A chemist made what was thought to be compound A or B. How cou...

Question

A chemist made what was thought to be compound A or B. How could coupling constants between H_a and H_b be used to distinguish between the two isomers? [Hint: Draw a chair conformation of each.]

Chair conformations of two isomers with labeled hydrogen atoms Ha and Hb for NMR coupling constant analysis.

Accepted Answer

All right. Hi everyone. So this question is asking us to use the coupling constant for H A and HB to distinguish between the following two isomers hint use chair confirmations of both molecules. So here we've got two different isomers of one turtle, two methyl cyclohexane. Now if we go ahead and compare both H A and HB in both of them in molecule A right or confirmation A both H A and HB are trans to each other, right? They are on opposite sides of the ring. Whereas in isomer B here H A and HB are cyst, they're on the same side. Now, that bit of information is crucial, right? Because recall that the coupling constant or in other words, the J value of two different coupled hydrogens depends on how those two hydrogens are oriented relative to each other. So it's important to note whether they are trans versus CS. Now, given that here, we've got two isomers of a substituted cyclohexane ring. Like the question is mentioning our best bet is always to draw a chair confirmation first because that way we can or we can understand better, excuse me, how these hydrogens are oriented relative to each other and we can better understand the dihedral angle between them. So here, right, it's important to also mention that you should always draw the most stable confirmation chair confirmation to be specific of whichever isomer you are considering. So if I go ahead and I draw the chair confirmation for isomer a here recall that there is such a thing as an equatorial preference. So the most stable chair confirmation is going to have both my church Beto group and my metal group in the equatorial positions. So when I do that right, I end up with my cyclo heine ring, first of all, because that's pretty important and that looks a little off. So I'm going to fix it. OK. So here is the foundation of the Cyclohexane ring, which definitely looks a little better than last time. And like I said before, right, both my tribual group and my meal group should be in the equatorial position in order to ensure that I have the most stable confirmation possible. Now, in addition to whether or not it's Axio versus equatorial, you have to consider the direction they're pointing in, right? Because our hydrogens H A and HB are trans, which means that our T group and our methyl group are also going to be trans, right? So here my budo group is depicted as a wedge, meaning it must be pointing upwards. So here I'm going to draw my torpedo group in a position that is both equatorial and pointing upwards. Now, as a consequence of that H A which is attached to the carbon that has Mittal group must be in the axial position. So therefore, right on the next carbon over carbon two, in this case, my methyl group is going to be equatorial, but it's going to be pointing downwards. And as a consequence of that, right hb on the same carbon is going to be axial but pointing upwards. So now let's go ahead and apply the same principle to ISOM or B over here. So the first one I drew is A and I'm going to move the label out of the way just to give myself a bit of space on the bottom of the screen. But first, I'm going to go ahead and draw the base of my cycle hexing ring which at this point, I sincerely hope has improved a bit in appearance. Alright. But now here's the thing because in isomer B, like we said previously, H A and HB were SIS and as a consequence of that, our tubule group and our Meho group are also sis and in this case, they're both pointing upwards because they're depicted as a wedge. Now. Well, ideally, right, equatorial preference states that ideally all of your substituent would be in the equatorial position. However, because both groups have to be pointing upwards in this scenario, they can't both be in the equatorial position. Therefore, because the total group is larger, the tubule group should occupy the equatorial position preferably, right. So in isomer B, my terror group is also going to be axial or equatorial, excuse me pointing upwards and H A as a consequence of that is going to be axial pointing downwards. Now, my Meho group in accordance to how Isomer B has been drawn must be pointing upwards, right. So my methyl group on the next carbon over must be in the axial position because that is how we can depict it pointing upwards. So as a consequence of that right HB is going to be equatorial and pointing downwards. So always make sure that your constituents are occupying the same face or opposite side, same side or opposite side of the ring as depicted in your initial drawing. But all this is to say right that at the very least now we can determine the dihedral angle between H A and HB in both isomers. So if we look at H A right H A and HB are both axial and pointing in opposite directions, therefore, the dihedral angle is going to be 180 degrees. Now recall that there is a chart of the most common hydrogen hydrogen interactions with their associated J values. And when we have two hydrogens coupled in a cyclohexane ring, and their dihedral angle is 180 degrees. Recall that their J value is around 15, it's around 15 for an axial axial interaction. Now, molecule B, on the other hand, molecule B on the other hand has a dihedral angle of about 60 degrees, right? Because H A is Axio and HB is equatorial. So with an Axio equatorial interaction, like the one in isomer B recall that the J value there is going to be around four. So that is how we can distinguish between the two isomers using only the coupling constant. Right. At long last, we made it. So essentially is number A would have a coupling constant of around 15 between H A and HB. Whereas isomer B would have a coupling constant of about 44 hnhb due to the orientation of the two hydrogens with respect to each other. So if you stuck around to the end, I thank you very much for watching and I hope you found this helpful.

A chemist made what was thought to be compound A or B. How could coupling constants between H_a and H_b be used to distinguish between the two isomers? [Hint: Draw a chair conformation of each.]

Key Concepts

Coupling Constants

Chair Conformation

Axial and Equatorial Positions

Watch next

A chemist made what was thought to be compound A or B. How could coupling constants between Ha and Hb be used to distinguish between the two isomers? [Hint: Draw a chair conformation of each.]

Key Concepts

Coupling Constants

Chair Conformation

Axial and Equatorial Positions

Watch next

A chemist made what was thought to be compound A or B. How could coupling constants between H_a and H_b be used to distinguish between the two isomers? [Hint: Draw a chair conformation of each.]