Topicity in Stereochemistry: Relationships & Examples

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

How can molecules be different from one another in their spatial orientation? In this lesson we will be learning about a stereochemical subject known as topicity by studying its definition and how the relationships between molecules are defined by using specific examples.

Relationships and Molecules

Are you one of those people that has a large family? When you go to family gatherings along with friends a lot of times the conversations come up of how you're related to everyone. For example, maybe you are trying to point out all your relatives to a friend. You mention that the young boy eating at the table is your younger brother, and that the the female playing cards in the kitchen is your second cousin.

Did you know that atoms within organic compounds can have relationships too, and be thought of as relatives of one another? Although we don't think about them in terms of brothers or cousins, the atoms within a molecule still have defined relationships to each other. This is the focal point of our current lesson, which deals with the concept of topicity relationships in stereochemistry. Let's discover how this subject works by looking carefully at its definition and then examining the different relationships.

Definition of Topicity

The subfield of stereochemistry is the branch of chemistry that's concerned with the three-dimensional or spatial orientation of compounds. Under the umbrella of stereochemistry, topicity seeks to define the specific relationship between atoms or groups of atoms (substituents) that are bonded to a molecule. It's very similar to how you define the relatives in your family based on their position in the family tree.

Topicity Relationships

The topicity of two or more substituents is classified as one of four different relationships, which we'll be looking at next. The four possible relationships are called homotopic, heterotopic, enantiotopic, and diastereotopic.

Homotopic Relationships

If two substituents within a molecule are equivalent (same) they are said to be homotopic. It would be similar to two siblings in a family being identical twins. A good example of this relationship would be the hydrogen atoms of methane. The hydrogens in methane are homotopic because they are all the same as one another, no matter how we manipulate the molecule.

The hydrogen atoms in methane all have a homotopic relationship to one another

Heterotopic Relationships

Let's say that we had two aromatic compounds and they differed in the way that the hydrogens were bonded to the benzene ring. Since the two hydrogens labeled in red are at different positions on the ring, these two hydrogens are heterotopic to one another. These are examples of constitutional isomers (same chemical formula, different atom connectivity).

An example of heterotopic hydrogens and constitutional isomers

Enantiotopic Relationships

Now let's consider a compound in which two hydrogens (labeled in red and blue) are bonded to a carbon that contains an alcohol group (-OH) and an aromatic ring (Ph group). Imagine if we replaced the red hydrogen with a generic atom, call it 'X' for instance. What would happen if we did the same thing for the blue hydrogen? What would be the relationship between the two modified compounds?

An example of enantiotopic hydrogen atoms

These two compounds would become enantiomers which are non-superimposable mirror images. For that reason, we would say that the two hydrogen atoms (blue and red) in the original molecule would be enantiotopic to one another. This type of analysis is sometimes referred to as a replacement test because it allows us to replace hydrogens on a molecule to determine their relationship relative to one another.

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