Essential Organic Chemistry
Paula Yurkanis Bruice
Chapter 6
Isomers and Stereochemistry
Disampaikan oleh : Dr. Sri Handayani
2013
Review of Isomerism
Isomers – Compounds that have the same
molecular formula but do not have identical
structures.
Constitutional Isomers – differ in the way
their atoms are connected.
Stereoisomers – differ in the way their
atoms are arrange in space.
Constitutional Isomers
6.1 Cis-trans isomers
Differ in the arrangement of their atoms in
space (cannot interconvert)- Alkenes.
6.1 Cis-trans isomers
 Cyclic structure.
6.2 Chirality
Chiral –
Nonsuperimposable on its
mirror image.
Achiral – Superimposable
on its mirror image.
If a molecule (or object) has
a mirror plane or an
inversion center, it cannot be
chiral.
Chiral or Achiral?
Brandy snifter
achiral
achiral
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Chiral or Achiral?
Shears
chiral
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Chiral or Achiral?
Beer mug
achiral
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Chiral or Achiral?
Hiking boot
chiral
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Chiral or Achiral?
Baseball glove
chiral
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Chiral or Achiral?
Boat propeller
chiral
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Chiral or Achiral?
Desk chair
achiral
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Chiral or Achiral?
School desk
chiral
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Chiral or Achiral?
cis-1,3-dimethylcyclopentane
achiral
mirror plane
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Chiral or Achiral?
trans-1,3-dimethylcyclopentane
chiral
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Chiral or Achiral?
1,1-dimethylcyclohexane
achiral
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6.3 Asymmetric centers
An asymmetric center is an atom that
is bonded to four different groups.
Chiral or Achiral?
Asymmetric
center
2-butanol
chiral
mirror image
Chiral or Achiral?
2-bromopropane
achiral
6.4 Isomers with one asymmetric center
6.4 Isomers with one asymmetric center
Enantiomers
A chiral compound and its mirror image are
called enantiomers.
2-butanol:
enantiomers
Enantiomers
Asparagine:
O
C
mirror
plane
OH
H2N C H
H2N
HO
C
O
H C NH2
CH2
CH2
C
C
O
L-asparagine
O
NH2
D-asparagine
(from asparagus)
(from vetch)
bitter taste
sweet taste
enantiomers
6.5 How to draw enantiomers
6.6 Naming enantiomers: the R,S system
Absolute Configuration
R and S
• Assign priorities to the remaining groups based
on atomic numbers.
• Clockwise (highest to lowest priority) R
• Counterclockwise S
HO
OH
H C
=
CH3
(R)-2-butanol
2
1
CH 2CH3
CH2CH3
C
CH3
3
Absolute Configuration
Assign priority:
• Atomic number of atom directly bonded.
• If the same atom is bonded, go to the next atom,
etc.
• Groups containing multiple bonds are treated as
though multiple atoms were attached:
N C
C O
=
C O
O C
C N
=
C N
N C
Absolute Configuration
Determine the absolute configuration of the
following compounds:
1
3
H3C
F
4
H
C
Cl
Br S 2
C
CH
DR 2 3
3
1
O
2
(CH3)2CHCH2
4
H
C
Cl
S
CH3CH2CH2CH2
1
3
4
H
1
H C
4
H
C
C N
CH3CH2 R 2
3
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Absolute Configuration
Determine the absolute configuration of the
following compounds:
CH3
CH3
1
2
CH3O CH
4
H
C
2
CH3
S
3
CH
CH3CH2
CH3O CH
CH3CH2
C
1
CH3
R
CH
3
OCH3
OH
1
CH2=CH
4
H
2
CH2=CH
4
H
C
2
CH3
S
CH3CH2
3
4
H
C
C CH
S
(CH3)2CH
1
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3
6.7 Chiral compounds are optically active
 Plane-polarized light is produced by passing
normal light through a polarizer.
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Optical Activity
 When plane-polarized light passes through a
solution of achiral molecules, the light emerges
from the solution with its plane of polarization
unchanged.
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Optical Activity
 However, when plane-polarized light passes
through a solution of a chiral compound, the light
emerges with its plane of polarization changed.
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Optical Activity
 Optical Activity – The ability of a compound to
rotate the plane of polarized light.
 A compound that rotates the plane of polarization
is said to be optically active.
 Chiral compounds are optically active and
achiral compounds are optically inactive.
 A polarimeter is used to make such measurements:
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