CfE Higher Chemistry

Unit 2: Nature’s Chemistry

Flavour

15/03/2019

Learning Outcomes :

Flavour 15/03/2019

• Classify the kind of molecules make up many flavour and aroma molecules.

• Investigate the structures, functional groups and names of these molecules.

Lesson Starter: (Past paper 2008)

Aldehydes (Alkanals) • Alkanals are a homologous series of aldehydes.

• Aldehydes contain the C=O functional group, this is called the CARBONYL group:

Ethanal

General Formula CnH2nO

Methanal

The R group is a hydrogen or an alkyl group.

Naming Branched Aldehydes • When naming an Aldehyde there is no need to number the

position of the carbonyl group. It is always on carbon number 1.

• Aldehydes have names ending in -al.

3-methylpentanal

C C C C C

CH3H

H H H

H H

H

H

H

O

Ketones (Alkanones) • Alkanones are a homologous series of ketones.

• Ketones also contain a CARBONYL functional group but in ketones it is in the middle of a carbon chain.

Propanone CH3COCH3

Butanone CH3COCH2CH3

General Formula CnH2nO

The Aldehydes and Ketones are isomers of each other!

The R groups are alkyl groups.

Naming Branched Ketones • The C=O carbon is given the lowest possible number in the

parent chain.

• When naming a Ketone it is sometimes necessary to number the position of the carbonyl group in order to distinguish between isomers.

• Ketones have names ending in -one.

C C C C C

CH3

H H

H H

H

H

H

H

H

O

3-methylpentan-2-one

Examples of Aldehydes

CH

O

CH

O

HO

OCH3

CH=CH CH

O

Benzaldehyde Vanillin Cinnamaldehyde(almonds) (vanilla beans) (cinnamon)

8

C

CHO H

CH OH

C

CH2OH

OH

OHH

H

C

OH

glucose

Examples of Ketones

9

CH2OH

OCH3

CH3

OOH

O

Cortisone

C

C

CHO H

C

O

H OH

C

CH2OH

OHH

H2OH

D-Fructose

butanedione Butter flavor

heptan -2- one Clove flavor

Practice Question

For each of the following molecules, classify them as either an aldehyde or ketone and name the molecule.

10

CH2 C CH3CH3

O

CH2 CC

OCH3

CH3

CH3 H

O

Practice Question

Draw the structural formulas for the following molecules:

A. 3-Methylpentanal

B. 2,3-Dichloropropanal

C. 3-Methylbutan-2-one

11

The chemistry of flavour

Can you identify any functional groups in the above molecules?

Notes: • Many flavour and aroma molecules are

aldehydes

• Oxygen from the air causes oxidation of food

• The oxidation of edible oils gives food a rancid flavour

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The chemistry of flavour

Molecules responsible for flavour in vegetables are

normally trapped inside the cell walls.

During cooking the cell walls are damaged for two

reasons:

• Chemical damage occurs as the cell walls, which are

made of cellulose, break down.

• Physical damage occurs as water inside the cells boils

forming steam and the cell walls break open.

A major issue in cooking is to retain molecules responsible

for flavour in the food – overcooking can result in loss of

these molecules.

One destination for lost flavour molecules is in the cooking

water.

This will occur if the flavour molecules are water-soluble.

If this is the case, many of the flavour molecules will be lost

down the drain when the cooking water is poured away.

The chemistry of flavour

Practice Question Naming compounds

C

H

H

H C

H

H

C

H

O

C

H

H

H C

H

H

C

H

O

C

H

H

H

C

O

C

H

H

H

C

H

H

H

C

O

C

H

H

H

C

H

H

H C

H

H

C

O

O

H

C

H

H

H C

H

H

C

O

O

H

C

H

H

H C

H

H

C

H

H

H

C

H

H

H C

H

H

C

H

H

H

C

H

H

H

C

H

C

H

H

C

H

H

H

C

H

C

H

H

C

H

H

H C C H

C

H

H

H C C H

C

H

H

H C

H

H

C

H

H

N

H

H

C

H

H

H C

H

H

C

H

H

N

H

H

C

H

H

H

C

O

O

C

H

H

H

C

H

H

H

C

O

O

C

H

H

H

H

O

C

O

C

H

C HH

H

N

H

H

H

O

C

O

C

H

C HH

H

N

H

H

Which compound shown is propyne?

propyne

aminopropane?

aminopropane

propanone?

propanone

propanoic acid?

propanoic acid

propanal?

propanal

2-aminopropanoic acid?

2-aminopropanic acid

propane?

propane

methylethanoate?

methylethanoate

propene?

propene

1 2 3 4 5 6 7 8 9

Success Criteria:

Next Lesson:

Many flavour and aroma molecules are aldehydes

Aldehydes and ketones both contain the carbonyl functional group.

Aldehydes and ketones can be identified from the ‘-al’ and ‘-one’ name endings.

Straight-chain and branched-chain aldehydes and ketones can be named from structural formulae.

Given the names of straight-chain or branched-chain aldehydes and ketones, structural formulae can be drawn and molecular formulae written.

Oxidation

Flavour 15/03/2019

CfE Higher Chemistry

Unit 2: Nature’s Chemistry

Oxidation

15/03/2019

Learning Outcomes :

Oxidation 15/03/2019

• Explain what is meant by the term Oxidation when referring to carbon compounds.

• Compare the oxidation reactions of Primary, Secondary and Tertiary alcohols.

Notes: For carbon compounds:

• Oxidation is an increase in the oxygen to hydrogen ratio

• Reduction is a decrease in the oxygen to hydrogen ratio

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Oxidation of food

Classification of Alcohols

• Primary Alcohols (1°) – the carbon atom to which the hydroxyl group is attached to is bonded to no more than one other carbon. The other bonds are to hydrogen atoms.

• Secondary Alcohols (2°) – the carbon atom to which the hydroxyl group is attached to is bonded to two other carbon atoms. Only one bond is to a hydrogen atom.

• Tertiary Alcohols (3°) – the carbon atom to which the hydroxyl group is attached to is bonded to three other carbon atoms. There are no bonds to hydrogen atoms.

Notes: • The oxidation of an organic compound causes the

oxygen to hydrogen ratio within the molecule to increase.

• This can be achieved by either removing hydrogen from the molecule or adding oxygen to it.

• Oxidation is – loss of electrons

- loss of hydrogen (H2)

- gain of oxygen ( O )

Oxidation of Alcohols

Notes: Primary alcohols can be oxidised in two stages.

The first stage changes the primary alcohol to an aldehyde.

This step involves the loss of hydrogen.

Primary Alcohol Aldehyde

Primary Alcohols

Oxidising Agent

• In the second stage of oxidation, the aldehyde is further oxidised to produce a carboxylic acid.

• This step involves the addition of oxygen.

Aldehyde Carboxylic acid

Oxidising Agent

Notes: • Secondary Alcohols can be oxidised in one

stage to produce ketones.

• This involves the addition of oxygen.

Secondary Alcohol Ketone

Secondary Alcohols

Oxidising Agent

Notes: • Tertiary Alcohols cannot be oxidised readily.

• This is because there is no H directly attached to the Carbon which contains the hydroxyl (OH) group to be removed.

Tertiary Alcohols

Oxidising Agents

In order to oxidise a primary or secondary alcohol an appropriate oxidising agent should be used. Commonly used oxidising agents are listed below:

• Acidified potassium dichromate solution

• Benedict’s solution / Fehling's solution

• Tollen’s solution

• Acidified potassium permanganate solution

• Heated solid copper (II) oxide

Success Criteria:

Next Lesson:

When applied to carbon compounds, oxidation results in an increase in the oxygen to hydrogen ratio

Primary alcohols are oxidised, first to aldehydes and then to carboxylic acids.

Secondary alcohols are oxidised to ketones.

Tertiary alcohols cannot be oxidised.

Oxidation Reactions

Oxidation 15/03/2019

CfE Higher Chemistry

Unit 2: Nature’s Chemistry

Oxidation Reactions

15/03/2019

Learning Outcomes :

Oxidation Reactions 15/03/2019

• Explain the oxidation reactions of Aldehydes and Ketones.

• Familiarise yourself with different oxidising agents that can be used in the lab.

• Identify the colour changes associated with the use of different oxidising agents.

Lesson Starter:

1. Which box shows the full structural formula of an alkanal?

2. Which box (or boxes) contain(s) an alkyne?

3. Which box (or boxes) contain(s) an alkene?

4. Which box (or boxes) contain(s) an alkanoic acid?

5. Which box (or boxes) contain(s) a ketone?

6. Which box (or boxes) contain(s) an alkanol?

Oxidation

Primary

Alcohol Secondary

Alcohol

Tertiary

Alcohol

Aldehyde Ketone

Carboxylic

acid No Reaction

No Reaction

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Experiment:

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Oxidation of Alcohols Expt 2.13

Notes:

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Experiment:

1. Before collecting the carbonyl compounds X and Y set up a water bath.

2. Add sulphuric acid to each of two test tubes to a depth of about 2 cm. Then

add potassium dichromate solution to both to give a total depth of about 3 cm

in each.

3. To one of these test tubes add about 5 drops of compound X and to the other

add about 5 drops of compound Y.

4. Label both test tubes, place both test tubes in the water bath and observe and

record any changes.

5. Add Benedict's solution to each of two test tubes to a depth of about 3 cm.

6. Repeat steps 3 and 4.

7. Add Tollens' reagent to each of two very clean test tubes to a depth of about 3

cm.

8. Repeat steps 3 and 4 and immediately after, wash the contents of the test

tubes down the drain with large amounts of water.

9. Record your observations.

Oxidation of Aldehydes and Ketones

Expt 2.14

Experiment: • Flammable, vapours irritate the eyes, skin and lungs, toxic by skin absorption

and by swallowing.

• Potassium dichromate is toxic if swallowed. It is carcinogenic and very toxic

by inhalation. It is also a skin sensitiser and is very toxic to the aquatic

environment.

• Sulphuric acid irritates the eyes.

• Benedict's solution contains copper salts and so is harmful if swallowed.

• Tollens' reagent contains diluted sodium hydroxide which irritates the skin

and eyes.

Hazards

Care

•Wear eye protection and immediately wash off any

chemical spillages on the skin.

•When working with Tollens' reagent and compounds

X and Y wear gloves.

Expt 2.14

Notes:

Oxidising Agents Colour Change

Acidified potassium dichromate • Acidified potassium dichromate

solution is used as an oxidising agent.

• The alcohol is added to the orange solution. On warming the solution, the orange dichromate ions are reduced to green Cr3+ ions.

The ion–electron equation shows that this reaction will only occur in the presence of H+ions. This explains why acidified

dichromate must be used.

Benedict's solution

• Benedict's solution can be used as an oxidising agent

• When an aldehyde is heated with Benedict's solution for a few minutes in a hot water bath, the blue solution slowly produces a red-orange precipitate of copper(I) oxide.

• Cu2+ ions are reduced to Cu+ ions.

Tollens' reagent

• Tollens' reagent can be used as an oxidising agent

• When an aldehyde is heated with Tollens' reagent for a few minutes in a hot water bath, the colourless solution slowly produces a silver mirror on the inside of the test tube.

• Ag+ ions are reduced to metallic silver.

Copper(II) oxide

• Copper(II) oxide is used as an oxidising agent

• The copper(II) oxide is heated strongly and the alcohol vapour is passed over it.

• When the alcohol is oxidised, the black copper(II) oxide is reduced to pinkish brown copper.

Cr2O7

2-(aq) ions reduced to Cr3+

Specific Tests for Aldehydes Acidified Potassium Dichromate

• oxidises primary and secondary alcohols.

• Oxidise aldehydes to carboxylic acids.

• It cannot be used as the specific test for aldehydes.

Benedict’s/ Fehling’s Solutions and Tollen’s Reagent

• Oxidise aldehydes to carboxylic acids.

• Will NOT oxidise an alcohol

Oxidation of Alcohols

Oxidising agent Colour Change Reaction

Acidified permanganate Purple Colourless MnO4- Mn2+

Hot copper(II) oxide Black copper oxide brown copper metal

Cu2+ + 2e Cu(s)

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Oxidising agent Colour Change Reaction

Acidified potassium dichromate H+ / Cr2O7

2-

Orange Blue/Green

Cr2O72- Cr3+

Fehling's / Benedict's Solution

Blue orange / red Cu2+ + 2e Cu(s)

Tollen’s Reagent Colourless Silver Ag+ + e Ag(s)

Oxidation of Aldehydes

Notes: • Hot copper(II) oxide or acidified dichromate(VI)

solutions can be used to oxidise: • primary alcohols to aldehydes and then to carboxylic

acids

• secondary alcohols to ketones

• During these reactions black copper(II) oxide forms a brown solid, and orange dichromate solution turns green.

• Tertiary alcohols cannot be oxidised using these oxidising agents.

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Notes: • Aldehydes, but not ketones, can be oxidised to

carboxylic acids.

• Oxidising agents can be used to differentiate between an aldehyde and a ketone.

• With an aldehyde:

• blue Fehling’s solution forms a brick red precipitate

• clear, colourless Tollens’ reagent forms a silver mirror

• orange acidified dichromate solution turns green

15/03/2019

Practice Question

1. Primary alcohols may be oxidised to carboxylic acids in two stages.

a) Draw the full structural formula for each product obtained by the oxidation of the following compound.

b) Name each of the products.

c) What colour change is observed when the compound is oxidised by acidified potassium dichromate solution?

Success Criteria:

Next Lesson:

In the laboratory, hot copper(II) oxide or acidified dichromate(VI) solutions can be used to oxidise primary and secondary alcohols.

Aldehydes, but not ketones, can be oxidised to carboxylic acids.

Fehling’s solution, Tollens’ reagent and acidified dichromate solution can be used to differentiate between an aldehyde and a ketone

Antioxidants

Oxidation Reactions 15/03/2019

CfE Higher Chemistry

Unit 2: Nature’s Chemistry

Antioxidants

15/03/2019

Learning Outcomes :

Antioxidants 15/03/2019

• Describe the result of the reaction of oxygen with edible oils.

• Explain what is meant by antioxidants.

• Write ion-electron equations for the oxidation of antioxidants.

Oxidation of Food • Oxidation of food can occur when food is exposed to

oxygen in the air.

• Foods which contain fats and oils are at a particularly high risk of oxidation.

• The oxidation of unsaturated oils and fats primarily takes place via a free-radical-mediated process and can lead to rancidity.

• Antioxidants are molecules that reduce the rate of oxidation reactions involving the transfer of electron(s) to an oxidising agent. Antioxidants are often added to foodstuffs to minimise oxidative damage.

Oxidation of food (2.15)

• When fats react with oxygen they are broken down,

causing:

• deterioration of flavour (rancidity)

• loss of colour

• loss of nutritional value

• a health risk from toxic oxidation products.

• As the fat decomposes and reacts with oxygen,

chemicals called peroxides are produced.

• These change into the substances characteristic of the

smell and soapy flavour of a rancid fat.

Antioxidants prevent the formation of peroxides and so

slow the process of the food 'going off'.

Some antioxidants react with oxygen itself and so

prevent the formation of peroxides.

Air-tight packaging, using inert gases like nitrogen,

vacuum packing and refrigeration can all be used to

delay the oxidation process. However, these can still be

inefficient and adding antioxidants can be an effective

way of extending the shelf life of a product.

Oxidation of food

Antioxidants in action

Oxidation occurs when the apple is left exposed to air

The apple is protected when dipped in orange juice containing the antioxidant vitamin C

Vitamin C (ascorbic acid)

• The antioxidant vitamin C can act as a

reducing agent (electron donor),

preventing oxidation (electron loss)

from the foodstuff.

C6H8O6 C6H6O6 + 2H+ + 2e-

Ascorbic acid Dehydroascorbic acid

Oxidative damage • Oxidation reactions can produce free radicals.

• A free radical is a highly reactive species containing an unpaired electron.

• Free radicals can damage food by removal of an electron.

• Antioxidant molecules ‘mop up’ free radicals to protect the foodstuff.

• The antioxidant molecule donates an electron to the potentially damaging free radical.

• A stable electron pair is formed, stabilising the free radical.

• The antioxidant itself becomes oxidised (loses an electron).

We will look at free radical reactions in more detail later.

Damaging free radical

Electron transferred

Antioxidant Antioxidant converted to a stable free radical

Neutralised free radical

Radical now in a stable pair

How does an antioxidant cancel out a free radical?

Antioxidant E-number Typical foods

Ascorbic acid

(vitamin C) E300

Beers, cut fruits, jams, dried potato. Helps to

prevent cut and pulped foods from going brown by

preventing oxidation reactions that cause the

discolouration. Can be added to foods, such as

potato, to replace vitamin C lost in processing.

Tocopherols E306

Oils, meat pies. Obtained from soya beans and

maize. Reduces oxidation of fatty acids and some

vitamins.

Butylated

hydroxyanisol

e (BHA)

E320

Oils, margarine, cheese, crisps. Helps to prevent the

reactions that break down fats and cause the food to

go rancid .

Citric acid E330

Jam, tinned fruit, biscuits, alcoholic drinks, cheese,

dried soup. Naturally-occuring in citrus fruits like

lemons. Helps to increase the anti-oxidant effects of

other substances. Helps to reduce the reactions that

can discolour fruits. May also be used to regulate pH

in jams and jellies.

typical antioxidants:

http://www.understandingfoodadditives.org

Antioxidants and health benefits • There may be health benefits from the use of antioxidants.

• Oxidation reactions in the body could be linked to the build-up of fatty deposits that cause blockages in arteries that can cause heart attacks.

• Antioxidants may be important in preventing this and there could also be a link with the prevention of certain cancers, arthritis and other conditions.

• The picture is not yet clear and a great deal of research needs to be undertaken.

http://www.understandingfoodadditives.org

Studies involving 230,000 men and women across the UK have shown

that there is no convincing proof that antioxidants have any effect on

how long people can live.

However 40% of women and 30% of men are reportedly taking these

supplements and spending over £333 million on them per year.

Free radicals in living cells • Free radicals are present in all living cell and are a part of the

cell processes. However excessive free radicals in our cells can attack the cell membranes (the outer coat of the cell). This attack causes cell and tissue damage.

• Radicals can also break strands of DNA (the genetic material in the cell). Some of the chemicals known to cause cancer, do so by forming free radicals.

• The imbalance between free radicals and antioxidants can lead to disease and ill health.

• The 4 main non-enzymatic antioxidants metalonin, α-tocopherol (Vitamin E), ascorbic acid (Vitamin C) and β-carotene (precursor for Vitamin A) can be found in a range of foods in our diet.

• However medical opinions are divided as regards the impact these antioxidants have our on general health.

Melatonin

This is a hormone which helps to regulate sleep in our

bodies. This compound can be termed as a terminal

antioxidant as once it has removed the free radicals it

has to be replaced.

OCH3

NH NH CH3

O

α-tocopherol

This is a form of vitamin E and can be found in

vegetable oil, nuts and seeds. It has been suggested

that it is good for the skin.

O

OH

CH3CH3

CH3

CH3

CH3CH3H H

CH3

CH3

Ascorbic Acid

This is also known as Vitamin C and is commonly found in

fruits and vegetables. It is one of the essential vitamins

and the human body is unable to synthesize it. It can be

easily oxidised and acts as a hydroxyl or superoxide anion

radical scavenger.

OO

OHOH

OHH

OH

CH3CH3

CH3CH3

CH3

CH3CH3

CH3CH3

CH3

β-carotene

This is a precursor to vitamin A. It is a highly red-orange

pigment found in plants and fruits. In particular it gives

carrots their orange colour. It helps human cells to

absorb vitamin A.

Notes: • Antioxidants are molecules that prevent

unwanted oxidation reactions occurring

• They are substances that are easily oxidised, and oxidise in place of the compounds they have been added to protect

• Antioxidants can be identified as the substance being oxidised in a redox equation

15/03/2019

Learning Outcomes :

Antioxidants 15/03/2019

• Oxygen reacts with edible oils giving the food a rancid flavour.

• Antioxidants are molecules which will prevent these oxidation reactions taking place.

• Ion-electron equations can be written for the oxidation of antioxidants.