Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

4. Alkanes and Cycloalkanes

Since double bonds and rings can’t rotate, we need different names for the different arrangements they can create. 

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concept

How to name different types of double bonds or rings

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Now that we know how to name double bonds, there's an extra complication that we have to consider, which is that double bonds, because of the lack of free rotation, are going to be able to arrange themselves in different ways. So now we have to take into account not only the fact that there's a double bond, it has priority. We're also gonna have to take into account the arrangement of that double bond. So let's go ahead and get started. So I'm sure you've heard of the word CIS and trans before CIS and Trans are names given to particular arrangements of double bonds or of rings. Okay, so basically, any time that you have no free rotation, okay, you're gonna have the possibility for CIS and trans. So, for example, um, a double bond. If two groups are oriented in the same position, remember that a double bond is is pretty like has a lot of regions of overlap, so it can't twist out of shape. So that means that the old one is stuck there in the same way a ring has the same problem. Where if I, for example, have a ring that looks like this. And then I have a group facing up, and I also have another group facing up. So notice that there's, like, a three D structure. Let's say this is one atom and another atom and see how they're both facing the same way. I can't actually rotate this one to the down position without breaking the ring, because that ring, in order to move one down, you have to snap. So this is CIS and trans of the words that we use for these arrangements that are stuck together. Okay, Onda. Like I said, these items exist because free rotation around pi bonds is impossible. Okay, so basically, the way that we that this works is that when two groups happened to be on the same side of the fence Okay, well, I'm going to talk about that is in a second when they're on the same side, that's gonna be sis. When they're on different sides of the fence, that's gonna be trans. So what kind of mysterious fence am I talking about? Well, let's say we have a double bond. The way that I like to split it up is I like to draw a dotted line right through the middle of the global bond. Okay, so that double bond is my fence. All right. And then what I say is, how are these groups related to each other on that double bond? So, for example, if I wanted to compare A and C, are they on the same side of the fence or different sides? While they're both on the same side, they're both on the top side. So the relationship between and see would be sis. What about the relationship between B and C? What would that relationship be? Well, those are on different sides, so that one would be trans. Okay. Got that so far. How about the relationship between A and B? How would if I were to say, what kind of relationship is that? Okay. And be actually, don't have assistant trans relationship. The reason is because they're on the same carbon. Okay, system. Trans Onley applies to Adams on other carbons, not on the ones that are on your own carbon. So and be would actually get a completely different type of name that we're not going to discuss here, But it wouldn't have to do Assistant trans So Assistant Trans on Lee has to do with the way that one Adam toe another relates to okay, the way that these relate to each other. Okay, so here I've got two different versions of two beauty. Mean, when I say that you shouldn't be shocked, you should know what I'm talking about because that means that's a butane with a double bond in the two position. Easy. Okay, but these air both not both the same molecule. In fact, they're gonna have different physical properties. They're going to behave differently and stuff like that. So what would be the names that we would give to these to show that they're distinctly different? The names that we would give would be what I would draw my fence and I would say, Okay, are these on the same side of the fence or different? Same. So this one would be cysts to beauty, and then this one would be trans to beauty. All right, And these are later on. We're gonna talk about them. There's something called stereo I summers, which means that they actually are connected the same way, but they're shaped differently. But for right now, just know that these are molecules that are locked in their position and they can't move
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Two groups coming off the same carbon never have a cis/trans relationship. In case you are wondering, they are called geminal (you don’t need to know this yet.) 

E/Z Nomenclature

The cis/trans nomenclature system is awesome, but it breaks down with multisubstituted alkenes. 

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concept

Why we need to use the E/Z naming system

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Now I want to talk about another naming system called E and Z. Okay, so I call this the easy naming system, and it's actually really easy. It's related to System Trans. The only thing is that you use the e and Z naming system when you have multi substituted Al Keane's. Okay, so I know you can barely see that Multi substituted. Even you can't see it and still doesn't make a whole lot of sense. My handwriting is terrible. Okay, so multi substituted al Queens. So let me give you an example. Let's go down to this example here, assigned CIS and trans to the following Calkins. I don't need names. I don't need full names. I just want to know if it's CIS and trans. So let's start off with this first one. Would that one be CIS or Trans? The answer is that this would be trans okay, because if I drew my fence along here, what you would see is that one group is on one side and one group is on the other. Does that make sense? Cool. How about two? What would that be To would be sis? Because both groups are on the same side. Okay. Now, lastly, what about three? What would you say about three? Okay, so three is actually a trick question. There is no assistant, Trans. Why? Because I have three different groups. I have one here. I have one here, so that could be Trans. But then I also have this one over here messing things up. Did I tell you how to figure out System trans when you have three things? No. So, basically, the E and Z naming system is gonna allow us to give unique names to try and Tetra substituted Calkins. What that means is that system trans Onley works if you have two substitutes. But if you have more than two assistant Trans breaks down like in example three example three. There's no way to use system trans there, so I have to use e and Z instead.

So the E/Z naming system allows us to name tri- and tetra- substituted alkenes.

The difference between E and Z

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concept

What does E and Z stand for?

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when I'm using NZ. All I do is this. I'm gonna choose the highest priority groups on both corners of the double bond. Okay. What does high priority mean? It means highest atomic mess. Okay, so I'm going to try to pick the side of the double bond. I'm gonna try to pick on each side double bond that Adam that has the highest atomic mass. That one's gonna get my priority. And then I'm going to figure out how those two high priority groups are related to each other. If they're related to each other Trans, then we're gonna sign the letter e. If they're related to each other as cysts, then we're gonna sign the letter Z. So basically, Z is just a fancy way of saying cysts with three or more substitutes. E is a fancy way of saying trans with three or more substitutes

Using the periodic table to assign priorities, trans = E, and cis = Z

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example

Assigning E/Z

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So I'm gonna go through the first one all by myself. I'm gonna like what I mean by myself is with you guys just guide you guys through it and then we'll see. Be okay. So first of all, let's look at f I have when I say that I have Thio do the highest party groups on both corners. What is a corner? I'm just talking about this being one corner and this being another Yes. So let's look at the green one first. Is there a side of this double bond that is going to have the higher priority? Is there one of those substitutes that's gonna have a prior party? And the answer is yes. Florian is gonna have a higher priority over this group here. Why? Because even though this group down here looks bigger, the flooring has a higher atomic mass right away. So what that means is flooring beats. Carbon flooring is further down. The periodic table on carbon is so this is going to be my high priority up here. Oops. Whatever This is gonna be my high priority. Appear now. Let's move to read. So for red, is there a way to assign which one is higher priority. And actually these air both exactly the same. They're both ch three. Okay, so this is an example where I actually have four different substitute Wint's. But since I have two of the same substitute Wint on a corner, I can't assign assistant Trans or NZ. Why? Because there's no way to distinguish thes from each other. They're both the same thing. So actually, this one is going to be not available. I can't assign CIS and Trans unless there's different things on both sides. Okay, so what I'm trying to say is that there's no way for me to know if this assistant Trans because this ch three is always going to be sis to the F and trans to the Ethel. This ch three is always gonna be sister the Ethel entrance, the F so it doesn't really matter. This one does not get designation
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example

Assigning E/Z

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Hey everyone. Now that we've done example A Let's take a look at example. B So here, when we're looking at this elk in the first thing we should do is find our corner. Carbons. Remember our corner Carbons are just are walkin carbons. We're going to make this one are blue corner carbon and this one are black corner carbon. The next thing we're gonna do is let's look at the blue one. What are the two groups that are attached to it? If we take a look, we know that there is a carbon here that's invisible And carbon has to make four bonds. So there is a hydrogen that's also present. If we look at carbon and hydrogen which has higher priority, carbon would have higher priorities. So we're going to highlight that. Now let's take a look at our black corner carbon here. What two groups is it attached to? Well, it's attached to an oxygen here and a carbon here of these two which has higher priority oxygen would have higher priority. So now that we determine the to higher priority groups, let's draw our fence And we see that our two higher priority groups are on the same side. Now, remember if we're dealing with a tri substituted or tetra substituted. All keen, then we use the E or Z designation. Since this is tri substituted. We're going to use your Z. And they're on the same side. So that would be sis and sis is related to Z. So this is a Zl keen Now echoing back to my french roots away that I remember, what she means is Z is Z same side. Okay, so that's a way that I tend to try to remember what is Aziz talking, how it's related to Sis. Remember if you are a Zl keen, the opposite of that would be an E. Talking. All right, So just keep this in mind when taking a look at any of these types of al keen structures to determine if they do in fact have an E or Z designation.
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Great! Now let’s combine this with what we’ve learned about functional groups (alkyl halides and alcohols) to name some more complex molecules. 

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Problem

Determine the IUPAC name of the following molecule 

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This was a great example of how the root chain must always have the functional group within it, even if it is shorter!

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Problem

Determine the IUPAC name of the following molecule 

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Hope you don’t hate me for throwing two alcohols in there! This is great practice though. 

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