Stereoisomers | Organic Chemistry 1

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 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 no 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

R / S label system:

A nomenclature system that refers to the spatial arrangement of atoms within a chiral molecular entity and its relevant stereochemical description. A clockwise direction is an R configuration (rectus, Latin for right), while a counterclockwise direction is an S configuration (sinister, Latin for left)
 

How to assign R or S 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 is equivalent to the same number of single bonds

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

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

   

Enantiomers: 

Stereoisomers which are mirror images of each other and which are not superimposable. 2 enantiomers of a given compound have identical physical properties (melting point, solubility, density ...) except for the direction of rotation of polarized light
 

Diastereomers: 

Stereoisomers which are not mirror images of each other and are therefore not superimposable. 2 diastereomers have different physical properties
 

How to identify enantiomers and diastereomers:

• 2 enantiomers have all their stereocenters of opposite in configuration: the enantiomer of a compound having a stereocenter of R configuration will have an S configuration for this same stereocenter
• 2 diastereomers have at least one stereocenter of the same configuration and another of opposite configuration

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:

A compound is said to be optically active when the plane of linearly polarized light rotates 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

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

Dextrorotatory vs. levorotatory:

• [α]_D ≠ 0°: molecules in solution are optically active
• [α]_D > 0°: plane-polarized light is rotated clockwise and the compound is said to be dextrorotatory. The rotation is marked (+) or d 
• [α]_D < 0°: plane-polarized light is rotated counterclockwise and the compound is said to be levorotatory. The rotation is merked (-) or l 

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

Meso compound:

An achiral compound ([α]_D = 0°) with 2 or more stereocenters. A meso compound usually has a plane of symmetry and is superimposable on the compound with 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 or racemate:

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

Enantiomeric excess (ee) or optical purity:

The measure of the amount of one enantiomer present in excess of the racemic mixture

ee = % of one enantiomer - % of the other enantiomer = $\frac{\mathrm{observed} {\left[\mathrm{\alpha }\right]}_{\mathrm{D}} \mathrm{of} \mathrm{the} \mathrm{mixture}}{{\left[\mathrm{\alpha }\right]}_{\mathrm{D}} \mathrm{of} \mathrm{the} \mathrm{pure} \mathrm{enantiomer}}$ x 100