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8/12/2019 stereoisomerism1-120824060050-phpapp01

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Stereoisomerism


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Occurs due to the restrictedrotation of C=C double bonds...

two forms CIS and TRANS

STRUCTURAL ISOMERISM

STEREOISOMERISM

GEOMETRICAL ISOMERISM

OPTICAL ISOMERISM

CHAIN ISOMERISM

Same molecular formula butdifferent structural formulae

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

Same molecularformula but atomsoccupy different

positions in space.

POSITION ISOMERISM

FUNCTIONAL GROUPISOMERISM


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Geometric isomers


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In alkenes

CIS Groups/atoms are on the

SAME SIDE of the double bond

TRANS Groups/atoms are on OPPOSITESIDES across the double bond


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RESTRICTED ROTATION OF C=C BONDS

Single covalent bonds can easily rotate . What appears to be a different structure isnot. 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


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RESTRICTED ROTATION OF C=C BONDS

C=C bonds have restricted rotation so the groups on either end of the bond arefrozen in one position; it isnt easy to flip between the two.

This produces two possibilities . The two structures cannot interchange easilyso the atoms in the two molecules occupy different positions in space .


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c i s t r a n s c i s t r a n s


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Isomerism

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

3-methylpentanehexane

cyclohexane

Are these constitutional isomers of hexane?No, differen t molecularformulae!! Are these constitutional isomers of cis but-2-ene?

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


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Stereoisomerism

Stereoisomers : Same molecular formulae,same connectivity; same constitutionalisomer. Different spatial orientation of the

bonds.

Are these stereoisomers of cis but-2-ene?

How does the connectivity differ between these two?


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Enantiomers and Diastereomers

Two kinds of Stereoisomers Enantiomers : stereoisomers which are mirror

objects of each other. Enantiomers aredifferent objects, not superimposable.

Diastereomers : stereoisomers which are notmirror objects of each other.

If a molecule has one or more tetrahedral carbonshaving four different substituents then enantiomers willoccur. If there are two or more such carbons then

diastereomers may also occur.


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Isomers, contain same atoms, same formula

Constitutional isomers, different

connectivities, bonding.

Stereoisomers, same connectivity,

different three dimensional orientationof bonds

Enantiomers, mirror objects Diastereomers, not mirror objects

Summary of Isomerism Concepts


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Mirror Objects Carbon with 4 different substituents. Weexpect enantiomers (mirror objects).

Reflect!

These are mirror objects. Are they the same thing just vieweddifferently ?? Can we superimpose them?

We cansuperimpose twoatoms. but not allfour atoms.

The mirro r plane still relates the two structures. Notice that we cancharacterize or name the molecules by putting the blue in the back, drawinga circle from purple, to red, to green. Clockwise on the right andcounterclockwise on the left. Arbitrarily call them R and S.

RS

Arrange bothstructures with thelight blue atomstowards the rear.

Notice how the reflection

is done, straight throughthe mirror!


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Recap: Tetrahedral Carbon with fourDifferent Substituents. Enantiomers

SimpleRotation,Same

SimpleRotation,Same

Mirror objects.Different, notsuperimposable.

Enantiomers


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

Reflection can giveany of the following

Can you locate the mirrorwhich would transform theoriginal molecule into eachmirror object?In the course of each reflection,

two substitutents are swapped.The other two remainunchanged.

What is common to each ofthese reflection operations?

All three of these structures are the same, just made bydifferent mirrors. The structures are superimposable.What rotations of the whole molecules are needed tosu erim ose the structures?

Again. all three objects on theright are the mirror object of thestructure above. They aredifferent views of the enantiomer .

A swap of two substituents isseen to be equivalent to areflection at the carbon atom.


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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 samestructure, superimposable ! Remember it does notmake any difference where the mirror is held for thereflection.

This molecule does not have an enantiomer; the

mirror object is superimposable on the original, thesame object.
http://localhost/var/www/apps/conversion/tmp/scratch_10/Chirality%20-%20Enantiomers.mp4


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Polarized light vibrates in one plane only, incontrast to ordinary light, which vibrates inall planes.

What causes such a rotation of the plane of polarized liight?

According to the vant Hoff theory, such an effect on the plane polarized lightis due to the presence of one or more chiral carbon atoms.
http://localhost/var/www/apps/conversion/tmp/scratch_10/POLARIZED%20LIGHT.mp4


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Fischer projection

A two-dimensional method of indicating the structure of an enantiomer.Horizontal lines indicate bonds extending forward from the paper and thevertical 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

CH 2 OH

H OH

CHO

CH 2 OH

convert toa Fischer

projection


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Glyceraldehyde(an aldotriose)

D (+) L (-)

C

C

CH 2

H OH

OH

OH

C

C

CH 2

OH H

OH

OH


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