What is a constitutional isomer? [with a nice chart]

What is a constitutional isomer?

HINT: It is the same thing as a structural isomer and configurational isomer.

Are you wondering if you have a constitutional isomer? There are two questions you need to ask yourself before we can answer this question. The first question: do the compounds have the same molecular formula? If they do not have the same molecular formula, then they are different compounds and not isomers. If they do have the same molecular formula, then they are isomers. The second question: what type of isomer are they? If all of the bonds and atoms of the two compounds you were looking at are in the same order, then they are stereoisomers. (There are all sorts of different types of stereoisomers, but we’ll deal with that later).  If they have different connectivity, meaning that the atoms and bonds are in a different order from each other, then you have constitutional isomers.

So, to answer the question what are constitutional isomers? They are compounds with the same molecular formula BUT different connectivity.  Let’s look at some examples of constitutional isomers.  Here is a typical problem you might see on an exam: draw all the constitutional isomers of C4H11Cl.

constitutional isomers

You can see here that all we are doing is putting different parts in a different arrangement.  First, we start with straight chain butane and put the chlorine in two different positions. Next, we start rearranging the carbon backbone, after each change pausing to move the chlorine around.  In this example, there is only one carbon backbone change that can take place, so we make that change, move the chlorine and have the right answer.  BEWARE: because this is a short chain, chlorine can only be in the 1 or 2 position….there is no 3 or 4 position since we also have to name it to have the lowest number possible.

This is 1-chloro, 2-methylpropane NOT 2-methyl, 3-chloroproane.

Professor Trick: are these compounds constitutional isomers?

The answer is no because they are the same molecule.

Professor Trick: Are these compounds constitutional isomers?

The answer is no because they have different formulas, which makes them different molecules.

Dr. Michael Pa got a bachelors degree in chemistry from Binghamton University, a masters degree in organic chemistry from the University of Arizona and a Ph.D. in organic chemistry from the University of Arizona. His research focus was on novel pain killers which were more potent than morphine but designed to have fewer side effects. There may even be a patent or two that came out of it. Prior to all of this, he was a chemist at Procter and Gamble. After all of that, he (briefly) worked as a post-doctoral assistant at Syracuse University, working on novel organic light-emitting diodes (OLEDs). In between, he did NOT compete at the 1996 Olympics, make the Atlanta Braves opening day roster, or become the head coach of the Indiana Pacers, as he had intended. #fail During this entire time, he always loved helping students, especially if they were struggling with organic chemistry. In 2006, Dr. Pa founded AceOrganicChem.com in order to make learning organic chemistry fast and easy. 14 years and about 60,000 students later we are still helping students to learn organic chemistry one reaction at a time at https://www.aceorganicchem.com