Atropisomers

These are exceptions to the rule: molecules that contain no chiral centers yet are chiral due to their inability to freely rotate.

Concept: Recognizing chiral molecules with zero chiral centers. 

3m
Video Transcript

Hey, everyone. So now that I knew in general terms how to predict what types of stereoisomers I'm going to get based on the number of chiral centers, now I want to start teaching you some of those exceptions to the rules. The first of those exceptions is a chiral compound that doesn't have any chiral centers at all and these are called atropisomers.
Atropisomers are these unusual compounds that contain no chiral centers, yet they actually are chiral due to their inability to freely rotate. Now, it really depends class by class, professor by professor how much emphasis they put on these compounds because they are unusual and they're not ones that we deal with a whole lot in orgo one. But sometimes your professor might just throw these in as a trick so that you'll forget about these kinds of compounds that can't rotate.
So here I have just a few, in fact, there's a few more, but these are the really common ones. So allenes, as you can allenes they're two double bonds together. They can't rotate because double bonds can't rotate.
Substituted biphenyls, I know you don't know what that is yet, but a biphenyl, just two phenyl groups that are attached in one spot, and they're substituted, meaning that they can't really rotate because those substituents that you can see sticking out kind of act as teeth. They lock it together so it can't fully rotate.
Trans-cyclooctene, this is actually a molecule that I was just recently talking about how it can form the cis version and the trans version. It turns out that the trans version, because it can't rotate, it can actually form a right-handed version of this twisted messed up thing and a left-handed version. Very peculiar looking molecule, but it is also chiral because of lack of rotation.
Then finally, BINAP, which I don't even remember exactly what it stands for, but it's very similar to the substituted biphenyl, except in this case an entire phenyl group or an entire benzene ring is acting as one of those teeth that's not going to let it rotate across that single bond.
All right? So all of these are going to be chiral even though they lack chiral centers. Like I said, this is not the usual. This is not the norm. These are unusual molecules that happen to display chirality. 

Allenes

Concept: Determining if allenes are chiral or not. 

3m
Video Transcript

So what I want to do is I want to teach you guys tests for the two most common ones, which are allenes and substituted biphenyls and the other ones don't really need rules so you're fine. You can just always assume that they're going to be chiral.
But it turns out that allenes and substituted biphenyls can be chiral or they can not be chiral depending on the rules that they follow. So let's look at this.
So what we're going to do is we're simply going to use a modified version of test two, which is the one for stereocenters to identify trigonal centers in the allene. But you're going to say okay, but there's two double bonds, which one do I look at. Well, this is the ghetto part. We're going to visualize the allene as just one big double bond. Maybe like squint a little bit and try to see that – try to ignore the carbon in the middle and just pretend that it's one big double bond. By the way, this is just my way of solving these, but I think it's helped a lot of students and I think it will help you guys.
If it's able to form E or Z isomers after visualizing it as a big double bond, that means that it's chiral. Remember that when I taught you guys about trigonal centers, I told you that they're actually achiral if they pass the test. But allenes are different. Just think about allenes as their own thing. Allenes are going to be chiral if they can form E or Z through this weird long double bond.
So I'll do the first one with you guys and then I'll let you guys do (b) and (c) on your own. So what I would do here is I would look at this compound and I would just ignore the C in the middle and just pretend that it's one big double bond. Then I would ask myself is this double bond able to form cis and trans or is it able to form E and Z? What do you guys think?
According to my rules for double bonds, this actually can not form E and Z or cis and trans because I have two of the same atom on one carbon, on one side so that means that no matter how much I switch these two groups, I'm never going to be able to get E and Z. So this one would be achiral.
So now what I want you guys to do is I want you guys to solve (b) and I want you guys to solve (c). 

Imagine that the allene is a big double bond. If it is able to form E or Z isomers, it is chiral

Substituted Biphenyls

Concept: Determining if substituted biphenyls are chiral or not. 

2m
Video Transcript

So these guys are going to have a pretty weird rule too. Basically, for substituted biphenyls what we look at is the substituents in the ortho position. Now ortho is a word we're not really going to use until orgo two very much, but just as a heads up what it means is next to or adjacent to. So basically what we're looking at is the substituents that are next to that single bond or that sigma bond that it's rotating around.
What I want to do is I want to figure out if any of the rings have two of the same group. If they have two of the same group, then it's not chiral. If it has basically two of the same group then that's going to be achiral. But if it has two different groups, then it's fine then it would be chiral, similar to the way that we did it for allenes.
Let's go ahead and do (a) together and then we'll have you guys do (b) and (c) on your own. So for (a) do I have two of the same groups on any of the rings in the ortho position? And the answer is no. These groups are different and these two groups are different. So that means this is going to be a chiral substituted biphenyl because of the fact that these groups – none of the rings have two of the same exact thing on one side.
So go ahead and try and figure out (b) and (c). 

Must meet two criteria to be chiral:

  1. All substituents are in the ortho- position
  2. None of the rings have two of the same group on them

*ortho- is a term we will use in Organic Chem 2. It means “adjacent to”, basically that the groups next to the primary carbon linking the two rings.  

Example: Is the the following substituted biphenyl chiral?

1m

Example: Is the the following substituted biphenyl chiral?

1m