ISOMERS

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ISOMERS
Learning Goals I will understand the difference
between structural isomers and stereoisomers
(diastereomers and enantiomers) and be able to
name/draw them accordingly
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TYPES OF ISOMERISM
STRUCTURAL ISOMERISM (CONSTITUTIONAL ISOMERS)
Same molecular formula but different structural
formulae
STEREOISOMERISM (CONFORMATIONAL CONFIGURATIONAL
ISOMERS)
GEOMETRICAL ISOMERISM (Diastereomers)
Occurs due to the restricted rotation of CC
double bonds... two forms - CIS and TRANS
Same molecular formula but atoms occupy different
positions in space.
OPTICAL ISOMERISM (Enantiomers)
Occurs when molecules have a chiral centre. Get
two non-superimposable mirror images.
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STRUCTURAL ISOMERS
caused by different arrangements of the carbon
skeleton similar chemical properties slightly
different physical properties more branching
lower boiling point
There are two structural isomers of C4H10. One is
a straight chain molecule where all the carbon
atoms are in a single row. The other is a
branched molecule where three carbon atoms are in
a row and one carbon atom sticks out of the main
chain.
BUTANE straight chain
2-METHYLPROPANE branched
C4H10
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STRUCTURAL ISOMERS
DIFFERENCES IN PROPERTIES Chemical Isomers show
similar chemical properties because the same
functional group is present. Physical Properties
such as density and boiling point show trends
according to the of the degree of
branching Boiling Point straight chain isomers
have higher values than branched ones the
greater the degree of branching the lower the
boiling point branching decreases the
effectiveness of intermolecular forces less
energy has to be put in to separate the molecules
- 0.5C straight chain
- 11.7C branched
greater branching lower boiling point
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Number of Possible Isomers of Alkanes
The more carbon atoms there are, the greater the
number of possible isomers
Section 14.3
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Drawing Structural Isomers You Try
There are 5 structural alkane isomers for C6H14.
Can you draw and name them all? Use molecular
model kits to help.
Section 14.3
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STEREOISOMERS
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
(Diastereomers) OPTICAL ISOMERISM
(Enantiomers)
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GEOMETRICAL ISOMERS (DIASTEREOMERS) IN ALKENES

• an example of stereoisomersim
• found in some, but not all, alkenes
• occurs due to the RESTRICTED ROTATION OF CC
  bonds
• get two forms....

CIS Groups/atoms are on the SAME SIDE of the
double bond
TRANS Groups/atoms are on OPPOSITE SIDES across
the double bond
Isomers - have different physical properties -
e.g. boiling points, density - have similar
chemical properties - in most cases
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DIASTEREOMERS
RESTRICTED ROTATION OF CC BONDS Single covalent
bonds can easily rotate. What appears to be a
different structure in an alkane is not. 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
Animation doesnt work in old versions of
Powerpoint
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DIASTEREOMERS
RESTRICTED ROTATION OF CC BONDS CC bonds have
restricted rotation so the groups on either end
of the bond are frozen in one position it
isnt easy to flip between the two.
Animation doesnt work in old versions of
Powerpoint
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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DIASTEREOMERS
How to tell if it exists
Two different atoms/groups attached
Two different atoms/groups attached
?
GEOMETRICAL ISOMERISM
?
Two similar atoms/groups attached
Two similar atoms/groups attached
Once you get two similar atoms/groups attached to
one end of a CC, you cannot have geometrical
isomerism
?
Two similar atoms/groups attached
Two different atoms/groups attached
Two different atoms/groups attached
Two different atoms/groups attached
?
GEOMETRICAL ISOMERISM
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DIASTEREOMERS
Isomerism in butene There are 3 structural
isomers of C4H8 that are alkenes. Of these ONLY
ONE exhibits geometrical isomerism.
BUT-1-ENE
2-METHYLPROPENE
trans BUT-2-ENE
cis BUT-2-ENE
YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE
CARBON ATOMS ARE IN A RING
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DIASTEREOMERS
RESTRICTED ROTATION OF CYCLOALKANES

• Cis or Trans?
• Name it!

c) Cis or Trans? d) Name it!
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OPTICAL ISOMERS (ENANTIOMERS)
Occurrence another form of stereoisomerism
occurs when compounds have non-superimposable
mirror images Isomers the two different
forms are known as optical isomers or
enantiomers they occur when molecules have a
chiral centre a chiral centre contains an
asymmetric carbon atom an asymmetric carbon
has four different atoms (or groups) arranged
tetrahedrally around it.
CHIRAL CENTRES
There are four different colours arranged
tetrahedrally about the carbon atom
2-chlorobutane exhibits optical isomerism because
the second carbon atom has four different
atoms/groups attached
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ENANTIOMERS

• two forms exist which are NON-SUPERIMPOSABLE
  MIRROR IMAGES of each other
• non-superimposable means you you cant stack
  one form exactly on top of the other
• ENANTIOMERS MUST BE
• non-superimposable mirror images ex.
  hands

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ENANTIOMERS
What is a non-superimposable mirror image?
Animation doesnt work in old versions of
Powerpoint
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Learning Check Classify each of the following
pairs as enantiomers or not enantiomers a)
b)
c)
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APPLICATION OF ISOMERS- THALIDOMIDE
The one obvious difference between optical
isomers is their response to plane polarised
light. However, some naturally occurring
molecules or specifically synthesised
pharmaceuticals show different chemical
reactivity. The drug, THALIDOMIDE is a chiral
molecule and can exist as two enantiomers. In
the 1960s it was used to treat anxiety and
morning sickness in pregnant women. Tragically,
many gave birth to children with deformities and
missing limbs. It turned out that only one of
the enantiomers (the structure on the right) was
effective and safe its optically active
counterpart was not. The major problem was that
during manufacture a mixture of the isomers was
produced. The drug was banned world-wide, but
not after tens of thousands of babies had been
affected.
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A Quote from Lewis Carrolls Alice Through the
Looking Glass
How would you like to live in
Looking-glass House, Kitty? I wonder if they'd
give you milk, there? Perhaps Looking-glass milk
isn't good to drink...
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The looking-glass world Alice entered would
indeed be a very different place, with
mirror-image DNA and enzymes and amino acids,
meaning that - as Alice suggested to her kitten -
looking-glass milk would almost certainly not be
good to drink. Its mirror-image molecules would
all be the wrong way around for her enzymes to
digest
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LEARNING CHECK Are the following pairs of
compounds consitutional isomers or stereoisomers?
Name all 6! a)
b)
c)
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Distinguishing Isomers Flowchart
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How Did We Do?
Learning Goals I will understand the difference
between structural isomers and stereoisomers
(diastereomers and enantiomers) and be able to
name/draw them accordingly