lab 5 - Steam Distillation of Monoterpenes(S-Carvone...

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Steam Distillation of Monoterpenes ( S )-(+)-Carvone and ( R )-(+)-Limonene from Caraway Seeds/Analysis of Products by Infrared Spectroscopy and TLC Mayank Kumar March 9, 2007 Methods and Background The purpose of the experiment was to isolate the essential oil, which contained ( S )-(+)-Carvone as its major component and ( R )-(+)-Limonene as a minor component, from caraway seeds by steam distillation and analyze and verify the mixture of products by infrared spectroscopy and thin-layer chromatography. Carvone is a type of terpene, a class of hydrocarbon compounds that is typically produced in plants. Terpenes are created from units of isoprenes, which contain 5 carbons; Carvone is a monoterpene, meaning it is made of two isoprenes and thus contains 10 carbons. In the biosynthesis of Carvone, mevalonate rearranges a double bond to from isoprene, which then reacts with another molecule of mevalonate by electrophilic attack to produce an intermediate that performs an intramolecular electrophilic attack to yield ( R )-(-)-Carvone and ( S )-(+)-Carvone (upon addition of oxygen). Carvone exists in two enantiomers, ( R )-(-)-Carvone, found in spearmint leaves, and ( S )-(+)- Carvone, found in caraway seeds. Enantiomers are types of stereoisomers, which are isomers having the same molecular skeleton, but different spatial arrangements of their atoms. Enantiomers are related as nonsuperimposable mirror images and are thus termed chiral. Specifically, in order for an atom in a molecule to be a chiral center (stereocenter), it must be bonded to four different substituent groups. ( R ) and ( S ) configuration are assigned to molecules following the Cahn-Ingold-Prelog (CIP) rules, which first order the four substituents based off molecular mass (where the substituent with the most mass equals 1 and the substituent with the least mass equals 4) and then determine configuration on the basis of clockwise ( R ) or counterclockwise ( S ) rotation, moving from 1 to 4. The two enantiomers of Carvone spatially differ in that the ( R ) enantiomer has a hydrogen coming “out” and a 1-methylethenyl group going “in” at the stereocenter, while the ( S ) enantiomer has the hydrogen going “in” and the 1- methylethenyl group coming “out.” Enantiomers also demonstrate identical physical properties, so the two enantiomers of Carvone would give the same infrared (IR) spectra, retention factor (R f ) values, boiling points, and nuclear magnetic resonance (NMR) spectra. However, the two enantiomers would have different optical rotations, as they rotate plane-polarized light the same amount of degrees, but in opposite directions (designated by + or -). If the two enantiomers were in a 1:1 mixture (racemic mixture), though, there would be no net rotation of plane-polarized
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light, resulting in an observed optical rotation of 0 °. Since the two enantiomers of Carvone smell differently to us, this indicates that the receptors on our olfactory neurons are chiral environments.
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