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CfE HigherChemistry

Unit 2: Natures Chemistry

Part 2: Oxidation, Proteins Fragrances and

Skin CareSummary Notes

Success CriteriaHillhead High SchoolChemistry Department

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Learning Intention Completed?

1. I can identify an amino acid and recognise the amino and carboxyl functional groups.

2. I can combine amino acids by forming amide links and know that this is a condensation reaction.

3. I understand that joining amino acids makes proteins and that the sequence of amino acids determines the properties of the protein formed.

4. I know that proteins are the major structural materials of animal tissue and that proteins are involved in the maintenance and regulation of life.

5. I know that enzymes are examples of proteins.6. I can draw the amino acids formed from the hydrolysis

of protein.7. I know what is meant by the term ‘essential amino

acid’.8. I can predict whether a molecule is likely to be fat/oil

soluble or water soluble by examining the functional groups present.

9. I can predict how volatile a molecule is likely to be by examining the size and the structure of the molecule.

10.

I can describe how heating a protein can change the structure of the protein.

11.

I can state whether an alcohol is primary, secondary or tertiary and whether it is likely to be oxidised.

12.

I can name common agents capable of oxidising alcohols and alkanals.

13.

I can name and draw the products formed when an alcohol or aldehyde is oxidised.

14.

I can describe the oxidation of a carbon compound in terms of the oxygen:hydrogen ratio.

15.

I can state the functional group of an antioxidant.

16.

I can define the term essential oil.

17.

I can identify and draw the terpene unit, isoprene.

18.

I can state whether or not a compound is a terpene.

19.

I can predict the products formed when a terpene is oxidised.

Hillhead High SchoolChemistry Department

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20.

I understand the cause of sunburn and the role of sun block in preventing sunburn.

21.

I know what a free radical is and can write equations for free radical reactions which include the initiation, propagation and termination steps.

22.

I know what a free radical scavenger is and can give examples of their use.

Key TermsTerm Definition

Amide link A group of atoms formed by the condensation polymerisation of amino acids in the formation on protein chains. The amide link is -CO-NH- and occurs


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between each pair of amino acid residues in the chain. Also called a peptide link.

Amino acid Compounds of the general formula H2NCHRCOOH – where R is, for example, H, CH3<C6H5CH2 – which link by condensation reaction to form proteins.

Essential amino acid

Amino acids which cannot be synthesised by animals and must be present in their diet.

Peptide link Same as an amide link but found in proteins exclusively.

Aldehyde Carbon compounds which contain the –CHO functional group. They are formed by oxidation of primary alcohols and they oxidise to form carboxylic acids. They are also known as alkanals.

Antioxidant Compounds that slow oxidation reactions. They are commonly added to food to prevent edible oils becoming rancid. Examples include vitamin E and C.

Carbonyl Group The functional group present in ketones, C=O (also present in aldehydes as part of their functional group –CHO).

Ketone Carbon compounds that contain the carbonyl group, C=O. They are formed from the oxidation of secondary alcohols. Unlike aldehydes ketones cannot be oxidised using mild oxidising agents.

Essential Oil Oils extracted from plants. They usually have distinctive smells, are non-polar, volatile and often contain carbon compounds known as terpenes.

Terpene Unsaturated compounds found in may plant oils. They are formed from the joining together of isoprene units.

Free radical Highly reactive atoms or molecules with unpaired electrons.

Free Radical scavenger

A compound added to plastics, cosmetics and foods to prevent free radical reactions. They react with free radicals to produce stable molecules. This terminates the reaction.

1. Proteins

Proteins are an essential part of our diet. We can obtain protein from many different sources including:

Fish Meat Eggs Cheese Pea and beans


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Proteins are the major structural material of all animal tissue. They can be either fibrous or globular.Protein Where

Found?Function Fibrous or

Globular?Collage Tendons,

muscle and bone

Structural support

Fibrous

Keratin Hair, skin and nails

Structural support

Fibrous

Myosin Muscles Helps muscles to contract

Insulin Pancreas Hormone which helps regulate blood sugar

Globular

Haemoglobin Red blood cells

Transports oxygen around the body

Globular

Immunoglobins

Blood, tears, aliva, skin

Fight infections

Globular

Amylase Saliva and pancreas

An enzyme that breaks down starch

Globular

Proteins are a type of polymer (long chain molecule) made up of thousands of amino acids joined together. The huge variety of proteins is because the amino acids can arrange themselves in many different ways. This is the basic structure of an amino acid:

Amino acids contain the amine or amino group –NH2 and the carboxyl group –COOH. For different amino acids the ‘R’ group changes.


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Some different amino acids are shown in the diagram to the left. They each have a different ‘R’ group attached to the central carbon.

Amino acids join together via a condensation reaction, whereby the protein is made as well as water.


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When the amino acids join together a new bond forms between the carbon atom of one amino acid and the nitrogen atom of the other. This area is now called a peptide link.The amino acids that join together may be different from one another resulting in differing repeating units along the protein polymer, as shown below.

Proteins can be broken down into amino acids via hydrolysis. In practice hydrolysis would be carried out with hydrochloric acid to avoid the reaction reversing.

Our bodies make amino acids every day, however there are some amino acids that cannot be made by the body and these are known as essential amino acids. These must be present within our diet and is the reason that having a


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balanced diet is so important. Only 8 amino acids are regarded as being essential for humans, with a further two essential in and during childhood.

2. The Chemistry of Cooking and the Oxidation of Food

The flavour of food is a combination of taste and aroma and the unique flavour of food is down to the combination of molecules present within them. Some common flavour molecules are shown below and their sources.Name Structure Where found?Limonene Lemons and

oranges

Furaneol Strawberries

Vanillin Vanilla pods


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Eugenol Bay leaves/ clove oil

Capsaicin Chilli peppers

Molecules that evaporate easily are said to be volatile and contribute towards aroma and hence flavour.An examination of the size and structure of a molecule will let us assess how volatile it is likely to be. Volatility is dependent upon the size of the molecule. A molecule with a mass of under 300 will be likely to be volatile, whereas larger molecules are not. The type of bonding present will also influence how volatile a molecule will be. If the molecule has strong intermolecular bonds it is likely to be less volatile than a molecule of similar mass which contains weaker intermolecular forces.

Flavour molecules will have different solubilities depending upon the functional groups present. Limonene does not dissolve in water, as water is a polar solvent and limonene is non-polar. Furaneol and vanillin are water soluble as they contain polar carbonyl and hydroxyl groups which can hydrogen bond to polar water molecules. Eugenol and capsaicin are less soluble in water due to the bulky non-polar carbon chains.

When we heat food the structure of it changes, this is particularly apparent when we heat proteins. This change is due to the shape of the protein chains changing, this is known as denaturing and also happens to enzymes. Proteins contain polar peptide links which hydrogen bond to other peptide links on neighbouring chains. When the protein is heated these intermolecular bonds break, making the protein uncoil and change shape.


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Enzymes are examples of globular proteins and work on the mechanism known as the ‘lock and key’ mechanism. Most of the time an enzyme will have a specific chemical reaction it can catalyse.

Enzymes are sensitive to pH and temperature and become denatured out with their optimum pH and temperature range.

Enzymes have optimum temperatures at approximately body temperature (37⁰C) and work best at neutral pH values (7).


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After these values the activity of the enzyme quickly decreases as the enzyme becomes denatured.

Oxidation

Preventing food from coming into contact with oxygen will maintain its freshness. This is because it prevents molecules from within the food reaction with oxygen and spoiling. This is known as oxidation. Organic oxidation is better described as increasing the oxygen atom to hydrogen atom ratio.

Oxidation of Alcohols

Alcohols are good sources of fuel as they burn to release energy. The balanced equation for the combustion of ethanol (a common alternative fuel) is shown below

C2H5OH (l) + 3O2 (g) 2CO2 (g) + 3H2O (l)This is complete oxidation of an alcohol. This occurs when there is a plentiful supply of oxygen and lots of energy is applied at the beginning of the reaction. A much milder form of oxidation of alcohols can occur which does not involve such a drastic change to the structure of the alcohol. The products made after oxidising an alcohol depends on the type of alcohol you start with. A summary of the products is included belowClassification of Alcohol

Example of alcohol Product

Primary

Ethanol ethanalSecondary Propan-2-ol

propanone


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Tertiary 2-methyl propan-2-ol No reaction!

Alcohols can be oxidised using a mild oxidising agent e.g. acidified dichromate solution. The dichromate ion (Cr2O7

2-) is orange in colour, as this gains electrons it turns to a blue-green colour due to the presence of Cr3+ ions. The equation that represents the reduction of the dichromate is shown below.

Cr2O72- +14H+ + 6e- 2Cr3+ + 7H2O

Oxidation of alcohols can also take place using hot copper(II) oxide. During the reaction the copper(II) oxide is reduced to copper. If ethanol was oxidised to ethanol the equation to represent it would look like this

CH3CH2OH + CuO CH3CHO + Cu + H20In summary:Primary alcohols oxidise to give alkanals.Secondary alcohols oxidise to make alkanones.Tertiary alcohols do not undergo oxidation (as they do not have a hydrogen atom attached to the carbon which is joined to the hydroxyl group).

Name Formula Structure


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Methanal CH2O

Ethanal C2H6O

Propanal C3H8O

Butanal C4H10O

The aldehydes (also known as the alkanals) all contain the carbonyl, C=O functional group. The functional group is always at the end of the molecule.

Name Formula StructurePropanone C3H8O

Butanone C4H10O


Carbonyl functional group

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The ketones (also known as the alkanones) also contain the carbonyl functional group, C=O. This time the carbonyl group is in the middle of the carbon chain.

Oxidation of Aldehydes

Aldehydes can be oxidised to carboxylic acids, whereas ketones cannot. There are several oxidising agents we can use to distinguish between aldehydes and ketones using this principle. The oxidising agents are listed in the table below, along with what would be observed when oxidising an aldehyde.Oxidising Agent

Observations Explanation

Acidified potassium dichromate

Orange blue-green Cr2O72- reduced to

Cr3+

Fehling’s (or Benedict’s) solution

Blue orange/red Cu2+ reduced to Cu+

Tollens’ reagent Colourless silver mirror

Ag+ reduced to Ag

When aldehydes oxidise they form carboxylic acids which contain the carboxyl group circled in the molecule of ethanoic acid below.

In Summary:Hillhead High SchoolChemistry Department

Carbonyl functional group

Carboxyl functional group

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Primary Alcohols Aldehydes Carboxylic AcidsSecondary Alcohols Ketones No reaction!Tertiary Alcohols No reaction!

Antioxidants

Many flavour and aroma molecules are aldehydes and ketones. When a food spoils these molecules are changed and our taste buds are sensitive to this change, detecting the food is off. For example aldehydes in food can be oxidised into carboxylic acids which we can taste and dislike. A common method to prevent oxidation of food is to add chemicals known as antioxidants. One of the most common antioxidants used is ascorbic acid, also known as vitamin C (C6H8O6). Ascorbic acid readily undergoes oxidation, thus saving the food itself from being oxidised.

C6H8O6 C6H6O6 + 2H+ + 2e-

Fragrances

Essential oils are concentrated extracts of the aroma compounds from plants. They are usually volatile, insoluble in water and composed of mixtures of organic compounds. For example lavendulol is an essential oil extracted from the lavender plant.

The most common compounds found in essential oils are terpenes. Terpenes are compounds based on isoprene (2-methylbuta-1,3-diene), which has the molecular formula C5H8.


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All terpenes contain isoprene units joined together. In general terpenes are based on lots of C5H8 units joining together.For example limonene, carvone and myrcene.

Terpenes can be oxidised, as shown below. The products from the oxidation of terpenes can also be responsible for aroma.

Skin Care

UV light provides enough energy to break covalent bonds and trigger reactions known as free radical chain reactions. These reactions can damage our skin causing aging of the skin and sunburn.Free radical reactions involve 3 key steps. In this example we will look at the reaction between hydrogen and chlorine. This reaction is a photochemical reaction, as it requires light to trigger the reaction.Hillhead High SchoolChemistry Department

Menthol (peppermint oil)

menthone

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1. InitiationThis is when the UV light breaks a covalent bond to form a free radical. A free radical is an atom containing an unpaired electron. This is represented by a next to the formula or symbol.

Cl – Cl Cl + Cl

The Cl-Cl bond is broken instead of the H-H bond as the Cl-Cl bond is weaker, hence easily broken.

2. PropagationThis is where the free radical made in step one then goes and reacts with the other reactant, in this case the hydrogen.

Cl + H-H HCl + H

The hydrogen free radical that has now been formed goes and reacts with the other reactant, in this case chlorine.

H + Cl-Cl HCl + ClThe chlorine free radical is regenerate and the reaction can start all over again. This is why the reaction can be described as a chain reaction. It will continue until one of the reactants run out or a free radical scavenger is added to ‘mop’ up unreacted free radicals.

3. TerminationFree radicals react with one another to produce stable products.

Cl + Cl Cl2H + H H2

H + Cl HCl


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Free radical scavengers are commonly used in anti-wrinkle cream to ‘mop’ up unreacted free radicals which cause damage and aging of the skin. Common free radical scavengers include Vitamin C and E.


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