1. ISOMERS
Learning Goals: I will understand the difference between structural isomers and stereoisomers (diastereomers and enantiomers) and be able to name/draw them accordingly

2. 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 C=C 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.

3. STRUCTURAL ISOMERS C4H10 BUTANE straight chain 2-METHYLPROPANE
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

4. DIFFERENCES IN PROPERTIES
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
- 0.5°C
straight chain
- 11.7°C
branched
greater branching
= lower boiling point

5. Number of Possible Isomers of Alkanes
The more carbon atoms there are, the greater the number of possible isomers
Section 14.3

6. 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

7. 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)

8. GEOMETRICAL ISOMERS (DIASTEREOMERS) IN ALKENES
an example of stereoisomersim
found in some, but not all, alkenes
occurs due to the RESTRICTED ROTATION OF C=C 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

9. RESTRICTED ROTATION OF C=C BONDS
DIASTEREOMERS
RESTRICTED ROTATION OF C=C 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 doesn’t work in old versions of Powerpoint

10. RESTRICTED ROTATION OF C=C BONDS
DIASTEREOMERS
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.
Animation doesn’t 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.

11.     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 C=C, 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

12. DIASTEREOMERS 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
* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING

13. RESTRICTED ROTATION OF CYCLOALKANES
DIASTEREOMERS
RESTRICTED ROTATION OF CYCLOALKANES
Cis or Trans?
Name it!
c) Cis or Trans?
d) Name it!

14. 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

15. ENANTIOMERS
two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
non-superimposable means you you can’t stack one form exactly on top of the other
ENANTIOMERS MUST BE:
non-superimposable mirror images ex. hands

16. What is a non-superimposable mirror image?
ENANTIOMERS
What is a non-superimposable mirror image?
Animation doesn’t work in old versions of Powerpoint

17. Learning Check: Classify each of the following pairs as enantiomers or not enantiomersb) c)

18. 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 1960’s 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.

19. A Quote from Lewis Carroll’s ‘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...”

20. 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

21. LEARNING CHECK: Are the following pairs of compounds consitutional isomers or stereoisomers? Name all 6! a) b) c)

22. Distinguishing Isomers Flowchart

23. 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