propyl butanoate 3) butyl pentanoate 8) C 3 C 2 CC 2 C 2 C 3 9) C 3 C 10) 4) methyl hexanoate 5) 6) pentyl methanoate 7) C 3 C 2 C 2 CC 3

22 Fats and ils Learning utcomes By the end of these lessons you will be able to: Explain the chemistry and structure of edible fats and oils. Explain the difference in melting points of fats and oils in terms of structural differences.

23 Fats in the Diet Fats provide more energy per gram than carbohydrates. Fat molecules are insoluble, and tend to group together and form a large droplet. This is how fat is stored in the adipose tissue. We store our extra energy as fat. The type of fat we eat is important. Animal fats contain important fat soluble vitamins. ils, are thought to be healthier than solid fats, as they are less likely to be deposited inside our arteries. owever, there is an ongoing debate about which fats are better for us. Polyunsaturated fats are considered to be less potentially harmful to the heart.

24 Fats and oils Naturally occuring Animal fat Vegetable oil Marine oil lard suet sunflower oil coconut oil cod liver oil whale oil

25 Fats and ils Fats and oils are a range of substances all based on glycerol, propan-1,2,3-triol. Natural fats and oils are a mixture of triglyceride compounds. 50% of your brain is fat! C C C Glycerol propan-1,2,3-triol Each - group can combine chemically with one carboxylic acid molecule. The resulting molecules are fats and oils. They are described as triglycerides. The hydrocarbon chain in each carboxylic acid can be from 4 to 24 C s long. The C s can be single bonded (saturated) or double bonded (unsaturated).

26 Fats and oils Both fats and oils are built from glycerol; an alcohol with three - groups. glycerol Systematic name is propane-1,2,3-triol

27 The other components of fat molecules are carboxylic acids. Long chain carboxylic acids are known as Fatty Acids. ne such fatty acid is Stearic acid: Stearic acid Systematic name is octadecanoic acid

28 Examples of Fatty Acids C C C 3 (C 2 ) 16 C Stearic Acid (suet, animal fat) Saturated 3 C C 2 C 2 C2 C 2 C2 C 2 C2 C 2 C2 C 2 C2 C 2 C2 C 2 C2 C 2 C C C C 3 (C 2 ) 7 C=C(C 2 ) 7 C leic Acid (olive oil) Unsaturated 3 C C 2 C 2 C2 C 2 C2 C C 2 C2 C 2 C2 C 2 C2 C 2 C C 2 C 2 C

29 The glycerol molecule and fatty acids form ester links. Fats and oils are ESTERS made from glycerol and long chain carboxylic acids

30 The formation of the ester links is an example of a condensation reaction. Formation / removal of water in the condensation reaction gives - The molecular formula shown above suggests that the fat molecule is shaped like an E, but the molecule is actually shaped more like this:

31 Fats are mainly built from carboxylic acids with C-C single bonds. (SATURATED) Stearic acid in beef fat ils have at least one C=C bonds in the carboxylic acids from which they are made. (UNSATURATED) leic acid in olive oil

32 il Double bonds in oil change the shape of the molecules. This makes the molecule less compact. Less tightly packed molecules result in weaker bonds between the molecules this makes oils liquid. Fat Fat molecules pack together more tightly, making fats solid at room temperature.

33 In practice both fats and oils are mixtures of esters containing both saturated and unsaturated compounds. Beef Fat live oil In general oils have a higher proportion of unsaturated molecules.

34 Notes: ow do the melting points of the fats compare with the melting points of the oils? What is meant by saturated and unsaturated? ow does the proportion of unsaturated molecules in an oil compare with that in a fat? Explain why fats are likely to have relatively high melting points and oils are likely to have relatively low melting points.

35 Unsaturation in fats and oils 1.Using a plastic pipette, add five drops of olive oil to 5 cm 3 of hexane in a conical flask. 2. Use a burette filled with a dilute solution of bromine water (0.02 mol dm 3) (armful and irritant). 3. Read the burette.

36 Unsaturation in fats and oils 4. Run the bromine water slowly into the oil solution. Shake vigorously after each addition. The yellow colour of bromine disappears as bromine reacts with the oil. Continue adding bromine water to produce a permanent yellow colour. 5. Read the burette. Subtract to find the volume of bromine water needed in the titration. 6. Repeat the experiment with: five drops of cooking oil (vegetable) and five drops of cooking oil (animal).

37 Fats and ils The degree of saturation in a fat or oil can be determined by the Iodine Number. (bromine can also be used). The iodine reacts with the C=C bonds, so the greater the iodine number, the greater the number of double bonds. Fat Av Iodine No Butter 40 Beef Fat 45 Lard 50 live il 80 Peanut il 100 Soya Bean il 180 Solid fats butter, beef fat & lard have low iodine numbers because they are more saturated than the unsaturated oils. Margarine is made from vegetable oils, butter from animal fats. ne reason why margarine spreads better! mega 3 fatty acids make up a large % of your brain s fat.

38 ydrogenation The addition of hydrogen to an unsaturated oil will harden the oil. This will Increase it s melting point. The hydrogen is added across the double bond. Used with margarine, otherwise margarine would be a liquid when taken out of the fridge.

39 Lesson Starter: ydrolysis of Esters 27/02/2018

40 Soaps and Emulsions Learning utcomes : explain how soaps are produced relate the cleansing action of soaps to the structure of the soap molecules.

41 Structures of fats and oils - Revision Fats and ils are esters of glycerol and long chain fatty acids. ydrolysis of a fat or oil produces glycerol (alcohol) and 3 carboxylic acids / fatty acids. C C C Glycerol part C C C R 1 R 2 R 3 Fatty acid part ydrolysis Fatty Acids + Glycerol (R 1, 2, 3 are long carbon chains, which can be the same or different)

42 ydrolysis To hydrolyse an ester we need to react it with water. owever in practice the ester is heated under reflux with either dilute acid or dilute alkali. When sodium hydroxide solution is used the ester is split up into the alkanol and the sodium salt of the acid. The alkanol can be removed by distillation and the alkanoic acid can be regenerated by reacting the sodium alkanoate with dilute hydrochloric acid. The equations for these reactions are: 27/02/2018

43 Soaps Soaps are salts of fatty acids. Soaps are formed by the alkaline hydrolysis of fats and oils by sodium or potassium hydroxide by boiling under reflux conditions: C C C C C C C + 3Na C + 3 C Na C -- C C C C Glyceryl tristearate Glycerol Sodium stearate (soap)

44 Structure of Soaps As most of the grime and dirt on skin, clothes and dishes tends to be trapped in oils and greases, water alone cannot rinse away the muck. If the oils/greases can be made to mix with water then it becomes easy to wash off, along with the grime. Soaps and detergents do this job in a clever way, due to the structure of the molecules: Sodium or potassium salts of long chain fatty acids really have two quite separate parts in terms of their bonding types a long hydrocarbon chain which is non polar and an ionic head. ydrophobic tail C - Na + ydrophilic head 27/02/2018

45 Structure of soap ydrophobic tail C - Na + ydrophilic head The long covalent hydrocarbon chain tail gives rise to the hydrophobic (water hating) and oil-soluble (non-polar) properties of the soap molecule (represented in yellow). The charged carboxylate group head (represented in blue) is attracted to water molecules (hydrophilic). In this way, soaps are composed of a hydrophilic head and a hydrophobic tail:

46 Cleansing action of soaps The following ball and stick diagram represents the initial interaction of soap on addition to water and material with a grease stain: (blue for hydrophilic head group) (yellow for hydrophobic tail group)

47 When the solution containing soap and water is agitated (stirred vigorously) the interactions of hydrophobicity and hydrophilicity become apparent. The hydrophobic, non-polar, tails burrow into the greasy, non-polar molecule like attracting like. In the same way the polar hydrophilic head groups are attracted to polar water molecules. The head groups all point up into the water at the top of the grease stain.

48 The attraction of the head group to the surrounding water, via polar-to-polar interactions, is so strong that it causes mechanical lift of the grease molecule away from the material on which it was deposited. The hydrophobic tails are anchored into the grease due to non-polar to non-polar attraction. In combination, these effects allow for the removal of the grease stain.

49

50 MICELLE C C C C C C C C Na + Na + Na + Na + Na + Na + Na+ Na grease particle

51 Success Criteria: Soaps I can explain how soaps are produced by alkaline hydrolysis of fats and oils I can relate the cleansing action of soaps to the hydrophobic and hydrophilic nature of soap molecules. Next Lesson: Emulsions

52 Lesson Starter:

53 Emulsions Learning utcomes : Describe the characteristics of an emulsion, and study the chemistry of typical emulsifier molecules.

54 Emulsifiers An emulsion contains small droplets of one liquid dispersed in an another liquid. Emulsions in food are mixtures of oil and water. To prevent oil and water components separating into layers, a soap-like molecule known as an emulsifier is added.

55 Experiment: Emulsifiers Expt 2.8 (b)

56 Emulsifier molecules Emulsifiers for use in food are commonly made by reacting edible oils with glycerol to form molecules in which either one or two fatty acid groups are linked to a glycerol backbone rather than the three normally found in edible oils. The one or two hydroxyl groups present in these molecules are hydrophilic whilst the fatty acid chains are hydrophobic. The presence of this emulsifier is shown on packaging by E-numbers, E471 and is one of the most common on food packaging.

57 Emulsifiers Mayonnaise contains oil and water. The emulsifier keeps these mixed and without it the oil and water separate.

58 Emulsifiers in food Emulsifiers are among the most frequently used types of food additives. They are used for many reasons: Emulsifiers can help to make a food appealing. They are used to aid in the processing of foods and also to help maintain quality and freshness. In low fat spreads, emulsifiers can help to prevent the growth of moulds which would happen if the oil and fat separated. Foods that Commonly Contain Emulsifiers Biscuits Toffees Bread Extruded snacks Breakfast cereals Cakes Desserts / mousses Soft drinks Chewing gum Frozen desserts Ice-cream Dried potato Chocolate coatings Margarine / low fat spreads Coffee whiteners Topping powders Peanut butter Caramels

59 Emulsifiers in food Foods that Commonly Contain Emulsifiers Biscuits Toffees Bread Extruded snacks Chewing gum Margarine / low fat spreads Breakfast cereals Frozen desserts Coffee whiteners Cakes Ice-cream Topping powders Desserts / mousses Dried potato Peanut butter Soft drinks Chocolate coatings Caramels

60 Success Criteria: I can explain how soaps are produced by alkaline hydrolysis of fats and oils I can relate the cleansing action of soaps to the hydrophobic and hydrophilic nature of soap molecules. I can define an emulsion. Soaps and Emulsions I can explain why emulsifiers are added to food. I can describe how emulsifiers are made and how they work Next Lesson: Proteins