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Chirality of molecules

Page history last edited by Fatima Al-Zahraa Itani 11 years, 1 month ago


  •       Stereoisomers are molecules of same molecular formula and similar structural formula.
  •       Stereoisomers differ in the way they rotate polarized light;

They can rotate polarized light either to the:

1-      Right =  Clockwise rotation   => (+) or (D) or (R)  D: Dextrorotatory   
OR  to the

2-      Left = Counterclockwise rotation => (-) or (L) or (S)        L: Levorotatory


1- An Optically inactive substance is the one that is not capable of rotating the plane of polarized light.




2- An optically active substance  is the one that is capable of rotating the plane of polarized light.



  •       They are type of stereoisomers.
  •       They are molecules that are not superimposable
  •       One molecule is mirror image to the second molecule.
  •       In enantiomers , the carbon atom is chiral ( bonded to 4 different groups of atoms)
  •       Note that if the carbon atom is bonded to 2 similar groups or more, this carbon is said Achiral.





Note: Rotate molecule II, either clockwise or anti-clockwise, it becomes superimposable with molecule I => molecule I and II are not enantiomers. => Optically inactive.


Characteristics of Enantiomers:

   Enantiomers have identical physical properties, except for the rotation of the plane of polarized light.

  •     Two enantiomers rotate plane-polarized light to an equal extent but in opposite directions.  Thus, if enantiomer A rotates polarized light +5°, the same concentration of enantiomer B rotates it –5°.


      Enantiomers have identical chemical properties except toward optically active reagents ( Same rate of reaction, same products, same catalyst, …)

      Examples about optically active reagents: enzymes, some catalysts…


(+) glucose is the main sugar in animals; it plays a unique role in animal metabolism.

(-) glucose can’t be metabolized by animals or fermented by yeasts.


How to determine whether the configuration of the carbon atom is R (+) or S (-)?



1-      The carbon atom should be Chiral (attached to 4 different groups).

2-      Apply the following sequence rule for the Chiral carbon:

Sequence rule:

1-      Number the groups attached to the carbon atom according to their priority. (The priority depends on the atomic number.)

2-      If two atoms are identical, compare the atoms to which these atoms are joined.

3-      If the direction is clockwise => R, and if the direction is anticlockwise => S.


If the group with lowest priority is pointed forward (toward you),

 Then if it’s:

1-      clockwise

       (To the right)=> S

2-      anticlockwise

(To the left) => R



Summary For Isomers

For more information and if you like to practice exercise about enantiomers, you can visit the following website:



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