Chirality – Isolation of Limonene from Citrus Fruits

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Chirality – Isolation of Limonene from Citrus Fruits
Introduction: Terpenes are a large and varied class of hydrocarbons produced by a wide variety of plants. Many essential oils belong to the terpene class. They are made up of two or more five carbon units that are called isoprenes. Limonene is a terpene that can be separated from orange peels almost 100% in the (R)-(+)-limonene form. Other citrus fruits like lemons, grapefruits, limes, and tangerines contain the compound as well. The purity of the oil extracted from the oranges can be determined by physical properties such as optical rotation and refractive index. In this experiment, limonene will be isolated by steam distillation. Normally, big molecules such as limonene require very high temperatures to boil. In the presence of water, liquids which are immiscible with water tend to boil below its normal boiling point. Steam distillation looks like this: Source:

Normal distillation would require extremely high temperatures and would be more time consuming. Distilling in the presence of water, however, helps to avoid this problem by keeping the temperature low.

Polarimetry measures how much a substance is able to interact with plane polarized light. Anisotropic crystalline solids and samples that have an excess of one enantiomer of a chiral molecule, can rotate the orientation of plane-polarized light. These substances are said to be optically active. In order to be optically active, the substance must have a chiral center. A chiral center consists of a carbon that has four different groups attached to it. A sample that has only one enantiomer of a chiral molecule is said to be optically pure. The compound can either may either rotate plane polarized light to the left or the right. The enantiomer that rotates light to the right, or clockwise when viewing in the direction of light propagation, is called the dextrorotatory (d) or (+) enantiomer, and the...
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