Monoterpene

Key takeaways

Monoterpene belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Monoterpene to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Monoterpene from memory before moving on to harder problems.

Reference excerpt

Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen functionality or missing a methyl group, are called monoterpenoids. Monoterpenes and monoterpenoids are diverse. They have relevance to the pharmaceutical, cosmetic, agricultural, and food industries.

Biosynthesis

Monoterpenes are derived biosynthetically from units of isopentenyl pyrophosphate, which is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. An alternative, unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP (methylerythritol phosphate, i.e. 2-methyl-D-erythritol-4-phosphate) pathway, which is initiated from C5 sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase. Geranyl pyrophosphate is the precursor to monoterpenes (and hence monoterpenoids). Elimination of the pyrophosphate group from geranyl pyrophosphate leads to the formation of acyclic monoterpenes such as ocimene and the myrcenes. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid geraniol. Additional rearrangements and oxidations provide compounds such as citral, citronellal, citronellol, linalool, and many others. Many monoterpenes found in marine organisms are halogenated, such as halomon.

Main examples

Bicyclic monoterpenes include carene, sabinene, camphene, and thujene. Camphor, borneol, eucalyptol and ascaridole are examples of bicyclic monoterpenoids containing ketone, alcohol, ether, and bridging peroxide functional groups, respectively. Umbellulone is another example of bicyclic monoterpene ketone.

Natural roles Monoterpenes are found in many parts of different plants, such as barks, heartwood, bark and leaves of coniferous trees, in vegetables, fruits and herbs. Essential oils are very rich in monoterpenes. Several monoterpenes produced by trees, such as linalool, hinokitiol, and ocimene have fungicidal and antibacterial activities and participate in wound healing. Some of these compounds are produced to protect the trees from insect attacks. Monoterpenes are emitted by forests and form aerosols that are proposed to serve as cloud condensation nuclei (CCN). Such aerosols can increase the brightness of clouds and cool the climate. Many monoterpenes have unique smell and flavor. For example, sabinene contributes to the spicy taste of black pepper, 3-carene gives cannabis an earthy taste and smell, citral has a lemon-like pleasant odor and contributes to the distinctive smell of citrus fruits, and thujene and carvacrol are responsible for the pungent flavors of summer savory and oregano, respectively. Monoterpenes are considered allelochemicals.

Uses Many monoterpenes are volatile compounds and some of them are well-known fragrants found in the essential oils of many plants. For example, camphor, citral, citronellol, geraniol, grapefruit mercaptan, eucalyptol, ocimene, myrcene, limonene, linalool, menthol, camphene and pinenes are used in perfumes and cosmetic products. Limonene and perillyl alcohol are used in cleaning products. Many monoterpenes are used as food flavors and food additives, such as bornyl acetate, citral, eucalyptol, menthol, hinokitiol, camphene and limonene. Menthol, hinokitiol and thymol are also used in oral hygiene products. Thymol also has antiseptic and disinfectant properties. Volatile monoterpenes produced by plants can attract or repel insects, thus some of them are used in insect repellents, such as citronellol, eucalyptol, limonene, linalool, hinokitiol, menthol and thymol. Ascaridole, camphor and eucalyptol are monoterpenes that have pharmaceutical use.

Health effects A study suggests that a range of floor cleaners with certain monoterpenes may cause indoor air pollution equivalent or exceeding the harm to respiratory tracts when the time is spent near a busy road. This is due to ozonolysis of monoterpenes like limonene leading to the production of atmospheric SOA. Another study suggests monoterpenes substantially affect ambient organic aerosol with uncertainties regarding environmental effects. In a review, scientists concluded that they hope that these "substances will be extensively studied and used in more and more in medicine". A 2013 study found that "Based on adverse effects and risk assessments, d-limonene may be regarded as a safe ingredient. However, the potential occurrence of skin irritation necessitates regulation of this chemical as an ingredient in cosmetics." According to a review, several studies showed "that some monoterpenes (e.g., pulegone, menthofuran, camphor, and limonene) and sesquiterpenes (e.g., zederone, germacrone) exhibited liver toxicity" and that i.a. intensive research on terpenes toxicity is needed.

See also Terpene Terpenoid Diterpene Sesquiterpene

References

Illustrations

Worked examples

Example 1 — a first encounter with Monoterpene

Start with the simplest possible case. Write down what Monoterpene claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Monoterpene before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Monoterpene ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Monoterpene

In research: Monoterpene appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Monoterpene in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Monoterpene is common in secondary-school and first-year university syllabi. It links to neighbouring topics Monoterpenes, Terpenes and terpenoids, Wood extracts, so understanding it makes those chapters shorter.
In everyday life: Look for Monoterpene outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Monoterpene in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Monoterpene means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Monoterpene out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Monoterpene in simple terms?

Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic).

Why does Monoterpene matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Monoterpene?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Monoterpene.