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Stereochemistry- Chapter 3. 1. Stereoisomerism 2. Chirality 3. Naming stereocenters - R/S configuration 4. Acyclic Molecules with 2 or more stereocenters 5. Cyclic Molecules with 2 or more stereocenters 6. Properties of Stereocenters 7. Optical activity

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1. Stereoisomerism 2. Chirality 3. Naming stereocenters - R/S configuration 4. Acyclic Molecules with 2 or more stereoc


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slide1

Stereochemistry- Chapter 3

1. Stereoisomerism

2. Chirality

3. Naming stereocenters - R/S configuration

4. Acyclic Molecules with 2 or more stereocenters

5. Cyclic Molecules with 2 or more stereocenters

6. Properties of Stereocenters

7. Optical activity

8. Separation of Enantiomers, Resolution

9. Significance of Chirality in the biological world

slide2

Isomers - same molecular formula - different compounds

constitutional isomers - different connectivity

stereoisomers - same connectivity

- different orientation in space

(recall cis/trans)

chirality handedness not superposable on its mirror image
Chirality handednessnot superposable on its mirror image

types:

planeimaginary plane through an object

one half is the mirror image of the other

symmetry = superposable

center identical parts on an axis

equidistant from a point

slide4

syn - plane of symmetry

anti - point of symmetry

.

Elements of Symmetry

Conformations of2,3-butanediol*

.

If symmetry is present, the substance is achiral.

*meso or R,S (later)

chiral center
common source of chirality - tetrahedral (sp3) carbon (atom) - bonded to 4 different groupsChiral Center

chiral center- carbon (atom) with 4 different groups

Enantiomers: stereoisomers

nonsuperposable mirror images

All chiral centers are stereocenters

Not all stereocenters are chiral centers

enantiomers
2-Butanol - 1chiral centerEnantiomers

different representations for this enantiomer

representation of mirror image

or enantiomer

enantiomers3
2-ChlorobutaneEnantiomers

How is handedness designated?

r s convention priority rules

increasing priority

increasing priority

R,S Convention - Priority rules

Each atom bonded to the chiral center assigned a priority by atomic number

higher atomic number, higher the priority

Same atoms bonded to the chiral center

look to the next set of atoms

priority assigned to 1st point of difference

naming chiral centers
Naming Chiral Centers

1. Locate the chiral center, prioritize four substituents

1 (highest) to 4 (lowest)

2. Orient molecule so that lowest priority (4) group is directed away ( behind )

3. Read three groups toward you (in front) (1) to (3)

Clockwise R configuration; counterclockwise S

slide15

Stereochemistry- Chapter 3

1. Stereoisomerism

2. Chirality

3. Naming stereocenters - R/S configuration

4. Acyclic Molecules with 2 or more stereocenters

5. Cyclic Molecules with 2 or more stereocenters

6. Properties of Stereocenters

7. Optical activity

8. Separation of Enantiomers, Resolution

9. Significance of Chirality in the biological world

slide16

Naproxen

Ibuprofen

S isomer particularly active, but R slowly converted to S

S isomer

slide17

R

S caraway/dill

Assign R/S to stereogenic carbon in coniine

R-(-)-coniine

poison hemlock

Golden pitcher plant

Assign R or S to carvone

spearmint

enantiomers diastereomers
molecule with 1 chiral center:

21 = 2 stereoisomers are possible

Enantiomers & Diastereomers

molecule with 2 chiral centers:

a max of 22 = 4 stereoisomers “possible”

molecule with n chiral centers:

2n = maximum stereoisomers are possible

slide19

2n

256 (ignore sugar)

enantiomers diastereomers1
2,3,4-trihydroxybutanal 2 chiral centers Enantiomers & Diastereomers

22 = 4 stereoisomers “possible” & exist

2 pairs of enantiomer

(Erythrose)

Diastereomers:stereoisomers that are not mirror images

enantiomers diastereomers2
2,3-Dihydroxybutanedioicacid (tartaric acid)

2n = 4 “possible”

but only three stereoisomers exist

symmetry plane-superposable

(same compound)

enantiomers

Meso compound: achiral but possessing 2 or more chiral centers

Enantiomers & Diastereomers
enantiomers diastereomers3
2-Methylcyclopentanol

diastereomers

top

bottom

enantiomers left right

Enantiomers & Diastereomers

cis-2-Methylcyclopentanol

trans-2-Methylcyclopentanol

enantiomers diastereomers4
1,2-cyclopentanediol

diastereomers

cis-1,2-cyclopentanediol

(a meso compound)

Enantiomers & Diastereomers

trans-1,2-cyclopentanediol

(enantiomers)

enantiomers diastereomers5
cis-3-methylcyclohexanolEnantiomers & Diastereomers

flip: axial-equatorial reverse

but still cis

enantiomers diastereomers6
trans-3-methylcyclohexanolEnantiomers & Diastereomers

flip: axial-equatorial reverse

but still trans

properties of stereoisomers
Enantiomers: identical physical and chemical properties inachiralenvironmentsProperties of Stereoisomers

m. pt. 174o 174o

pK1 2.98 2.98

Diastereomers: different compounds different physical and chemical properties

m. pt. 146o 174o

pK1 3.23 2.98

plane polarized light
Light vibrating in all planes  to direction of propagation

Plane-polarized light:light vibrating only in parallel planes

Plane-Polarized Light

optical activity

Plane-polarized lightthe vector sum of left and right circularly polarized light

optically activity enantiomers chiral interact with circularly polarized light
rotating the plane one way with R center

and opposite way with S

Optically ActivityEnantiomers (chiral) interact with circularly polarized light

result: rotation of plane-polarized light clockwise (+)

or counterclockwise (-)

plane polarized light polarimeter
Plane-Polarized Light (polarimeter)

Change in the polarized plane?

achiral

sample

no change in the plane

plane polarized light polarimeter1

rotates the plane

Plane-Polarized Light (polarimeter)

Change in the polarized plane?

CHIRAL

slide31

Stereochemistry- Chapter 3

1. Stereoisomerism

2. Chirality

3. Naming stereocenters - R/S configuration

4. Acyclic Molecules with 2 or more stereocenters

5. Cyclic Molecules with 2 or more stereocenters

6. Properties of Stereocenters

7. Optical activity

8. Separation of Enantiomers, Resolution

9. Significance of Chirality in the biological world

optical activity
observed rotation:, degrees a compound rotates polarized light -dextrorotatory (+) right

- levorotatory (-)left

T

specific rotation []D =

S

R

Optical Activity

( )-(+)-lactic acid ( )-(-)-lactic acid

slide33

Example: 0.5g (-)-epinephrine-HCl in 10mL H2O measured in 20 cm cell (25o/D) obs = -5.0o, []D =?

25o

R-enantiomer is (-); R or S above?

[a] = deg (cm2g-1 )

optical activity1
Racemic mixture: equal amounts of (+) and (-) enantiomers - rotation is 0oOptical Activity

For a 50/50 mixture of S and R, = ?

0o

slide35

21o

(S)-(+)-2-bromobutane, []D=+23.1o

21o

But from the obs, []D=+9.2?

Mixisbetween 100% S and 50/50(S/R)

optical purity

 the sample has70%S and30%R

It’s not pure; possibly someR present!

If some R, what percent?

+23.1o > +9.2o < 0o

= 40%

40% excess= 40%S

+ (60%S/Rmixture)

40% excess= 40%S + (30%S + 30%R)

slide37

6 - 4

6 + 4

ee = x100%

rt

[]D of (+)-2-butanol = +13.5o; obs sample = ?

obs

+13.5pure

obs

pure

opt pure = = ee

20% =

e.g. 6g of (+)-2-butanol plus 4g of (-)-2-butanol, ee = ?

= 20%

obs = (.20)(+13.5%) = +2.7o

enantiomeric excess
Example:A commercial synthesis of naproxen (Aleve) gives the S enantiomer in 97% ee.

What are the percentages of the R & S in this mixture?

Enantiomeric Excess

100% sample =

97%S + (3%S and R)

97%S + (1.5%S+1.5%R)

98.5%S + 1.5%R

resolution separation of enantiomers
One strategy: convert enantiomeric pair into 2 diastereomers Resolution - separation of enantiomers

diastereomers - different compounds

different physical properties

Common - reaction forming salt

separate diastereomers

remove :B

leaves pure enantiomers

4

slide41

----resolved----

Resolution by acid-base reactions

Pure-Sb

racemic

mix

slide42
Examples of enantiomerically pure bases

CH

=CH

CH

=CH

2

2

[

]

[

]

Resolution

H

H

H

H

H

H

N

HO

N

H

HO

H

CH3O

N

N

(+)-Cinchonine

(-)-Quinine

23

= +228

D

25

= -165

D

slide43

racemic bases with chiral acids like:

[]D = -127o HCCl3 from Strycnos seeds (S nux-vomica)

brucine

Strychnine no methoxy groups

slide44

R

pure

enantiomer

R

S

S

[] = 0

R

S

+

S

S

S

S

enantiomeric mixture

Resolution

slide45

[]25 = -8.2

[]25 = +8.2

D

D

R

S

[] = 0

S

enantiomeric mixture

pure

enantiomer

Resolution

slide46

lipase

>69%ee

50/50 mix

R-Enzyme

slide47

lipase

>69%ee

A 50/50 enantiomeric mixture of esters

forms R-acid and recover S-ester.

R-Enzyme

slide48
Enzymes as resolving agents

racemic mix ethyl ester of (S)- and (R)-naproxin

(R)-ester - no effect

(S)-now acid

different functional gp.

slide49

carbohydrates

deoxynucleic acid

amino acids

CHEMICAL &ENGINEERING NEWS Oct 23, 2000, pg 55

Chiral Drugs Sales top $100 Billion

proteins
proteins are long chains of amino acids covalently bonded by amide bonds formed between the carboxyl group of one amino acid and the amino group of another amino acidProteins

Chapter 5