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In accordance with the chemical classification of elements and compounds, limonene is classified as a hydrocarbon of a cyclic terpene. Its physical features are as follows: it is colourless liquid with a smell of lemon, and of room temperatures in its normal condition.
The lemon odour was the key reason why this compound was called Limonene, and all the citrus fruits contain it in their outer layer, which is responsible for the smell of these fruits. In fact, it is a chiral molecule and as Burton (2007) emphasized: since it is common with such forms biological sources create lone enantiomer: the principal industrial source, citrus fruit contains D-limonene (+)-limonene, which is (R)-enantioner. (Burton 2007, p. 350)
Considering the chemical terminology, as it is stated by Kodama and Okubo (1977): limonene may be featured as the aromatic hydrocarbon along with such compounds as arenes, alkenes and alkyne-based compounds which are composed of carbon and hydrogen. This compound is described as pure. (Kodama, Okubo, et.al. 1977)
Terpenes in general are regarded as the large class of hydrocarbons which are produced by plants. However, some insects are also produced by insects, and this natural origin of limonene may be regarded as the perfect way of limonene isolation. Achiral objects are objects that are identical to their mirror image EnantiomerIn stereochemistry. Limonene is derived from isoprene, and isoprenoids are a large and structurally diverse family of compounds which acts as an essential part in plants as hormones, photosynthetic pigments, electron carriers, and membrane components and they also serve in communication and defence. Even though they are normally synthesized through condensations of the five carbon(c-5) compounds isopentenyl diphosphate (IPP) and its allylic isomer dimenthylallyl diphosphate (DMAPP), two different and autonomous biosynthetic routes to these precursors exist in plants. In accordance with Pakdela and Panteaa (2001, p. 99), the following statement should be emphasized:
The cytosolic pathway to IPP starts from acetyl-CoA and proceeds through the classical intermediate mevalonic acid to make available precursors for the biosynthesis of sesquiterpenes and triterpenes.
In the light of this statement, it should be stated that the plastidial passageway is initiated by the transketolase-type condensation of pyruvate (carbon 2 and 3) and glyceraldehydes-3 phosphate to 1 -deoxyxylulose-5-phosphate (DXP), followed by the isomerisation and reduction of this intermediate to 2-c-methylerythritol-4 -phosphate, formation of the cytidine 5D-diphosphate derivative, phosphorylation at C2,and cyclization to 2-C-methylerythritol-2. This plastidial passageway provides precursors for the biosyhthesis of isoprene and genes programming each enzymes of the passageway, up to formation of the cyclic diphosphate. These have been isolated from plants and from eubacteria in which the passageway also operates. (Matura, 2002)
As for the chemical features of limonene, it should be stated that it is a stable terpene which is generally distilled without any additional decomposition. However, it may crack to a form a isoprene is the temperatures are elevated. In moist air it may be oxidised to carveol and carvone (Simonsen, 1947). Limonene occurs naturally as the (R)-enantiomer, but racemises to dipentene at 300c. (Turner, Harris, 1952)
If it is warmed with mineral acid it may be isomerized to the diene a-terpinene component, which will be easily oxidised to p-cymene an aromatic hydrocarbon. In the active organic form it can be extracted of an orange by the means of filtration process with CO2 and dry ice. (Wainman, Junfeng, 2000)
Bibliography
Burton. A Lemon-Fresh Ozone Environmental Health Perspectives Vol. 7 no 115. 2007. p 350.
Kodama, R., A. Okubo, E. Araki, K. Noda, H. Ide, and T. Ikeda. Studies on d-limonene as a gallstone solubilizer. (VII). Effects on development of mouse fetuses and offsprings. Oyo Yakuri. Vol. 13. No. 6., pp. 863-873. 1977.
Matura M. Oxidized citrus oil (R-limonene): a frequent skin sensitizer in Europe . J. Am. Acad. Dermatol. Vol. 47 No. 5., pp. 70914. 2002.
Pakdela, H. D. Panteaa and C. Roy. Production of dl-limonene by vacuum pyrolysis of used tires . J Anal Appl Pyrolysis Vol. 57,. No. 1., pp. 91107. 2001.
Simonsen, J. The Terpenes. 2nd edn. Cambridge University Press. 1947.
Turner, E.E., M. M. Harris. Organic Chemistry. London: Longmans, Green & Co. 1952.
Wainman, T., Z. Junfeng.Ozone and Limonene in Indoor Air: A Source of Submicron Particle Exposure. Environmental Health Perspectives Vol 12. No 108. 2000. p. 1139.
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