Topicity EtOH & Definitions (webpage)

From WikidChem

Topicity_etoh_&_definitions_(webpage) This slide discusses stereotopicity—the arrangement of one atom with respect to another atom within a molecule. During this slide, Professor McBride demonstrated stereotopicity using an ethanol molecule. Specifically, he looked at how the five hydrogen sites relate to one another.

Note-it might be helpful to build the ethanol molecule and attempt to follow along, especially if you were not in class.

It is important to remember the four layers of molecular structure: composition, constitution, configuration, and conformation. Stereotopicity deals with the middle two. Any two hydrogen atoms attached to the ethanol carbon without the alcohol functional group are constitutionally and configurationally homotopic. This is because there is free rotation around the single bond between the two carbons. If the carbon atom is rotated around the bond's axis so that one hydrogen atom is in the other’s location, the atom does not change in any noticeable way-thus it is homotopic.

However, if one compares the two hydrogens attached to the alcohol-group carbon, the result is different. If the carbon atom is rotated so that one hydrogen is in the location held previously by the other hydrogen, the spatial location of the OH group has moved—and the molecule, while constitutionally homotopic (the atoms and the bonds have not changed, and the same atoms are still connected by the same bonds), is configurationally heterotopic (because the arrangement of all the atoms in space has changed) The difference might seem subtle, so it might be useful to build an ethanol molecule and see for yourself.

In order to talk about enantiotopicity and diastereotopicity, it is useful to assume for a moment that the single bond between the carbons of ethanol cannot be rotated. Now, if two hydrogens on the carbon atom without the OH group are compared to each other, they are configurationally heterotopic, because they differ in position in space to the OH functional group on the other carbon. How can you determine if the two hydrogens are diastereotopic or enantiotopic? Picture two molecules-one where one of the hydrogens has been replaced by fluorine, and one where the other hydrogen has been replaced by fluorine. If the two molecules are mirror images of each other, then the hydrogens are enantiotopic, but if the two molecules are not mirror images, then the two hydrogens are diastereotopic.

• Imagining the H in question to be replaced by F is a sensible device. Another way to think about it is just to remove the H in question (but not change the geometry of the atom to which it was attached). This way one is considering the place ("topos") that the H occupies. - JMM

While stereotopicity specifically compares the location of one atom to another atom in a molecule, the entire molecule must be considered. For instance, two hydrogen atoms might be connected to an achiral carbon, but if they are different in relation to other atoms far away in the molecule, they are heterotopic.