Constitutional Isomers

Constitutional isomers, also called structural isomers, are compounds that share the same molecular formula but have different atomic connectivity.


What exactly is a constitutional isomer? Constitutional isomers are basically the anagrams of O-Chem. Let’s say you’re given the molecular formula C5H12. Ignoring stereochemistry, how many different compounds can we draw using bondline? 


Pentane isomers

We can actually only draw three. Notice that all we’re really doing with carbon-only compounds is changing the branching patterns. If we try to draw any others, we’ll get duplicates or have the wrong number of atoms. Let’s see how different connectivity patterns can give us different functional groups. Let’s draw all the structures for C4H10O: 


Butanol isomers

Check it out! There are two alcohols and two ethers. That difference in bonding pattern results in the alcohol compounds’ relatively high acidity relative to the ethers. Unfortunately, there’s no systematic way to tell exactly how many constitutional isomers exist just based on a chemical formula. 

When drawing isomers of a molecule, chemical formula is key! An easy way to tell the chemical formula of a molecule is to check for rings and pi-bonds. Basically, a ring means that the molecule has one degree of unsaturation, and each pi-bond also means that the molecule has one degree of unsaturation. Check out my post or the video above for a refresher on Index of Hydrogen Deficiency.

Constitutional isomers vs conformational isomers: 

Constitutional isomers have different atomic connectivity irrespective of bond rotation, and conformational isomers have the same atomic connectivity and different bond rotation. Let’s see if we can tell the difference: 


Constitutional vs conformational

In the top pair, the only thing that has taken place is a flip in chair conformation. Notice that the alcohol and chlorine are still facing the same directions after the flip. These are conformational isomers. The bottom two, however, have the same chemical formula but different connectivity. There is an extra carbon between the alcohol and chlorine in the compound on the right. These are constitutional isomers. 

So, that’s it for this overview of constitutional isomers! Remember that you might be asked to distinguish between constitutional isomers vs. stereoisomers, so check out my videos to learn how to do that. Good luck studying.