Chiral Molecules

Chapter 12.6

Definition of Isomer* Structural *

• Molecules with the same formula but different structure and characteristics

Example C3H7OH

1-propanol 2-propanol

• Molecules that have the same structural components but are mirror images from each other

• Like left hand, right hand or right shoe, left shoe

Definition of Isomer * Stereoisomers/Enantiomers *

Enantiomers of Molecules

• They do not match up…no matter how they are rotated

Rotated On Top

Biological Importance

• In the 3D environment of the body, enantiomers are considered two different molecules: only one version works! Like your

right glove doesn’t fit your left hand

Carbon Stereoisomers

• For carbon to form a stereoisomer – it has to bond to 4 different substituents

• Below: orange, purple, green and white

Carbon Stereoisomer

• If Carbon does not have 4 different substituents, it can be rotated and made to overlap…giving an identical molecule

Chirality

• A carbon with 4 different substituents is called a chiral carbon, often indicated by a *

• It can come in the form of two different enantiomers

Fischer Projection

• Molecular drawing that shows chirality• Chiral carbons are indicated as crosses

SH SH H OH HO H

NH2 NH2

Fischer Projection• Horizontal lines – forward bonds in 3-D model• Vertical lines – bonds that point away in 3-D

Mark all Carbons that are chiral

Which of the molecules are correctly drawn as Fischer Projections?

A B C CHO COOH COH

H3C H HO CH3 H3C OH CHO CHO OH No – not chiral

2 CHO attached to CYes

4 different substituentsNo – not chiral

2 OH attached to C

Drawing Rules for Fischer

• Molecular axis has to be vertical with highest oxidized Carbon on top

• All chiral carbons become crossed lines• Non-chiral carbons are written condensed• Draw the molecule below as a Fischer Projection

D and L Enantiomers

• The handedness of enantiomers is indicated as either L –Left or D- Right with reference to certain substituents

D and L Enantiomers• In larger carbohydrates, the hydroxyl furthest

from the top is used for D and L