Estereoq

ESTEREOQUÍMICA

Isomers

stereoisomersconstitutionalisomers

diastereomersenantiomers

Molecular Chirality: Enantiomers

A molecule is chiral if its two mirror image forms are not superposable upon one another.

A molecule is achiral if its two mirror image forms are superposable.

Chirality

BrCl

H

F

Bromochlorofluoromethane is chiral

It cannot be superimposed point for point on its mirror image.

BrCl

H

F


H

ClBr

FTo show nonsuperimposability, rotate this model 180° around a vertical axis.

BrCl

H

F


H

ClBr

F

Another look

are enantiomers with respect to each other

and

nonsuperimposable mirror images are called enantiomers

Enantiomers

Chlorodifluoromethaneis achiral

Chlorodifluoromethaneis achiral

The two structures are mirror images, but are not enantiomers, because they can be superimposed on each other.

The Chirality Center

a carbon atom with fourdifferent groups attached to it

also called:chiral centerasymmetric centerstereocenterstereogenic center

The Chirality Center

w

x y

z

C

A molecule with a single chirality center is chiral.

Bromochlorofluoromethane is an example.

Chirality and chirality centers

Cl F

Br

H

C

A molecule with a single chirality center is chiral.

2-Butanol is another example.

Chirality and chirality centers

CH3

OH

H

C CH2CH3

Examples of molecules with 1 chirality center

CH3

C

CH2CH3

CH2CH2CH2CH3CH3CH2CH2

a chiral alkane


Linalool, a naturally occurring chiral alcohol

OH


1,2-Epoxypropane: a chirality centercan be part of a ring

O

H2C CHCH3

attached to the chirality center are:

—H

—CH3

—OCH2

—CH2O


Limonene: a chirality center can be part of a ring

CH3

H C

CH3

CH2

attached to thechirality center are:

—H

—CH2CH2

—CH2CH=

—C=


Chiral as a result of isotopic substitution

CH3CD

T

H

A molecule with a single chirality centermust be chiral.

But, a molecule with two or more chirality centers may be chiral

or it may not.

Properties of Chiral Molecules:

Optical Activity

A substance is optically active if it rotates the plane of polarized light.

In order for a substance to exhibit opticalactivity, it must be chiral and one enantiomer must be present in excess of the other.

Optical Activity

Light

has wave properties

periodic increase and decrease in amplitude of wave

Light

optical activity is usually measured using light having a wavelength of 589 nm

this is the wavelength of the yellow light from a sodium lamp and is called the D line of sodium

Polarized light

ordinary (nonpolarized) light consists of many beams vibrating in different planes

plane-polarized light consists of only those beams that vibrate in the same plane

Nicol prism

Polarization of light

Rotation of plane-polarized light

Absoluteand

Relative Configuration

Relative configuration compares the arrangement of atoms in space of one compound with those of another.

Absolute configuration is the precise arrangement of atoms in space.

Configuration

Relative configuration compares the arrangement of atoms in space of one compound with those of another.

until the 1950s, all configurations were relative

Absolute configuration is the precise arrangement of atoms in space.

we can now determine the absolute configuration of almost any compound

Configuration

No bonds are made or broken at the chirality center

in this experiment. Therefore, when (+)-3-buten-2-ol

and (+)-2-butanol have the same sign of rotation, the

arrangement of atoms in space is analogous. The two

have the same relative configuration.

CH3CHCH2CH3

OH

Pd

[] + 33.2° [] + 13.5°

Relative configuration

CH3CHCH

OH

CH2

HHO

H OH H2, Pd

HHOH2, Pd

H OH

Two possibilities

But in the absence of additional information, we can't tell which structure corresponds to(+)-3-buten-2-ol, and which one to (–)-3-buten-2-ol.

HHO

H OH H2, Pd

HHOH2, Pd

H OH

Two possibilities

Nor can we tell which structure corresponds to(+)-2-butanol, and which one to (–)-2-butanol.

HHO

H OH H2, Pd

HHOH2, Pd

H OH

Absolute configurations

[] +33.2°[] +13.5°

[] –13.5° [] –33.2°

Not all compounds that have the same relative

configuration have the same sign of rotation. No bonds

are made or broken at the chirality center in the

reaction shown, so the relative positions of the atoms

are the same. Yet the sign of rotation changes.

CH3CH2CHCH2Br

CH3

HBr

[] -5.8° [] + 4.0°

Relative configuration

CH3CH2CHCH2OH

CH3

The Cahn Ingold PrelogR-S Notational System

1. need rules for ranking substituents at chirality center in order of decreasing precedence

2. need convention for orienting molecule so that order of appearance of substituents can be compared with rank

The system that is used was devised by R. S. Cahn, Sir Christopher Ingold, and V. Prelog.

Two requirements for a systemfor specifying absolute configuration

1. Rank the substituents at the chirality center according to same rules used in E-Z notation.

2. Orient the molecule so that lowest-ranked substituent points away from you.

The Cahn-Ingold-Prelog Rules

43

2

1

Example

4 3

2

1

Order of decreasing rank:4 > 3 > 2 > 1

• 1. Rank the substituents at the chirality center according to same rules used in E-Z notation.

• 2. Orient the molecule so that lowest-ranked substituent points away from you.

• 3. If the order of decreasing precedence traces a clockwise path, the absolute configuration is R. If the path is counterclockwise, the configuration is S.

The Cahn-Ingold-Prelog Rules

43

2

1

Example

4 3

2

1

Order of decreasing rank:4 3 2

clockwise

R

counterclockwise

S

C OH

H3C

HCH3CH2

Enantiomers of 2-butanol CHO

CH3

HCH2CH3

(S)-2-Butanol (R)-2-Butanol

Very important! Two different compounds with the same sign of rotation need not have the

same configuration.

Verify this statement by doing Problem. All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?

HH3C

H

H

Chirality center in a ring

R

—CH2C=C > —CH2CH2 > —CH3 > —H

Fischer Projections

Purpose of Fischer projections is to show configuration at chirality center without necessity of drawing wedges and dashes or using models.

Rules for Fischer projections

Arrange the molecule so that horizontal bonds at chirality center point toward you and vertical bonds point away from you.

Br Cl

F

H

Rules for Fischer projections

Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.

Br Cl

F

H

Physical Properties of Enantiomers

Same: melting point, boiling point, density, etc

Different: properties that depend on shape of molecule

(biological-physiological properties) can bedifferent

Physical properties of enantiomers

O O

CH3 CH3

H3C H3CCH2 CH2

Odor (–)-Carvonespearmint oil

(+)-Carvonecaraway seed oil

Ibuprofen is chiral, but normally sold asa racemic mixture. The S enantiomer is the one responsible for its analgesic and antiinflammatory properties.

Chiral drugs

CH2CH(CH3)2

HH3C

C

O

C

HO

Chiral Moleculeswith

Two Chirality Centers

How many stereoisomers when a particular molecule contains two chirality centers?

2,3-Dihydroxybutanoic acid

What are all the possible R and S combinations of the two chirality centers in this molecule?

O

CH3CHCHCOH

HO OH

23

Carbon-2 R R S SCarbon-3 R S R S


4 Combinations = 4 Stereoisomers

O

CH3CHCHCOH

HO OH

23



4 Combinations = 4 Stereoisomers

What is the relationship between these stereoisomers?

O

CH3CHCHCOH

HO OH

23



O

CH3CHCHCOH

HO OH

23


enantiomers: 2R,3R and 2S,3S2R,3S and 2S,3R

HO

CO2H

CH3

H

OHHR

R

CO2H

CH3

H

HHO

OH

S

S

CO2H

H

CH3

HO

HHO

R

S

CO2H

CH3

H OH

OHHR

S

enantiomersenantiomers


[] = -9.5° [] = +9.5°

[] = -17.8°[] = +17.8°


O

CH3CHCHCOH

HO OH

23


but not all relationships are enantiomeric

stereoisomers that are not enantiomers are diastereomers

Isomers

stereoisomersconstitutionalisomers

diastereomersenantiomers

HO

CO2H

CH3

H

OHHR

R

CO2H

CH3

H

HHO

OH

S

S


CO2H

H

CH3

HO

HHO

R

S

CO2H

CH3

H OH

OHHR

S


diastereomersdiastereomers

[] = -9.5° [] = +9.5°

[] = -17.8°[] = +17.8°

CO2H

CH3

Fischer Projections

recall for Fischer projection: horizontal bonds point toward you; vertical bonds point away

staggered conformation does not have correct orientation of bonds for Fischer projection

Fischer projections

transform molecule to eclipsed conformation in order to construct Fischer projection

Fischer projections

CO2H

CH3

OH

OH

H

H

S SR R

Two chirality centers in a ring

nonsuperposable mirror images; enantiomers

trans-1-Bromo-1-chlorocyclopropane

S RS R


nonsuperposable mirror images; enantiomers

cis-1-Bromo-1-chlorocyclopropane

S RS R


stereoisomers that are notenantiomers; diastereomers

cis-1-Bromo-1-chloro-cyclopropane

trans-1-Bromo-1-chloro-cyclopropane

Estereoq

Entertainment & Humor

Transcript of Estereoq