Higher Chemistry | Unit 2 Part 2

Proteins, oxidation of food, cooking, fragrances, skincare

Proteins

Natural polymers made by joining together amino acid molecules. Major structural materials of animal tissue, and are essential for the maintenance and regulation of life processes.

There are two classes of proteins:

- Fibrous proteins are made of long, straight chains tightly packed together, forming an insoluble solid structure. eg. main structural protein in animals, keratin, elastin, collagen.

- Globular proteins are made when long chains fold in on themselves and are held by intermolecular forces (hydrogen bonding). Often water-soluble. eg. hormones, enzymes, haemoglobin.

Essential amino acid

An amino acid that cannot be synthesised in the body and must be consumed in diet.

Amino acid

A monomer that when joined together synthesises a protein. All amino acids contain an amine (-NH2) and a carboxyl (-COOH) functional group.

How are proteins made?

- Formed by the condensation reaction between amino acids, where the amine and carboxyl group join together with the elimination of water.

- Link formed is a peptide or amide link (-CONH-)

Hydrolysis of proteins

During digestion, protein chains are broken down into their constituent amino acids. They can also be hydrolysed in a lab using acids and alkalis.

Structure of proteins

- Long chain molecules can twist to form spirals, sheets and other complex shapes which are kept by intermolecular bonds.

- Hydrogen bonds are broken when heated, and the protein changes shape (ie. it is denatured). This change in shape causes the texture of food to change.

Chromatography

A method of identifying separated amino acids. Different amino acids will travel different distances.

Aldehydes and ketones

- Homologous series both containing the carbonyl group (C=O), and are isomers of one another.

- Aldehydes are named systematically -al. Carbonyl groups will be on the first carbon atom. Used in food products or essential oils due to distinct flavours and aromas.

- Ketones are named systematically -one. Carbonyl groups will not be on the first carbon atom. Ketones over 5 carbons long must be named with the position of the C=O.

Oxidation of alcohols

- A reaction in which there is an increase in the ratio of oxygen:hydrogen (or a decrease in hydrogen:oxygen).

- Primary alcohol → aldehyde → carboxylic acid

- Secondary alcohol → ketone

- Tertiary alcohol → X (position of OH and branch prevents =O from being formed)

Reduction to alcohols

- A reaction in which there is a decrease in the ratio of oxygen:hydrogen (or an increase in hydrogen:oxygen).

- Carboxylic acid → aldehyde → primary alcohol

- Ketone → secondary alcohol

Oxidising agents of alcohols

Ketones cannot oxidise further, and so will not react with oxidising agents. Alcohols and aldehydes react with:

- Acidified potassium dichromate; orange → green (all)

- Hot copper (II) oxide; black → copper/brown (all)

- Tollens’ reagent; clear → silver mirror (aldehyde → carboxylic acid)

- Fehling’s solution; blue → brick-red (aldehyde → carboxylic acid)

Reducing agents of alcohols

Lithium aluminium hydride (LiAlH4) is commonly used.

Antioxidant

- Molecules that are easily oxidised, and therefore prevent unwanted oxidation reactions from happening by being oxidised themselves (ie. reducing agents)

- Denoted as an E number from E300-E399.

- Vitamin C is an example of an antioxidant. Used in food products, skincare, cosmetics.

Food reacting with oxygen

Oxidation occurs when oxygen reacts with edible oils in food, giving it a rancid flavour as the food goes off.

Flavour molecules

- Tend to be small molecules with low boiling points and high volatility due to their weaker LDFs. Many flavour and aroma molecules are aldehydes.

- Depending on the functional groups present on the molecule, some flavour molecules are more soluble in water than oil, and vice-versa.

Essential oils

- Concentrated extracts of the volatile, non-water soluble aroma compounds from plants and are mixtures of organic compounds.

- Used in perfumes, cosmetics, cleaning products, and flavourings.

- Steam distillation can extract essential oils from natural substances.

Terpenes

- Unsaturated hydrocarbons which are key components in most essential oils.

- Formed by joining isoprene (2-methylbuta-1,3-diene) units which always have 5 carbons.

- Components in fruit and floral flavours and aromas.

- Can be oxidised within plants to form terpenoids which can contain OH or COOH functional groups. Terpenoids are responsible for distinctive spice aromas.

Effect of sunlight

- UV light is high-energy light present in sunlight. Exposure to UV light can result in molecules gaining sufficient energy for bonds to be broken (skin cancer, sunburn, aging).

- Sunscreen prevents UV light from reaching the skin.

Free radicals

Atoms or molecules formed when UV light breaks bonds, and have unpaired electrons (a single free electron), and so are highly reactive.

Free radical action

- Free radicals are involved in the halogenation of alkanes. Normally, alkanes will not undergo reactions as they are saturated, but under UV light, free radical action occurs.

- Initiation: UV light has enough energy to break bonds between atoms, and free radicals are formed. eg. Cl2 → Cl· + Cl·

- Propagation: Radicals react with the alkane, forming alkyl radicals, which react with more molecules, and so on (chain reaction). eg. Cl + CH4 → HCl + CH3·, then CH3· + Cl2 → CH3Cl + Cl·

- Termination: When 2 radicals collide and form one unreactive, stable molecule, terminating the reaction. eg. CH3· + Cl· → CH3Cl

Free radical scavengers

- Molecules which can react with free radicals to form stable molecules, preventing chain reactions.

- Used in skincare, cosmetics, food preservatives. Natural free radical scavengers can act as antioxidants. Used in polymers to terminate oxidation reactions.