ch05 estereoquimica

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Organic Chemistry

4th Edition

Paula Yurkanis Bruice

Irene Lee

Case Western Reserve UniversityCleveland, OH

2004, Prentice Hall

Chapter 5

Stereochemistry

The Arrangement of

Atoms in Space;

The Stereochemistry of

Addition Reactions


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Isomers

Nonidentical compounds having the same molecular

formula


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Cis-Trans Isomers


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Achiral compounds have superimposable mirror images

Chiral compounds have nonsuperimposable mirror

images


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Enantiomers

nonsuperimposable mirror-image molecules


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Drawing Enantiomers

Perspective formula

Fischer projection


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A stereocenter (stereogenic center) is an atom at which

the interchange of two groups produces a stereoisomer


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Naming Enantiomers

Rank the groups (atoms) bonded to the chirality center

The R,Ssystem of nomenclature


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Orient the lowest priority (4) away from you

Clockwise = Rconfiguration

Counterclockwise = Sconfiguration


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Naming from the Perspective Formula

1

23

4

1. Rank the groups bonded to the asymmetric carbon

2. If the group (or atom) with the lowest priority is

bonded by hatched wedge,


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3. If necessary, rotate the molecule so that the lowest

priority group (or atom) is bonded by a hatched wedge

4. You can draw group 1 to group 2, passing group 4,

but never 3


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Naming from the Fischer Projection

1. Rank the groups (or atom) that are bonded to the asymmetric

carbon and draw an arrow with the highest priority to the lowestpriority

Cl

H

CH2CH2CH3CH3CH2 (R)-3-chlorohexane

2. If the lowest priority is on a horizontal bond, the naming is

opposite to the direction of the arrow

CH3

CH2CH3

OHH (S)-2-butanol


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A Fischer projection can only be rotated 180 in the plane

of the paper to yield the same molecule

3. The arrow can go from group 1 to 2, passing group 4, but not

group 3

CO2H

OH

CH3H

(S)-lactic acid


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Chiral compounds are optically active; they rotate the

plane of polarized light.

Clockwise (+) Counterclockwise (-)

Different from R,Sconfiguration

Achiral compounds do not rotate the plane of polarized

light. They are optically inactive.


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A polarizer measures the degree of optical rotation of a

compound The observed rotation (a)

lxc

=

aa

Tis the temp in C

is the wavelength

a is the measured rotation in degrees

lis the path length in decimeters

cis the concentration in grams per mL

[ ] rotationspecificT=

a

Each optically active compound has a characteristic specific

rotation


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A racemic mixture, which contains an equal amount of

the two enantiomers, is optically inactive

optical purity = observed specific rotation

specific rotation of the pure enantiomer

enantiomeric excess =excess of a single enantiomer

entire mixture

I ith th hi l b i f


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Diastereomers are stereoisomers that are not enantiomers

Isomers with more than one chiral carbon: a maximum of

2n stereoisomers can be obtained

CH3CHCHCH3

Cl OH

* *


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Identification of Asymmetric Carbons in

Cyclic Compounds

H

CH3

H

Br*

*these two groups

are different

H

CH3

H

Br

H

Br

CH3

H

H

CH3

H

Br

CH3

H

H

Br

cis-1-bromo-3-methylcyclohexane trans-1-bromo-3-methylcyclohexane


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Meso Compounds

Have two or more asymmetric carbons and a plane of

symmetry

They are achiral molecules


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As long as any one conformer of a compound has a

plane of symmetry, the compound will be achiral

plane ofsymmetry

plane of

symmetry


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The R,Snomenclature of isomers with more than one

asymmetric carbon

R ti f d th t t i t i


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Reactions of compounds that contain an asymmetric

carbon.

CH2CH2CH2Cl

CH3CH2 CH3

H

CH2CH2CH2OH

CH3CH2 CH3

HOH-

No reaction at the asymmetric carbon; both the reagent and theproduct have the same relative configuration.

CH

CH3CH2CH2 CH3

H

CH2 CH2CH3

CH3CH2CH2 CH3

H

H2

Pd/C

If a reaction breaks a bond at the asymmetric carbon, you need to

know the reaction mechanism in order to predict the relative

configuration of the product.

R l i f R i Mi


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Resolution of a Racemic Mixture

(R)-acid (S)-acid

enantiomers

(S)-base(R,S)-salt (S,S)-salt

diastereomers

(R,S)-salt (S,S)-salt

HCl HCl

(S)-baseH+

+

(R)-acid

(S)-baseH+

+

(S)-acid

Di i i ti f E ti b


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Discrimination of Enantiomers by

Biological Molecules

T i l i A i t d ith


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Terminologies Associated with

Stereochemistry

pro-R-hydrogen

pro-S-hydrogen

Enantiotopic hydrogens have the same chemical

reactivity and cannot be distinguished by achiral agents,

but they are not chemically equivalent toward chiral

reagents


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Diastereotopic hydrogens do not have the same reactivity

with achiral reagents


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Atoms other than carbon can be asymmetric

N+CH3CH2CH2 CH2CH3

H

CH3

N+

CH2CH2CH3CH3CH2H

CH3

Br- Br-

O

POCH2CH3

H

O

PCH3CH2O OCH3

HCH3O


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Amine Inversion


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A regioselective reaction: preferential formation of one

constitutional isomer

A stereoselective reaction: preferential formation of a

stereoisomer


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A stereospecific reaction: each stereoisomeric reactant

produces a different stereoisomeric product or a different

set of products

All stereospecific reactions are stereoselective

Not all stereoselective reactions are stereospecific

St h i t f El t hili


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Stereochemistry of Electrophilic

Addition Reactions of Alkenes

What is the absolute configuration of the product?


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Addition reactions that form one asymmetric carbon

CH CH


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CH3CH2C CH2 HBr CH3CH2CHCH2Br

2-methyl-1-butene

CH3

+peroxide

CH3

*

1-bromo-2-methylbutane

Addition reactions that form an additional asymmetric


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Addition reactions that form an additional asymmetric

carbon

Addition reactions that form two asymmetric carbons


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A carbocation reaction intermediate


Two substituents added to the same side of the double bond: syn

Two substituents added to opposite sides of the double bond: anti

C C

CH3CH2 CH2CH3

CH3H3C

+ HCl CH3CH2CH

CH3

CCH2CH3

Cl

CH3

C C

CH3CH2

H3C

CH2CH3

CH3

H

C C

C C

CH3CH2

H3C

Cl

CH2CH3

CH3

H

Cl

CH3CH2

CH2CH3

HCl

H3C

CH3 C C

CH3CH2

CH2CH3

HCH3

H3C

Cl



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A radical reaction intermediate


Stereochemistry of Hydrogen Addition


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Stereochemistry of Hydrogen Addition

(Syn Addition)

H d ti f C li Alk


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Hydrogenation of Cyclic Alkenes

CH(CH3)2

H3C

+ H2Pt/C

H

CH3

H

CH(CH3)2

H3C

H

CH(CH3)2

H

Only cis isomers are obtained with alkenes containing

fewer than eight ring atoms

Both cis and trans isomers are possible for rings

containing eight or more ring atoms

Stereochemistry of Hydroboration


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Stereochemistry of HydroborationOxidation


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Addition reactions that form a bromonium ion

intermediate (anti addition)


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