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Asymmetry
resulting in an object not being superimposable upon its mirror image. chemical
chirality is usually associated with a
carbon atom having four
different
substituents. Two isomers which are mirror images of one another are
called enantiomers; . Towards symmetrical environments, enantiomers behave
identically, but towards other chiral molecules they are different. Thus
they will have identical melting points, but may have different tastes and
smells.
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Here is what Louis Pasteur (1822 - 1895), wrote to a friend in 1851:
" The
molecular structures of the two tartaric acids are asymmetric, and on
the other hand, they are rigorously the same, with the sole difference of
showing
asymmetry
in opposite senses. Are the atoms of the right acid grouped on the spirals
of a right-handed helix, or placed on the solid angles of an irregular
tetrahedron, or disposed according to some particular asymmetric grouping
or other? We cannot answer these questions.''
Perhaps you would understand this quotation better if you knew what Pasteur
was investigating. when we shine light through a polarizer, most of the light
is blocked off. The only light which gets through is light which is vibrating
in the right direction. If we put another polarizer in the way, at right
angles to the first, no light gets through at all. (If this is new information,
stop here, and refer to polarized light, then come back here to continue
reading.)
If there is a suitable chemical between the two filters, one which twists
the light, the chemical will twist the light around so that some of it can
get through the second filter. Tartaric acid in solution makes light rotate,
or twist, but Louis Pasteur had a problem. The absolutely chemically identical
racemic acid did nothing of the sort when it was dissolved and tested. It
had no effect on the light at all. when he examined the tartaric acid, Pasteur
found
asymmetrical
crystals, with all of the crystals having the same asymmetry. Racemic
acid was different: half of the crystals looked just like tartaric acid,
but the other half were the exact opposite. They were mirror images of the
tartaric acid crystals.
Pasteur separated out the ''tartaric''
crystals from the racemic
acid, and sure enough, a solution of these crystals rotated the light, just
like tartaric acid, while the crystals left behind in his fossicking also
rotated light, but in the opposite direction. So Pasteur shouted the French
equivalent of ''Ringol'' and rushed off to write to an old friend about his
marvellous discovery. Since that time, we have learned about many other chemicals
which can turn up in two different forms: most of them are carban-containing,
so we should not be surprised that the chemicals in our bodies are often
of one handedness or the other.
Written by Peter Macinnis
©WebsterWorld Pty Ltd/contributors 2002