Explain isomerism: State any 4 characteristics of isomerism.

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jchumzrz eNotes educator| Certified Educator

Here are four characteristics of isomerism, starting with the general definition of what isomerism is:

  1. The word isomerism is used in organic chemistry to refer to when multiple compounds with the same molecular formula have different physical forms due to the spatial arrangement of their atoms. This means that two compounds can be made up of the same number of atoms of the same elements, but that the atoms in each compound can be arranged in distinct ways that give rise to some notable differences. There are several types of isomerism that can take place in organic chemistry.
  2. Structural isomerism is the easiest type of isomerism to understand. It arises due to the difference in connectivity of the component atoms in each compound. An example of two structural isomers are the alcohols 1-propanol and isopropyl alcohol. Both alcohols have the formula C3H8O, but in 1-propanol the hydroxyl group is attached to an end carbon and in isopropyl alcohol the hydroxyl group is attached to the central carbon (see the third link in the references for an image of these isomers).
  3. Another type of isomerism is stereoisomerism, which is usually divided into two main subcategories. The first subcategory is called geometric (or cis/trans) isomerism. In organic chemistry, this type of isomerism almost always arises due to restriction of rotation in a molecule from a double bond (often a carbon-carbon double bond). Once a double bond is placed in certain molecules, the variation in orientation of atoms on either side of the double bond gives rise to isomers rather than different rotational configurations of the same compound (due to restriction of rotation). An example of geometric stereoisomerism is 1,2-dichloroethene. The two chlorine groups can be on the same side of the plane formed by the double bond, resulting in cis-1,2-dichloroethene, or they can be on opposite sides resulting in trans-1,2-dichlorethene. Although it is easy to mix them up in simplified skeletal depictions, these two compounds are not identical (click "Geometric Isomerism" in the first reference below for more examples and visuals).
  4. Optical isomerism is the second type of stereoisomerism. It is perhaps the hardest to understand and recognize. This type of isomerism is also sometimes called "Chirality," which is derived from the Greek word for hand. This is because hands are a good example of how optical isomerism works. If you hold up both of your hands in front of your face to look at them, you will quickly notice that the left and right hands are mirror images of one another. In terms of spatial orientation, however, your left and right hands are not identical. The proof of this is that you cannot rotate your right hand in space such that it could lie on top of your left hand with all of the fingers and thumbs lined up. This is how chirality works on a molecular level; two molecules are isomers (and not identical) because they are mirror reflections of each other across a plane. An example of optical isomerism is 2-butanol, which has two different forms that are mirror images of one another. Chirality is very important in biochemistry and medicine because sometimes one optical isomer is therapeutic and the other is deadly. On top of that, optical isomers have very similar physical properties and are hard to separate. For more examples and depictions of optical isomers, click "Optical Isomerism" in the first reference link below.