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Re: A question about mirrors



From: Michael Edmiston <edmiston@BLUFFTON.EDU>
[snip]
Thank you for your detailed answer. I understand your reasoning
but...

Two points here. One about communication with an "intelligent being," and
the other about whether there are more "opposites."

(1) Rather than discuss the rotations and reflections as John Denker did,
another way to approach this is from the stereochemistry of molecules. If
we assume the "intelligent being" has never seen a mirror, but knows some
chemistry, and knows some astronomy, we can define mirror images and right
and left handedness by chemistry and astronomy. I will assume we have had
sufficient dialogue with this "intelligent being" that we can discuss
chemicals... that is, the being understands what we mean when we say
carbon,
hydrogen, chemical bond, etc. If not, you'll have to reach that level
first.

Take a carbon-containing molecule in which at least one carbon atom has
four
other atoms bonded to it, and one (and only one) of the carbon atoms has
four "different" atoms attached. A simple example would be CHClFBr
(flourochlorobromomethane).

The geometry around this carbon is tetrahedral. This molecule is also
chiral (handed). Construct two model molecules with balls and sticks, or
whatever. Depending upon how you assembled the two models, they might be
identical (can be stacked so they are superimposed on each other)...
otherwise they will be "mirror images" of each other. Whichever they are
(identical or mirrors) one molecule can be converted to the other form by
switching the positions of any two attachments. Assemble them so they are
not superimposable on each other. We now have two objects that are
"mirror
images" of each other. This could serve as one type of definition of
"mirror image."
This is where I object. Why do you say that the alien must only get two
isomers? Yes, I know that it may quite easily be demonstrated for this
particular case *but* the demonstration would only be valid (with obvious
changes) for a molecule which may be broken down into asymetrical
tetrahedral subunits or maybe even any ball-and-stick model. But I gather
that you are saying
that,
I believe the reduction of other objects into "chiral
centers" is also possible and would follow the same logic.
Crucially - how?
Apologies to those of you who have not taken an organic chemistry course
and
do not have experience with the fascinating subject of stereochemistry.
Fortunately, I know enough at least to follow your argument. But (as a
digression) I read somewhere that this model used to show stereoisomers
(tetrahedral ball-and stick) is a simplification and that in reality the
adjectives chiral and achiral are fuzzier.
This of course does not alter the main thread.
Regards,
Abhishek Roy



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