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A Few Things From the
Organic Unit, Followed up
1
Geometric Isomers in Organic
Molecules
• The only type of geometric isomer “type” we will
discuss is cis-trans isomerism
– With TM complexes, we had cis-trans isomerism in
square planar and octahedral complexes
• Ligands were either 90° (cis) or 180° (trans) apart from one
another
– Cis = “same side”; trans = “across from one another”
• In organic molecules, cis-trans isomers
can occur in certain alkenes
– Alkene  a double bond is necessary (but
not sufficient)
2
Example of a cis-trans isomer pair
(see p. 920 in Tro and Table 20.9 for properties)
Cis = “same side of double bond”
Trans = “opposite sides of double bond”
(across the double bond)
3
The prior two compounds are only different
because rotation about a db is hindered—a p
bond would need to break
From Tro
(Section 10.7;
1st semester)
When is cis-trans isomerism possible?
• Consider the following “generalized”
alkene with substituents
• To have cis-trans isomers, both of the
following must be true:
–AB
AND
–DE
Alternate view:
If either A = B OR D = E, then NO cis-trans
isomer pair. (Flipping vertically would yield
same [superimposable] structure: not
isomers)
D
A
C
C
B
Can’t be
same
E
AND
Can’t be
same
5
6
Which of the following compounds
could exhibit geometric isomerism?
a)
c)
CH2=CHCH3
CCl2=CF2
b)
CH3CH=CHCH3
d)
CHCl=CF2
HINT: Redraw structures as on prior
slide to better “see” the four
“things” attached! (See board)
Copyright © 2011 Pearson Education, Inc.
Which of the following compounds
could exhibit geometric isomerism?
a)
c)
CH2=CHCH3
CCl2=CF2
Copyright © 2011 Pearson Education, Inc.
b)
CH3CH=CHCH3
d)
CHCl=CF2
See doc camera examples from text
9
Optical Isomerism in molecules with a
tetrahedral carbon
• Review from metal complexes unit (next
slide)
10
Chirality
• Any molecule with a nonsuperimposable mirror
image is said to be chiral
• A pair of (such) nonsuperimposable mirror
images are called optical isomers or
enantiomers
Tro: Chemistry: A Molecular Approach, 2/e
11
Copyright  2011 Pearson Education, Inc.
Optical Activity
• Enantiomers have all the same physical
properties except one – the direction they rotate
the plane of plane-polarized light
– each one of the enantiomers will rotate the plane the
same amount, but in opposite directions
– that’s why enantiomers are called “optical isomers”
12
Tro: Chemistry: A Molecular Approach, 2/e
Optical Isomerism in molecules with a
tetrahedral carbon
• Any carbon with 4 different substituents will be a
chiral center (chirality center, technically)
• Thus, if a molecule has exactly one chiral carbon
•
(center), it will have an optical isomer
If any two (or more) of the substituents are
identical, the center will not be chiral and the
mirror image will be superimposable (not isomers)
Tro: Chemistry: A Molecular Approach, 2/e
13
Copyright  2011 Pearson Education, Inc.
To tell if two mirror image structures are
superimposable, rotate structure and compare
See board for way to “rotate” on paper.
Tro: Chemistry: A Molecular Approach, 2/e
14
Copyright  2011 Pearson Education, Inc.
Which of the following can exhibit
optical isomerism?
a)
b)
c)
d)
Copyright © 2011 Pearson Education, Inc.
Which of the following can exhibit
optical isomerism?
Same 
a)
Same 
no optical
isomer
c)
no optical
isomer
(left C)
Same 
no optical
isomer
right C)
b)
d)
None are the same on left!
All three are same on right
 will be an optical isomer
(left C is a chiral(ity) center;
exactly one in molecule)
Copyright © 2011 Pearson Education, Inc.
Same 
no optical
isomer
(left C)
Same 
no optical
isomer
right C)
See doc camera examples from text
17
Which functional groups are present in
the molecule penicillin shown below?
OH
O
OH
OH
HO
O
OH
OH
OH
OH
O
OH
O
OH
O
a)
b)
c)
d)
e)
Alcohol
Amine
Ester
Carboxylic acid
All of the above
Copyright © 2011 Pearson Education, Inc.
NH 2
O
OH
Which functional groups are present in
the molecule penicillin shown below?
OH
O
OH
OH
HO
O
OH
OH
OH
OH
O
OH
O
OH
O
a)
b)
c)
d)
e)
Alcohol
Amine
Ester
Carboxylic acid
All of the above
Copyright © 2011 Pearson Education, Inc.
NH 2
O
OH