Carvone, 2-methyl-5-(1-methylethenyl)-2-cyclohexene-1-one,
a
naturally occuring monocyclic mono
terpene ketone,
exists as a strongly scented
oil in several species
of edible plants.
Though most know it as a primary component in the flavor
and odor of
spearmint,
caraway, and
dill, carvone
holds a special place in the hearts and syllabi of first
semester
organic chemistry students everywhere.
----------
The
professor walks in, carrying his normal small basket
of props for the class. He says nothing, instead drawing
a
structure on the overhead. "Aww, man!", most of the
class thinks, expecting that this means a pop quiz.
But then, instead of instructing the class to name
functional groups or, *gasp*, the full
IUPAC name,
the professor passes around a small vial containing a
clear, pale-yellow thin fluid. "Smell it", he instructs.
Most of the class readily recognizes it as something
minty, a few even get it correct, as spearmint.
"You've all just sniffed
carvone, the main
constituent of spearmint oil, which I've drawn on
the overhead.". He then shuffles through his
basket-o'-goodies, and passes around another vial,
which looks similar to the first.
Fewer people recognize this smell, a sort of oily
odor with a hint of mint-like spiciness, though a
few say it smells vaguely like dill.
"Dill works, though you'll find it in a much higher
concentration in caraway seeds", the professor
points out. "You've all just sniffed
carvone,
which I've drawn on the
overhead.".
The class looks confused, and even the strange guy
in the back (who everyone suspects has just a
tad too much "experiential knowlege" of
organic chemistry) looks up and cocks his head to
the side like a confused puppy.
"In 1849,
Louis Pasteur cooled a solution of
racemic acid in a dish on his window sill, allowing
it to crystalize...", the professor begins, in his
explanation of a property of
molecules that the
class will soon come to hate.
----------
The basic structure (see below) of Carvone has
an extremely interesting property. The same
chemical structure, with the same arrangement
of
bonds, can form in two distinct ways,
allowing for a type of
stereoisomer called an
enantiomer. These relate to one another
in the same way a left and right hand do - They
have all the same parts, and look mostly alike,
but no matter how hard someone tries (short of
turning it inside-out), a left-handed glove will
not fit a right hand.
This property has numerous consequences,
particularly for
carbon-based life forms as
exist on Earth. A typical
in vitro
synthesis produces a mix of both
isomers,
called a
racemate (Pasteur's "
racemic acid"
actually consisted of a mix of the isomers of
tartaric acid). An
in vivo synthesis, such
as a human cell might use to produce a given substance,
will almost always produce a single isomer rather
than a racemate. As a result, ingestion of the
R form may have different effects than ingestion
of the S form. The most well known example of
this occurred with the drug
thalidomide, used in
the 1960's for morning sickness. The S enantiomer
of thalidomide would correctly alleviate nausea,
while the R version caused severe
birth defects
in the growing
fetus.
Carvone has no particular medicinal activity in humans.
However, the sharply different taste and smell of the
two enantiomers make it almost ubiquitously used
as an example in any organic chemistry class.
----------
CH3 CH3
/ /
/ /
HC====C HC====C
/ \ / \
/ \ / \
H2C C====O H2C C====O
\ / \ /
\ / \ /
C----CH2 C----CH2
/ . . \
/ . . \
H3C----C\ H H3C----C\ H
\\ \\
\\ \\
CH2 CH2
S-(+)-carvone R-(-)-carvone
caraway, dill spearmint
oily minty
----------
Properties:
Boiling point ............ 230°C @ 101.3kPa
Flash point ............... 88°C (R), 95°C (S)
Vapor pressure ....... 0.0533kPa @ 20°C
Specific gravity ......... 0.956 - 0.960 @ 25°C
Refractive index ...... 1.495 - 1.499 @ 20°C
Optical rotation ........ 61° (R-, S+)
Soluble in .................
ethanol,
methanol,
propylene glycol
Insoluble in ...............
glycerin,
water
LD
50 (oral, rat) ......... 1640 mg/kg
Sources:
The Merck Index, 12th ed.
McMurray's Organic Chemistry, 4th ed.
http://hexane.chem.uiuc.edu/cyberprof/public/chemistry/102x/Lecture/lect21c.html
http://www.ims-global.com/insight/news_story/0301/news_story_030116.htm
http://www.ensia.inra.fr/~courtois/fidel/Lisbon/flaves/sec2/sec222.html
http://www.thegoodscentscompany.com/data/rw1002741.html
http://www.uiowa.edu/~chemsafe/MSDS/(R)-(-)-Carvone.html
http://www.uh.edu/engines/epi604.htm