Hadepe Stereochemistry

8/6/2019 Hadepe Stereochemistry

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Stereochemistry

Prof. Dr. Harno Dwi Pranowo

Austrian-Indonesian Center for Computational Chemistry

Chemistry Department, FMIPA UGM


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Stereoisomers differ only in the way the atoms are oriented in space.They have identical IUPAC names (except for a prefix like cis ortrans).

They always have the same functional group(s).


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Stereochemistry

refers to the three-dimensional structure of a molecule.


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Although everything has a mirror image, mirror images

may or may not be superimposable.

Some molecules are like hands. Left and right hands are

mirror images, but they are not identical, or

superimposable.


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We can now consider several molecules to determine

whether or not they are chiral.


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A and B are stereoisomersspecifically, they are

enantiomers.

A carbon atom with four different groups is a tetrahedral

stereogenic center.


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In general, a molecule with no stereogenic centers will notbe chiral.

With one stereogenic center, a molecule will always be

chiral.


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How to Locate Stereogenic Centers?

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Always omit from consideration all C atoms that cannot

be tetrahedral stereogenic centers. These include

CH2 and CH3 groups

Any sp orsp2 hybridized C


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Larger organic molecules can have two, three or even

hundreds of stereogenic centers.


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Stereogenic Centers in Rings

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How To Draw Enantiomers?

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Enantiomers in Nature

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Naming enantiomers with the prefixes R orS is called

the Cahn-Ingold-Prelog system.

To designate enantiomers as R orS, priorities must be

assigned to each group bonded to the stereogeniccenter. The atom of highest atomic number gets the

highest priority (1).

Labeling Stereogenic Centers with RorS


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If two atoms on a stereogenic center are the same,

assign priority based on the atomic number of the atoms

bonded to these atoms. One atom of higher atomic

number determines the higher priority.


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To assign a priority to an atom that is part of a multiple bond,

treat a multiply bonded atom as an equivalent number of

singly bonded atoms. For example, the C of a C=O is

considered to be bonded to two O atoms (phantom atoms).

Other common multiple bonds are drawn below:


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Pay attention to these examples

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Ifthe lowest priority group is NOT at

the back

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R and S Assignments in Compounds with Two

or More Stereogenic Centers.

One stereoisomer of 2,3-

dibromopentane

The complete name is (2S,3R)-2,3-dibromopentane


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Stereoisomers of2,3-dibromopentane

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For a molecule with n stereogenic centers, the maximumnumber of stereoisomers is 2n.


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Let us now consider the stereoisomers of 2,3-dibromobutane.

Since this molecule has two stereogenic centers, the maximum number

of stereoisomers is 4.


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A meso compound is an achiral compound that

contains stereogenic centers. C is a meso compound.


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Compound C contains a plane of symmetry, and is

achiral.

Meso compounds contain a plane of symmetry so thatthey possess two identical halves.


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Disubstituted Cycloalkanes

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Consider 1,3-dibromocyclopentane. Since it has two

stereogenic centers, it has a maximum of four stereoisomers.


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The cis isomer is superimposable on its mirror image,

making the images identical. Thus, A is an achiralmeso compound.


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The trans isomer is not superimposable on its mirror image,

labeled C, making B and C different compounds. B and C are

enantiomers.


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Configurational Isomers of Dienes

The Z isomer has the high-priority groups on the sameside


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Review: Types ofIsomers

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Determining the relationship between a pair

ofmolecules

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Optical Activity

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The rotation of polarized light can be clockwise or

anticlockwise. If the rotation is clockwise, the compound is called

dextrorotatory. The rotation is labeled dor (+).

If the rotation is counterclockwise, the compound is

called levorotatory. The rotation is labeled lor (-).

Two enantiomers rotate plane-polarized light to an equal

extent but in opposite directions. Thus, if enantiomer A

rotates polarized light +5, the same concentration of

enantiomer B rotates it 5.

No relationship exists between R and S prefixes and the(+) and (-) designations that indicate optical rotation.


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Resolution of a Racemic Mixture

React a racemic mixture with a chiral compound to form

diastereomers, which can be separated.


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ChromatographicResolution of Enantiomers


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Enantiomers can be dangerously different!

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Discrimination of Enantiomers by BiologicalMolecules


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Stereochemistry of Electrophilic AdditionReactions of Alkenes

What is the absolute configuration of the product?


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Addition reactions that form one asymmetric carbon

Hadepe Stereochemistry

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