The Science of Essential Oils: What They Are Made Of and Why It Matters

Essential oils may seem like simple aromatic extracts, but beneath their delicate fragrance lies a complex world of molecular architecture, bioactivity, and botanical intelligence. At Delune, we don’t just believe in the beauty of scent—we honour the chemistry behind it. This deep dive into the science of essential oils will help you understand how these natural compounds work, why they’re effective, and what sets truly therapeutic oils apart from synthetic imitations.

What Are Essential Oils, Really?

Essential oils are volatile, aromatic compounds extracted from plants. Volatility means they evaporate quickly, which is what allows them to disperse through the air—and be absorbed when you inhale them. But they're far more than fragrance. They’re made up of hundreds of bioactive chemical constituents that can interact with your body’s systems, from the skin and bloodstream to the limbic brain.

Essential oils are made up of hundreds of bioactive chemical constituents. While many are built from a fundamental five-carbon unit called isoprene (C₅H₈), which forms the basis for the large class of compounds known as **terpenes** (including monoterpenes and sesquiterpenes), other important constituents belong to different chemical families with distinct structures and origins.

The Key Chemical Groups in Essential Oils

Each essential oil contains a mix of constituents from several chemical families. Here’s a breakdown of the most important ones:

1. Monoterpenes (C₁₀H₁₆)

Monoterpenes are the most common group found in essential oils. Chemically, they consist of two isoprene units and are highly volatile—meaning they evaporate quickly and are often the top notes you smell first in a blend. Examples include limonene, myrcene, and pinene.

Therapeutic properties: Monoterpenes are known for being:

• Anti-inflammatory
• Antiseptic
• Antiviral
• Antibacterial
• Decongestant

You’ll find them in oils like lemon, orange, grapefruit, pine, and frankincense. They stimulate circulation and lymphatic drainage and can support respiratory health.

Stability warning: Monoterpenes are highly reactive to light, heat, and air. Citrus oils, which are rich in limonene, degrade quickly—typically within six months. This oxidative breakdown can make them ineffective for therapy and even cause skin sensitization. That’s why at Delune, we always recommend storing high-monoterpene oils in cool, dark places and replacing them regularly.

2. Sesquiterpenes (C₁₅H₂₄)

Made of three isoprene units, sesquiterpenes are larger and heavier than monoterpenes, making them more stable and slower to evaporate. This means they often show up in base notes, lingering longer on skin and fabric.

Therapeutic properties:

• Anti-inflammatory (stronger and longer-lasting than monoterpenes)
• Anti-septic
• Hormone balancing
• Emotionally grounding

Sesquiterpenes are deeply grounding and calming, influencing deeper body systems and potentially impacting neurological and endocrine function, including areas like the pineal and pituitary glands, contributing to their balancing effects. Oils like chamomile matricaria, yarrow, vetiver, and spikenard are rich in sesquiterpenes.

Interesting note: Blue essential oils get their colour from a sesquiterpene called azulene. It acts like a topical anaesthetic, calming inflammation with a sensation similar to silk being smoothed over the skin.

3. Phenols

Phenols are potent, aromatic compounds. They’re often sharp, spicy, or medicinal in aroma and have powerful effects on the body. Found in oils like clove, cinnamon, and thyme thymol chemotype.

Therapeutic properties:

• Antiseptic
• Antibacterial
• Immunostimulant
• Nervous system stimulant

Use with caution: Phenols are very aggressive compounds. When overused, they can irritate the skin and mucous membranes, especially in the nose and lungs. The liver also struggles to process them efficiently, and they tend to accumulate over time. Oils high in phenols should be diluted to 1% or less and avoided in sensitive individuals, children, or during pregnancy.

4. Aldehydes

Aldehydes are compounds with a fresh, lemony, and slightly metallic scent. Found in oils like citronella, lemon balm (Melissa), and lemongrass, they tend to be uplifting and emotionally clearing while also physically calming.

Therapeutic properties:

• Antiseptic
• Anti-inflammatory
• Disinfectant
• Sedative yet emotionally uplifting

Considerations: Aldehydes are unstable and can oxidise quickly. Like phenols, they must be used in low concentrations (ideally around 1%). High levels of aldehydes can cause skin reactions or make the oil rancid. If your lemongrass or Melissa oil smells harsh or sour, it’s time to replace it.

5. Ketones

Ketones are complex, therapeutic compounds found in oils such as rosemary (ct camphor), peppermint, spike lavender, and turmeric.

Therapeutic properties:

• Mucolytic (breaks down mucus)
• Wound healing
• Scar tissue regeneration

Safety concerns: Ketones are among the more controversial groups due to their potential toxicity. Many oils high in ketones appear on safety watch lists, especially for people with epilepsy, diabetes, or liver conditions. Some, like pulegone (found in pennyroyal and sage), are neurotoxic and potentially fatal at high doses.

Certain ketones can also affect insulin regulation and may contribute to hypoglycemia, particularly in susceptible individuals or at high doses. Oils high in ketones should never be used casually or without proper dilution. However, in small doses and in the right hands, they can be powerfully regenerative—such as in helichrysum for scar healing.

6. Esters

Esters form when an acid and an alcohol combine. They are fruity, sweet-smelling, and generally very gentle—making them favourites for emotional and skin therapies.

Found in: lavender (linalyl acetate), bergamot, Roman chamomile

Therapeutic properties:

• Antispasmodic
• Sedative
• Anti-inflammatory
• Skin soothing

Esters are very skin-friendly and ideal for sleep blends, anxiety support, and muscle relaxation. Their sweetness also makes them helpful in children’s blends. However, some oils (like wintergreen) contain esters like methyl salicylate, which—though helpful for pain relief—can thin the blood and must be avoided if on anticoagulants.

7. Lactones & Coumarins

This small family of constituents includes complex molecules often found in trace amounts but with outsized effects. Lactones and coumarins are found in oils like cassia, elecampane, and tonka bean.

Therapeutic properties:

• Expectorant
• Anti-inflammatory
• Spasmolytic (relieve cramps)

Risks: Coumarins are photosensitising, which means they can cause pigmentation and skin sensitivity when exposed to UV light. This is especially true in bergamot oil—unless it is furanocoumarin-free (FCF). Lactones can be neurotoxic in high doses and are best used under professional guidance only.

In perfumery, coumarins add a smooth, hay-like softness. In cassia, they deliver spice and warmth. But in high doses, some can be toxic to the liver or kidneys.

Chemotypes: One Plant, Many Profiles

Ever noticed that not all lavender smells the same? That’s due to chemotypes. A chemotype is a variant of a plant species with a different chemical profile, influenced by geography, climate, and cultivation. For example:

• Lavandula angustifolia: Calming, safe, low ketone content
• Lavandula stoechas: High ketones, not suitable during pregnancy

This is why we always specify species and chemotype in our product descriptions at Delune—because quality starts with clarity.

Understanding Headspace: The Invisible Signature of a Plant

In 1977, researchers discovered that the chemical profile of a flower changes depending on whether it’s been picked. The headspace—the scent released from a living plant—is often different than what’s captured in distilled oil.

Factors like time of day, harvesting methods, and flower colour can alter an oil’s chemical makeup. For instance, tuberose shows higher levels of limonene at night than during the day. And even different rose colours have different molecular profiles. Bulgarian Rose Otto, for instance, is rich in nerol-geraniol, while Red Chrysler roses contain high levels of citronellol and even unique compounds like 3,5-dimethoxy toluene.

Why This All Matters: Nature Isn’t a One-Note Song

Isolating just one “active ingredient” from a plant often results in a product lacking the complex synergy found in nature. While isolates have valuable applications, this natural synergy is often why whole essential oils offer a unique, full-spectrum therapeutic experience compared to single compounds.

When you use a Delune pure essential oil, you're not just using a chemical. You're inviting in the whole plant’s intelligence—the aroma, the chemistry, the energetics, and the soul. That’s what makes it therapeutic.

That’s what makes it Delune.

Important Safety Note

Understanding the chemistry of essential oils highlights the importance of safe usage. Always dilute essential oils properly before applying to the skin, follow recommended guidelines for inhalation, and be aware of specific cautions for sensitive individuals, children, pregnant or nursing women, or those with pre-existing medical conditions (including epilepsy, diabetes, or liver issues). When in doubt, consult with a qualified aromatherapy practitioner or healthcare provider.