Course Code: BP401T
Course Title: Pharmaceutical Organic Chemistry III
Stereoisomerism
Session Objectives
By the end of this session, students will be able to:
➢ Explain the types of stereoisomerism
➢ Geometrical isomerism
➢ Optical isomerism
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STEREOISOMERISM
Molecules have the SAME MOLECULAR FORMULA but the atoms are joined to
each other in a DIFFERENT SPACIAL ARRANGEMENT – they occupy a different
position in 3-dimensional space.
There are two types…
•GEOMETRICAL ISOMERISM
•OPTICAL ISOMERISM
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GEOMETRICAL ISOMERISM IN ALKENES
Introduction
• an example of stereoisomerism
• found in some, but not all, alkenes
• occurs due to the RESTRICTED ROTATION OF C=C bonds
• get two forms…
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GEOMETRICAL ISOMERISM IN ALKENES
Introduction
• an example of stereoisomerism
• found in some, but not all, alkenes
• occurs due to the RESTRICTED ROTATION OF C=C bonds
• get two forms…
CIS (Z)
Groups/atoms are on the TRANS (E)
SAME SIDE of the double Groups/atoms are on
bond OPPOSITE SIDES across
the double bond
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GEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different
structure is not. It looks like it but, due to the way structures are
written out, they are the same.
ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION
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GEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
C=C bonds have restricted rotation so the groups on either end of the
bond are ‘frozen’ in one position; it isn’t easy to flip between the two.
This produces two possibilities. The two structures cannot interchange easily
so the atoms in the two molecules occupy different positions in space.
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GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAME side of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITE sides of C=C bond
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GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAME side of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITE sides of C=C bond
To determine priority, the Cahn, Ingold and Prelog convention is used.
eg C2H5 > CH3 > H and I > Br > Cl > F > C > H
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GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAME side of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITE sides of C=C bond
To determine priority, the Cahn, Ingold and Prelog convention is used.
eg C2H5 > CH3 > H and I > Br > Cl > F > C > H
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GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
E / Z Z (zusammen) higher priority groups / atoms on
the SAME side of C=C bond
E (entgegen) higher priority groups / atoms on
OPPOSITE sides of C=C bond
To determine priority, the Cahn, Ingold and Prelog convention is used.
eg C2H5 > CH3 > H and I > Br > Cl > F > C > H
E Z Z E
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GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
CIS / Should only be used when there are two H’s and two
TRANS non-hydrogen groups attached to each carbon.
cis non-hydrogen groups / atoms on the
SAME side of C=C bond
trans non-hydrogen groups / atoms on
OPPOSITE sides of C=C bond
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GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
CIS / Should only be used when there are two H’s and two
TRANS non-hydrogen groups attached to each carbon.
cis non-hydrogen groups / atoms on the
SAME side of C=C bond
trans non-hydrogen groups / atoms on
OPPOSITE sides of C=C bond
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GEOMETRICAL ISOMERISM IN ALKENES
E/Z or CIS-TRANS
CIS / Should only be used when there are two H’s and two
TRANS non-hydrogen groups attached to each carbon.
cis non-hydrogen groups / atoms on the
SAME side of C=C bond
trans non-hydrogen groups / atoms on
OPPOSITE sides of C=C bond
cis trans cis trans
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GEOMETRICAL ISOMERISM
Isomerism in butene
There are 3 structural isomers of C4H8 that are alkenes*. Of
these ONLY ONE exhibits geometrical isomerism.
but-1-ene cis but-2-ene trans but-2-ene 2-methylpropene
(Z) but-2-ene (E) but-2-ene
*
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GEOMETRICAL ISOMERISM
How to tell if it exists
Two different Two different
atoms/groups atoms/groups GEOMETRICAL ISOMERISM
attached attached
Two similar Two similar
atoms/groups atoms/groups
Once you get two similar
attached attached
atoms/groups attached to
one end of a C=C, you
Two similar Two different cannot have geometrical
atoms/groups atoms/groups isomerism
attached attached
Two different Two different
atoms/groups atoms/groups GEOMETRICAL ISOMERISM
attached attached
summary
• Stereo isomers are molecules have the SAME MOLECULAR FORMULA but the
atoms are joined to each other in a DIFFERENT SPACIAL ARRANGEMENT – they
occupy a different position in 3-dimensional space.
• Stereoisomers are of two types: Geometrical isomerism & Optical isomerism
• Geometrical isomerism occurs due to the RESTRICTED ROTATION OF C=C bonds
• C=C bonds have restricted rotation so the groups on either end of the bond are
‘frozen’ in one position; it isn’t easy to flip between the two.
• Groups/atoms are on the SAME SIDE of the double bond – CIS (Z)
• Groups/atoms are on OPPOSITE SIDES across the double bond – TRANS (E)
• Higher priority groups / atoms on the SAME side of C=C bond Z (zusammen)
• higher priority groups / atoms on OPPOSITE sides of C=C bond E(entgegen
Disclaimer
All data and content provided in this presentation are
taken from the reference books, internet – websites
and links, for informational purposes only.
Thank You
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