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Chirality or Handedness

The world you encounter is chiral or has handedness.  The foods you eat, the medicine you take, the molecules that you are made of are chiral. Just as you can identify a right or left hand, you need to be able to identify a right or left handed molecule.

How to determine if an object or molecule is chiral?

The ultimate test.

    • An object (or molecule) is chiral if it is not super-imposable on its mirror image.

Lets examine what this means by looking at an example.  Do you think the letter "A" is chiral?  Let us see if we can prove or disprove!  To test if the letter "A" is chiral we would need to generate its mirror image and then see if this mirror image would occupy the same space (i.e. super-imposable) as the original.

Clearly it is super-imposable so it is not chiral.

Now try the letter "R"!

 

Letters from the alphabet are easy since we only have to worry about 2 dimensions.  With molecules we need to worry about 3 dimensions.

Action
  1. Click the "Synchronize" check box below then examine and rotate the mirror images of alanine.  These mirror images are related to one another as enantiomers.
  2. Next uncheck "Synchronize" check box and rotate the image on the right to match up as many atoms as possible with the image on the left. Are they superimposable?

No they are not superimpoasable.  These mirror images are related to one another as enantiomers.  Our hands are enantiomers of one another.

Synchronize

 M
I
R
R
O
R

D-Alanine   L-Alanine

 

    • An object (or molecule) is chiral if it does not have a plane of symmetry

This is just as good as the ultimate test above.  If you can draw a plane of symmetry through a molecule in which you have the same atoms and configuration on each side of the plane, then the molecule is not chiral.