Physical Properties of enantiomers:

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Physical Properties of enantiomers EVERYTHING
identical except
Every enantiomer will rotate the plane of
polarized light either right () or left (-). The
extent and sense of rotation cannot be predicted
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The polarimeter
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Specific rotation a

• Angle of rotation a is determined by
• nature of the compound
• concentration
• size of the sampler (polarimeter cell)
• Temperature
• wavelength
• In order to obtain a physical value independent
  from
• experimental details lets define
• a

a(measured)
(Conc) x (Cell lenght)
a25 can be either positive () or negative (-)

D
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REMEMBER Clockwise and counterclockwise
rotation is just a formalism to unequivocally
describe a structure. In principle it has
NOTHING to do with optical rotation of polarized
light as observed in a polarimeter. Only
experiment will be able to tell if , for
example, the enantiomer S will be levorotatory
(-)S or dextrorotatory ()S.
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Racemate compound (a 5050 mixture of the two
enantiomers)
optically inactive !!!
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Enantiomeric excess
One enantiomerically pure substance has by
definition 100 enantiomeric excess. Pure
(S)-()-2-butanol has a specific rotation of
13.52 deg. A mixture of the two enantiomers
with a predominance of the S Enantiomer will have
a spec. rot. of less than 13.52 but more than
0
moles one enant. moles other enant.
enantiomeric excess
x 100

total moles of both OR
observed specific
rotation enantiomeric excess
x
100
specific rotation of a pure enantiomer
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Molecules with more stereocenters n
stereocenters 2n stereoisomers
BUT
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Diastereoisomers Enantiomers
Not superposable But mirror image
Not superposable But not mirror image
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Lets consider now 2,3-dibromobutane CH3-HCBr-H
CBr-CH3 2 stereocenters 4 stereoisomers
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Meso form (optically inactive)
mirror plane
The two molecules are in fact superposable they
are identical
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Nomenclature
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Nomenclature
R
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Nomenclature
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Nomenclature
also R
So (2R,3R)-2,3-dibromobutane
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Fisher Projections Formulae
Vertical lines behind the plane Horizontal lines
out of plane
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• Rules
• Rotation in plane only by 180
• No flip over

Very useful for rapidly testing superposability