Cumulative Electrocyclic Problems

Concept: Concept: Two Steps to Predicting Any Electrocyclic Products

6m
Video Transcript

In this video we're going to put it all together and talk cumulatively about both thermal and photochemical electrocyclic reactions, so guys, the reason I put this page together is because one I wanted to just give you an overview of what the expectations are for how to determine stereochemistry with thermals and photochemical reactions, electrocyclic reactions but also I kept thinking to myself like there's got to be an easier way, maybe I can come up with a cheat sheet that will help my students to come up with these conclusions a little faster than having to draw the Mo's from scratch every time. So, what I've done here is I'm gonna, I'm going to present this cheat sheet to you and it's going to be up to you to figure out how helpful it is or not, if you think that you're going to be doing lots of electrocyclic reactions this semester it might be a good idea to learn this sheet, if you think that it's just a very small thing that you need to learn how to do and it's not a huge focus for your class then we just stick to the old method we're already doing because you know that that works every time. So, let's go ahead and just go through the steps, so the first thing you always need to do is figure out the rotation, if it's going to be conrotatory or disrotatory, we already have gone through the process of figuring this out manually, which means that you obtain the HOMO through a combination of drawing your molecular orbitals and then the activation type, either heat or light, and then you can figure out, you can look at the orbitals and you can figure out how they rotate. So, that's already what you're used to doing and that's perfectly acceptable, you can do that every time if you want but I have a shortcut and that shortcut is to use this little summary chart and with the summary chart does is it tells you the rotation the conclusion of the rotation based on either how many pi bonds you're starting or not either but both the number of pi bonds your starting and the type of activation that you have. So, for example, if you happen to have an even number of pi bonds in your polyene and if you're using thermal activation, heat activation then you can just automatically memorize that it's going to be conrotatory and then as you change them the, the conclusion changes. Now, memorizing this whole table I think would be really just dumb, I don't think that that's a good thing to do because I think that you're going to mess up and forget it but I do have a little abbreviation that might help that I thought up called etc. So, you know how you say etc, etc. So, e.t.c is etc. So, you could think that if you have an even number of pi bonds with thermal activation it's going to be conrotatory and that's etcetera and then if any of the other things change then you can just base, what your knowledge off of it etc. So, for example, let's say that instead of an even number it's an odd number but I still have thermal activation? Well, I would think, okay? etcetera, odd numbers thermal activation then that must be disrotatory because I'm changing one thing, if you're changing two things at the same time then it goes back to conrotatory. So, if it actually turns out to be odd and photochemical then I would say? Well, that's both of them are opposites that must be back to a conrotatory, okay? So, just, it's up to you to figure out if you want to use this or not, I am going to make you practice it one time but after that if you never want to do it again, that's fine. So, now let's say that you figured out Conor tutorial purses disrotatory depending on, which one you use depending on, regardless of the method you know it. Now, step two is to determine the stereochemistry, so the way you determine the stereochemistry is first of all just draw the structure in 3d and draw out the rotation and then figure out if they should be going cis or trans to each other. So, that's what we've already done perfectly acceptable way to do things, you can totally do that every time if you want to. but I made another chart for this step as well in case it can help you and maybe it does maybe it doesn't it really you should determine based on how many of these you're going to have to do this semester and what this one says is that it also gives you the conclusion of whether it's going to be cis or trans based on the rotation, which you should have figured out in step one, and then based on one new idea, which is the pi bond either being the same or different. Now, what I mean by same or different is that same means that they're both either cis or they're both trans, different means that one cis and one is trans, right? And again, the conclusion of cis or trans is going to change based on, if you change one of those things. So, once again, I don't think you should memorize this whole chart because then you're going to too many things going on in your head but you could memorize this one really stupid phrase that I thought of which is, if it's the same, DIS is CIS kind of like DIS instead of this, I'm just pulling out a little bit of my slang here and if it's the same DIS, this disrotatory is CIS. So, you could think about and you could say okay, if I already know it's going to be disrotatory and my bonds are cis to, are both cis then I know it's going to be a CIS or if they're both trans they're still gonna beat CIS because they're the same, but if they're, if they're different or if it's conrotatory they just think it's the other one, okay? So, basically for both of those you have kind of starting point and then you can change your answer based on how many of those variables are changing, okay? So guys, I don't know if this will be helpful or not but at least I wanted to try to help you guys and if you just decide to use the old method, that's totally fine, let's move to a practice problem.

Problem: Use the summary charts to predict the product of the following reactions. If there is more than one isomer possible, draw them

11m

Problem: Use the summary charts to predict the product of the following reactions. If there is more than one isomer possible, draw them

6m

Problem: Predict the product for the following reaction. 

4m

Cumulative Electrocyclic Problems Additional Practice Problems

Account for the difference in the products of the following reactions:

Watch Solution

Draw the product of each of the following reactions:

Watch Solution

Draw the product of each of the following reactions: 

Watch Solution

a. Identify the mode of ring closure for each of the following electrocyclic reactions.

b. Are the indicated hydrogens cis or trans?

Watch Solution

Which of the following are correct? Correct any false statements.

a. A conjugated diene with an even number of double bonds undergoes conrotatory ring closure under thermal conditions.

b. A conjugated diene with an antisymmetric HOMO undergoes conrotatory ring closure under thermal conditions.

c. A conjugated diene with an odd number of double bonds has a symmetric HOMO.

Watch Solution

a. Under thermal conditions, will ring closure of (2E, 4Z, 6Z, 8E )-2,4,6,8-decatetraene be conrotatory or disrotatory?

b. Will the product have the cis or the trans configuration?

c. Under photochemical conditions, will ring closure be conrotatory or disrotatory?

d. Will the product have the cis or the trans configuration?

Watch Solution

a. For conjugated systems with two, three, four, five, six, and seven conjugated  bonds, construct quick MOs (just draw the lobes at the ends of the conjugated system as they are drawn on pages 1274 and 1275 ) to show whether the HOMO is symmetric or antisymmetric.

b. Using these drawings, convince yourself that the Woodward–Hoffmann rules in Table 28.1 are valid.

Watch Solution