Ch. 1 - A Review of General ChemistryWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

“How are the following two molecules related to each other?”

Does this sound familiar? This is one of the most important questions you will have to answer in Organic Chemistry 1. 

Definition of Constitutional Isomers

Concept #1: What is a constitutional isomer?


So, what are constitutional isomers? They're compounds that have the same exact molecular formulas meaning that both the compound will have all the same atoms in them but what is different about them is their conductivity so go ahead and write that down, conductivity what does that mean? Conductivity has to do with the way the atoms are bonded to each other so for example if my first molecule has atoms A linked to B linked to A or bonded to A, B and C, my second molecule might have the same atoms A, B and C but they're arranged differently so it will be A is connected to C and then C is connected to B that would be an example of a constitutional isomer. For this course what you're going to have to be able to do is you're going to have to be able to look at two different compounds and tell what is that relationship? Are they the same compound? Meaning that everything is exactly the same in terms of the molecular formula and the conductivity or are they completely different compounds meaning that they have different atoms entirely? And then there's this other category which is are they constitutional isomers? Which means that they have the same atoms but they're connected differently and it can be really tricky to differentiate which one is which and that's why I made this nice little flow chart for you guys to follow, alright?

Constitutional isomers are molecules with identical atoms but different connectivity

Constitutional Isomer Flowchart

Concept #2: Using the flowchart to determine isometric relationships.


So, imagine that you're given this question right here, this is in the exam how are the following two compounds related? Are they identical, constitutional, isomers or different compounds? where do you even begin? Well it turns out that this question is I think a little beyond your level, all you guys are probably a little bit stumped by this question you might think that you know the right answer and you might be right but most likely didn't get to the answer in a systematic way so what I want to teach you guys the systematic way to get these problems right every single time, alright? So, we're going to come up back up to that question what I actually want to do is go through these steps right here and these steps will guide you through constitutional isomer questions, OK? And notice that it uses IHD that's why I had to teach you the IHD first because that's the easiest way to tell different between different types of compounds so my first question that I have to have to ask myself makes sense it's is just are all the atoms the same, OK? Because remember I told you that in order to be a constitutional isomer all atoms have to be exactly the same so the way we tell a lot of students what they do is that they start counting every single atom all in the entire molecule and they compared to the other one and then obviously they see if they're the same or not but what I would recommend is don't count the hydrogens, OK? Count only non-hydrogen atoms and then count the IHB in both compounds the reason why is because IHD is actually just a measurement of hydrogen, remember that IHD would tell you how many hydrogen atoms are missing so if and IHD happens to be much easier to calculate than number of hydrogens you know why? Because if you have to calculate number of hydrogen that means you're looking at a bond line structure you're going to have to first put all the hydrogens in there and then you're going to have so many hydrogen that you might miscount and that happens all the time with students so what I prefer is hey instead of counting each out at a time just use the IHD instead, the IHD is a placeholder for number of hydrogens so now check this out if those two numbers are not exactly the same so if I have a different amount of non-hydrogen atoms or a different amount IHD in both the atoms in the both of the molecules then these are going to be different compounds, why are they different compounds? Well because if they don't have the same atoms then they're just automatically different, OK? A compound can only be the same as another compound if it has the same exact atoms inside it, OK? So that would be the answer for this question up here if we found that they had different amounts of carbon which is a non-hydrogen atom or different amounts of IHD but let's keep going and see what happens, if they happen to line up meaning that the non-hydrogen atoms and IHD are exactly the same and both then we go to step two, OK? What is step two? Well step two makes sense as well now I'm going to ask myself are all the atoms connected exactly the same way, OK? Now it turns out that most of the time this is going to be a very easy question to answer, most of the time you're going to see your two compounds and they're going to look very different for example, one of them is a square and one of them is a four-carbon chain would that be the same thing? Would it be the same thing about a square on one side and a four-carbon chain on the other side would you say they're connected the same way? Absolutely not, they were completely different the only way that they can be connected the same is if every atom is connected to the same atom on the other molecule, alright? So, most of the time common sense is just going to tell me yes or no, either they look the same or they don't but sometimes you get a situation like up here where these two molecules above me they look like they are kind of similar but maybe they're rotated I don't know, OK? So, for this one we're going to want to use a systematic method, OK? And what I always say is to look for what I call a landmark atom, OK? Now this is not a word they're going to find in your book so don't look for Landmark atom it isn't anything it's just a word that I used to say look for something that stands out, OK? So, you look for something that stands out that you can compare to everything that you can compare and both molecules, OK? So, for example maybe if your compound has an oxygen in it then you look at where the oxygen is in the first one and where the oxygen is in the second one and see are they in the same place, if your oxygen has I mean if your compound has something else like a ring in it maybe you look at the ring and you say is the ring in the same place in both, OK? If they're not exactly the same then that means that you have constitutional isomers, why? Because that means that we got to step two which means all the atoms and the IHD was the same but then step two what we said is that they are not exactly the same in the way they're connected so that's constitutional isomers that's the definition of a constitutional isomer, OK? What if they are the same though? So, let's say that means that now all atoms are the same and they're connected exactly the same though those are just going to be identical compounds, does that make sense? And those are my three options.

Example #1: How are the following two compounds related?

Are they different molecules, constitutional isomers, or identical? 

Pair A

Pair B

Example #2: How are the following two compounds related?

Is there a choice we can rule out immediately? 

Example #3: How are the following two compounds related?

So now you should have a pretty good idea of how to solve questions asking for molecular relationship.