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Chirality is a term used to describe the three-dimensional orientation of a molecule in space. This term is derived from the Greek word meaning “hand” or “handedness.” Just as you have two hands, chiral molecules exist in two forms distinguished as either the right-hand or the left-hand. The two forms are non-superimposable mirror images, meaning that if you place one on top of the other, it is impossible to line them up exactly. It may be helpful to visualize this principle by simply looking at your hands. When your hands are placed side by side, you can see that they are mirror images of each other. However, when you place one hand on top of the other, they do not line up. No matter how you turn your hands, you can never get them to line up exactly.

In order to be considered chiral, one or more carbon atoms in the molecule must be considered asymmetrical, meaning it is bonded to four different groups. When this occurs, a chiral atom can assume either of the two “hands.” Each hand has different chemical properties, which affects their physiologic interactions in the body. This makes chirality a very important principle to understand.

One hand is produced predominantly in nature. However, in a laboratory environment, the ratio of right to left handed molecules is always 50/50 because their structural similarity makes it difficult to control for one hand over the other. Most essential oil constituents are chiral. Take limonene for example, a chiral constituent found in high amounts in citrus essential oils. There is one chiral center in limonene. However, just the handedness of this one carbon atom greatly affects the properties of the oil. For instance, one hand of limonene gives a citrus aroma while the other smells more like pine.

The ratio of right to left handed constituents can be determined through a special type of gas chromatography. Although not commonly performed on a batch-to-batch basis, this testing method can provide key insights into the purity of an essential oil by ensuring that no synthetic constituents are present.

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