CfE Higher Chemistry Unit 2: Natures Chemistry

Esters, Fats and Oils, Soaps

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Lesson Starter: Name / Draw these molecules

a) b)

c) d)

e) 2-methylpentan-1-olf) Pentanoic acid

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Learning Outcomes :

Esters

• 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

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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 (-COO- or -OCO-)

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.

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EstersEsters are formed when an alcohol reacts with a carboxylic acid, water is also produced during this reaction.

Carboxylic acid + Alcohol → Ester + water

+ → + H2O

Ester link

The functional group of an ester is the ester link

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Naming EstersThe name of the ester is made up of two parts the alcohol (__yl) part and the carboxylic acid (___anoate) part.

ethyl propanoate made by reacting ethanol with propanoic acid

C

H

C

H

O C

O

C

H

H

H

C

H

H

H

H

H

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Naming Esters

methanol + methanoic acidà methyl + watermethanoate

CH

H

H

OH C

O

OH H C

H

H

H

O C

O

H

Ester link

Studies have revealed that the oxygen atom in the main chain of the ester always comes from the alcohol.

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Practice QuestionName 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. Octanol & Octanoic acid

7. Ethanoic acid & Heptanol

Propyl ethanoate

Butyl pentanoate

Methyl propanoate

Pentyl methanoate

Butyl butanoate

Octyl octanoate

Heptyl ethanoate

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Practice Questions

Now try the practice questions on the worksheet.

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Making Esters

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Esters can be prepared by reaction of an alcohol with a carboxylic acid:

R O H H OC R'

O

+ R OC R'

O+ H2O

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 mixture2) using concentrated sulphuric acid as a catalyst.

The presence of the concentrated sulphuric acid also increases the yieldof ester.

Experiment:

Acid

Alcohol

Collect an experiment work

card from the front.

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Making esters

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?

Notes: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.

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Properties of Esters

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:

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Name Shortened Structural Formula

Odour/Flavour

CH3(CH2)4CH3 BananaMethyl Butanoate Pineapple

3-Methylbutyl Butanoate CH3(CH2)2(CH2)2CH(CH3)2 Apple

CH3COOC3H7 PearMethyl-1-butyl ethanoate CH3COOCH(CH3)C4H9 Banana

2-Methylpropyl methanoate

Raspberry

C3H7COOC5H11 Apricot, StrawberryBenzyl ethanoate CH3COOCH2C6H5 Peach, flowers

Methyl 2-aminobenzoate C6H4(NH2)COOCH3 GrapesBenzyl butanoate C3H7COOCH2C6H5 Cherry

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Uses of esters: Solvents

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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.

Uses of esters: DecafinationEthyl 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.

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Experiment:

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Decafination

Caffeine (C8H10N4O2) 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.

Uses of esters

.

Caffeine is more soluble in theorganic solvent ethyl ethanoatethan in water, so we will extractcaffeine into the organic solvent toseparate it from glucose, tannins,and other water soluble compoundsusing a separating funnel.

The ethyl ethanoate portions can becombined and the ethyl ethanoateremoved by evaporation to leavethe caffeine

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Hydrolysis of Esters

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Alcohol + Carboxylic Acid ⇌ Ester + Water

Condensation

Hydrolysis

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

O

O R H

O

H+

Bonds broken

Ester + Water

+R C

O

HO

RH

O

Bonds formedCarboxylic Acid + Alcohol

Hydrolysis of an Ester• To reverse the formation of ester (ie hydrolyse it)

we need to react the ester with water.

• However 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.

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Starter Questions

1) ethyl ethanoate

2) propyl butanoate

3) butyl pentanoate

4) methyl hexanoate

5)

6) pentyl methanoate

7) CH3CH2CH2OOCCH3

8) CH3CH2OOCCH2CH2CH3

9) CH3OOCH

10)

Name the products formed by hydrolysis of each of the following compounds:

Fats and OilsLearning Outcomes

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.

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. Oils, are thought to be healthier than solid fats, as they are less likely to be deposited inside our arteries. However, there is an ongoing debate about which fats are better for us.

• Polyunsaturated fats are considered to be less potentially harmful to the heart.

Fats and oils

Naturally occuring

Animal fat Vegetable oil Marine oil

lardsuet

sunflower oilcoconut oil

cod liver oilwhale oil

Fats and OilsFats 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 yourbrain is fat!

Each -OH 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).

C

C

C

O

O

O

H

H H

H H

H

H

H

Glycerolpropan-1,2,3-triol

Fats and oils

glycerol

Systematic name is propane-1,2,3-triol

Both fats and oils are built from glycerol; an alcohol with three -OH groups.

Stearic acid

Systematic name is octadecanoic acid

The other components of fat molecules are carboxylic acids. Long chain carboxylic acids are known as Fatty Acids.One such fatty acid is Stearic acid:

Examples of Fatty AcidsCH3(CH2)16COOH

CH3(CH2)7CH=CH(CH2)7COOH

Stearic Acid (suet, animal fat) Saturated

Oleic Acid (olive oil) Unsaturated

C17H35COOH

C17H33COOH

CH2CH2

CH2CH2

CHCH

CH2CH2

CH2CH2

CH2CH2

CH2C

O

OHCH2

CH2CH2

CH3

CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2C

O

OH

CH3

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

glycerol and long chain carboxylic acids

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 actuallyshaped more like this:

Fats are mainly built from carboxylic acids with C-C single bonds. (SATURATED)

Oils have at least one C=C bonds in the carboxylic acids from which they are made. (UNSATURATED)

Stearic acid in beef fat

Oleic acid in olive oil

Oil

Fat

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 molecules pack together more tightly, making fats solid at room temperature.

In practice both fats and oils are mixtures of esters containing both saturated and unsaturated compounds.

Beef Fat

Olive oil

In general oils have a higher proportion of unsaturated molecules.

Notes:• How do the melting points of the fats compare

with the melting points of the oils?

• What is meant by saturated and unsaturated?

• How 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.

Unsaturation in fats and oils1.Using a plastic pipette, add five drops of olive oil to 5 cm3 of hexane in a conical flask.

2. Use a burette filled with a dilute solution of bromine water (0.02 mol dm–3) (Harmful and irritant).

3. Read the burette.

Unsaturation in fats and oils4. 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).

Fats and OilsThe degree of saturation in a fat or oil can be determined by theIodine 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.

Omega 3 fatty acids make up a large % of your brain’s fat.

Solid fats – butter, beef fat & lard have low iodine numbers because they are more saturated than the unsaturated oils.

Fat Av Iodine No

Butter 40Beef Fat 45

Lard 50Olive Oil 80

Peanut Oil 100Soya Bean

Oil180

Margarine is made from vegetable oils, butter from animal fats. One reason why margarine spreads better!

Hydrogenation• 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 wouldbe a liquid when taken out of the fridge.

Lesson Starter: Hydrolysis of Esters

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Learning Outcomes :

Soaps and Emulsions

• explain how soaps are produced • relate the cleansing action of soaps to the structure of the

soap molecules.

Structures of fats and oils - Revision• Fats and Oils are esters of glycerol and long chain fatty acids.• Hydrolysis of a fat or oil produces glycerol (alcohol) and 3

carboxylic acids / fatty acids.

(R 1, 2, 3 are long carbon chains, which can be the same or different)

Glycerol part

Fatty acid part

Hydrolysis

Fatty Acids + Glycerol

C

C

C

O

O

O

H

H C

H C

C

H

H

O

R1

O

R2

O

R3

Hydrolysis

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• To hydrolyse an ester we need to react it with water.

• However 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:

SoapsSoaps 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:

Glyceryl tristearate

+ 3NaOH + 3 Na+

GlycerolSodium stearate

(soap)

C

C

C

O

O

O

H

H C

H C

C

H

H

OC17

H35

OC17

H35

OC17

H35

C

C

C

O

O

O

H

H H

H H

H

H

H

C17H35COO--

Structure of Soaps

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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’.

COO- Na +Hydrophobic

tailHydrophilic

head

Structure of soapThe 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:

COO- Na +Hydrophobic

tailHydrophilic

head

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)

Cleansing action of soaps

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.

The attraction of the head group to the surrounding water,via polar-to-polar interactions, is so strong that it causesmechanical lift of the grease molecule away from the materialon which it was deposited. The hydrophobic tails are anchoredinto the grease due to non-polar to non-polar attraction. Incombination, these effects allow for the removal of thegrease stain.

MICELLE

OOC

COO

OOC

COO

OOC

OOC

COO

COO

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

-

-

-

-

-

-

-

-

greaseparticle

O HH

O HH

O HH

O HH

O HH

O HH

O HH

O HH

OHH

OHH

OHH

OHH

OHH

OHH

OHH

O HH

O HH

O HH

Success Criteria:

Next Lesson:

ü I can explain how soaps are produced by alkaline hydrolysis of fats and oils

ü I can relate the cleansing action of soaps to the hydrophobic and hydrophilic nature of soap molecules.

Emulsions

Soaps

Lesson Starter:

Learning Outcomes :

Emulsions

• Describe the characteristics of an emulsion, and study thechemistry of typical emulsifier molecules.

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, asoap-like molecule known as an emulsifier is added.

Experiment: EmulsifiersExpt 2.8 (b)

Emulsifier moleculesEmulsifiers for use in food are commonly made by reacting edible oilswith glycerol to form molecules in which either one or two fatty acidgroups are linked to a glycerol backbone rather than the three normallyfound in edible oils.

The one or two hydroxyl groups present in these molecules arehydrophilic 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.

EmulsifiersMayonnaise contains oil and water.The emulsifier keeps these mixed and without it the oil andwater separate.

Emulsifiers in foodEmulsifiers are among the most frequently used types of food additives. They are used for many reasons:

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

• 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.

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

Success Criteria:

Next Lesson:

• 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.• I can explain why emulsifiers are added to food.• I can describe how emulsifiers are made and how they

work

Proteins

Soaps and Emulsions