Staff Site Universitas Negeri Yogyakarta KO II Stereokimia

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?
2-butanol

Asymmetric
center

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

Asparagine:
O

C

Enantiomers
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

2

1

CH3O CH

4
H

C
2
CH3
S
3
CH

CH3CH2

CH3O CH
CH3CH2

C
1
CH3
R
3
CH
OCH3

OH
1

CH2=CH

4
H

2

CH2=CH

4
H

C
2
CH3
S
3
CH3CH2

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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