Tetrahedral Stereo

The dimension of a molecule can be interpreted topologically, based on the connections of the consisting atoms, or spatially, based on their Cartesian coordinates.

• Stereoisomers

  • Special cases
  • Representation in 0D, 2D and 3D

• Chirality

  • Cahn-Ingold-Prelog priority rules
    

Stereoisomers

Molecules with same connectivity but different spatial arrangement are called stereoisomers.

Stereoisomer types:

• Enantiomers: Molecules that are non-superimposable, complete mirror images of each other.

• Diastereomers: Stereoisomers that are not enantiomers.

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Special cases

• Non-carbon tetrahedral stereocenters

  • Ammonium and phosphonium salts
    
  • Amine oxides and phosphanones
    
  • Phosphanes
    
    In this case the lone pair of phosphorus atom is considered as the fourth ligand.
  • Phosphates and phosphonates
    
  • Sulfoxides
    
  • An atom *in a ring * is a tetrahedral center, if
  • the central atom has 2 different kinds of ligands outside the ring, and
  • the graph invariant of the ring is not the same in the two sides of the central atom, or

  • the graph invariant of the ring *is the same * in the two sides of the central atom, but

    • the ring contains even numbers of atoms (including the parity central atom), or
    • there is an atom with nonzero parity in the opposite side of the ring:

        <img src="images/download/attachments/1806808/cyclobutane_diol.png" alt="images/download/attachments/1806808/cyclobutane_diol.png"/><img src="images/download/attachments/1806808/p002.png" alt="images/download/attachments/1806808/p002.png"/>
      

• Nitrogen atoms in a ring is tetrahedral stereo center, if

  • they are bridgehead atoms.
    
  • N is a tetrahedral stereo center in a 3 membered ring,
  • if the graph invariant of the ring is not the same in the two sides of the nitrogen atom.
    

Representation in 0D, 2D and 3D

• **0D: ** Stereoinformation is defined in 0 dimension by parity.

• **2D: ** Stereoinformation in 2 dimension is defined by wedge, hatch or wiggly bond types.

• **3D: ** Stereoinformation in 3 dimension is defined by the coordinates.
  

Chirality

An atom in the molecule around which the ligands are arranged so that interchange of two ligands leads to stereoisomer is called stereocenter * or *stereogenic center . Chirality appears in stereoisomerism which is due to tetrahedral stereogenic centers. These centers can have point chirality. The ligands of the chiral center are assigned a priority based on the Cahn-Ingold-Prelog priority rules. Each chiral center is then labeled by R or S based on the orientation of the assigned numbers. The center is oriented so that the lowest-priority is pointed away from the viewer. If the priority of the remaining three substituents decreases clockwise, it is labeled R , otherwise, if it decreases counter clockwise, it is S .

R-bromo(chloro)iodomethane	S-bromo(chloro)iodomethane

Pseudo-asymmetric atoms which are invariant on reflection in a mirror are labeled by r/s descriptors:

Cahn-Ingold-Prelog priority rules

Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Eds:Henri A. Favre, Warren H. Powell) Chapter P-9 Specification of Configuration and Conformation Royal Society of Chemistry

Wikipedia: Cahn-Ingold-Prelog priority rules.