What chemistry lies behind fragrances and protecting skin from the Sun?
Essential oils and terpenes as components of fragrances, the structure of terpenes built from isoprene units, the oxidation of terpenes, and how UV light damages skin and how sunblocks and free-radical scavengers protect it.
An SQA Higher Chemistry answer on fragrances and skincare, covering essential oils and terpenes built from isoprene units, the oxidation of terpenes to flavour and aroma compounds, how ultraviolet light damages skin, and how sunblocks and free-radical scavengers provide protection.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this key area is asking
The SQA wants you to describe essential oils and terpenes, explain how terpenes are built from isoprene units, describe the oxidation of terpenes, and explain how ultraviolet light damages skin and how sunblocks and free-radical scavengers protect against it. The free-radical chain reaction (initiation, propagation, termination) is a regular exam item that must be reproduced in the right order.
Essential oils and terpenes
Terpenes are found in the fragrances of fruits, herbs and spices, which is why essential oils are used in perfumes and flavourings.
Oxidation of terpenes
UV light, free radicals and skin damage
Free radicals react in three stages:
- Initiation: radicals are produced, often when UV light breaks a bond, for example .
- Propagation: a radical reacts with a molecule to form a product and a new radical, so the chain continues.
- Termination: two radicals combine, removing the unpaired electrons and stopping the chain.
Worked example: identifying chain-reaction steps
Protecting the skin
Two main strategies protect the skin:
- Sunblocks contain molecules that absorb or reflect UV light before it can reach and damage the skin.
- Free-radical scavengers (such as the antioxidants vitamin C and vitamin E in skincare products) react with free radicals, removing them in termination steps and stopping the damaging chain reactions.
Examples in context
Free-radical chemistry connects skincare to food spoilage and even to plastics. The antioxidants added to sunscreens and anti-ageing creams, such as vitamin E, are the same kind of free-radical scavengers added to packaged foods to slow rancidity and to plastics to stop them degrading in sunlight. Terpenes are big business in perfumery: limonene gives citrus fruits their smell and is extracted from orange peel for cleaning products and fragrances, while pinene gives pine its scent. The link between UV exposure and free-radical damage is the scientific basis of public-health advice to use a high-SPF sunblock, which both reflects and absorbs UV before it can initiate the chain reactions that age skin and raise the risk of skin cancer.
Try this
Q1. Name the small molecule from which terpenes are built. [1 mark]
- Cue. Isoprene (2-methylbuta-1,3-diene).
Q2. Explain how a free-radical scavenger protects the skin. [2 marks]
- Cue. It reacts with free radicals and removes them (in termination), stopping the chain reactions that damage skin.
Q3. Name the step in a free-radical chain reaction in which UV light breaks a bond to make radicals. [1 mark]
- Cue. Initiation.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20193 marksTerpenes are a class of compounds found in essential oils. (a) Name the small unsaturated molecule from which terpenes are built. (b) Explain why terpenes can be oxidised. (c) State one use of essential oils.Show worked answer β
Markers reward the building unit, the reason terpenes react with oxidising agents, and a valid use.
(a) Terpenes are built from isoprene units (isoprene is 2-methylbuta-1,3-diene).
(b) Terpenes contain carbon-to-carbon double bonds, which are sites of unsaturation that can be oxidised. The oxidation produces some of the aldehydes, ketones and carboxylic acids that contribute to flavour and aroma.
(c) Essential oils are used in perfumes, cosmetics or as flavourings (any one).
A common loss is naming isoprene without the systematic name when asked, or failing to link oxidation to the carbon-to-carbon double bonds.
SQA Higher 20223 marksFree radicals can damage skin. (a) Define a free radical. (b) Name the three steps of a free-radical chain reaction. (c) Explain how a free-radical scavenger in a skincare product reduces the damage.Show worked answer β
A 3 mark answer needs the definition, the three named steps, and the scavenger's action.
(a) A free radical is a highly reactive atom or molecule with an unpaired electron.
(b) The three steps are initiation (radicals are produced, often by UV light), propagation (radicals react and regenerate more radicals) and termination (radicals combine, ending the chain).
(c) A free-radical scavenger reacts with the free radicals and removes them (often by combining in a termination step), so the damaging propagation chain reactions are stopped before they can harm the skin.
Markers reward all three correctly ordered steps and linking the scavenger to termination or removal of radicals.
Related dot points
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An SQA Higher Chemistry answer on esters, fats and oils, covering the condensation reaction that forms an ester from an alcohol and carboxylic acid, the ester link, hydrolysis, and the structure of fats and oils as glycerol esters of saturated and unsaturated fatty acids.
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Sources & how we know this
- SQA Higher Chemistry Course Specification β SQA (2018)