Stereoisomerism(1)

Stereoisomerism

Occurs due to the restricted rotation of C=C double bonds... two forms… CIS and TRANS

STRUCTURAL ISOMERISM

STEREOISOMERISM

GEOMETRICAL ISOMERISM

OPTICAL ISOMERISM

CHAIN ISOMERISM

Same molecular formula but different structural

formulae

Occurs when molecules have a chiral centre. Get two non-superimposable mirror images.

Same molecular formula but

atoms occupy different

positions in space.

POSITION ISOMERISM

FUNCTIONAL GROUP

ISOMERISM

Geometric isomers

In alkenes

CISGroups/atoms are on theSAME SIDE of the double

bond

TRANSGroups/atoms are on

OPPOSITE SIDES across the double bond

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

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.

cis trans cis trans

Isomerism

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

3-methylpentanehexanecyclohexane

Are these constitutional isomers of hexane?

No, different molecular formulae!!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.

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

Summary of Isomerism Concepts

Mirror Objects – Carbon with 4 different substituents. We expect enantiomers (mirror

objects).Reflect!

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.

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.

RS

Arrange both structures with the light blue atoms towards the rear….

Notice how the reflection is done, straight through the mirror!

Recap: Tetrahedral Carbon with four Different Substituents.

Enantiomers

Simple Rotation, Same

Simple Rotation, Same

Mirror objects. Different, not superimposable.

Enantiomers

But the reflection might have been done differently. Position the mirror

differently….Reflection can giveany of the following…

Can you locate the mirror which would transform the original molecule into each mirror object?

In the course of each reflection, two substitutents are swapped. The other two remain unchanged.

What is common to each of these reflection operations?

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?

Again. all three objects on the right are the mirror object of the structure above. They are different views of the enantiomer.

A swap of two substituents is seen to be equivalent to a reflection at the carbon atom.

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

CH OH

CH2OH

H OHCHO

CH2OH

convert to a Fischerprojection

Glyceraldehyde(an aldotriose)

D (+) L (-)

C

C

CH2

H OH

OH

OH

C

C

CH2

OH H

OH

OH