Steam Distillation of Monoterpenes (
)-(+)-Carvone and (
Caraway Seeds. Analysis of Products by Infrared Spectroscopy and TLC.
March 3, 2008
Methods and Background
The purpose of the experiment was to isolate the essential oil, which contained
)-(+)-Carvone as its major component and (
)-(+)-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 exists in two enantiomers, (
)-(-)-Carvone, found in spearmint leaves,
)-(+)-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
Specifically, in order for an atom in a molecule to be a chiral center
(stereocenter), it must be bonded to four different substituent groups.
) and (
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 (
) or counterclockwise (
moving from 1 to 4.
The two enantiomers of Carvone spatially differ in that the (
enantiomer has a hydrogen coming “out” and a 1-methylethenyl group going “in” at the
stereocenter, while the (
) 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
) values, boiling points, and nuclear magnetic resonance (NMR)
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 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.
Steam distillation is used to separate and purify volatile organic compounds, such
as Carvone, that are immiscible or nearly immiscible with water; typically, volatile
organic liquids and water are co-distilled in this process.
Steam distillation is also useful