Stereoisomers | Organic Chemistry 1

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

Chirality

Asymmetric atom (or stereocenter or chirality center): 

Atom connected to 4 different substituents. They are sometimes marked with an asterick

 

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

 

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

 


​​​​​​​Chiral molecule:

Molecule that is not superimposable on its mirror image
Molecules with only 1 stereocenter are always chiral. Molecules with more than 1 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) Classify the subst. according to the atomic number of the atoms directly bonded to the chirality center
     2) If equal, look at the 2nd, then the 3rd ... atom far away from the chirality center
     3) Multiple covalent bond equals the same number of single bonds

- The group of lowest should point 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. 2 enantiomers have opposite configurations in all chirality centers. 2 enantiomers of a given compound have identical properties (melting point, solubilities, densities ...)
 

Diastereomers: 

Stereoisomers that are not mirror images of each other. At least one of their chirality center has an opposite configuration, but not all of them. 2 diastereomers have different properties

 

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

Optical activity:

A compound is said to be optically active when the linearly polarized light is being rotated when it is passing through it. Chiral molecules are optically active

 

Specific rotation [α]D:

Angle of rotation (in degrees) of the plane of polarization of a ray of monochromatic light that passes through a substance in solution

[α]D​​​​​​​ ≠ 0°: molecule in solution is optically active
​​​​​​​[α]D > 0°: the plane-polarized light is rotated clockwise ⇒ the solution is said to be dextrorotary
[α]D < 0°: the plane-polarized light is rotated counterclockwise ⇒ the solution is said to be levorotary

[α]D of 2 enantiomers: same absolute value, opposite sign
[α]D of 2 diastereomers: different absolute value, same or opposite sign

Meso Compound

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:
​​​​​​​

There is a plane of symmetry

 

The 2 compounds with opposite configurations at all chirality centers are superimposable (after a rotation of 180°)

 

Racemic Mixture and Enantiomeric Excess

Racemic mixture or racemate:

50/50 mixture of the 2 enantiomers of a given compound. Racemates exhibit zero optical rotation, [α]D = 0°
A sample of a single enantiomer is said to be enantiomerically pure
 

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