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


Asymmetric atom (or stereocenter or chirality center): 

An atom connected to 4 different substituents. They are sometimes marked with a star. For n stereogenic centers, the maximum of different stereoisomers is 2n

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:

A molecule that is not superimposable upon 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 do not have a stereocenter or 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 but plane of symmetry:
the molecule is achiral

Absolute Configuration R / S

R / S label system:

A nomenclature system to distinguish asymmetric centers

How to assign R or configuration:

  1. Identify the 4 atoms attached to the chirality center and prioritize them by atomic number
    - if 2 atoms are identical, make a list of substituents, and look for the first point of difference
    - a multiple bond equals the same number of single bonds

  2. Orient the molecule with the lowest priority group backwards (dashed line)

  3. Identify the direction of rotation of the 1-2-3 sequence: clockwise is R, counterclockwise is S


Enantiomers vs. Diastereomers


Stereoisomers that are nonsuperimposable mirror images of each other. 2 enantiomers have their stereocenters opposite in configuration. 2 enantiomers of a given compound have identical physical properties (melting point, solubility, density ...) except for the direction of rotation of polarized light


Stereoisomers that are not mirror images of each other. At least one stereocenter of 2 diastereomers has the same configuration and one is opposite. 2 diastereomers have different physical properties


nonsuperimposable mirror images
opposite configuration for all C*
⇒ enantiomers


not mirror images
opposite configuration for 1 C*
same configuration for 1 C*
⇒ diastereomers

Optical Activity

Optical activity:

A compound is said to be optically active when the plane of linearly polarized light is rotated as it passes through a solution of that compound. Chiral molecules are optically active


Specific rotation [α]D:

The angle of rotation of the plane of polarization of a ray of monochromatic light passing through a solution of a substance

[α]D = αl x c 

[α]D: specific rotation (usually given as °)
α: observed rotation (in °)
l: length of sample tube (in dm)
c: concentration (in g.mL-1)


Dextrorotatory vs. levorotatory:

[α]D ≠ 0°: molecules in solution are optically active
[α]D > 0°: the plane-polarized light is rotated clockwise and the compound is called dextrorotatory. The rotation is labeled (+) or d 
[α]D < 0°: the plane-polarized light is rotated counterclockwise and the compound is called levorotatory. The roation is labeled (-) or 

There is no relationship between a R or S compound and a d or l compound

2 enantiomers rotate polarized light with the same extent but in the opposite direction: their [α]D have the same absolute value but of opposite sign. [α]D of 2 diastereomers have a different absolute value and an identical or opposite sign

Meso Compounds

Meso compound:

An achiral compound ([α]D = 0°) with 2 or more stereocenters. A meso compound generally has a plane of symmetry and is superimposable on the compound having opposite configurations at all stereocenters


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

1. It has a plane of symmetry

2. It is superimposable on the compound having opposite configurations at all stereocenters (after a rotation of 180°):

Racemic Mixture and Enantiomeric Excess

Racemic mixture or racemate:

The 50/50 mixture of 2 enantiomers of a given compound. A racemic mixture is optically inactive: [α]D = 0°. A sample of a single enantiomer is said to be enantiomerically pure

Enantiomeric excess (ee) or optical purity:

The measure of the amount of an enantiomer present in excess relative to the the racemic mixture

ee = % of one enantiomer - % of the other enantiomer = observed [α]D of the mixture[α]D of the pure enantiomer x 100