Stereoisomers | Organic Chemistry 1

Stereoisomerism concept is studied in this chapter: the chirality of chemical compounds, the absolute configuration of a stereocenter, the types of stereoisomers (enantiomers, diastereomers, meso compound), the optical activity of stereoisomers, the racemic mixture and enantiomeric excess.

Chirality

Asymmetric atom (or stereocenter or chirality center) = atom that is connected to four different substituent groups. They are sometimes denoted by an asterick.

 

The central C is connected to 4 different substituents:
this is an asymmetric carbon.

 

The central C is connected to only 3 different substituents (2 methyl groups):
this is not an asymmetric carbon.

 

 

Chiral molecule = molecule that is not superimposable on its mirror image.

Molecules with one stereocenter are always chiral.
Molecules with more than one stereocenter are chiral if they do not contain a plane of symmetry.
If they have a plane of symmetry, they are said to be achiral.

 

2 stereocenters + No plane of symmetry:
the molecule is chiral

 

2 stereocenters + Plane of symmetry:
the molecule is achiral

 

Absolute Configuration R/S

R/S system is a nomenclature system for the asymmetric centers.
 

How to determine if an asymmetric center is R or S?

- Look at the 4 substituents and assign priorities according to Cahn-Ingold-Prelog rules:
1) Rank the subst. according to the atomic number of the atoms directly bonded to the chirality center
2) If equal look the 2nd, the the third atoms away from the chirality center
3) Multiple bonded atoms are equivalent to the same number of single bonded atoms

- The group of lowest priority is pointing directly back, away from you

- If a curved arrow drown from priority 1 to 2 to 3 is clockwise: R configuration; counterclockwise: S configuration
 

Enantiomers vs. Diastereomers

Enantiomers = stereoisomers that are mirror images of each other and are non-superposable.
Enantiomers have opposite configurations at all chirality centers.
 

Diastereomers = stereoisomers that are not mirror images of each other.
At least one of their chirality center has an opposite configuration, but not all.

 

non-superposable mirror images
opposite configurations at all C*
= enantiomers

 

not mirror images
opposite configuration of 1 C*
same configuration of 1 C*
= diastereomers

 

 

Optical Activity

Chiral molecules are optically active: they interact with plane-polarized light.
When a molecule is optically active, its specific rotation [α]D is different from 0°.
[α]D can be + (the solution is said to be dextrogyre) or - (the solution is said to be levogyre).
 

2 enantiomers have the same absolute value for their [α]D but opposite sign.
2 diastereomers have a different absolute value for their [α]D and same or opposite sign.

Meso Compound

Meso compounds = compounds that are achiral, yet contain chirality centers.
They do not have optical activity: [α]D = 0°.
A meso compound has a plane of symmetry and is superimposable on the compound with opposite configurations at all chirality centers.

 

(1R,3S)-cyclohexane-1,3-diol is a meso compound:
 

- (1R,3S)-cyclohexane-1,3-diol has a plane of symmetry:

- (1R,3S)-cyclohexane-1,3-diol is superimposable on the cpd with opposite configurations at all chirality centers (after a rotation of 180°):

 

 

 

Racemic Mixture and Enantiomeric Excess

2 enantiomers of a given compound have identical properties (melting point, solubilities, densities ...).

2 diastereomers have different properties.
 

A sample of a single enantiomer is said to be enantiomerically pure. 

A 50/50 mixture of the 2 enantiomers of a given cpd is called racemic mixture or racemate.

Racemates show zero optical rotation, [α]D = 0°.
 

Enantiomeric excess or optical purity (ee):
 

ee = # moles of 1 enantio. - # moles of the other enantio.# moles of both enantiomers x 100 = observed [α]D of the mixture[α]D of the pure enantiomer x 100