Enantiomers are stereoisomers that exhibit a property known as chirality. The word chiral comes from the Greek which means hand like. Our hands are mirror images of each other which are not identical. If they were then we would not need a right hand and left hand glove. We can prove that they are not identical by trying to lay one hand on top of the other palms up. When we attempt to do this, we observe that the thumbs fingers do not lie on top of one another. We say that they are non-superimposible upon one another. Since they are not the same and yet are mirror images of each other, they are said to exhibit chirality. Any object whose mirror image is not identical to it (in other words non-superimposible) will be chiral. Other examples besides our left and right hand would include ears, shoes (have you ever tried to put your right shoe on your left foot?), gloves (tried wearing a right hand glove on the left hand?), and a car. All of these exhibit chirality.

Other objects are said not to exhibit chirality. They have mirror images that can be superimposed upon one another. In other words such an object is identical with its mirror image. A baseball bat, a hammer, most socks all do not exhibit chirality and are referred to as achiral objects.

Molecules like objects are capable of exhibiting chirality. These chiral molecules have certain characteristics which are different from other molecules that do not exhibit chirality. Chiral molecules have the following characteristics:

1. Chiral molecules will have their mirror images non-superimposible as we saw with all chiral objects above
2. Chiral molecules must have no internal plane of symmetry. Molecules that do have an internal plane of symmetry will be able to be split in half and each half will be a mirror image of the other. An example of this is 2-Propanol (Fig 1 below). If we construct a plane that cuts the molecule in half that would run through the Hydrogen atom of the middle carbon and the Hydroxyl group, the two halves would be mirror images of one another (See Fig 1 below). Such a molecule would be achiral not capable of exhibiting chirality.

   

   On the other hand, 2-Butanol does not have an internal plane of symmetry(See Fig 2 below). Constructing a plane that runs through the molecule at any part of the molecule will not result in the two halves being mirror images of one another. Such a molecule such as 2-Butanol is said to be chiral. If we construct the mirror image of 2-Butanol, and then tried to superimpose it on each other it cannot be done. They are non-superimposible mirror images and constitute an enantiomeric pair.

   

3. Chiral molecules will have one stereogenic center carbon. A stereogenic center carbon is a carbon that has four non-equivalent groups bonded to it. In our example above with 2-Propanol (See Fig 1 above), there are no stereogenic centers in the molecule. Each carbon has less than four non-equivalent groups bonded to it. The two end carbons each have three Hydrogens each of which are equivalent (the same). The middle carbon has two Hydrogens besides the two methyl groups bonded to it. Since 2-Propanol has no stereogenic center carbon, it is not expected to exhibit chirality. It is achiral.

   On the other hand, 2-Butanol (See Fig 2 above) does have a stereogenic center carbon which is the C-2 carbon (carbon #2). The number 2 carbon will have a Hydrogen, a Hydroxyl, a methyl, and an ethyl group bonded to it. Such a molecule would be chiral possessing no internal plane of symmetry and having a non-superimposible mirror image. Its mirror image and itself would constitute an enantiomeric pair.

   Let's give you an opportunity to determine which of the following compounds will be chiral or achiral. If chiral identify the stereogenic center:

   • 2-Bromopentane
   • 4-Heptanol
   • 2-methyl-1-butanol
   • 1-chloro-2-methylbutane
   • 2-propanol

   Once you have determined which of the above are chiral and have identified its stereogenic center carbon, then you might check for the correct answers

   Return to Stereochemistry menu

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   R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

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   Acknowledgements:

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   Acknowledgements

   Send Comments to R.H. Logan:

   rhl7460@dcccd.edu

   All textual content copyrighted (c) 1997
   R.H. Logan, Instructor of Chemistry, DCCCD
   All Rights reserved
   
   

   Revised: 2/9/2008

   The following are the correct answers:

   

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