The document discusses different types of isomerism including structural isomerism, stereoisomerism, and constitutional isomerism. It explains that stereoisomers have the same molecular formula and connectivity but different spatial orientations of bonds. Specifically, it defines enantiomers as stereoisomers that are non-superimposable mirror images, and diastereomers as stereoisomers that are not mirror images. The document also discusses restricted rotation of double bonds leading to cis-trans isomers and the use of Fischer projections to represent stereoisomers.

1. Stereoisomerism

2. CHAIN ISOMERISM
STRUCTURAL ISOMERISM POSITION ISOMERISM
Same molecular formula but
different structural formulae FUNCTIONAL GROUP
ISOMERISM
GEOMETRICAL ISOMERISM
Occurs due to the restricted
STEREOISOMERISM rotation of C=C double bonds...
two forms… CIS and TRANS
Same molecular
formula but atoms
occupy different
positions in space. OPTICAL ISOMERISM
Occurs when molecules have a
chiral centre. Get two non-
superimposable mirror images.

3. Geometric isomers

4. In alkenes
CIS TRANS
Groups/atoms are on the Groups/atoms are on OPPOSITE
SAME SIDE of the double bond SIDES across the double bond

5. 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

6. 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.

7. cis trans cis trans

8. Isomerism
• Constitutional Isomers: Same atoms but
linked (bonded) together differently.
Spatial orientation not important.
hexane 3-methylpentane
cyclohexane
No, different molecular
Are these constitutional isomers of isomers of hexane?
Are these constitutional cis but-2-ene?
formulae!!
Not this one! It is 2-butene. Cis / trans
does not matter.

9. Stereoisomerism
• Stereoisomers: Same molecular
formulae, same connectivity; same
constitutional isomer. Different spatial
orientation of the bonds.
Are these stereoisomers of cis but-2-ene?
How does the connectivity differ between these two?

10. Enantiomers and Diastereomers
Two kinds of Stereoisomers
– Enantiomers: stereoisomers which are mirror
objects of each other. Enantiomers are
different objects, not superimposable.
– Diastereomers: stereoisomers which are not
mirror objects of each other.
If a molecule has one or more tetrahedral carbons
having four different substituents then enantiomers will
occur. If there are two or more such carbons then
diastereomers may also occur.

11. Summary of Isomerism Concepts
Isomers, contain same atoms, same formula
Constitutional isomers, different Stereoisomers, same
connectivities, bonding. connectivity, different three dimensional
orientation of bonds
Enantiomers, mirror objects Diastereomers, not mirror objects

12. Mirror Objects – Carbon with 4 different substituents. We
expect enantiomers (mirror objects).
Reflect!
The mirror plane still relates the two structures. Notice that we can
characterize or name the molecules by putting the blue in the back, drawing
a circle from purple, to red, to green. Clockwise on the right and
counterclockwise on the left. Arbitrarily call them R and S.Notice how the reflection
is done, straight through
the mirror!
Arrange both
structures with the
light blue atoms
towards the rear….
These are mirror objects. Are they the same thing just viewed
differently ?? Can we superimpose them?
We can
superimpose two
atoms. but not all
S R
four atoms.

13. Recap: Tetrahedral Carbon with four
Different Substituents. Enantiomers
Mirror objects.
Different, not
superimposable.
Simple Simple
Rotation, Sa Enantiomers Rotation, Sa
me me

14. But the reflection might have been done
differently. Position the mirror differently….
Reflection can give
any of the following…
Again. all three objects on the
right Can the mirror object of the
are you locate the mirror
structure above. They eachthe
which would transform
What is common to are of
different reflectionthe into each
original molecule
these views of operations?
In themirror object?
enantiomer. of each
course
reflection, two substitutents are
A swap of two substituents is
swapped.be equivalent to a
seen to The other two remain All three of these structures are the same, just made by
unchanged.at the carbon atom. different mirrors. The structures are superimposable.
reflection
What rotations of the whole molecules are needed to
superimpose the structures?

15. Now Superimposable mirror objects:
Tetrahedral Carbon with at least two identical
substituents.
Reflection can interchange the two red substituents.
Clearly interchanging the two reds leads to the same
structure, superimposable! Remember it does not
make any difference where the mirror is held for the
reflection.
This molecule does not have an enantiomer; the
mirror object is superimposable on the original, the
same object.

16. Polarized light vibrates in one plane only, in
contrast to ordinary light, which vibrates in
all planes.
What causes such a rotation of the plane of polarized liight?
According to the van’t Hoff theory, such an effect on the plane polarized light
is due to the presence of one or more chiral carbon atoms.

17. Fischer projection
A two-dimensional method of indicating the structure of an enantiomer.
Horizontal lines indicate bonds extending forward from the paper and the
vertical lines indicate bonds extending backward from the paper.
The formulas are always written with the aldehyde(or ketone) group)-the
most highly oxidized-at the top.
convert to
CHO a Fischer CHO
projection
H C OH H OH
CH2 OH
CH2 OH

18. Glyceraldehyde
(an aldotriose)
H H O
O
C C
H C OH HO C H
CH2 OH CH2 OH
D (+) L (-)