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IQ5 What are the properties of organic acids and bases?
IQ5 What are the properties of organic acids and bases?
5.1 investigate the structural formulae, properties and functional group including:
a) primary, secondary and tertiary alcohols
b) aldehydes and ketones
c) amines and amides
d) carboxylic acids
5.2 explain the properties within and between the homologous series of carboxylic acids amines and amides with reference to the intermolecular and intramolecular bonding present
5.3 investigate the production, in a school laboratory, of simple esters
5.4 investigate the differences between an organic acid and organic base
5.5 investigate the structure and action of
a) soaps and
b) detergents
5.6 draft and construct flow charts to show reaction pathways for chemical synthesis, including those that involve more than one step
5.1 investigate the structural formulae, properties and functional groups including primary, secondary and tertiary alcohols, aldehydes and ketones, amines and amides, and carboxylic acids.
The high melting points and boiling points in alkanols are due to hydrogen bonding between the molecules. Alcohols only form one set of H-bonding per pair of molecules because of the 90 degree bond angle (puts strain on the arrangement.)
The double bonded oxygen in aldehydes and ketones (carbonyl bond) is another source of polarity but as there is no O-H bond, there can only be dipole-dipole interactions between adjacent molecules.
The location of the double bond within the molecule of a ketone, increases the polarity of the bond, but only slightly, so ketones have a slightly higher boiling point than their corresponding aldehyde (propanal 48oC vs propanone 56oC, butanal 75oC vs butanone 80oC).
The ability of the COOH group in carboxylic acids to be involved in two hydrogen bonds gives an alkanoic acid an even higher boiling point than that of a similar sized alkanol. Two hydrogen bonds can occur between a pair of alkanoic acid molecules as shown below.
Size is also important as larger molecules require additional energy for motion. So we would expect an increase in melting point and boiling point as we increase molecular weight (ie, increase the length of the chain or number of carbons) and increase the number of electrons per molecule.
The presence of nitrogen in amines and amides also affects both the chemical reactivity of the molecules and their physical properties. Compete the table below (Aylward & Findlay, 2008, pp. 102-112):
View videos:
TASK 5.1.1
Complete the table
5.2 explain the properties within and between the homologous series of carboxylic acids, amines and amides, with reference to the inter-molecular and intra-molecular bonding present
Carboxylic acids
The carboxylic acid functional group, -COOH, in alkanoic acids can lose a hydrogen ion and behave as a weak acid.
The diagram shows a common way to illustrate the characteristic structure of the members of a homologous series, in this case, carboxylic (or alkanoic, or organic) acids. The R group represents a carbon chain of unspecified length (for our purposes, between 0 and 7 carbons). This is a skeletal formula as the carbon atom within the functional group (COOH) is not labelled with a C. In this type of formula the junction of bonds with no letter indicating the atom present is assumed to be a carbon and hydrogens bonded directly to carbon atoms are not shown.
However, whilst you may see this short-hand version, or a condensed structural formula, eg CH3CH2COOH, in different text books or web sites, it is best in HSC Chemistry Exams to always draw full (or graphic) structural formulae where possible.
The functional group is the key to the chemical properties of the compound, but it is also relevant when discussing the physical properties.
In the diagram of the general formula for an acid above, there is a non-polar region, indicated by the R, and two other features: C=O and C-O-H. Both these groups contain polar covalent bonds, with the C=O bond especially polar, and both affect the bonding within the members of the carboxylic acid homologous series, but also between carboxylic acids and other organic compounds.
Intermolecular forces between molecules of the same homologous series (eg acids) cause the chemical and physical properties we observe - these properties will be similar within the homologous series because they have the same functional group(s), but will change slightly within the series because of the different numbers of carbon atoms - mass, size, number of intermolecular forces)
Density (intermolecular forces between carboxylic acid molecules produce properties - these will be similar within the homologous series because they have the same functional group(s), but will change slightly within the series because of the different numbers of carbon atoms - mass, size, number of intermolecular forces): Density is a function of how tightly packed the molecules are in the solid or liquid phase. Whilst stronger intermolecular forces, ie hydrogen bonds, may hold molecules closer together than the weaker dispersion forces, side branches can affect the density as the molecules may not be as tightly packed. Comparing a secondary and tertiary alcohol of comparable molar mass, we find the tertiary alcohol, with the side branch, has a lower density (ρ):
2-butanol ρ = 0.802 g.cm-3
2-methyl-2-propanol ρ = 0.781 g.cm-3
Melting Point (intermolecular forces between molecules produce physical properties - these will be similar within the homologous series because they have the same functional group(s), but will change slightly within the series because of the different numbers of carbon atoms - mass, size, number of intermolecular forces):
Melting point indicates the transition from solid to liquid. This requires sufficient energy to overcome the forces between molecules which are keeping them packed together, allowing the molecules to move past one another in the liquid state. Intermolecular forces must be broken here, so the stronger the forces, the more energy will be required and hence the higher the melting point/boiling points. Hydrogen bonding between molecules is the intermolecular force that requires the most energy for separation and hence the higher mp for the carboxylic acids and amides.
Aldehydes and ketones are slightly lower as there are no hydrogen bonds, only dipole-dipole interactions.
Amines and amides contain nitrogen atoms which are also highly electronegative and form polar bonds with carbon atoms, however as the C-N bond is not as strongly polar as the C-O bond, the hydrogen bonding in amines and amides is weaker than that in a corresponding alcohol.
Boiling Point (intermolecular forces between molecules produce physical properties - these will be similar within the homologous series because they have the same functional group(s), but will change slightly within the series because of the different numbers of carbon atoms - mass, size, number of intermolecular forces):
Boiling point indicates the transition from liquid to gas. This involves sufficient energy to overcome the forces between molecules which are holding the molecules close enough for them to move past one another in the liquid state, allowing each molecule to move independently ie as a gas. Intermolecular forces must be overcome here, so again, the stronger the forces, the more energy will be required and hence the higher the boiling point. Hydrogen bonding between molecules requires the most energy for separation, producing the higher bpt for the carboxylic acids and amides.
Aldehydes and ketones are slightly lower as there are no hydrogen bonds, only dipole-dipole interactions. In amines, the presence of the N atom provides opportunities for H-bonding and even more so for amides, where dimers, similar to those between carboxylic acid molecules, can form.
Solubility in water (interactions occur between molecules of an homologous series and other molecules eg water):
Our solubility rule is like dissolves like. Molecules with some polarity will interact with polar water molecules and will show some degree of miscibility (see image below).
Solubility in water is particularly high for acids, alcohols and amines, less so for aldehydes and ketones.
However, unless the functional group occurs several times throughout the molecule, the presence of a hydrocarbon chain will not attract water molecules, the solubility of polar molecules will decrease with chain length as shown in Table 9.3 below
Solubility in non-polar solvents (interactions occur between molecules of an homologous series and other molecules eg tetrachloromethane):
Based on our solubility rule of like dissolves like, molecules with long non-polar regions can interact with other non-polar molecules.
Even with a polar head, the longer the non-polar tail, the more miscible the substance is in a non-polar solvent. Long molecules with a polar head and non-polar tail can act as surfactants (see soap section).
Brønsted-Lowry classification (interactions occur between molecules of an homologous series and other molecules eg acids and bases):
Carboxylic acids have the ability to donate a proton. The highly polar O-H bond can be made to ionise, fulfilling the definition of a Bronsted-Lowry acid, although organic acids are weak acids.
Amines can accept a proton (a hydrogen ion) attaching to the nitrogen non-bonding pair, making them Bronsted-Lowry bases.
View videos:
TASK 5.2.1
1. Compare and explain the solubility of aldehydes and ketones in water (Hegarty, 2018, p. 56). (3 marks)
2. Draw labelled diagrams to show the intermolecular forces between neighbouring ketone molecules and between a ketone and water (Hegarty, 2018, p. 58). (4 marks)
5.3 investigate the differences between an organic acid and organic base.
While we have had some experience with organic acids and know the acidic nature of carboxylic acids, it is also important to be aware of the fact that the amines are good organic bases.
The best way to analyse organic substances to distinguish between their acidic, neutral or basic nature is to use our Brønsted-Lowry or Lewis definitions of acids and bases. If we can find a proton which could be donated (or a lone pair of electrons which could be accepted), we have an acidic substance (eg Figure 42). Carboxylic acids can donate a proton, according to the Brønsted-Lowry definition of acids as shown below. Ethanoic acid can donate a proton (H+) to form the ethanoate ion.
Likewise if we find a structure which could accept a proton, we have a basic substance. The presence of the lone pair on the nitrogen atom of an amine is an indicator that these organic compounds could attract and accept a proton and so act as a BL base (eg Figure 43). Methylamine can accept a proton (H+) to form the protonated methylamine ion (methyl ammonium).
Although we can generalise that most organic acids and bases are weak, to determine the actual strength we need to look at the Ka and Kb values.
View video:
- OC#27 https://www.youtube.com/watch?v=Vw8J-8MzOgc [9.27]
TASK 5.3.1
1. Use an equation to show why butyric acid (butanoic acid) fulfils the Brønsted-Lowry criteria to act as an acid. (2 marks)
2. Primary, secondary and tertiary amines can act as Brønsted-Lowry bases. Use a diagram, or series of diagrams, to explain why this is the case. (3 marks)
3. Proteins are made up of amino acids. A peptide bond is formed between adjacent amino acids. Draw a diagram to show the formation of a peptide bond using two (named) amino acids.
4. The four nitrogenous bases which make up rungs in the DNA spiral staircase are shown below (Davis, Disney, & Smith, 2018, p. 350). Circle and identify any functional groups you recognise and show why any one of these bases can be classified as a base. (4 marks)
5.4 investigate the production, in a school laboratory, of simple esters
ESTERIFICATION OF CARBOXYLIC ACIDS
Esterification is the reaction between alcohols and carboxylic acids to make esters.
Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulfuric acid.
The reaction occurs between an acid RCOOH and an alcohol R'OH (where R and R' are carbon-hydrogen groups eg methyl, ethyl etc, general name is alkyl group; R and R'can be the same or different).
One example of an ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is CH3COOCH2CH3
The ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid (-ic acids form -ate salts eg sulfuric acid forms sulfate salts, -oic acids form -oate salts eg ethanoic acid forms ethanoate salts). The "ethyl" bit comes from the ethyl group (from ethanol) on the end.
The acid is named by counting up the total number of carbon atoms in the chain - including the one in the -COOH group. So, for example, CH3CH2COOH is propanoic acid, and CH3CH2COO- is the propanoate group.
Note: You can find more about naming acids and esters by following this link.
TASK 5.4.1
Name these esters
Making esters
Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulfuric acid.
The reaction occurs between an acid RCOOH and an alcohol R'OH (where R and R' are carbon-hydrogen groups eg methyl, ethyl etc, general name is alkyl group; R and R'can be the same or different).
Carboxylic acids and alcohols are often warmed together in the presence of a few drops of concentrated sulphuric acid in order to observe the smell of the esters formed.
As an equilibrium reaction
The esterification reaction is both slow and reversible, ie it is an EQUILIBRIUM reaction, and so an equilibrium expression can be written for it.
NOTE: all substances are liquids, so the liquids are included in the expression. This only happens when all are liquids.
Le Chatelier's Principle applies to reaction conditions: How could you make more? Why is refluxing apparatus used?
Concentrated sulfuric acid is used as a catalyst, and has a dual role:
- Speeds up the reaction.
- Acts as a dehydrating agent, forcing the equilibrium to the right and resulting in a greater yield of ester.
Only very small amounts of sulfuric acid are used, as large amounts would be wasteful, uneconomical, and would complicate the final separation processes. Therefore, the addition of the acid does not have a considerable impact on the yield of ester.
In addition to the catalyst, esterification reactions are usually carried out at temperatures near the boiling point of the alcohol being reacted, which also increases the reaction rate.
Small esters are formed faster than bigger ones.
To make a small ester like ethyl ethanoate, you can gently heat a mixture of ethanoic acid and ethanol in the presence of concentrated sulphuric acid, and distil off the ester as soon as it is formed.This prevents the reverse reaction happening. It works well because the ester has the lowest boiling point of anything present. The ester is the only thing in the mixture which doesn't form hydrogen bonds, and so it has the weakest intermolecular forces.
Larger esters tend to form more slowly. In these cases, it may be necessary to heat the reaction mixture under reflux for some time to produce an equilibrium mixture. The ester can be separated from the carboxylic acid, alcohol, water and sulfuric acid in the mixture by fractional distillation.
- Refluxing: The process of heating a reaction mixture in a vessel with a condenser tube attached.
- Esterification reactions are refluxed to prevent:
- The build-up of pressure that occurs with a closed vessel reaction.
- The loss of volatile components.
- By preventing the escape of volatile components that are flammable, refluxing improves the safety of esterification reactions.
Esters.pptx
A note on smell
The ester's smell is often masked or distorted by the smell of the carboxylic acid. A simple way of detecting the smell of the ester is to pour the mixture into some water in a small beaker. Esters are virtually insoluble in water and tend to form a thin layer on the surface. Excess acid and alcohol both dissolve and are under the ester layer. Small esters like ethyl ethanoate smell like typical organic solvents (ethyl ethanoate is a common solvent in, for example, glues. As the esters get bigger, the smells tend towards artificial fruit flavouring.
VIEW videos
- OC#26: https://www.youtube.com/watch?v=NxsCoaEDKV8&list=PLeFSFSJ9WqSDRKZVcK33Gtz60RsJ5_t7Q&index=32 [12.43]
- Esterification: https://www.youtube.com/watch?v=MKUsNNsK5aI&list=PLB755F0C836855EAB&index=68 7.36
- Esterification mechanism: http://www.youtube.com/watch?v=aKDPN8tTyOM 9.10
PRAC 5.4.1 Making an ester
TASK 5.4.2
PAST HSC QS (QS WITH NO MARKS INDICATED ARE MULTIPLE CHOICE. HSC EXAMS IN EARLIER YEARS HAD 15 MC QS, LATER YEARS AND CURRENTLY THERE ARE 20.)
2017 Q4; Q27 5
2016 Q3, Q22A 1; 22B 2; Q22C 2
2015 Q9
2014 Q26A 1; Q26A 1; 26B 5
2013 Q18; Q21B 4
2012 Q21A 2; Q21B 4; Q31A 3; 31B 1; 31C 1
2011 Q6; Q8
2010 Q22A 1; Q22B 2 ; Q22C 3
2009 Q10; Q16 3
2008 Q5; Q11
2007 Q23 3
2006 Q7
2005 Q6; Q20 7
2004 Q1
5.5 investigate the structure and action of
a) soaps
First, some terms:
- Glycerol is propan-1,2,3-triol
- Fatty acids are long-chain hydrocarbons with a carboxylic (-COOH) group at one end.
- A triglyceride is the general name for a compound containing three fatty acid molecules (from below: left diagram, fatty acids are the ones that are shown horizontally bonded) joined to one glycerol molecule (same diagram, shown vertically bonded).
- Triglycerides are esters (same diagram: they have -O-C=O bonding arrangement). Fats and oils are both types of triglycerides.
- Fats are solid at room temperature due to all single C–C bonds in the R group (eg (CH2)14 group below), therefore they are said to be saturated.
- Oils are liquid at room temperature due to the presence of many double C=C bonds in the R group i.e. unsaturated.
Experiment 5.5.1 Surface Tension
Soaps and detergents are surfactants (surface active agents), which are chemical substances which decrease the surface tension of water.
View video, then investigate some effects of the surface tension of water:
- Seven Science Experiments with Surface Tension https://www.youtube.com/watch?v=WsksFbFZeeU [3.27]
Production of a soap: Saponification
- Saponification is the conversion of fats and oils (forms of triglycerides, esters) in a basic solution to produce glycerol and a salt of a fatty acid. The salts of long chain alkanoic acids (fatty acids) are called soaps. They can lather in water and dissolve oils and grease which are normally immiscible in (do not mix with) water.
- The negatively charged hydroxide ion of the base (e.g. NaOH) attacks the partially positive carbonyl (C=O) carbon of fats/oils, leading to bond breaking between the glycerol and the carboxyl group present in fats/oils.
- The metal ion from the base (e.g. Na+ ) is attracted to the carboxylate produced after cleavage of fats/oils, forming a salt of fatty acid (soap).
- If the fatty acid was oleic acid and the base KOH, then the soap molecule produced would be potassium oleate,
- C18H33O2-K+
- Remember the general reaction as:
fatty acid plus alkali gives soap plus glycerol.
Structure of a soap
Soaps consist of a charged region (anionic head) and a non-polar region (tail) as shown in the diagram below (Davis, Disney, & Smith, 2018, p. 353).
Cleaning Action of Soaps
In order to understand the action of soap as a cleaning product we need to look at its chemical structure. We also need to understand the term surfactant. Surface active agents (surfactants) are chemical substances which decrease the surface tension of water. This enables them to “dissolve” dirt and grease. Cleaning agents such as soap are surfactants.
Soap has a long tail made up of many carbon-carbon bonds (with hydrogens attached). This part of the molecule is hydrophobic (water-fearing). However, because it is non-polar, it will dissolve other non-polar substances like oil or wax.
The other end of the soap, the head, is anionic. This end readily dissolves in water, it is hydrophilic.
As one part of the soap is dissolved in water while the other end is attached to oil, the soap molecule forms a bridge between the oil and the water. Hence it is able to move oily substances away from fabrics, fibres, ceramics or even skin. Several soap molecules attack the oily substance, lifting it away from the fabric. More soap molecules come in, surrounding the oil molecule with their tails attached to the oil and their heads sticking out forming a structure known as a micelle.
This keeps the oil molecules separate from one another as each is surrounded by a sphere of negative “heads” each repelling other similar complexes. The decrease in surface tension allows the water to combine with the oil molecules.
Soap, water and oil together form an emulsion with the soap acting as an emulsifier
An emulsion is formed when two or more normally immiscible substances form a stable mixture which does not separate. It is not a solution, as none of the molecules are associated with each other. Instead, one of the liquids is evenly dispersed (spread) throughout the other through the action of another chemical called an emulsifier.
Each emulsion has two different types. For example, if the two liquids were oil and water, the two emulsions would be:
-oil suspended in water e.g. sorbolene cream
-water suspended in oil e.g. mayonnaise
The use of soap as a cleaning agent is dependent on its ability to emulsify dirt/grease in water. Most dirt is non-polar and oils/grease are non-polar long-chain hydrocarbons. Water is polar, thus it is immiscible with dirt and grease.
When soap is placed in water and agitated, its ions dissociate (the positive ion takes no part in the cleaning action). The negative fatty acid ions, called the surfactant (surface acting agent), do not disperse evenly. Instead, they form clumps with the polar –COOH heads sticking outwards. The –COOH groups form hydrogen bonds with water. This spherical structure is called a micelle.
Non-polar grease/dirt molecules are surrounded and dissolved by the non-polar lipophilic (fat-loving) core of the micelle, thus forming an emulsion between water and oil. This emulsion keeps the grease molecules suspended, so it can then be carried away by water.
SOAP AS AN EMULSIFIER
An emulsion is a mixture formed when two or more normally immiscible substances form a stable mixture which does not separate. It is not to be confused with a solution, as none of the molecules are associated with each other. Instead, one of the liquids is evenly dispersed throughout the other through the action of another chemical called an emulsifier. Each emulsion has two different types. For example, if the two liquids were oil and water, the two emulsions would be:
-oil suspended in water e.g. sorbolene cream
-water suspended in oil e.g. mayonnaise
The use of soap as a cleaning agent is dependent on its ability to emulsify dirt/grease in water. Most dirt is non-polar and oils/grease are non-polar long-chain hydrocarbons. Water is polar, thus it is immiscible with dirt and grease.
When soap is placed in water and agitated, its ions dissociate (the positive ion takes no part in the cleaning action). The negative fatty acid ions, called the surfactant (surface acting agent), do not disperse evenly. Instead, they form clumps with the polar –COOH heads sticking outwards. The –COOH groups form hydrogen bonds with water. This spherical structure is called a micelle.
Non-polar grease/dirt molecules are surrounded and dissolved by the non-polar lipophilic (fat-loving) core of the micelle, thus forming an emulsion between water and oil. This emulsion keeps the grease molecules suspended, which can be carried away by water.
View videos:
EXPERIMENT 5.5.2a Saponification
Introduction
Saponification is the process of making soap. The ingredients to make soap (fat or oil, plus NaOH) are readily obtainable and cheap. The history of soap-making goes back many centuries and we can speculate that soap was discovered by accident, perhaps when hot animal fat dripped into the alkaline ashes of a wood fire during cooking.
Aim: To prepare soap via saponification and compare its properties with a commercial soap and a detergent.
Risk assessment:
- Warning: Wear safety glasses!
- Warning: Sodium hydroxide is caustic. Avoid skin contact. Clean up spills immediately.
- Observe proper lab attire to prevent caustic and corrosive sodium hydroxide from entering eyes and causing irritation and burns.
- Clean up any spillage and make sure the laboratory is well ventilated.
- There is a risk of spitting when the mixture is being heated. Observe continuous stirring to prevent the occurrence of explosion or solution spitting vigorously causing burns to skin and eyes
- Do not use the soap on skin as there may be excess NaOH.
Reagents:
- Sodium hydroxide (NaOH) pellets 1g weighed on watch glass
- Coconut or olive oil 5mL
- NaCl (solid) 18g weighed on filter paper
- deionised water 50mL
- ethanol 10mL
Equipment:
- watch glass
- 10mL measuring cylinder
- 100mL beaker
- stirring rod
- pipette
- Bunsen burner
- Tripod + wire gauze mat
- electronic balance
- paper towel
Method 1
1 Weigh 4 g of olive oil into a 150 mL beaker.
2 Add 10 mL of water, 1 g of NaOH and 10 mL of ethanol to the beaker.
3 Set up a water bath using a tripod, wire gauze, 250 mL beaker and Bunsen burner.
4 Heat the reaction mixture in the water bath and stir the mixture every five minutes. Note: After about 30 minutes of heating, the mixture will suddenly become creamy and then start to curdle. This signals that soap has been made.
5 While you are waiting, prepare a saturated salt solution for Step 7. Dissolve about 18 g of NaCl in 50 mL of water. Also set up a filtration apparatus for Step 7.
6 Stop heating after the soap forms. Allow the solution to cool by leaving it to stand for a few minutes.
7. Add the NaCl solution and bring back to boil, stirring continuously for another couple of minutes.
8. Decant the liquid layer on top of the curds (soap) and wash the raw soap with deionised water, using a wash bottle. Spoon out the rinsed soap onto a pad of paper towels. Gently pat to absorb excess water. .
Keep the soap for the succeeding analysis.
EXPERIMENT 5.5.2b Testing the soap
Comparison of prepared soap, commercial soap and detergent
Equipment:
- 9 test tubes
- test tube rack
- 10mL measuring cylinder
- vegetable oil 3mL
- pipette
- soap (from previous experiment)
- commercial soap and detergent (very small amount of each)
- hard water 30mL
- deionised water 100mL
Method:
- Transfer one-third of the soap into each of test tubes 1, 2 and 3.
- Transfer about the same amount of commercial soap into test tubes 4, 5 and 6.
- Transfer about the same amount of detergent into test tubes 7, 8 and 9.
- Add 10 mL of deionised water to test tubes 1, 4 and 7 using measuring cylinder.
- Add 3mL oil (pipette) and 10mL deionised water (measuring cylinder) to test tubes 2, 5 and 8.
- Add 10mL of hard water to test tubes 3, 6 and 9 using measuring cylinder
- Shake each test tube vigorously (stoppered with thumb) for the same amount of time
- Record whether an emulsion or separate layer was formed and height and density of the foam.
Results:
- The synthesised soap was found to foam less compared to commercial soap and detergent, which can be attributed to its simple composition (commercial soap and detergent contain a larger number of and more sophisticated ingredients).
- Detergent and lab soap will both form an emulsification.
- Detergent will produce more foam than lab soap.
5.5 investigate the structure and action of
b) detergents.
Comparing Soaps and Detergents
Synthetic surfactants include detergents. These differ from soaps both in terms of their structure and chemical composition, and their action in water.
Molecular Structure
Both soaps and detergents have a similar structure consisting of a hydrophobic tail and hydrophilic head, however soaps have an anionic head but synthetic detergents can have either an anionic, cationic or non-ionic (polar) head.
Chemical Composition
Soaps are the sodium or potassium salts of long chain fatty acids, detergents are hydrocarbons with either a sulfate (anionic), ammonium (cationic) or ethoxy (non-ionic) head.
Detergents are artificial soaps, invented during the 1940’s and have largely replaced soap in cleaning because they are more powerful emulsifiers and because they can be modified to specialised applications.While soaps are made from animal and vegetable oils and manufactured via saponification (heating triglycerides with a strong base), detergents are derived from petroleum. An alkanol from petroleum is reacted with sulfuric acid to a sulfur-containing acid, which is then reacted with a strong base.
Effect in hard water
Hard water contains significant concentrations of magnesium and calcium ions. Soap will not‘lather’ in hard water as it forms a precipitate ‘scum’ which accumulates on surfaces. Detergents will lather in hard water and they do not form precipitates.
Use detergent on a toothpick instead of dropping from the bottle.
Now quickly draw the "loaded" toothpick through the water. Do balls of milk form?
Anionic detergents: The original and still most widely used group of detergents. The structure of the molecules is very similar to natural soaps. They have a long non-polar tail and an anionic head (a sulfonate C-O-SO2-O-, similar to SO42-).
They work just like soaps but are slightly more effective.
They produce a lot of foam, which has no particular impact on their cleaning ability. They are mainly used for cleaning glass and ceramic dishes (dishwashing liquids) and clothing (laundry detergents), because these have negative surface charges that repel the anionic detergents, and so can be easily washed away, carrying the emulsified grease/dirt with them.
Cationic detergents: These have a region which is a derivative of ammonium (with one H on the NH4+ replaced with alkyl (carbon chain) groups. One or two of these are long chains.) The long chains are non-polar while the nitrogen region is water soluble.
They emulsify fats well, but are unsuitable for glass and ceramics, because they are attracted to their negative surface charges and so do not wash away easily.
They are commonly used for plastics, hair conditioners and fabric softeners because they are absorbed into the fibres, which reduces friction and static, thus causing fabric and hair to appear ‘fluffy’.
Other uses include germicides, mouthwash and antiseptic soaps, owing to their germicidal properties.They are also a component of many disinfectants and antiseptics.
Non-ionic detergents:
They have a similar tail to the other types of surfactants but the head is different (again!)
Non-ionic detergents do not form ions in a solution at all. Instead, along the hydrocarbon chain they contain hydrophilic groups (such as oxygen atoms, alcohol groups or polysaccharides), which form hydrogen bonds with water.
They produce less foam and are used for car-washing, cosmetics and dishwashing. One disadvantage is their loss of solubility in warm water.
Review activity for detergents
View videos:
- OC#28 https://www.youtube.com/watch?v=0tfV57LyqXE [17.18]
- Micelles https://www.youtube.com/watch?v=NjZDTiV2s_w 1.52
- Chemistry 101: How does soap work https://www.youtube.com/watch?v=g4Y6WKmW3pY 7.12
- Emulsions https://www.youtube.com/watch?v=Xe7WSl4EewI&list=PLeFSFSJ9WqSCW3lJ-NCLBmUIh1Os8fK3U&index=15 6.00
5.5.2 Saponification and test of the product
EXPERIMENT 1: Making the soap
Aim: To prepare soap via saponification and compare its properties with a commercial soap and detergent.
Reagents:
- Sodium hydroxide (NaOH) pellets 5g weighed on watch glass
- Coconut oil 5mL
- NaCl (solid) 10g weighed on filter paper
- 1 M NaCl solution 50mL
- deionised water 30mL
- food colouring 2-3 drops
Equipment:
- watch glass
- 10mL measuring cylinder
- 100mL beaker
- stirring rod
- pipette
- Bunsen burner
- Tripod + wire gauze mat
- electronic balance
- paper towel
Risk assessment:
- Observe proper lab attire to prevent caustic and corrosive sodium hydroxide from entering eyes and causing irritation and burns.
- Clean up any spillage and make sure the laboratory is well ventilated.
- There is a risk of spitting when the mixture is being heated. Observe continuous stirring to prevent the occurrence of explosion or solution spitting vigorously causing burns to skin and eyes
- Do not use the soap produced on skin as there may be excess NaOH.
Method
Soap Preparation (Saponification)
- Weigh and transfer 5g NaOH pellets to a 100-mL beaker.
- Dissolve the NaOH by adding 30 mL deionised water into the beaker.
- Add 5mL of coconut oil
- Gently boil the mixture using Bunsen burner for about 10 minutes with constant stirring.
- When the layer of oil has disappeared, allow the solution to cool by leaving it to stand for a few minutes.
- Add 10 g of solid NaCl (measured using electronic balance) to the solution and and bring back to boil, stirring continuously for another couple of minutes.
- Add 2-3 drops of food colouring.
- Decant the liquid layer on top of the curds (soap) and wash the remaining solid 2-3 times with NaCl solution.
- Tip the soap onto a paper towel and allow it to dry overnight.
- Keep the soap for the succeeding analysis.
Results:
Conclusion:
- Soap was produced via saponification reaction
EXPERIMENT 2: Testing the soap
Comparison of prepared soap, commercial soap and detergent
Equipment:
- 9 test tubes
- test tube rack
- 10mL measuring cylinder
- vegetable oil 3mL
- pipette
- soap (from previous experiment)
- commercial soap and detergent (very small amount of each)
- hard water 30mL
- deionised water 100mL
Method:
- Transfer one-third of the soap into each of test tubes 1, 2 and 3.
- Transfer about the same amount of commercial soap into test tubes 4, 5 and 6.
- Transfer about the same amount of detergent into test tubes 7, 8 and 9.
- Add 10 mL of deionised water to test tubes 1, 4 and 7 using measuring cylinder.
- Add 3mL oil (pipette) and 10mL deionised water (measuring cylinder) to test tubes 2, 5 and 8.
- Add 10mL of hard water to test tubes 3, 6 and 9 using measuring cylinder
- Shake each test tube vigorously (stoppered with thumb) for the same amount of time
- Record whether an emulsion or separate layer was formed and height and density of the foam.
Results:
- The synthesised soap was found to foam less compared to commercial soap and detergent, which can be attributed to its simple composition (commercial soap and detergent contain a larger number of and more sophisticated ingredients).
- Detergent and lab soap will both form an emulsification.
- Detergent will produce more foam than lab soap.
Prac 5.5.3 Soap as an emulsifier
Aim: To demonstrate the effect of soap as an emulsifier
Equipment:
- 2 x 250-mL large conical flasks
- Vegetable oil
- Distilled water
- Commercial soap
Method:
- Transfer about 50 mL distilled water to a large conical flask (Flask 1).
- Add about 50 mL vegetable oil to flask, shake and record the observation
- Obtain another conical flask (Flask 2) and add about 50 mL distilled water.
- Pour out 50 mL vegetable oil into the flask.
- Add 4-5 drops of liquid soap/detergent or a few flakes of solid soap.
- Shake and record the observation
Results/Observations: (see diagrams below)
Conclusion: What evidence suggests an emulsion has been produced?
- A cloudy layer of tiny oil droplets was observed upon addition of soap or detergent in the water + oil mixture, indicating that emulsion has occurred and that soap was an effective emulsifier.
- The oil and water formed a homogenous mixture with no distinct oil-water layers
TASK 5.5.1
PAST HSC QS
2017 Q31AI 2, Q31AII 3
2016 Q31D 6
2015 Q31BI 3; Q31BII 4
2012 Q34E 7
2010 Q32DII 2; 32DII 3
2009 Q27DII 2; Q27DIII 2
2006 Q29BIII 3
5.6 draft and construct flow charts to show reaction pathways for chemical synthesis, including those that involve more than one step.
We now have a very large number of organic substances which can be converted into other organic compounds. A flow chart is a convenient way of summarising this information and allowing us, at a glance, to identify the key functional groups and organic series and how they are related to one another.
We call halogenoalkanes haloalkanes
Test yourself with this blank
View Videos:
- Organic reaction pathways https://www.youtube.com/watch?v=bBXnowZAaFA [2.31]
- OC#29 https://www.youtube.com/watch?v=zgHMoG8bGEU&list=PLeFSFSJ9WqSDRKZVcK33Gtz60RsJ5_t7Q&index=35 [10.45]
TASK 5.6.1
1. Devise reaction pathways for the following reactions (Chan, et al., 2019, p. 370) (12 marks)
a) Ethanamine from ethene
b) Butan-1-ol from butane
c) Pentanoic acid from 1-chloropentane
d) Methanoic acid from methane
e) Ethanol from ethene
f) Propyl methanoate from propane and methane
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