What is Limonene?

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Molecular structure of Limonene

Limonene is from a family of hydrocarbons produced by plants-known as ‘terpenes’ possessing a strong smell of oranges. Limonene is an orange terpene. Terpenes are present in essential oils, especially citrus oils, which in turn can be used for many things such as natural food flavorings, fragrances and aromatherapy. Limonene is a colorless liquid hydrocarbon . It is used in chemical synthesis as a precursor to carvone (member of a family of chemicals called ‘terpenoids’) and as a renewably-based solvent in cleaning products. Limonene takes its name from the lemon, as the rind of the lemon, like other citrus fruits, contains considerable amounts of this compound, which contributes to their odor.


Limonene is a chemical compound that can manifest in two ways, as l-limonene or d-limonene. The two are mirror images of one another chemically, and have the same properties, but different scents. The "d" version typically smells like a citrus fruit, and is used in food grade products, cleaning products, and beauty products, while "l" tends to have a more sour turpentine-like scent mixed with pine, and is used primarily as an alternative solvent for cleaning products. Both versions can be produced as technical grade, and are produced in the same manner, but chemically separated by scientists. Technical grade limonene has been used in cleaning products since the 1990s. The name comes from "lemon" and "lime," since it is most commonly found in the rinds of citrus fruit.


What are the Properties of Limonene?

  • Limonene is an alkene, meaning its functional group is C=C, its suffix is-ene and its formula is CnH2n.
  • The Molecular formula is C10H16.
  • Its full name is 1-methyl-4-prop-1-en-2-yl-cyclo hexane.
  • Limonene melts at -95.2 °C and it boils at 176 °C, therefore Limonene is liquid at room temperature.
  • It is lighter than water and insoluble in water, unless a surfactant is added.
  • D-limonene is considered a combustible liquid. Limonene is a cyclic terpene.
  • The flash point (lowest temperature at which that chemical, in liquid form, will produce combustible vapors that will ignite with the introduction of an ignition source) is 119 °F (48 °C), which classifies it as a flammable product. This means it is also classified as a hazardous material for disposal purposes.
  • Biological properties: Recent research suggests that limonene is a selective adenosine A2A receptor (an adenosine receptor, but also denotes the human gene encoding it) agonist (chemical that binds to a receptor of a cell and triggers a response by that cell). It also increases in-vitro cytosolic (liquid found inside cells) cyclic adenosine monophosphate (cAMP) concentrations by activation of protein kinase A. Protein kinase A (PKA) refers to a family of enzymes whose activity is dependent on the level of cyclic AMP (cAMP) in the cell, in cell biology.


How is the Biosynthesis of Limonene done?

Limonene is formed from geranyl pyrophosphate (an intermediate in the important cellular metabolic pathway present in all higher eukaryotes and many bacteria), via cyclization of a neryl (Nerol is a monoterpene found in many essential oils such as lemongrass) carbocation ( ion with a positively-charged carbon atom) or its equivalent as shown. The final step involves loss of a proton from the cation to form the alkene.


What is the Chemistry of Limonene?

  • Limonene is a relatively stable terpene and can be distilled without decomposition, although at elevated temperatures it cracks to form isoprene.
  • It oxidizes easily in moist air to produce carveol (a natural unsaturated, monocyclic monoterpenoid alcohol that is a constituent of spearmint essential oil), carvone ( found naturally in many essential oils, but is most abundant in the oils from seeds of caraway plant and dill), and limonene oxide.
  • With sulfur, it undergoes dehydrogenation to p-cymene. Cymene, or p-cymene, is a naturally occurring aromatic organic compound.


How is Technical grade Limonene oil prepared?

The most common source it is extracted from is orange rind. The rinds are juiced first to create a food grade limonene product, and then they are sent into a steam extractor. The extractor presses out more oils. The steam is compressed down, and the oil floats to the top where it can be collected. This oil is technical grade.


What are the Reactions of Limonene?

  • Limonene occurs naturally as the (R)-enantiomer. An enantiomer is one of two stereoisomers that are mirror images of each other that are "non-superposable" (not identical), much as one's left and right hands are "the same" but opposite.
  • When warmed with mineral acid (inorganic acid), limonene isomerizes to the conjugated diene (compounds with alternating single and multiple bonds) α-terpinene (which can also easily be converted to p-cymene). The terpinenes are three isomeric hydrocarbons that are classified as terpenes. It is possible to effect reaction at one of the double bonds selectively.
  • The most widely practiced conversion of limonene is to carvone. Carvone is a member of a family of chemicals called terpenoids.


What are the Uses of Limonene?

  • The chief use of D-limonene is as a pioneer to carvone.
  • Limonene is common in cosmetic products. As the main odor constituent of citrus (plant family Rutaceae), D-limonene is used in food manufacturing and some medicines, e.g. as a flavoring to mask the bitter taste of alkaloids, and as a fragrant in perfumery; it is also used as botanical insecticide.
  • Particularly, the (R)-(+) enantiomer is most active as an insecticide. It is added to cleaning products such as hand cleansers to give a lemon-orange fragrance.
  • In contrast, L-limonene has a piney, turpentine-like odor.
  • Limonene is increasingly being used as a solvent for cleaning purposes, such as the removal of oil from machine parts, as it is produced from a renewable source (citrus oil, as a byproduct of orange juice manufacture).
  • It also serves as a paint stripper when applied to painted wood and is also useful as a fragrant alternative to turpentine.
  • Limonene is also used as a solvent in some model airplane (flying or non-flying models of existing or imaginary aircraft) glues.
  • All-natural commercial air fresheners, with air propellants, containing limonene are used by philatelists to remove self-adhesive postage stamps from envelope paper. Philately is the study of stamps and postal history and other related items.
  • As it is combustible, limonene has also been considered as a biofuel.
  • Limonene can be used to dissolve polystyrene (organic compound), and is a more ecologically friendly substitute for acetone.
  • In preparing tissues for histology (study of the microscopic anatomy of cells and tissues of plants and animals) , D-limonene is often used as a less toxic substitute for xylene when clearing dehydrated specimens. Clearing agents are liquids miscible with alcohols (such as ethanol or isopropanol) and with melted paraffin wax, in which specimens are embedded to facilitate cutting of thin sections for microscopy.


What are the Health effects of Limonene?

  • Limonene and its oxidation products are skin and respiratory irritants, and limonene-1,2-oxide (formed by aerial oxidation) is a known skin sensitizer. Most reported cases of irritation have involved long-term industrial exposure to the pure compound, e.g. during degreasing or the preparation of paints.
  • A study of patients presenting dermatitis showed that 3% were sensitized to limonene.
  • Even though, once thought to cause renal cancer in rats, limonene now is considered by some researchers to be a significant chemo preventive agent with potential value as a dietary anti-cancer tool in humans.
  • There is no evidence for carcinogenicity or geno toxicity in humans.
  • The International Agency for Research on Cancer (IARC) classifies D-limonene under Class 3: “not classifiable as to its carcinogenicity to humans”.
  • The Carcinogenic Potency Project, however, estimates that it causes human cancer on a level roughly equivalent to that caused by exposure to caffeic acid via dietary coffee intake.
  • No information is available on the health effects of inhalation exposure to D-limonene in humans, and no long-term inhalation studies have been conducted in laboratory animals.
  • D-Limonene is biodegradable, but due to its low flash point, it must be treated as hazardous waste for disposal.
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