enantiomers: non superimposible mirror images

chirality

Me

EtBr

H

Me

EtBr

H

enantiomers: non superimposible mirror images

“stereogenic center”

Not all mirror images are chiral. These are the same (not chiral)

Me

EtBr

H

Me

EtBr

H

Me

EtEt

H

Me

EtEt

H

formalism: wedge (forward)

dashed (back)

enantiomershave same mp, bp, NMR, IR, etc

How to tell if molecules are chiral? Clue: many (but not all*) chiral carbons have 4 different

things attached

R1

R2R3

R4

Me

BrEt

Cl

N

OH

EtOH

NH2

R1

R2R3

R1

chiral

achiral

* exceptions to be discussed

When plane-polarized light interacts with a chiral molecule, the plane of polarization of the light is rotated to the left or right.

This effect is called optical rotation and the molecule is said to be optically active.

Optical activity is measured using a polarimeter. This device contains a light source, a polarizer to produce the plane-polarized light, a sample cell, and an analyzer to determine the amount of rotation.

Cahn Ingold Prelog rules

1) Highest atomic #2) then by isotope atomic weight3) then by substituents, one atom at a time

prioritize by:

CH3

HD CH3

a

b

c

d

4) if atoms are equal, go to first point of difference

HD

bc

d

Brthis carbonis connected to C,C,H,H

Higher priority: this carbonis connected to C,C,C,H

a

HD

bc

d

this carbonis connected to C,C,H,H

Higher priority: this carbonis connected to Cl, C, H, H

a

Cl

Cl

HD

bc

d

Br

Higher priority: this carbonis connected to Br,C,H,H

this carbonis connected to C,C,C,H

a

5) multiple bonds

HH3C

bc

da H

H3C

HH

~

HH3C

bc

da H

H3C C

C

~

C C

C

C

HH3C

b

c

d

Oa

HH3C

O

OOH

~O

O O

H

b

c

d

a

View the molecule with the lowest priority group in the back

a

bc

clockwise

R

(Latin, rectus)

b

c

a

db

ac

counterclockwise

S

(Latin, sinister-left)

opposite enantiomer:

CH3

Br H

CH3

Br Ha

b

c

d

CH3

C3H7 Br

c

ab

R

CH3Br

higher priority

higher priority

Br

I

higher priorityhigher priority

same rules for alkenes

(E) (Z)Pile of Latvian Mittens

The Latvians are knitting 4,500 mittens for the participants of the NATO summit there (2006)!

See here: www.rigasummit.lv/en/?id=newsin&nid=115

Chiral molecules are not necessarily optically active

= 0

(50% Left, 50% Right)

Racemates are not optically active

= 0CH3

Br H

CH3

Br H

CH3

Br H

CH3

Br H

CH3

BrH

CH3

BrH

CH3

BrH

CH3

BrH

L + R L + L

What happens if an enantiomer interacts with another element of chirality?

diastereomers

What happens if an enantiomer interacts with another element of chirality?

diastereomershave different mp, bp, NMR, IR, etc

ClC3H7

BrH3CFC2H5

R R

ClC3H7

FH3CBrC2H5

R S

O O

R-carvonespearment

S-carvonecaraway

diastereomeric pairs up the nose

diastereomers

R Br

IR

R Br

IS

R Br

IR

R Br

IS

easy to see in ring compounds

easy to see in acyclic compounds if we are consistent in our drawings

meso compoundsMeso compounds contain an internal mirror plane (or center of symmetry) which divides the two halves of the molecule which are mirror images of each other. NOT OPTICALLY ACTIVE!

Vitruvian man = meso man. He has symmetrically substituted elements of chirality, equal and opposite

AA

BBCC

A

AC

B

BC

recognizing planes and centers of symmetry

AA

BBCC

R S

plane of symmetry

A

AC

B

BC

R S

center of symmetry

in general, look for R & S stereocenters in thesame molecule attached to exactly the same thing

meso compounds

R Me

MeS

R Me

MeR

SMe

Me R

easy to see in ring compounds

easy to see in acyclic compounds if we are consistent in our drawings

R

Me

Me

S

meso compounds

s

CH3

HBr

Cl(R)