4.6 CHIRALITY:

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4.6 CHIRALITY handedness in Molecules
Many molecules exist in right-handed and
left-handed forms
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Mirror Images
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A MIRROR IMAGE IS A REFLECTION
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Your left hand is a mirror image of your right
hand
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Some mirror images are superimposable
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Some mirror images are not superimposable
(nonsuperimposable)
Nonsuperimposable mirror images have right
handed and left-handed forms stereoisomers.
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Molecules have mirror images too.
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Chirality
Pronounced kye-rality like reality
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IN A MOLECULE
A carbon that is tetrahedral (sp3) and has
four different groups attached is called...
a chiral center
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Another name is .
stereocenter
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A molecule with one stereocenter is chiral.
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Its mirror images are not superimposable
mirror
rotate
this molecule is chiral
note that the fluorine and bromine
are interchanged in the mirror image
non-superimposable mirror images
do interchanges in class
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Practice with models

• Make a molecule with a stereocenter
• Make its mirror image
• Try to superimpose them
• Are they the same or different molecules?
• Interchange two of the atoms.
• Now try to superimpose them.
• Are they the same or different molecules?

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PLANES OF SYMMETRY
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A SYMMETRIC OBJECT HAS A PLANE OF SYMMETRY
- ALSO CALLED A MIRROR PLANE
plane of symmetry
mirror plane
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IF AN OBJECT HAS A PLANE OF SYMMETRY, ITS MIRROR
IMAGE WILL BE IDENTICAL
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IDENTICAL MIRROR IMAGES WILL SUPERIMPOSE
(MATCH EXACTLY WHEN PLACED ON TOP OF EACH OTHER)
IF AN OBJECT HAS AN IDENTICAL MIRROR IMAGE
ITS. ACHIRAL
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CHIRAL
An object without symmetry is
no symmetry
The mirror image of a chiral object is different
and will not superimpose on the original object.
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WHICH OBJECTS ARE SYMMETRIC ?
WHICH OBJECTS ARE ACHIRAL ?
(mirror image is identical)
sym (outside)
achiral
sym
achiral
sym
sym
achiral
achiral
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ACHIRAL
The plane of the paper is a plane of symmetry
F
F
C
l
C
l
B
r
B
r
C
l
C
l
TWO IDENTICAL GROUPS RENDERS A TETRAHEDRAL
CARBON ACHIRAL
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TWO VIEWS OF THE PLANE OF SYMMETRY
plane of symmetry
F
F
side view
edge view
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ACHIRAL
A CARBON ATOM WITH THREE SIMILAR GROUPS IS
ALSO ACHIRAL ..
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plane of symmetry
side view
edge view
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The whole secret of the study of nature lies
in learning how to use ones eye.
George Sand, author