Stereochemistry: How to determine Chirality of a Molecule

Chemistry Tutorial

Stereochemistry: How to determine Chirality of a Molecule

Intro

There are priority rules in order to determine whether a molecule is of the R-configuration or S-configuration.
1) Atoms with the higher atomic number have higher priority. For example, Nitrogen has a higher atomic number than Carbon so it gets priority.
2) If a decision cannot be reached (because both atoms attached to the central carbon are the same) then go to the next atom.
3) Put the lowest priority towards the back (away from you). Usually, it is hydrogen by itself so it gets 4th priority.
4) If your priorities are in a clockwise direction (ranging from 1 – 4, with 4 being the lowest) then the molecule is of R configuration. If your priorities are in a counterclockwise direction (ranging from 1-4, with 4 being the lowest) then the molecule is of S configuration
5) Your central carbon is the chiral carbon that has 4 bonds attached to it.
6) Double bonds count as two bonds to the same atom.

Sample Problem

      OH  O  
      |   || -> (double bond)
H3C - C - C - OH
      |
      H 

Solution

Steps to Solve Problem:
1) The Chiral Carbon in this problem is the carbon that has the OH, CO2H, H, and CH3 attached to it.
2) The OH gets first priority because OH has an atomic number of 16 whereas Carbon has an atomic number of 12 and Hydrogen has an atomic number of 1.
3) Now you have two carbons and a Hydrogen to assign. Since hydrogen has an atomic number of one it always comes last, so it gets fourth priority which is often ignored when determining chirality.
4) To decide which Carbon gets second priority look at the atoms attached to the carbon. The carbon on the right has an OH and 2 Oxygens attached to it. Remember, double bonds count as two atoms. Whereas the carbon on the left has 3 Hydrogen’s attached to it
5) Since Oxygen has a higher atomic number than Hydrogen, the Carbon on the right gets second priority.
6) The carbon on the left gets third priority.
7) Since the numbers on the molecule are going in a clockwise direction, ignoring Hydrogen, the molecule is an R-configuration.

  (1) OH  O  
      |   || (2)
H3C - C - C - OH
 (3)  |
      H (4)


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