Simone Lawson
Limonene, a colorless liquid aliphatic hydrocarbon, is a major component of the aromatic scents and resins found in numerous coniferous and broadleaved trees. It is also the main fragrance of citrus peels and is used in various products such as perfumes, aftershaves, bath products, cleaning agents, and insecticides. Limonene has two optical isomers, d-limonene and l-limonene, which have different properties and uses. This raises the question: how many constitutionally inequivalent carbons does each isomer of limonene have?
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What You'll Learn
Limonene's isomers and their sources
Limonene is a colorless liquid aliphatic hydrocarbon classified as a cyclic monoterpene. It is a major component of the essential oil of citrus fruit peels, such as orange peel oil. The (+)-isomer, which is more common in nature, has a fragrance similar to oranges and is used as a flavoring agent in food manufacturing. It is also used in chemical synthesis as a precursor to carvone and as a renewable solvent in cleaning products.
The less common (−)-isomer has a piny, turpentine-like odor and is found in the edible parts of plants such as caraway, dill, and bergamot orange plants. Both isomers have well-recognized flavors and aromas. The R)-enantiomer, also known as the (+)-isomer or D-limonene, is the one typically found in citrus oils. The (S)-isomer, also known as the (–)-isomer or L-limonene, is produced by coniferous trees and the aforementioned edible plants.
Limonene is formed from geranyl pyrophosphate through the cyclization of a neryl carbocation or its equivalent. The final step involves the loss of a proton from the cation to form the alkene. It is used in various applications, including as a paint stripper, a fragrant alternative to turpentine, a solvent in some model airplane glues and paints, and as an adhesive remover. Limonene is also used in personal care products such as perfumes, aftershaves, bath products, and other products where its fragrance is desirable.
In terms of health effects, limonene applied to the skin may cause irritation from contact dermatitis, but it is otherwise considered safe for human use. There is no evidence that limonene in citrus fruits or supplements affects the onset or progression of cancer.
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Uses of limonene
Limonene, a terpene commonly found in citrus fruits, has a wide range of uses across various industries. Here are some of the key uses of limonene:
Fragrance and Aromatherapy
Limonene is known for its fresh citrus scent, making it a popular ingredient in perfumes, cosmetics, and personal care products like soaps, lotions, and aftershaves. It is also used in aromatherapy to promote calming and relaxing effects, helping to reduce stress and anxiety.
Food and Beverage Flavoring
The distinctive citrus flavour of limonene is utilized as a natural flavouring agent in the food and beverage industry. It is added to products such as sodas, candies, desserts, and other food items to impart a lemon or orange flavour.
Cleaning and Household Products
Limonene is a key component in many cleaning products, including detergents, hand cleansers, and air fresheners. It helps to dissolve oils and remove adhesives due to its solvent properties. Its pleasant aroma also makes it an effective ingredient in deodorants and air fresheners.
Insect Repellent and Pesticides
Due to its strong aroma, limonene is used as a natural insect repellent and insecticide. It is an active ingredient in eco-friendly pest control products, helping to repel insects without the use of harmful chemicals.
Medical and Therapeutic Applications
Limonene has been studied for its potential therapeutic properties, including anti-inflammatory, antioxidant, and immune-boosting effects. It has also been investigated for its potential to prevent or treat certain types of cancer and heart disease. However, more research is needed to fully understand its efficacy and impact on human health.
Skin Care and Cosmetics
Limonene is used in skin care products due to its refreshing fragrance and potential skin benefits. It is said to brighten the skin by reducing hyperpigmentation and dark spots. However, caution should be exercised as direct application to the skin may cause irritation in some individuals.
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Limonene's chemical properties
Limonene is a colorless liquid aliphatic hydrocarbon classified as a cyclic monoterpene. It is a major component in the essential oil of citrus fruit peels. The (+)-isomer, occurring more commonly in nature as the fragrance of oranges, is a flavoring agent in food manufacturing. It is also used in chemical synthesis as a precursor to carvone and as a renewables-based solvent in cleaning products. The less common (−)-isomer has a piny, turpentine-like odor and is found in the edible parts of plants such as caraway, dill, and bergamot orange plants. Limonene is a chiral molecule, and biological sources produce one enantiomer: the principal industrial source, citrus fruit, contains (+)-limonene (d-limonene), which is the (R)-enantiomer.
Limonene is formed from geranyl pyrophosphate, via cyclization of a neryl carbocation or its equivalent. The final step involves the loss of a proton from the cation to form the alkene. It is a major constituent of oil of citrus rind, dill oil, and oil of cumin, neroli, bergamot, and caraway. It is also used as a paint stripper and is useful as a fragrant alternative to turpentine. Limonene is also used as a solvent in some model airplane glues and as a constituent in some paints. Commercial air fresheners, with air propellants, containing limonene are used by stamp collectors to remove self-adhesive postage stamps from envelope paper. Limonene is also used as a solvent for fused filament fabrication-based 3D printing.
Limonene is known for its strong aroma and has various health benefits, including antibacterial properties and potential anticancer effects. It is an anti-oxidant and has chemopreventive and therapeutic activities, including activity against spontaneous and chemically induced rodent mammary, skin, liver, lung, and forestomach tumors. It is also used in medicines, such as a flavoring agent to mask the bitter taste of alkaloids, and as a fragrance in perfumery, aftershave lotions, bath products, and other personal care products. (+)-Limonene is also used as a botanical insecticide and in organic herbicides. It is added to cleaning products, such as hand cleansers, to give a lemon or orange fragrance and for its ability to dissolve oils.
Limonene is combustible and has been considered as a biofuel. It oxidizes easily in moist air to produce carveol, carvone, and limonene oxide. With sulfur, it undergoes dehydrogenation to p-cymene. Limonene occurs commonly as the (R)-enantiomer, but racemizes at 300 °C. When warmed with mineral acid, limonene isomerizes to the conjugated diene α-terpinene (which can also easily be converted to p-cymene). It is a mild skin and eye irritant. Ingestion of 20 g of d-limonene caused diarrhea and a temporary increase in protein in the urine (proteinuria) in five male volunteers. Air levels of d-limonene may irritate the eyes and airways of some people, especially when the levels build up indoors.
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Limonene's oxidation reactions
Limonene is a colorless liquid aliphatic hydrocarbon and a cyclic monoterpene. It is a major component in the essential oil of citrus fruit peels. The oxidation reactions of limonene are of particular interest due to its high aerosol yield and two chemically different double bonds, an endocyclic and an exocyclic double bond.
Limonene oxidizes easily in moist air to produce carveol, carvone, and limonene oxide. The most widely practiced conversion of limonene is to carvone, which involves a three-step reaction. The first step involves the regioselective addition of nitrosyl chloride across the trisubstituted double bond. The oxidation of limonene can also lead to the formation of oligomers, such as C19H30O5, C18H28O6, and C19H28O7, which are believed to be esters, aldol reaction products, or hemiacetals.
The oxidation of limonene by ozone and hydroxyl radicals has been studied extensively, and it has been found that the initial reaction predominantly occurs at the endocyclic double bond. The first-generation oxidation products may be unsaturated and highly reactive, leading to further oxidation. The oxidation pathways can be influenced by the presence of other reagents, such as an OH scavenger, which suppresses the OH reaction pathways and allows oxidation only via ozonolysis.
R-Limonene, a common fragrance terpene, autoxidizes when exposed to air, and the oxidation products can cause contact allergies in some individuals. Both oxidized R-limonene and oxidized linalool have been patch-tested, showing positive reactions in dermatitis patients. Limonene is also used in various domestic and industrial products, such as cleaning agents, fragrances, and solvents, which can contribute to exposure and potential allergic reactions.
In summary, the oxidation reactions of limonene are diverse and can lead to the formation of various compounds, including carvone, carveol, limonene oxide, oligomers, and carboxylic acids. These reactions are important to understand due to the widespread use of limonene in nature and industry, as well as the potential health implications associated with its oxidation products.
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Limonene as a substitute for other compounds
Limonene is a colourless liquid aliphatic hydrocarbon and a cyclic monoterpene. It is a major component in the essential oil of citrus fruit peels. The (+)-isomer is the more common form of limonene, occurring in nature as the fragrance of oranges. This variant is used as a flavouring agent in food manufacturing and as a fragrance in products like perfumes, aftershaves, and bath products. It is also used in chemical synthesis as a precursor to carvone and as a solvent in cleaning products.
The less common (−)-isomer has a piney, turpentine-like odour and is found in the edible parts of plants like caraway, dill, and bergamot oranges. This form of limonene is used as a solvent for cleaning purposes, such as an adhesive remover, and as a fragrant alternative to turpentine. Limonene is also used as a paint stripper, a solvent in some model airplane glues, and as a constituent in some paints.
Due to worker safety concerns, volatile organic compound regulations, price fluctuations, and unpredictable availability, formulators have sought to reduce or replace limonene in their products. One challenge in replacing limonene is finding an alternative that meets the solvency needs and performance expectations of users. Sta-Sol® ME concentrates, for instance, are proven extenders or substitutes for limonene in cleaning and degreasing applications. Another alternative to limonene is dipentene, which is mainly composed of terpene hydrocarbons and is used as a substitute in household and industrial cleaners, solvents, pesticides, and oil extraction.
Limonene is also used in histology and histopathology as a substitute for xylene when clearing dehydrated specimens. Its advantages as a substitute include being less toxic and miscible with alcohols and melted paraffin wax, which is useful for embedding specimens to facilitate cutting thin sections for microscopy.
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