Wrapping Up: Subdivision of Isomers
Isomers(different compounds with same molecular formula)
Constitutional Isomers
(isomers whose atoms have a different
connectivity)
Stereoisomers(isomers that have the same
connectivity but differ in spatial arrangement of their
atoms)
Enantiomers(stereoisomers that are
nonsuperimposable mirror
images of each other)
Diastereomers(stereoisomers that areNOT mirror images of
each other)
Determining the relationship between
molecules• If the molecular formula is different they are not
isomers
Example: benzene vs. cyclohexane
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Determining the relationship between
molecules• If the molecular formula is identical, but they are
connected differently they are structural isomers
Example: cis-1,2 vs. cis-1,3-dimethylcyclohexane
Example: diethyl ether vs. 1-butanol
Determining the relationship between
molecules• If the connectivity is identical, but they differ in
the 3-D arrangement of atoms in space they are steroisomers.
Determining the relationship between
molecules• If the molecules are not superimposable and
mirror images, they are enantiomers:
Determining the relationship between
molecules• If the molecules are not superimposable nor are
they mirror images, they are diastereomers:
Determining the relationship between
molecules• In molecules with multiple stereocenters, this
may be difficult to visualize.
• Simply, if every R, S center flips between the molecules you are comparing, they are enantiomers:
Determining the relationship between
molecules• If at least one center remains the same and at
least one other flips, they are diastereomers!
Physical Properties of Stereoisomers• Enantiomers have identical physical properties
(e.g. melting point, boiling point, refractive index, solubility etc.)
Compound bp (oC) mp (oC)
(R)-2-Butanol 99.5
(S)-2-Butanol 99.5
(+)-(R,R)-Tartaric Acid
168 – 170
(–)-(S,S)-Tartaric Acid 168 – 170
(+/–)-Tartaric Acid 210 – 212
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Physical Properties of Stereoisomers• Enantiomers also:
o Have the same chemical properties (except reaction/interactions with chiral substances)
o Show different behavior only when they interact with other chiral substances (enzymes)
o Rotate plane-polarized light in equally in opposite directions - this property of enantiomers is called optical activity
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Optical Activity• The property possessed by chiral substances of
rotating the plane of polarization of plane-polarized light
• The electric field (like the magnetic field) of light is oscillating in all possible planes
• When this light passes through a polarizer (Polaroid lens), we get plane-polarized light (oscillating in only one plane)
Polaroidlens
Optical Activity – Measuring on
Polarimeter• Polarimeter – instrument to measure optical
activitya = observed optical
rotation
Optical Activity – Measuring on
Polarimeter
Optical Activity – Measuring on
Polarimeter
Optical Activity – Measuring on
Polarimeter
Optical Activity – Calculating Specific
Rotation
D[] =
25 c x
temperature
wavelength of light(e.g. D-line of Na lamp, l = 589.6 nm)
concentration of sample solutionin g/mL
length of cellin dm (1 dm = 10 cm)
ℓ
observed rotation
Optical Activity – Calculating Specific
Rotation• The value of a depends on the particular
experiment (since there are different concentrations with each run)
• But specific rotation [aD] should be the same regardless of the concentration
Optical Activity – Calculating Specific
Rotation• Two enantiomers should have the same value of
specific rotation, but the signs (+/-) are opposite
mirror
HO
*
CH2CH3
CH3
H
[] = + 13.5o25
D
OH
*
H3CH2C
CH3
H
[] = 13.5o25
D
Optical Activity – Calculating Specific
Rotation• An equimolar mixture of two enantiomers is called
a racemic mixture (or racemate or racemic form)
• A racemic mixture causes no net rotation of plane-polarized light
HC2H5
CH3
OH
(R)-2-Butanol
HC2H5
H3C
HO
(S)-2-Butanol(if present)
rotation
equal & opposite rotation by the enantiomer
Racemic Forms and Enantiomeric Excess• A sample of an optically active substance that
consists of a single enantiomer is said to be enantiomerically pure or to have an enantiomeric excess of 100%
• An enantiomerically pure sample of (S)-(+)-2-butanol shows a specific rotation of +13.52
• A sample of (S)-(+)-2-butanol that contains less than an equimolar amount of (R)-(–)-2-butanol will show a specific rotation that is less than 13.52 but greater than zero
Calculating Enantiomeric Excess or Optical
Purity• Also known as optical purity
• It can also be caluclated from optical rotation data
% enantiomericexcess
moles of oneenantiomer
moles of otherenantiomer
total moles of both enantiomers
= x 100
% enantiomericexcess *
observed specific rotation
specific rotation of thepure enantiomers
= x 100
Calculating Enantiomeric Excess or Optical
Purity• A mixture of the 2-butanol enantiomers showed
a specific rotation of +6.76o
• The specific rotation of pure (S)-2-butanol is +13.5o
• The enantiomeric excess of the (S)-(+)-2-butanol is 50%
% enantiomericexcess *
+6.76+13.52
= x 100 = 50%
Calculating Enantiomeric Excess or Optical
Purity• It is important to know what this means. Since
any R impurity will ‘cancel’ the rotation of an equal amount of S:
• A sample with an ee of 50% S is actually 50% pure S and 50% racemic R/S.
• The total S enantiomer in the sample is actually 75%!
Physical Properties of Diastereomers• Stereoisomers that are not enantiomers
• Unlike enantiomers, diastereomers usually have substantially different chemical and physical properties
Other Chirality In Organic Chemistry
R1
SiR2
R4 R3
R1
GeR2
H R3
R1
NR2
R4 R3
X SR2 R1
O
Other Chirality In Organic Chemistry
P(Ph)2
P(Ph)2
(Ph)2P
(Ph)2P
(S)-BINAP (R)-BINAP
enantiomers
Other Chirality In Organic Chemistry
enantiomers
C C
Cl
H
CCl
HCC
Cl
H
CCl
H
mirror
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