Isomers are compounds which have the same molecular formula (i.e. the same number of carbons, hydrogens and oxygens etc.) but the arrangement of atoms in space differs. This different arrangement of atoms can lead to significantly different chemical properties of a compound for example ethanol, C2H6O has a boiling point of 78 °C whereas dimethyl ether, an isomer of ethanol and also having molecular formula C2H6O has a boiling point of –25 °C. This is known as structural isomerism and therefore ethanol and dimethyl ether are structural isomers of each other.
In addition to knowing the molecular formula and which isomer we have, we also need to consider how the atoms in the molecule are spatially arranged because molecules are not flat. The arrangement of the atoms in space can have a significant effect on the properties of the compound for example its boiling point or physical characteristics. Fumaric acid (C4H4O4) has a melting point of 299–300 °C, whereas maleic acid (C4H4O4) has a melting point of 140–142 °C. The only difference is how the substituents are arranged on either side of the double bond (i.e. are they on the same side or on different sides). Isomers which have identical constitution but are only differing in the arrangement of their atoms in space are called stereoisomers. Stereoisomers which are mirror images of each other are known as chiral compounds and stereoisomers which are not mirror images of each other as diastereoisomers.
Chiral compounds are important as chirality is present in nature. If a compound is chiral it has a non-superimposable mirror image. Hands and feet as examples of chiral things; if you try and put your left hand in your right glove it will not fit. One everyday example of chirality is the smell of oranges and lemons. Both the molecules responsible for the smell have the same molecular formula and arrangement of atoms in 2-dimensional space, but in 3-dimensional space they are quite different.
Chemists have only recently started to have an appreciation of the importance of 3-dimensional spatial arrangement of atoms. One famous incident was the development of the anti morning-sickness drug Thalidomide, which was marketed in 1957. When it was created it was a mixture of enantiomers; one enantiomer cured morning sickness whereas the other produced birth defects. Nowadays, all drugs which are marketed have to be subjected to rigorous testing to determine exactly which diastereomer (or enantiomer depending if it is the mirror image or not) is active and reactions need to be developed which ensure that only the desired compound is developed, which was the aim of this project.
The project involved taking a known compound containing atoms for which we already knew the spatial arrangement and then reacting it with another compound to generate one of two products. We tried to bias the reaction towards making one product over another by using different additives (to help the reaction run faster) and running the reaction at a range of temperatures. The project was successful with us being able to bias the reaction more towards one product than another, therefore allowing us to choose which product we would like to have from a reaction. This is important because it allows chemists to make bespoke molecules for specific uses and, in the case of medicinal chemistry, reduces the potential risk of harmful side-effects caused by having a mixture of molecules with different spatial arrangements, which was the problem with Thalidomide.
Robert Arnold