StereochemistryChiral Molecules

StereochemistryStereosiomers are molecules

Same formula and connectivityDiffer by arrangement in space onlyMolecules are non-superimposable

Not identical

Achiral and Chiral CarbonTetracarbon with 4 different atoms/groups

attached chrial center or chiral carbonF

BrI Cl

Cl

I F

Br

*switch position of 2 atoms --> not the same molecule

Test for Chirality: Planes of Symmetrymirror plane: an imaginary plane that bisects

a molecule in such a way that two halves of the molecule are mirror images of each otherMolecules with plan of symmetry are achiral

Chiral MoleculesHas no plane of symmetryHave non-superimposable mirror image

Chiral MoleculesMolecules that have no plane of symmetryMolecules that have non-superimposable

mirror imageYour left hand and right hand

Which one of these compounds having plane of symmetry?

methylcyclohexane1,3-dimethylcylcohexane

ISOMERSDifferrnet compounds with

same molecular structure

Constitutional isomersisomers whose atoms have a

dif ferent connectivity Stereoisomersisomers that have the same connectivity

but that differ in the arrangement of their atoms in space

Enantiomersstereoisomers that are nonsuperimposable

mirror images of each other

Diastereoisomersstereoisomers that are not mirror image

of each other

StereoisomersEnantiomers: stereoisomers whose molecules

are nonsuperimposable mirror image of each otherExact MW, BP, MP ect …except how it rotates

in polarized light

BrH

*

mirror

BrH

*

StereoisomerDiastereoisomers: stereoisomers whose

molecules are not mirror image of each other

mirror

Cl H

Cl H

Cl H

H Cl

Nomenclature of Enantiomers: The R,S-SystemEach chiral carbon in a chiral molecule is

designated a “R” (Rectus: right) or “S” (Sinitrus: left) configuration

Rules: Assign priorities to atoms directly attached to chiral carbon based on Atomic number# 1 = highest atomic ## 4 = lowest atomic #

Nomenclature of Enantiomers: The R,S-System

If there’s a tie (e.g two carbons) move to the next carbon atom until tie is broken

If there’s a double bond = 2 single bondsWith the lowest priority group (#4) point

away from you, rotate 14Clockwise R configurationCounterclockwise S configuration

Nomenclature of Enantiomers: The R,S-SystemVinyl group, is of higher

priority than the isopropyl group

CH

CH2

CH(CH3)2

ExamplesAssign (R) or (S) designation to each of the

following compounds

Cl

OH O

*I

O Cl

*

HBr

Molecules with Multiples Chiral CentersThe maximum number of stereiosomers can

be predicted 2n

n = number of chirality centers

Br

Br

*

*

Naming compounds with multiple chirality centerFollowed same rules for naming R,SDesignate position of chiral carbon

BrH

BrH

2,3-dibromobutane

RR

SS

enantiomers (configuration inverted at all chrial centers

SR

RS

enantiomers

diastereiosomers (configuration inverted at some,but not all, chiral centers

ExamplesDraw alll possible stereoisomers and provide

an appropriate name for each stereoisomer

Cl

Br

H

H

Fisher Projection FormulasShow three dimension structure

Compound with several chiral centersUsed carelessly, these projection formulas can

easily lead to incorrect conclusion

BrH

BrH

=

CH3

CH3

HBr

BrH

=

CH3

CH3

HBr

BrH

ExamplesDraw all stereoisomers of

H

ClH

Cl

Meso compoundsA structure with two chirality centers does

not always have four possible stereoisomers.achiral occurred within chiral molecules when

carbon is inverted Look for internal plane of symmetry

Properties of EnantiomersRecall: the molecules of enatiomers are not

superposable one on the otherEnantiomers have identical M.P, B.P Different physical properties and direction

which they rotate plane-polarized light Same amount in rotation but opposition direction

Different reaction rate when interact with another chiral moleucle

Plane-Polarized lightWhen regular light beam

is passed through a polarizer, all of the light waves, except those whose electromagnetic fields ossicllate in a single direction, are filter out

plane-polarized light optically active

Polarimeter

A substance that is rotated in a clockwise rotation

α (measured of degree) is positive (+)Dextrorotatory

A substance that is rotated in a counterclockwise rotation

α is negative (-)levorotatory

c l=

the specif ic rotation

the observed rotation

c = the concentration of solution in grams per millilers of solution (ordensity in g/ml-1 for neat liquid

l = the length of the tube in centimeters

Specific RotationDepends on the temperature and the

wavelength of light that is employed

H OH HO H

R - 2 - Butanol

25

D= -13.53o

S - 2 - Butanol

25

D= +13.53o

Specific RotationThe direction of rotation of plane-polarized

light is often incorporated into the names of optically active compoundsNo obvious correlation exists between the (R)

and (S) configurations of enantiomers and the direction ([(+) or (-)] in which they rotate plane-polarized lightCl

CH3H

Cl

CH3H

(R)- (-)-1-chloro-2-methylbutane

25

D= -1.64o

(S)-(+)-1-chloro-2-methylbutane

25

D= + 1.64o

Racemic MixturesAn equimolar mixture of two enantiomers is

called a racemic mixture (racemate, racemic form)No net rotation of plane-polarized light50:50 mixtureResulted from chemical reaction of achiral

molecule

H3CH2C

O

CH3

H2

catalyst H3CH2C CH3 H3CH2C CH3

OHH HO H

+

R S

Enantiomeric excessA sample of an optically active substance that

consists of a single enantiomers is said to be enantiomerically pure or to have an enantiomeric excess of 100%

% enantiomeric excess (ee) =moles of one enantiomer - moles of other enantiomer

total moles of both enantiomers

x 100

% enantiomeric excess (ee) =observed specif ic rotation

specif ic rotation of the pure enantiomerx 100

Resolution: Separating EnantiomersA common way of separation uses the

conversion into diastereomers, that are not mirror images of each otherRecrystallization or chromatography

Stereoisomerism of cyclic compounds Is trans-1,2-dimethylcyclopentane superposable on its mirror

image Is cis-1,2-dimethylcyclopentane superposable on its mirror image? Is cis-1,2-dimethylcyclopentane a chiral molecule? Would cis-1,2-dimethylcylcopentane show optical activity? What is the stereoisomeric relationship between 1 and 3

H

Me

Me

H

Me

H

H

Me

H

Me

H

Me

Enantiomer Meso1 2

3

cis-1,4-dimethylcyclohexane tran-1,4-dimethylcyclohexane

diastereoisomer

cis-1,3-dimethylcyclohexane trans-1,3-dimethylcyclohexane

cis-1,2-dimethylcyclohexane tran-1,2-dimethylcyclohexane

Si Ge N S

R4 R3

R1 R2R1 R2

R4 R4 R2R3 R3 R1

OR1 R2