Classification of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers

In organic chemistry, we may likewise find ourselves puzzling over questions like, “how are these two (or more) molecules related”?  And much like family terminology, remembering the differences between constitutional isomers, stereoisomers, enantiomers, and the like can be difficult at first.

In this article, we try to show how to answer questions such as:

• Are these two molecules isomers? (and what are isomers, ayway?)
• Are these two isomers constitutional isomers or stereoisomers (and what’s the difference?)
• Are these two stereoisomers enantiomers or diastereomers (and what does that mean?) 

Thankfully the answer to each of these questions is very clear cut, and I hope that you will find that with practice (and a few vivid examples) are easier to remember than the whole third-cousin versus second-cousin-once-removed thing.

Where We Begin By Stating The Obvious

A molecule can be several types of isomer at the same time, depending on which molecule you are comparing it to. 

To use our family analogy: the terms “brother”, “sister”, “mother”, “daughter” are words that describe relationships between (at least) two people. You can be a daughter (to your mom), a sister (to your brother), a cousin (to your aunt & uncles’ children), and “not related” (to me) all at the same time.

To ask whether you are a daughter OR a sister makes no sense without the context of including the person “to WHOM” you share that relationship.

So it is with molecules. A molecule can be a constitutional isomer, diastereomer, enantiomer, and more (or none!), all at the same time to different molecules, depending on which other molecule(s) you are comparing it to. 

There are three important distinctions to learn, and we will go through them each in turn.

• A given pair of molecules can be isomers OR non-isomers
• A given pair of isomers can be constitutional isomers OR stereoisomers
• A given pair of stereoisomers can be enantiomers OR diastereomers

(on exams especially, there’s always the possibility that a “given pair of molecules” is actually the same molecule, drawn differently. We’ll cover that possibility too).

The flowchart maps out like this:

One key difference between families and molecules:

Through circumstances I will leave to the reader to figure out, it is possible for someone to simultaneously be both a father and a brother to the same individual.

Thankfully, we have no such problems in organic chemistry. Two molecules might be stereoisomers of each other, but they can’t be stereoisomers and constitutional isomers of each other. The distinctions are clear.

Case 1: Non-Isomers vs. Isomers

Isomers are two (or more) molecules that share the same molecular formula. 

For some molecular formulae, no isomers exist.   For example, there is only one possible isomer for CH₄ (methane), C₂H₆ (ethane) and propane (C₃H₈), and only two are possible for C₄H₁₀ (2-methylpropane and n-butane).

As the number of carbon atoms increases, however, so does the number of possible isomers.  For dodecane (C₁₂H₂₆), 355 isomers are possible. And it only goes up from there!

Despite sharing the same molecular formulae, isomers may have very different physical properties, such as boiling point, melting point, and chemical reactivity.

Take cyclohexane (b.p. 63 °C) and 1-hexene (80 °C) which both have the molecular formula C₆H₁₂. No matter how different their physical properties, or reactivities, their common molecular formula makes them isomers of each other.

Likewise, propionic acid and 1-hydroxy-2-propanone share the same molecular formula, C₃H₆O₂, making them isomers of each other (but not isomers of cyclohexane or 1-hexene, of course!).

This leads us to the next question. Let’s say that two given molecules are isomers. What kind of isomer are they?

Case 2. Constitutional Isomers vs. Stereoisomers

Isomers divide neatly in to two categories: constitutional isomers (different connectivity) and stereoisomers (same connectivity, different arrangement in space). So what does that actually mean?

Constitutional Isomers: Different Connectivities

Constitutional isomers have the same molecular formula, but different connectivities.

The same parts, but arranged in different ways. 

Constitutional isomers have the same empirical formulae but their core IUPAC names are different.

Stereoisomers: Different Arrangement In Space

There is only one way to connect C₆H₁₂together to form cyclohexane, and only one way to connect the same atoms together to get 1-hexene.

But there are two ways to connect C₆H₁₂to give molecules with the names 2-hexene, and 3-methyl-1-pentene! And four ways to connect C₆H₁₂ to give 1-ethyl-2-methylcyclopropane!