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Terpinene
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| Names | |||
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| IUPAC names
α: 4-Methyl-1-(1-methylethyl)-1,3-cyclohexadiene
β: 4-Methylene-1-(1-methylethyl)cyclohexene γ: 4-Methyl-1-(1-methylethyl)-1,4-cyclohexadiene δ: 1-Methyl-4-(propan-2-ylidene)cyclohex-1-ene | |||
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3D model (JSmol)
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| ECHA InfoCard | 100.029.440 | ||
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| Properties | |||
| C10H16 | |||
| Molar mass | 136.238 g·mol−1 | ||
| Density | α: 0.8375 g/cm3 β: 0.838 g/cm3 γ: 0.853 g/cm3 | ||
| Melting point | α: 60-61 °C | ||
| Boiling point | α: 173.5-174.8 °C β: 173-174 °C γ: 183 °C | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The terpinenes are a group of isomeric hydrocarbons that are classified as monoterpenes. They each have the same molecular formula and carbon framework, but they differ in the position of carbon-carbon double bonds. α-Terpinene has been isolated from cardamom and marjoram oils, and from other natural sources. β-Terpinene has no known natural source but has been prepared from sabinene. γ-Terpinene and δ-terpinene (also known as terpinolene) have been isolated from a variety of plant sources. They are all colorless liquids with a turpentine-like odor.[1]
Production and uses
[edit]α-Terpinene is produced industrially by acid-catalyzed rearrangement of α-pinene. It has perfume and flavoring properties but is mainly used to confer pleasant odor to industrial fluids. Hydrogenation gives the saturated derivative p-menthane.[1]
Biosynthesis of α-terpinene
[edit]
The biosynthesis of α-terpinene and other terpenoids starts with the isomerization of geranyl pyrophosphate to linalyl pyrophosphate (LPP). LPP then forms a resonance-stabilized cation by loss of the pyrophosphate group. Cyclization is then completed thanks to this more favorable stereochemistry of the LPP cation, yielding a terpinyl cation.[3] Finally, a 1,2-hydride shift via a Wagner-Meerwein rearrangement produces the terpinen-4-yl cation. It is the loss of a hydrogen from this cation that generates α-terpinene.
Plants that produce terpinene
[edit]References
[edit]- ^ a b M. Eggersdorfer (2005). "Terpenes". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a26_205. ISBN 3-527-30673-0.
- ^ Dewick, P. M. (2009). Medicinal Natural Products: A Biosynthetic Approach. United Kingdom: John Wiley & Sons. pp. 187–197.
- ^ Christianson, David W. (2017). "Structural and Chemical Biology of Terpenoid Cyclases". Chemical Reviews. 117 (17): 11570–11648. doi:10.1021/acs.chemrev.7b00287. PMC 5599884. PMID 28841019.
- ^ Li, Rong; Zi-Tao Jiang (2004). "Chemical composition of the essential oil of Cuminum cyminum L. from China". Flavour and Fragrance Journal. 19 (4): 311–313. doi:10.1002/ffj.1302.
- ^ Wang, Lu; Wang, Z; Zhang, H; Li, X; Zhang, H; et al. (2009). "Ultrasonic nebulization extraction coupled with headspace single drop microextraction and gas chromatography–mass spectrometry for analysis of the essential oil in Cuminum cyminum L.". Analytica Chimica Acta. 647 (1): 72–77. Bibcode:2009AcAC..647...72W. doi:10.1016/j.aca.2009.05.030. PMID 19576388.
- ^ Iacobellis, Nicola S.; Lo Cantore, P; Capasso, F; Senatore, F; et al. (2005). "Antibacterial Activity of Cuminum cyminum L. and Carum carvi L. Essential Oils". Journal of Agricultural and Food Chemistry. 53 (1): 57–61. Bibcode:2005JAFC...53...57I. doi:10.1021/jf0487351. PMID 15631509.
- ^ Hillig, Karl W (October 2004). "A chemotaxonomic analysis of terpenoid variation in Cannabis". Biochemical Systematics and Ecology. 32 (10): 875–891. Bibcode:2004BioSE..32..875H. doi:10.1016/j.bse.2004.04.004. ISSN 0305-1978.
- ^ Shahwar, Muhammad Khuram; El-Ghorab, Ahmed Hassan; Anjum, Faqir Muhammad; Butt, Masood Sadiq; Hussain, Shahzad; Nadeem, Muhammad (2012-07-01). "Characterization of Coriander (Coriandrum sativum L.) Seeds and Leaves: Volatile and Non Volatile Extracts". International Journal of Food Properties. 15 (4): 736–747. doi:10.1080/10942912.2010.500068. ISSN 1094-2912.
Terpinene
View on Grokipediafrom Grokipedia
Chemistry
Structure and isomers
Terpinenes constitute a group of isomeric monoterpenes characterized by the molecular formula C10H16 and a core cyclohexadiene carbon framework based on the p-menthane skeleton, consisting of a six-membered ring substituted with a methyl group and an isopropyl group.[1] These hydrocarbons differ primarily in the positioning and configuration of their two carbon-carbon double bonds, which dictate their distinct chemical behaviors and natural occurrences.[8] The four principal isomers are α-terpinene (1-methyl-4-(propan-2-yl)cyclohexa-1,3-diene), featuring conjugated double bonds between carbons 1-2 and 3-4; β-terpinene (4-methylidene-1-(propan-2-yl)cyclohex-1-ene), with an endocyclic double bond at position 1-2 and an exocyclic methylene group at position 4, though it is rarely found in nature and primarily synthetic; γ-terpinene (1-methyl-4-(propan-2-yl)cyclohexa-1,4-diene), exhibiting isolated endocyclic double bonds at positions 1-2 and 4-5 in a 1,4-diene arrangement with methyl at carbon 1 and isopropyl at carbon 4; and δ-terpinene (also known as terpinolene; 1-methyl-4-(propan-2-ylidene)cyclohex-1-ene), which incorporates one endocyclic double bond at 1-2 and an exocyclic double bond at the isopropylidene group on carbon 4.[9][10][11] In α-terpinene, the conjugated diene system enhances reactivity compared to the isolated double bonds in γ-terpinene or the mixed system in β-terpinene, while δ-terpinene's exocyclic unsaturation introduces additional strain and volatility.[12] These structural variations result from differences in double bond localization within the shared carbon skeleton, leading to conjugated versus isolated or semi-conjugated systems that influence stability, spectroscopic properties, and biosynthetic pathways.[13] The naming conventions originated in the late 19th century, when German chemist Otto Wallach and contemporaries systematically isolated and characterized terpenes from essential oils; for instance, α-terpinene was identified in the 1880s from sources like cajeput and eucalyptus oils during early studies on monoterpene fractionation.[14]Physical and chemical properties
Terpinenes are colorless to pale yellow liquids at room temperature, with melting points below 25 °C for α-terpinene, around -10 °C for γ-terpinene, and similarly low for β-terpinene, rendering all isomers liquid under ambient conditions.[1][3][15] Their boiling points are in the range of 173–183 °C at standard pressure, specifically 173–175 °C for α-terpinene, 173–174 °C for β-terpinene, and 181–183 °C for γ-terpinene, reflecting their volatility as monoterpenes.[1][3][15] Densities are approximately 0.83–0.85 g/cm³ across the isomers, with values of 0.833–0.838 g/cm³ for α-terpinene, 0.83 g/cm³ for β-terpinene, and 0.841–0.845 g/cm³ for γ-terpinene; refractive indices range from 1.472 to 1.480, such as 1.475–1.480 for α-terpinene and 1.472–1.478 for γ-terpinene.[1][3][15] Solubility is limited in water (e.g., 0.00868 mg/mL for γ-terpinene at 22 °C and 1.9 mg/L for β-terpinene at 25 °C) but high in organic solvents like ethanol and fixed oils.[3][15]| Isomer | Boiling Point (°C) | Density (g/cm³) | Refractive Index | Water Solubility (mg/mL at ~25 °C) |
|---|---|---|---|---|
| α-Terpinene | 173–175 | 0.833–0.838 | 1.475–1.480 | Insoluble |
| β-Terpinene | 173–174 | 0.83 | 1.475 | 0.0019 |
| γ-Terpinene | 181–183 | 0.841–0.845 | 1.472–1.478 | 0.00868 |
Occurrence in nature
Sources in plants
Terpinene, particularly its γ-isomer, is prominently produced in several plant families, including Lamiaceae, Apiaceae, Myrtaceae, and Theaceae, where it serves as a key monoterpene component in essential oils and glandular trichomes.[28][29] In the Lamiaceae family, species such as Origanum majorana (marjoram) and Origanum vulgare (oregano) accumulate significant levels of γ-terpinene, often as a precursor to phenolic compounds like carvacrol.[30] Similarly, Apiaceae plants like Cuminum cyminum (cumin) and Trachyspermum ammi (ajowan) feature terpinene in their seed oils, contributing to their characteristic aromas.[28] Myrtaceae representatives, including Melaleuca alternifolia (tea tree) and various Eucalyptus species, produce terpinenes in leaf volatiles, with γ-terpinene comprising a substantial portion.[31] In the Theaceae family, Camellia sinensis (tea plant) contains γ-terpinene in its essential oils, typically around 7%.[32] Notable concentrations of γ-terpinene have been documented in specific species, underscoring their natural abundance. In Origanum majorana, γ-terpinene reaches up to 25.73% of the essential oil composition, varying by geographic origin and environmental factors.[33] Melaleuca alternifolia essential oil contains 10-28% terpinene, primarily γ- and α-isomers, which dominate the monoterpene fraction.[34] In contrast, Citrus species from the Rutaceae family, such as Citrus unshiu and Citrus reticulata, produce low amounts of γ-terpinene, typically 0.2-1.2% in peel oils, often alongside dominant limonene.[35][36] Ecologically, terpinenes function as defense compounds in these plants, deterring herbivores and pathogens through antimicrobial and repellent properties that disrupt insect feeding and microbial growth.[37] They also contribute to plant aromas, acting as volatile signals to attract pollinators and seed dispersers, thereby enhancing reproductive success in natural habitats. These terpinene-producing plants predominate in Mediterranean, Australian, and tropical regions, reflecting adaptations to diverse climates. Lamiaceae and Apiaceae species thrive in the Mediterranean Basin's dry summers, while Myrtaceae like Melaleuca and Eucalyptus are native to Australia's arid and subtropical zones, and Citrus extends into tropical areas.[38][39]Content in essential oils
Terpinenes, particularly the α-, β-, and γ-isomers, are prominent monoterpene hydrocarbons in many essential oils, contributing to their aroma and potential bioactivity. In tea tree oil derived from Melaleuca alternifolia, total terpinenes account for 10-28% of the composition, with γ-terpinene being the dominant isomer at 10-28% and α-terpinene ranging from 5-13%. This distribution aligns with international standards for high-quality tea tree oil, where these isomers support the oil's characteristic profile alongside terpinen-4-ol.[20][40] In cumin essential oil from Cuminum cyminum seeds, γ-terpinene concentrations vary from 13.7% to 23.3%, often comprising a significant portion of the monoterpene fraction and influencing the oil's spicy, earthy scent. This isomer frequently appears alongside cuminaldehyde and p-cymene as one of the major components.[41] Similarly, in essential oils from certain Eucalyptus species, such as E. globulus and E. radiata, α-terpinene levels are typically below 1% and co-occur with dominant components like 1,8-cineole and α-pinene.[42] For Lippia alba essential oil, γ-terpinene contents are generally lower, around 2-10% depending on the chemotype, but can contribute notably in carvacrol- or thymol-dominant variants.[43] The variability in terpinene concentrations within these essential oils is influenced by several factors. Environmental conditions, including soil nutrient levels (e.g., calcium and potassium availability), climate (temperature and altitude), and harvest timing, can alter terpene biosynthesis and oil yield. For instance, higher organic matter in soil and moderate temperatures often correlate with elevated terpinene levels. Genetic differences among cultivars and chemotypes further modulate composition, with some strains selectively accumulating specific isomers.[44][45] Quantification of terpinenes in essential oils relies on standardized analytical techniques, primarily gas chromatography-mass spectrometry (GC-MS). This method separates and identifies isomers based on retention times and mass spectra, enabling precise measurement of relative percentages with detection limits suitable for trace analysis. GC-MS protocols often incorporate internal standards and flame ionization detection (FID) for validation, ensuring reproducibility across samples.[46][47]Biosynthesis
Metabolic pathway
The biosynthesis of terpinene in plants primarily occurs in plastids through the methylerythritol phosphate (MEP) pathway, which generates the C5 precursors isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP) from glyceraldehyde 3-phosphate and pyruvate. These precursors condense head-to-tail, catalyzed by geranyl diphosphate synthase, to form the C10 intermediate geranyl pyrophosphate (GPP), the universal precursor for monoterpenes including terpinene isomers. Although the mevalonate (MVA) pathway in the cytosol can also produce IPP and DMAPP, the MEP pathway predominates for monoterpene synthesis in plants.[48][49][50] The key steps involve the cyclization of GPP by monoterpene synthases (TPS), which initiate ionization to form a linalyl diphosphate intermediate, followed by ring closure to an α-terpinyl cation; this cation can then lead to limonene or terpinene isomers as alternative products via different deprotonation or rearrangement paths, with pyrophosphate (PPi) elimination. For example, α-terpinene arises via deprotonation of the α-terpinyl cation to form the conjugated 1,3-diene system, often as a direct product alongside limonene in synthase reactions. Terpene synthases, such as those in the TPS-b subfamily, catalyze these transformations.[51][52][53] Isomer-specific routes diverge at the carbocation stage: γ-terpinene arises from the terpinen-4-yl cation (derived via hydride shift from the α-terpinyl cation) via deprotonation, potentially routing through sabinene (formed by a 1,2-hydride shift from the α-terpinyl cation), whereas α-terpinene involves deprotonation to position the endocyclic double bonds in the 1,3-configuration. These variations highlight how subtle enzyme active site differences direct product specificity in plants like carrots and thyme.[51][50][53] The overall reaction can be summarized as parallel products from a shared intermediate:
[52][51]