Tài liệu về stereoisomerism

Stereoisomerism


C HA IN ISOMER ISM

STRUCTURAL ISO MERISM

POSITIO N ISOM ER ISM

Same mol ecular formula but dif ferent structura l
formulae
FUNC TIO NA L GROUP ISOMER ISM

GEO METRIC AL ISOMERISM

STEREO ISO MERISM

Oc curs due to the restricted rotati on o f C=C do uble
bo nds ... two forms… CIS and TRANS

Same mo lecular formula but ato ms
o ccupy dif ferent pos itio ns in
spac e.

OP TIC AL ISOMERISM

Oc curs when mo lecules have a c hiral centre. Get two
no n- superimpo s able mirror imag es.


Geometric isomers

In alkenes

CIS

TRANS

Groups/atoms are on the

Groups/atoms are on OPPOSITE SIDES across the double

SAME SIDE of the double bond

bond


RESTRICTED ROTATION OF C=C B ONDS

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 BEC AUSE C- C BONDS HAVE ‘FREE’ ROTATIO N


RESTRICTED ROTATION OF C=C B ONDS

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.


cis

trans

cis

trans


Isomerism

•

Constitutional Isomers: Same atoms but linked (bonded) together differently.
Spatial orientation not important.

hexane

3-methylpentane

cyclohexane

No, different molecular formulae!!
Are these constitutional isomers of hexane?
Are these constitutional isomers of cis but-2-ene?

Not this one! It is 2-butene. Cis / trans does not matter.

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?


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.


Summary of Isomerism Concepts

Isomers, contain same atoms, same formula

Constitutional isomers, different connectivities, bonding.

Stereoisomers, same connectivity, different three dimensional orientation
of bonds

Enantiomers, mirror objects

Diastereomers, not mirror objects


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 four atoms.
S

R

Recap: Tetrahedral Carbon with four Different Substituents. Enantiomers

Mirror objects. Different, not
superimposable.

Simple Rotation, Same

Enantiomers

Simple Rotation, Same


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 are the mirror object of
Can you
locate
theare
mirror
which
would
transform
the structure
above.
They

different
views
of the
theis original
molecule
each reflection
mirror object?
What
common
to eachinto
of these
enantiomer
.
operations?
A swap of two substituents is seen to be equivalent
Into
thea course
of each
twoatom.
substitutents are
reflection
at reflection,
the carbon
swapped. The other two remain unchanged.
All three of these structures are the same, just made by different mirrors. The structures are

superimposable. What rotations of the whole molecules are needed to superimpose the structures?


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.


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.


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.

CHO
H

C

OH

CH2OH

convert to 
a Fischer
projection

CHO
H
OH
CH2 OH


Glyceraldehyde
(an aldotriose)

H

H

O

C

C
H

C

OH

CH2 OH
D (+)

O

HO

C

H

CH2 OH
L (-)