The subdiscipline of chemistry that studies this is called stereochemistry. As we progress through this blog, the differences in chemical properties of the stereoisomers will become increasingly apparent. This will have very profound impacts on the uses and applications of chemical compounds, particularly organic compounds. Amongst these are compounds of pharmaceutical importance, whose stereochemistry could determine the fate of the patients. A particular subfield that deals with this is called pharmaceutical stereochemistry, which we will get to learn more about in no time.
Let us return to general stereochemistry, given the numerous ways in which stereoisomers can be formed, there are also many ways to classify stereoisomers into different categories. Keep in mind that in a group of stereoisomers, more than one of these categories could apply. In fact, some of these classifications have overlaps, so it is best if we do not think of these classifications as mutually exclusive groups, but rather different ways to classify isomers [2].
The first type of classification is based on interconvertibility of the isomers amongst themselves: conformational and configurational isomers. Conformational stereoisomers (or conformers for short) are stereoisomers whose difference in spatial arrangement of their atoms are due to rotation about carbon-carbon (C–C) single bonds. Most conformers can easily interconvert to one another amongst themselves at room temperature. Configurational isomers, one the other hand, do not usually interconvert at room temperature since this will involve breaking and reforming of bonds [1].
A quick example would be to look at molecules (a), (b), and (c). Molecules (a) and (b) can interconvert between each other by simply rotating the carbon single bond. However, molecules (a) and (b) cannot convert to molecule (c) without breaking the carbon-substituent bonds and rearranging the substituents. Hence, molecules (a) and (b) are conformers, while molecule (c) is a configurational isomer to molecules (a) and (b).
The second type of classification is based on mirror images. If a molecule is non-superimposable on its mirror image, the molecule and its mirror image are called enantiomers. This only applies to a very specific type of molecules called chiral molecules, which neither have a plane of symmetry nor a center of symmetry. Enantiomers can interact with plane polarized light. If an enantiomer rotates plane polarized light by a specific angle in a clockwise direction, its mirror image rotates plane polarized light by that same angle in an anticlockwise direction [3].
Chiral molecules often have an asymmetric carbon atom (also known as chiral carbon) which is a carbon atom that is attached to four different types of atoms or groups of atoms. In a molecule with multiple chiral carbons, it is possible to have up to 2n stereoisomers [8]. Stereoisomers which share the same configurations on at least one (but not all) chiral carbon and are not mirror images of each other are called diastereomers. Diastereomers whose configurations on all, but one, chiral carbons are the same have a special name, epimers. Some of you may have noticed that in fact, all conformers are diastereomers, but not all diastereomers are conformers.
Amongst diastereomers that are configurational isomers, we have cis-trans isomers which are isomers of at least di-substituted compounds whose functional groups may be on the same side (cis) or the opposite sides of the carbon chain (trans). While the term is often used on molecules with one or more double bonds, cis-trans isomerism can apply to molecules with only single bonds too [6, 7]. For molecules with alkene group, an alternative convention called E-Z isomerism exists to prevent confusion in nomenclature. The details of the convention will be covered later.
Moreover, amongst conformers, there are very specific types of isomers. that are due to hindered rotation about a single bond or hindered inversion of bond angle. Those due to a hindered rotation is called atropisomers, while those due to a hindered inversion of bond angle is called akamptisomers [5]. Details of these two types of stereoisomers will be covered later.
Fun fact: the first akamptisomers were only discovered this year (2018)!
References
1. Burrows, A., Holman, J., Parsons, A., Piling, G., and Price, G. (2017) Isomerism and stereochemistry, in Chemistry³: introducing inorganic, organic and physical chemistry 3rd ed. Oxford University Press. Oxford.
2. Fujita, S. (2016) Classification of stereoisomers. Flowcharts without and with the intermediate concept of RS-stereoisomers for mediating between enantiomers and stereoisomers. Tetrahedron: Asymmetry 27, 43–62.
3. Hardinger, S. Stereochemistry Tutorials: Classification of Isomers. Organic Chemistry at UCLA.
4. Kropp, A. Stereoisomers: Definition, Types & Examples. Study.com. Study.com.
5. Krämer, K. (2018, May 22) First new form of isomerism discovered in 50 years will be the last. Chemistry World. The Royal Society of Chemistry.
6. Nguyen, T., and Clark, J. (2016, December 16) Physical Properties of Alkenes. Chemistry LibreTexts. Libretexts.
7. Patel, E., and Aworanti, I. (2017, November 6) Absolute Configuration: R-S Sequence Rules. Chemistry LibreTexts. Libretexts.
8. Reusch, W. (2013, May 5) Designating the Configuration of Chiral Centers. Stereoisomers.
You must be logged in to post a comment.