Test 3:Disubstituted Cycloalkanes - Online Tutor, Practice Problems & Exam Prep

Chirality tests for disubstituted cycloalkanes include three main types: geminal, para, and middle disubstituted. Geminal disubstituted compounds are always achiral due to identical substituents on the same carbon. Para disubstituted compounds are also achiral if on even-numbered rings. Middle disubstituted compounds depend on the orientation; cis configurations yield meso compounds, while trans configurations are chiral. A notable exception is cis-1,2-cyclohexane, which some argue is chiral due to chair conformation, while others claim it is achiral due to equilibrium between conformations.

Three types of disubstituted cycloalkanes

At the beginning of this lesson, I told you guys that there were going to be 3 tests for chirality. I've already taught you 2 of them. The first was the internal line of symmetry, the second was finding stereo centers, and now I'm going to teach you the third one. And it turns out that the third one is more of a shortcut than a new rule. So, it turns out that this third test has to do only with disubstituted cycloalkanes. Okay. What are those? That just basically means a ring that has 2 identical things coming off of it. Okay? And these happen to be really commonly tested molecules in homework and in exams. So if we can figure out a way to figure these out even quicker, that's going to save us time in the long run. That's going to be smart. So, these are really just shortcuts. What that means is that for all of these, you could use the other two rules, but if we just memorize these shortcuts, it's just going to make it even faster for us when we see these. Okay? So I want to go ahead and talk about them. There's basically 3 different types of rings that we deal with on a regular basis.

There's what I call gem disubstituted. 'Gem' is short for the word, and later on what we're going to learn about the word 'geminal' is that it means 2 things coming off the same carbon. So if you ever see geminal disubstituted, that is always achiral. You don't have to give it a test. You don't need to look for a line of symmetry. Nothing. It's just always going to be achiral. And the reason for that is that there's no chiral center. Okay? If you think about it, both of these R groups here are going to be exactly the same no matter what. So there's no chiral center so this is always going to be achiral. Easy, right?

Then let's go off to the other extreme, which is para disubstituted. Now 'para' is a word that we're going to use a lot more in Organic Chemistry II and what it means is 'across from'. Okay? So, it basically means or opposite to. Wow. That just my handwriting is getting worse and worse. 'Across from' or also 'opposite' something that you can just memorize. You don't have to test it. You can just say, hey, if it's paradisubstituted, if they're across from each other perfectly, then it's also going to be achiral. I don't need to test anything. Now something to keep in mind that you have to be careful about is that this is only possible on even-numbered rings. Okay? The reason is, if you have, let's say, a 5-membered ring, there are no two positions that are perfectly across from each other. This one, ideally, the position that would be across from it would be here, but there's nothing, no position there. So this is only possible on even-numbered rings, like 4-membered, 6-membered, 8-membered, stuff like that. If you have 5 or 7, it's not going to work or 3, it's not going to work. All right. So cool. We've already knocked out 2 huge categories: gem-disubstituted, para-disubstituted. They're always achiral. Okay.

What about in between? In between, we're calling that middle disubstituted. That just means somewhere in the middle between gem and somewhere between para. Basically, somewhere in between 0 degrees or 180 degrees, basically if you're thinking of it as a circle. Well, for these, it's actually going to depend where the groups are facing. If the groups are facing cis, then the answer is always going to be mesoachiral. I already showed you an example of one of these when we did the when I showed you guys the meso compound and I showed you how a ring could be a meso compound. Okay. So if they're cis, it's going to be mesoachiral. If they're trans, then it's always going to be chiral. You don't have to test it. You don't have to do RS, anything like that. Just say it's going to be chiral for sure.

Cis-1,2-Disubstituted Cyclohexane:A controversial exception.

There is one exception, but there's like a huge disclaimer here that I have to give. Okay? And what this is is that this happens to be controversial. And I know that that's a really weird word to use for organic chemistry because it's like organic chemistry is such an old science. Like, how could there be controversy about this? It's like, is this political? Well, it's not like republicans think one thing and democrats think another. It's that some professors tend to be renegades and think their own things versus what the book says. So according to your book, this is actually achiral. Okay. So according to your book, this exception here of cis 1,2-cyclohexane, which I'm going to draw for you, which is actually this right here. Okay? According to your book, this is achiral. Okay? So it would fit into this rule that we have right there about if it's cis 1,2-cyclohexane, then it would be achiral. But according to some professors, it's actually chiral. I've seen that at the same university, 2 professors So let me show you where the controversy lies so that you guys will understand. It turns out that remember that cyclohexane doesn't exist in a planar structure. It actually exists as a chair conformation. So what that means is that I'm just going to draw cis 1,2-cyclohexane. Okay? If I have 2 groups facing up like this, okay, it looks like it has a plane of symmetry. It looks like it has a perfect plane of symmetry, so this should be achiral. But it turns out that what it really looks like is a chair. So what that means is that if they're both facing cis and if they're 1,2 apart from each other, then that means that one of them will always be axial and one of them will always be equatorial. Remember how to figure this out? Basically, they both want to go to the same face of the ring, so and they both want to be cis and that's you one, two, so one of them is going to be axial and one of them is going to be equatorial. Is there a plane of symmetry now? Not really. If you draw it like this, there's no plane of symmetry because if you cut it down the middle, you're going to have 2 different positions. Okay? So the professors who say that it's chiral will go by this argument. We'll say it's chiral because the positions are different. Now the professors that say that it's achiral, which is actually the majority of professors, will point to the fact that this is in equilibrium. So what that means is that even though, sure, one of the chairs is chiro by itself, but it doesn't just exist as one chair. It exists as an equilibrium of 2 chairs. Okay. So what that means is that on average, you always have one axial and one equatorial and they kind of cancel out. So the professors who say that it's achiral point to this equilibrium. Okay? Now what's your job? Your job is if your professor made a big deal out of this in class or even if maybe they never talked about it at all, maybe it would be safe to just go to their office hour and just say, Hey professor, What do you think about 1,2 Cis Cyclohexane? Is that chiral or is that achiral? Because the last thing you want is for this to show up on your test and you don't know which one to pick because you don't know. Now just so you know, either argument is kind of defensible. You could either say you could defend that it's chiral, you could defend that it's achiral with these 2 different arguments. But just so you know, the majority of your book and the majority of people rest in the achiral argument. They say that because of this is in equilibrium, then it's achiral overall even though the structure by itself is chiral. But like I said, I wouldn't be pointing this out unless I had seen professors test the other way and actually say that it's chiral. Okay? So this is going to be kind of up to you guys to just do a little bit of homework with your professor and just say, hey, is this chiral or achiral? And then they can let you know and then you memorize it that way. Are you guys cool with that? Long explanation. Sorry, but I just want to give you the heads up. But other than that, if it's not that very special case, we're going to go with these rules right here, which is cis and trans, that's all you need to know. So what I want you guys to do is for test 3, I want you guys to just do these answer these next four questions and then we'll go ahead and I'll answer them. Alright? So go for it.

Is the following disubstituted cyclopentane chiral?

Alright. So a is going to be what we call the middle disubstituted. Okay. And is this cis or is it trans? Going to be achiral. See how easy that was? Cool. Let's move on.

Is the following disubstituted cyclobutane chiral?

Alright. So b is one of those even numbered rings that can form para. Okay. So it's para dis substituted. In this case, do I worry about cis and trans? No, it doesn't matter. It could be cis. It could be trans. I don't care because it's always going to be achiral. So you don't even worry about it. It's achiral. Just trust me on that. If you really want to know, there is an internal line of symmetry here. Okay? So it doesn't matter whether they're cis or trans, there's always going to be an internal line of symmetry there, so it's achiral. Alright? So let's move on.

Is the following disubstituted cyclopropane chiral?

So C is also going to be what we call middle disubstituted. The reason it's middle is that it's not geminal; they're not both on the same side and it's not para, so it's somewhere in the middle. This one is middle trans, so that means that this one is always going to be chiral. No other test needed. It's middle and it's trans. All right. So let's go on.

Is the following disubstituted cyclohexane chiral?

Alright. So the answer to this one was actually to ask your professor. And the reason is because this is that one exception where I have a cis1,2-cyclohexane. Obviously, it's disubstituted cyclohexane. So what that means is that it really just depends on how your professor tends to see it. I would say, just so you guys know, about 90% of professors will say this is achiral. But I have seen those grumpy professors who are in the 10 percent that insist that it's chiral. If you wanted to, let's say you're too lazy to go ask your professor, then let's go ahead and assume it's achiral unless your professor specifically said something in class about it being chiral. I also know that, for example, for graduate tests like MCAT and PCAT, this will be considered achiral. Right now I just want you to get the highest grade. That's why I'm saying ask your professor. Later on, when you're studying for MCAT, PCAT, or whatever graduate school, we're going to just say it's achiral. Hope that clears things up. Let's go ahead and move on to the next topic.

Here’s what students ask on this topic:

What are the three main types of disubstituted cycloalkanes and their chirality rules?

The three main types of disubstituted cycloalkanes are geminal, para, and middle disubstituted. Geminal disubstituted compounds have two identical substituents on the same carbon and are always achiral. Para disubstituted compounds are achiral if they are on even-numbered rings, as the substituents are perfectly opposite each other. Middle disubstituted compounds depend on the orientation of the substituents: cis configurations yield meso compounds (achiral), while trans configurations are chiral.

Why are geminal disubstituted cycloalkanes always achiral?

Geminal disubstituted cycloalkanes are always achiral because the two substituents are identical and attached to the same carbon atom. This lack of difference between the substituents means there is no chiral center, making the molecule achiral. The internal symmetry of the molecule ensures that it cannot have non-superimposable mirror images.

How does the chair conformation of cyclohexane affect the chirality of cis-1,2-cyclohexane?

The chair conformation of cyclohexane affects the chirality of cis-1,2-cyclohexane because it places one substituent in an axial position and the other in an equatorial position. This disrupts the plane of symmetry, making the molecule appear chiral. However, due to the rapid interconversion between chair conformations, the molecule is often considered achiral in equilibrium, as the different conformations average out.

What is the difference between cis and trans configurations in middle disubstituted cycloalkanes regarding chirality?

In middle disubstituted cycloalkanes, the chirality depends on whether the substituents are in cis or trans configurations. Cis configurations, where the substituents are on the same side of the ring, result in meso compounds, which are achiral due to an internal plane of symmetry. Trans configurations, where the substituents are on opposite sides of the ring, result in chiral compounds, as there is no plane of symmetry.

Why is there controversy over the chirality of cis-1,2-cyclohexane?

The controversy over the chirality of cis-1,2-cyclohexane arises because some professors argue that the chair conformation, which places one substituent axial and the other equatorial, lacks a plane of symmetry, making it chiral. Others argue that the rapid interconversion between chair conformations results in an equilibrium state that averages out the positions, making the molecule achiral. Most textbooks and professors consider it achiral due to this equilibrium.