Limonene - oil from citrus peels

Figure 1: Orange peels and limonene. The peels of oranges contain oil. This oil is mostly made up of limonene. On the right: D- and L- limonene; mirror images of each other, but with very different properties.

Have you ever used a citrus degreaser? How does that stuff work? It turns out that the peels of citrus plants contain oil! Not the slimy black stuff that comes from million-year-old compressed algae, but a much fruitier substance. The majority (~90%) of orange peel oil is made up of a compound called limonene. This compound has a duality to its nature that is very important. Think about hands: there are left hands and right hands, which, though they are made of the same parts (four fingers, a thumb, and a palm), are not identical. Rather, hards are mirror images of each other. Limonene is just the same – there is a “left” limonene and a “right” limonene. In chemical language, these are called D-limonene and L-limonene. Our noses are particularly tuned to the difference between these two: D-limonene smells like citrus, while L-limonene smells like pine or turpentine. Limonene is used extensively in the perfume industry, as a component of degreasers, as a natural insect and pest deterrent, and even as a building block for renewable, biodegradable plastics [1]. Fantastic!

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[1] Alternating Copolymerization of Limonene Oxide and Carbon Dioxide. Christopher M. Byrne, Scott D. Allen, Emil B. Lobkovsky, and Geoffrey W. Coates*. Journal of the American Chemical Society 2004 126 (37), 11404-11405 https://pubs.acs.org/doi/abs/10.1021/ja0472580.