Humulene

Some plants have a special smell.

This smell comes from hops. It helps make beer smell good. Many plants like pine trees have it too. It is in the air around us. It can also come from orange trees. Some plants like ginger have it. This smell can even help keep bugs away.

It is found on every part of the Earth. It is a tiny part of plant oils. This smell is very interesting.

Have you ever smelled the fresh scent of hops?

This smell comes from a substance called humulene. It is a part of the oil in hops plants. This plant is used to make beer. Humulene helps give many beers a "hoppy" smell. It can make up 40% of the oil in hops.

Many other plants have humulene too. You can find it in pine trees and orange orchards. It is in sage and ginger. It is even found in sunflower fields. Scientists study it in labs. They want to see if it helps stop swelling in the body. In Brazil, researchers found it helps keep mosquitoes away.

Humulene also lives in the air. Plants let it out into the sky. Once it is in the air, it reacts with ozone. Ozone is a gas in our atmosphere. This happens when there is sunlight. This change helps make tiny bits in the air called aerosols.

Scientists can also make humulene in a lab. They use many different ways to build it. They use tools to join parts together to make the shape of the molecule.

Have you ever smelled the fresh scent of hops?

This smell comes from a special substance called humulene. It is a part of the oil in the flowering cones of the hops plant. This plant is called Humulus lupulus. Humulene can make up as much as 40% of that oil. It is a very important part of the brewing process. When making beer, humulene and its reaction products give many drinks their "hoppy" aroma. Scientists found that a specific part of humulene helps create this scent.

Humulene is a type of molecule called a sesquiterpene. This means it is built from three isoprene units. It has a shape with an 11-membered ring. This ring is a circle made of atoms. Inside the ring, there are three double bonds. These bonds are spots where the atoms connect in a certain way. One of these bonds is more reactive than the others. This means it is more likely to change during a reaction. Scientists use computers to study these different shapes.

Many different plants around the world make humulene. It is found on every continent. You can find it in pine trees and orange orchards. It is also in sunflower fields and tobacco. Some plants have a lot of it. For example, it is in 29.9% of the oil from mint. It is also in 10% of the oil from the Chinese laurel tree. It is found in sage and ginger too. It even helps give Vietnamese coriander its taste.

Scientists can also make humulene in a laboratory. They do not just find it in nature. They use many different ways to build it. One way is called the McMurry synthesis. This uses a special reaction with titanium. Another way is the Takahashi synthesis. This uses a method called intramolecular alkylation. There is also the Suginome synthesis and the de Groot synthesis. The de Groot method uses oil from eucalyptus trees. These methods help scientists study how the molecule works.

Humulene also plays a role in our atmosphere. Plants release it into the air. This substance is called a biogenic volatile organic compound. Once it is in the sky, it reacts with ozone. This happens when there is sunlight. This process is called photooxidation. The reaction can create tiny bits in the air called aerosols. These aerosols form when the humulene reacts with ozone. This happens very quickly in the air.

Humulene is a naturally occurring chemical compound known as a monocyclic sesquiterpene. Its chemical formula is C15H24. It is characterized by a large, 11-membered ring structure. This molecule is built from three isoprene units. Within this ring, there are three nonconjugated C=C double bonds. Two of these bonds are triply substituted, while one is doubly substituted. Humulene is a vital component in many aromatic plants. It plays a major role in the scents and flavors we experience in nature.

In nature, plants create humulene through a biological process called biosynthesis. This process starts with a substance called farnesyl diphosphate, or FPP. Specific enzymes called sesquiterpene synthesis enzymes act as catalysts to transform FPP into humulene. Scientists can also recreate this process in a laboratory. One method is called the Corey synthesis, which prepares allylic stannane from farnesol. Other methods exist to build the molecule's macrocycle. The McMurry synthesis uses a titanium-catalyzed carbonyl coupling reaction. The Takahashi synthesis uses intramolecular alkylation of an allyl halide by a protected cyanohydrin anion. There is also the Suginome synthesis and the de Groot synthesis, which uses eucalyptus oil.

Humulene is most famous for its presence in the hops plant, *Humulus lupulus*. It is a primary component of the essential oil found in the flowering cones of these plants. The concentration of humulene can vary depending on the specific variety of the plant. In some cases, it can make up as much as 40% of the essential oil. Humulene is also an isomer of $\beta$-caryophyllene. This means they have the same chemical formula but different arrangements of atoms. Because of this, the two are often found together in many aromatic plants.

Many different plants across all continents produce $\alpha$-humulene. It is found in the essential oils of common sage, known as *Salvia officinalis*. It is also present in Japanese spicebush, or *Lindera aggregata*. You can find it in ginseng species and the ginger family, known as Zingiberaceae. In *Mentha spicata*, humulene can make up 29.9% of the essential oils. It accounts for 10% of the leaf oil in the Chinese laurel tree, *Litsea mushaensis*. It is also found in the leaf extract of *Cordia verbenacea*, a bush from coastal tropical South America. Finally, it contributes to the taste of Vietnamese coriander and the aroma of cannabis.

Humulene is very important in the brewing industry. When making beer, humulene and its reaction products create a "hoppy" aroma. Noble hop varieties tend to have higher levels of humulene. In contrast, some bitter hop varieties contain lower levels. During the brewing process, humulene undergoes chemical changes to form multiple epoxides. A scientific study used gas chromatography–mass spectrometry to analyze these samples. A trained sensory panel helped determine the results. They found that the hydrolysis products of humulene epoxide II specifically produce that classic "hoppy" scent.

Beyond flavor, humulene has interesting properties in science and medicine. Researchers are studying the molecule in laboratory settings for potential anti-inflammatory effects. In 2015, researchers in Brazil discovered a different use for it. They identified $\alpha$-humulene as an active contributor to the insect repellent properties of *Commiphora leptophloeos* leaf oil. Specifically, it helps repel the yellow fever mosquito, *Aedes aegypti*. This shows how a single molecule can serve many different purposes in the natural world.

Humulene also affects the chemistry of our atmosphere. It is a biogenic volatile organic compound, which means plants emit it into the air. Once in the atmosphere, it has a high potential for secondary organic aerosol formation. This happens when humulene reacts quickly with ozone in the presence of sunlight. This process is called photooxidation. Humulene has a very high reaction rate coefficient of $1.17 \times 10^{-14} \text{ cm}^3 \text{ molecule}^{-1} \text{ s}^{-1}$. Because it has three double bonds, it can form first-, second-, and third-generation products. These products can condense to form aerosols. At typical ozone levels, the lifetime of $\alpha$-humulene is only about 2 minutes. However, its first-generation products last about 1 hour, and second-generation products last about 12.5 hours.