The Chemistry of Citrus Fruit: the Terpene Limonene

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Where does that lemony smell come from? A chemical compound called limonene is responsible for the pungent aroma we associate with citrus fruit. Image by gleangenie

Citrus fruits offer both pronounced flavor and pungent aroma, but how?

The chemical responsible for the pungency is the terpene limonene – more specifically, the dextrorotatory enantiomer¹, d-limonene. Although modern terminology is switching from D and L notation to S and R, we’ve chosen the older terminology, more often associated with limonene.

What is Limonene?

Limonene is useful commercially in environmentally-safe cleaning formulations, as a solvent, and as raw material in the synthesis of other chemicals, such as carvone.

This is a ball-and-stick model of the chemical structure of d-limonene. The black balls are carbon atoms, the white balls are hydrogen atoms and the sticks are bonds (double sticks are double bonds). Image by Ben Mills & Jynto

This chemical is also useful as an insecticide and for assorted other purposes. Manufacturers separate limonene from the fruit’s peel by means of steam distillation, or a centrifugal process.

Lab Synthesis of Limonene from Isoprene

Labs can prepare limonene (a mixture of both enantiomers) from two molecules of isoprene (a starting material associated with the manufacture of tires).

Since this process involves the joining together of two molecules, with nothing eliminated, it is an addition reaction – classified as a Diels-Alder reaction.

Limonene in Nature

In nature, terpenes are not derived from isoprene. Rather, they come from isopentenyl pyrophosphate.

Isopentenyl pyrophosphate has the structural formula:

H₂C=C(CH₃)CH₂CH₂-OP(O)O-O-P(O)OH

Isopentenyl pyrophosphate is derived beforehand by a complex series of metabolic steps identified as the mevalonate pathway.

This is a stick model of D-Limonene, a simplified representation. The vertices represent carbon atoms. Hydrogen atoms are implied, not drawn. The dashed line means the part at the bottom lies behind the plane of the paper, while the remainder of the molecule lies in the plane. Image by Vincent Summers, all rights reserved.

D-Limonene to L-Carvone

Levorotatory carvone (l-carvone) is a substance that exists in many essential oils, but a lab can use dextrorotatory limonene (d-limonene) to prepare l-carvone as well – typically for flavoring or fragrance.

First, they react d-limonene with nitrosyl chloride to form d-limonene nitrosochloride.

The nitrosochloride then reacts with potassium hydroxide to form l-carvoxime.

The oxime is further reacted with dilute acid, giving l-carvone.

L-carvone is a terpenoid (terpene derivative) with the aroma of spearmint. Interestingly, however, d-carvone does not smell like a variety of mint – but like caraway.

Limonene as Raw Material

When processing citrus fruits to harvest limonene, according to the Florida Chemical Company, Inc. website, nothing from the fruit used (generally oranges) goes to waste – not the pulp beneath the peel, not the seeds. Processors use vacuum distillation to produce food-grade limonene, and steam distillation to produce technical-grade limonene – for use in cleaners and so on.

Diels-Alder reaction: Isoprene to Limonene. Copyright image by Vincent Summers, all rights reserved.

Chemistry of Limonene

Limonene is a terpene (the word ‘terpene’ comes from ‘turpentine’) – but this pungent chemical doesn’t smell like turpentine.

It’s the source of citrus flavor and fragrance – whether in desserts as a food-grade chemical, or in cleansers – or to produce other flavors and fragrances via the use of chemical reactions.

Love the smell of citrus in cleansers and flavorings? Now you know where it comes from.

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