Download CHEM 210 Chapter 5 Wrap-up

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 are
NOT mirror images of
each other)
Determining the relationship between molecules
• If the molecular formula is different they are not
isomers
Example: benzene vs. cyclohexane
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
© 2014 by John Wiley & Sons, Inc. All
rights reserved.
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
(R)-2-Butanol
(S)-2-Butanol
(+)-(R,R)-Tartaric Acid
(–)-(S,S)-Tartaric Acid
(+/–)-Tartaric Acid
© 2014 by John Wiley & Sons, Inc. All
rights reserved.
bp (oC)
99.5
mp (oC)
99.5
168 – 170
168 – 170
210 – 212
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 planepolarized 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)
Polaroid
lens
Optical Activity – Measuring on Polarimeter
• Polarimeter – instrument to measure optical
activity
a = observed optical
rotation
Optical Activity – Measuring on Polarimeter
Optical Activity – Measuring on Polarimeter
Optical Activity – Measuring on Polarimeter
Optical Activity – Calculating Specific Rotation
observed
rotation
temperature
25
[a]D =
wavelength of light
(e.g. D-line of Na
lamp,
l = 589.6 nm)
c
concentration of
sample solution
in g/mL
a
x
ℓ
length of cell
in dm
(1 dm = 10 cm)
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
CH3
CH3
*
H
HO
25
*
CH2CH3
[a] = + 13.5
D
o
H3CH2C
25
mirror
H
OH
[a] =  13.5
D
o
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 planepolarized light
rotation
H
CH3
OH
C2H5
(R)-2-Butanol
H 3C
H
HO
C 2H 5
(S)-2-Butanol
(if present)
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
% enantiomeric
=
excess *
observed specific rotation
specific rotation of the
pure 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%
% enantiomeric
=
excess *
+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
R4
R1
R3
Si
H
R2
R1
R4
R1
R2
Ge
R2
R2
R3
N
R3
X
R1
S
O
Other Chirality In Organic Chemistry
Other Chirality In Organic Chemistry
mirror
H
C
Cl
C
C
H
H
Cl
Cl
enantiomers
H
C
C
C
Cl