Chemical Formulas, Structures and Constitutional Isomers
In the General chemistry courses, when solving different problems like the ones in stoichiometry and gas laws, we often use the following notations for representing organic molecules:
CH4, C3H8, C6H6, C4H10O
These are called chemical formulas which show us the composition of the molecule, i.e. what atoms and how many there are in the molecule but they don’t tell us anything about the structure of the molecule. They do not show how the atoms are connected.
For example, butane, represented as C4H10, can have several structures that satisfy the chemical formula:
Both have four carbons and ten hydrogens but they are connected differently and they are said to be constitutional or structural isomers.
Another example can be the formula C2H6O that would work for ethanol-the drinking alcohol or dimethyl ether:
Having the same molecular mass, these two have completely different physical and chemical properties:
So, keep in mind, the connectivity of atoms is crucial in defining the properties of a molecule. This is particularly relevant and obvious in Organic Chemistry as, essentially, there are no limits to connecting carbon atoms differently and synthesizing new molecules. This is why a large part of your organic chemistry class is going to be learning how to distinguish between a group of molecules with similar structures, but yet so much different properties.
To summarize and formulate:
Constitutional (structural) isomers are compounds with the same formula but different connectivity.
Below are a few more examples of constitutional isomers:
Constitutional isomers and relationship of molecules:
You might be asked in your test to determine whether a pair of molecules are constitutional isomers or the same compound or maybe even another type of isomers like stereoisomers. For these types of questions, the first thing is keeping in mind the definition for constitutional isomers – they have to have the same chemical formula without which it is irrelevant to discuss any possibility of isomerism.
The second and trickier part is keeping in mind that molecules are free to move around (and they do!) and just because we draw them in one way, doesn’t mean that is the only way of showing an accurate structure for that compound.
For example; what is the relationship between the following two molecules?
They may look different, but they both represent the same compound. It is as simple as remembering the analogy with a marker. Just because we flip it upside down, doesn’t mean we are getting a different marker:
Another important factor to remember for the entire duration of the organic chemistry class is that there is a free rotation around the single bonds and because of this the same molecule can adopt different conformations. You can compare this to the following representation:
Same goes for a molecule; unless there is a bond(s) broken during any process, we are working with the same molecule. They are different conformations resulting from rotation around a single bond(s) but because this rotation and interconversion happen very fast, most often we consider them as being the same compound.
For example, what is the relationship between these two compounds?
They might look like different molecules, but all that happened is a rotation around Ca-Cb bond:
What is the best way to determine if molecules are constitutional isomers?
The easiest way of determining if molecules are constitutional isomers is to quickly count the number of carbons and the degree of unsaturation (Hydrogen Deficiency Index). If all the atoms are the same and molecules have the same HDI, then they are constitutional isomers.
However, to be absolutely sure, especially for large molecules, we need to name the molecules according to the IUPAC nomenclature rules. We will cover this later on, but for now, try to follow the principles and keep in mind the little tricks to better visualize organic molecules.
Check out this 60-question, Multiple-Choice Quiz with a 2-hour Video Solution covering Lewis Structures, Resonance structures, Localized and Delocalized Lone Pairs, Bond-line structures, Functional Groups, Formal Charges, Curved Arrows, and Constitutional Isomers.
Molecular Representations Quiz
For question 2 with OH, are you saying none of them are const isomers? I though B,D, and F were
I mean B and D
Yes, B and D are constitutional isomers and so is A since they all have the same number of identical atoms and the same degree of HDI i.e. no ring or multiple bonds.
F, on the other hand, is the same molecule as A flipped by 180o like the markers in the mentioned in the post.
There are a lot more of these in the “Molecular Representations” quiz.
can you please clarify why D would be a constitutional isomer?
Constitutional isomers can have the same or different functional groups. As long as the molecular formula is the same (C3H8O in this case) but the atomic connectivity is different, they would be classified as constitutional isomers. For example, A and B have the same functional group (OH) located at different points on the carbon skeleton. On the other hand, D is an ether and therefore, the functional group is different. However, because it has the same molecular formula, it is still a constitutional isomer to the alcohols.