REVIEW TO DATE

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REVIEW TO DATE
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Is there a double bond?
YES
NO
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CONFORMATIONS
Different internal arangements of the atoms in a
molecule that differ by rotation(s) about one or
more of the single bonds.
Conformations are NOT ISOMERS they are different
arrangements of the SAME MOLECULE.
The connection pattern of the atoms does not
change.
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FREE ROTATION
MOST ATOMS ATTACHED BY A SINGLE BOND ARE FREE TO
ROTATE AROUND THE BOND
THAT IS - THE TWO ENDS OF THE BOND CAN ROTATE
RELATIVE TO EACH OTHER
THE CARBON AND THREE HYDROGEN ATOMS ON THE RIGHT
END OF THE BOND CAN SPIN RELATIVE TO THE CARBON
AND OTHER THREE HYDROGENS ON THE LEFT END OF THE
BOND
fix
rotate
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ETHANE
NEWMAN PROJECTIONS
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TWO CONFORMATIONS OF ETHANE
EXTREMES
ECLIPSED
STAGGERED
H
H
H
H
H
H
H
H
H
H
H
H
NEWMAN PROJECTIONS
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ETHANE
CONFORMATIONS DIFFER IN ENERGY
12.1 kJ/mol
eclipsed
3 Kcal/mol
ENERGY
staggered
0 kJ/mol
0 Kcal/mol
The staggered conformation has a lower energy.
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LONGER CHAINS
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DECANE
BALL AND STICK MODEL
SPACEFILLING MODEL
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DEGREE OF SUBSTITUTION
.. how do you designate specific carbon
atoms within a structure ?
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DEGREE OF SUBSTITUTION
C
C
PRIMARY CARBON
SECONDARY CARBON
CONNECTED TO TWO OTHER CARBONS
CONNECTED TO ONEOTHER CARBON
QUATERNARY CARBON
TERTIARY CARBON
CONNECTED TO THREE OTHER CARBONS
CONNECTED TO FOUR OTHER CARBONS
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EXAMPLE
A hydrocarbon containing carbon atoms
with differing degrees of substitution
PRIMARY
QUATERNARY
TERTIARY
SECONDARY
All of the methyl groups (CH3) are primary.
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IDENTIFYING FUNCTIONAL GROUPS
Circle and name the functional groups.
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CYCLOHEXANE
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CHEMICAL WHIMSEY
BOAT
CHAIR
sessel
boot
German
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CHAIR CONFORMATION
axial
equatorial
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BOAT CONFORMATION
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bowsprit-flagpole
STERIC STRAIN
CYCLOHEXANE BOAT
note eclipsing on side of boat
TORSIONAL STRAIN
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ALKENES
cis and trans
E and Z
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cis AND trans ISOMERS
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CIS / TRANS ISOMERS
Because there is no rotation about a
carbon-carbon bond, isomers are possible.
cis
trans
substituents on the same side of main chain
substituents on opposite sides of main chain
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TWO IDENTICAL SUBSTITUENTS
If an alkene has two identical substituents on
one of the double bond carbons, cis / trans ( or
E / Z ) isomers are not possible.
all of these compounds are identical
no cis / trans isomers
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NAMING cis / trans ISOMERS OF ALKENES
main chain stays on same side of double bond cis
main chain crosses to other side of double bond
trans
cis-3-hexene
trans-3-hexene
notice that these prefixes are in italics
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E / Z SYSTEM OF NOMENCLATURE
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E/Z SYSTEM OF NOMENCLATURE
To avoid the confusion between what the
main chain is doing and the relationship of two
similar groups .. the IUPAC invented the E/Z
system.
cis ?
F
Cl
trans ?
I
H
This system also allows alkenes like the one
above to be classified .. an
impossibility with cis / trans.
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E / Z NOMENCLATURE
In this system the two groups attached to each
carbon are assigned a priority ( 1 or 2 ). If
priority 1 groups are both on same side of double
bond
Z isomer zusammen together (in German)
same side
opposite sides
Z
E
If priority 1 groups on opposite sides of double
bond
E isomer entgegen opposite (in German)
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ASSIGNING PRIORITIES
1. Look at the atoms attached to each carbon of
the double bond. 2. The atom of higher atomic
number has higher (1) priority.
1
1
example
I gt Br
F gt H
2
2
Since the 1s are on the same side, this compound
is Z
(Z)-1-bromo-2-fluoro-1-iodoethene
notice use of parentheses
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3. If you cant decide using the first atoms
attached, go out to the next atoms attached.
If there are non-equivalent paths, always
follow the path with atoms of higher atomic
number.
Once you find a difference, you can stop.
path goes to F not to H
1
1
comparison stops here
2
2
path goes to C not to H
This molecule has Z configuration.
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4. The atoms in double bonds are replicated at
either end of the double bond.
2
1
Then, when comparing groups, follow the path of
highest priority as before.
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MORE THAN ONE DOUBLE BOND
DIENES AND POLYENES
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STEREOGENIC CARBON ATOMS
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STEREOGENIC CARBONS
( called chiral carbons in older literature )
This is one type of .
stereocenter
. others are possible
A stereogenic carbon is tetrahedral and has
four different groups attached.
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Is there a double bond?
YES
NO
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Configuration the arrangement of the groups or
atoms about the
chirality center
The Cahn-Ingold-Prelog convention R or S
configuration

• Rank the group priority (Atomic Mass,1 being
  largest 4 being smallest)
• Set the 4th group away from you
• Clockwise R counterclockwise S

clockwise
R configuration
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(No Transcript)
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The atoms in double bonds are replicated at
either end of the double bond.
2
1
Then, when comparing groups, follow the path of
highest priority as before.
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Bromochlorofluoroiodomethane
1
1
4
4
2
3
3
2
R
S
Enantiomers
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How Many Stereoisomers Are Possible?
maximum number of stereoisomers 2n, where n
number of stereocenters (sterogenic carbons)
sometimes fewer than this number will exist
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22 4 stereoisomers



23 8 stereoisomers
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2-Bromo-3-chlorobutane
mirror
S
R
R
S
enantiomers 1
diastereomers
S
S
R
R
enantiomers 2
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Models of the Isomers of 2-Bromo-3-chlorobutane
(methyl groups are reduced to a single red atom)
Br
Cl
enantiomers-1
CH3
H
diastereomers
enantiomers-2
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MESO ISOMER
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2,3-Dichlorobutane
( Replace the Bromine with a second Chlorine )


(2n 4)
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MESO ISOMER
mirror
plane of symmetry
S
R
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2,3-Dichlorobutane
enantiomers
meso
diastereomers
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Models of the Isomers of 2,3-Dichlorobutane
(methyl groups are reduced to a single red atom)
Cl
CH3
meso
H
diastereomers
enantiomers
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CONCLUSION
2n the maximum
number
When a molecule has 1. multiple stereocenters
and 2. there is a possibility of an arrangement
with a plane of symmetry you
will not always find all of the 2n stereoisomers
that are possible.
Some of the stereoisomers may be meso
isomers, and their mirror images will be
superimposable (identical) - this will eliminate
at least one of the possible stereoisomers, and
sometimes more.
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FISCHER PROJECTIONS
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OH
H
ORIENTATION OF THE MAIN CHAIN AND THE
SUBSTITUENTS IN A FISCHER PROJECTION
OH
H
H
OH
continuation of the main chain
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rotate 90o
CH3
Cl
H
main chain in red
H
orient main chain vertically
Br
CH3
H
Cl
Br
convert to Fischer Projection
H
CH3