Condensed Structures

What if you want to describe a molecule, but you have nothing but a keyboard? (No fancy pictures or drawings). This is was actually a big dilemma in the chemistry world, which is why we now have condensed structure. 

Condensed Rules

Concept: How to interpret condensed structures.

7m
Video Transcript

Hey, guys, so now we're going to talk about another way to represent organic structures and that's called the condensed structure. The condensed method is a common way to describe the connectivity, connectivity – how a molecule is connected, of a molecule using only text.
My speculation is that the logic behind this is that professors and chemists wanted a way to represent these molecules using only a text editor. Back in the day, there weren't these fancy drawing programs. In fact, all they had was typewriters. And they wanted some way to be able to notate what a molecule looked like without having to draw fancy structures. That's where they came upon the condensed structure.
Now, one thing that's important for this class is that you're going to have to really learn how to interconvert between bondline and condensed very quickly. The reason is because your professor, just to trick you, will use both structures interchangeably. One question might be in bondline, another question might be in condensed. Typically, just from my years of tutoring experience, I know that students really don't like to think in condensed structure because it's kind of tricky.
So what I'm going to do today is I'm going to really show you how to interpret it. So let's go ahead and look at – there's two different types. Let's start off with the full condensed structure.
The full condensed structure is literally only text. As you can see, we're taking a bondline structure and we're going to represent it in just only one line of text. Now a lot of this is straight forward meaning that you can see that this first CH3 here is represented by that CH3 there. Then you think, “Oh, this isn't so bad.”
The confusing part is that parentheses can be used to represent different things depending on the subscripts in front of them. Let's go ahead and look at that.
The first interesting parenthesis that you should know is this one that I've indicated in blue which is a CH2 plus a subscript. Notice that I have CH2 bracket 3. What that means is that I have a repeating unit. That's going to happen over and over and over again. When I have CH2 with a 3 that doesn't mean I have 3 CH2's sticking out of one place. What it means is I have 3 CH2's attached in order, like in a line. Does that make sense so far?
The reason that saves us a lot of time is because some of these molecules can get really, really long. Imagine having a molecule that's 100 carbons long. Do you want to write 100 CH2's? No, it's way easier just to put CH2 in parenthesis with a 100 underneath it and that means I'm going to repeat this unit 100 times. Cool, so far? Awesome.
So let's keep going. Then we have this red one. The red one is parenthesis alone. If you have parenthesis alone, that indicates that you have a branch coming off of the chain. Now, what's going to be interesting here is that sometimes this parenthesis is optional, meaning that your professor may not always be so nice as to put that parenthesis there to say that there's a branch. Sometimes you're just going to have to know.
Alright, let's look at the logic here. I have a carbon right here and that carbon is attached to two other things. It's attached to one-half of the chain on the left and one-half of the chain on the right. By the way, this carbon, let's go ahead and locate it. It's this one right here. Let's just make sure that we know what we're talking about. That carbon has a part of the chain on the left and a part of the chain on the right. But it's also supposed to have two other things coming off of it because remember carbon wants to have four bonds. So what are those two other things?
Well, there's an H. That means that I have one H sticking off there. And I also have an OH. Now, notice that in this case I was nice and I put it in parenthesis. What that meant was it's very easy to say, “Oh, the OH is the thing that's coming off here. Done. I have my four bonds.” Perfect.
But sometimes you might see it as just CHOH. And then what you would need to know is that okay, one of them the H is going in one direction, the OH is going in the other, but you know that both of them are attached to that carbon because remember that that carbon needs four bonds. That's kind of the way that we think about it. You always think in terms of how can I make carbon have four bonds.
Finally, as you can see, there's one more type of parenthesis. Sorry about getting a little sidetracked, but I wanted you guys to see an example of that.
And then the last one is that if I have something else that's in parenthesis other than CH2 and it has a number in front of it. So, in this case, I'm giving you CH3 with a 2 in the magenta brackets. What that means is that these things are not attached in a line. They're both attached to the same carbon.
Basically, the only time that I have things in a line repeating is if it's CH2. That would mean that it would be linear. So go ahead and write that down. Linear meaning that it's all in one line. But if I have two things that are not CH3 or three, right here, these would indicate branching because of the fact that both of these CH3's must be coming off of one carbon and what that would be is they would be attached to this one right here.
Now let's look at the logic with this one. Let's look at this carbon. Where is that carbon? That carbon is right here. That carbon needs to have four things attached to it. Well, first of all, it has the whole left part of the chain, easy. And then it has, let's look at it. Let's see what's after the carbon. After the carbon, I have an H and I have a CH3 and then I have another CH3 because it says 2. Those are the three other things that are attached to that carbon. This would be bond two, bond three and bond four.
If you were to go ahead and draw that, you would see that I have an H here and then I have CH3 here and then I have CH3 here. Do you get that? At the end of the day, the carbon still has four bonds, so it's fine. But if you ever draw a structure, if you ever translate a structure that does not give carbon four bonds, then you know you made a mistake. 

The condensed structure shows us the connectivity of the molecule. The use of parentheses is important:

  • Parentheses with no subscripts: Branch on the chain.
  • Parenthesis with subscripts: Multiple branches on a chain.
  • CH2 within parentheses + subscripts: Repeating CH2 units within a chain.

Condensed Mixed Structures

Example: How to draw condensed mixed structures.

6m
Video Transcript

So now let's look at one that's a little bit easier and that's the condensed mixed structure. The condensed mixed structure is one in which I have a ring. So I'm just going to put here rings. The reason is because rings are very, very difficult, if you think about it, rings would be very difficult to draw in a test because a ring could be a certain shape. It's two-dimensional. And most text editors don't give you the ability to draw up and across, it's only to go across. Think that you're on a typewriter, would you be able to type a ring? No.
So what you do with the condensed mixed structure is that the ring is the only thing that stays as bondline and then everything else that's branching off of it goes to condensed. As you can see what I do is I take this thing, convert it over here and now all of the branches coming off of the ring become a condensed formula. Does that make sense so far? Cool.
I also wanted to talk about a few other things right here. Notice that – let's start off with this one because it's kind of easy.
What I have is, think about it, I have parenthesis. Does it have CH2 in parenthesis? No. It's something other than CH2, right? It's actually CH3. That must mean what? Does it mean that all the CH3's are in a line? No, that would only be if it was CH2. What it means is that all three CH3's are coming off of the same carbon. Does that make sense?
That's what we see right here. I have carbon and then CH3, CH3, CH3. Does that make sense? Cool. I hope so.
Now let's go to one that's a little bit trickier. That is this one over on the left-hand side. So now what is this weird line that I drew? Where did that come from? This line is a pretend line, so don't pay attention to it too much. All I'm trying to do is separate the right side from the left side to show you how we draw it differently.
Now because condensed structure has to do with connection. It has to do with what is connected to what. When you're on the left-hand side of a mixed structure, you always have to draw in reverse. You have to put the letters backwards. The reason is because I have to show exactly what order they're connected in.
So let's look at this. This right here is an O attached to a CH3. Would you agree with that? Cool. But if I just write – instead of writing this let's say that I wrote O, CH3 like that, would that be correct? That would actually be incorrect. So go ahead and put an X on that. That would be wrong. The reason is because it looks like the O is attached to one of the H's. That doesn't make a lot of sense.
The way we need to show this is that the O is actually connected to the carbon first and then the carbon is connected to three H's, right? So then the way that we draw it is O, C and then H3 after that. Does that make sense? This only applies when you're on the left side because if you're on the right side then it just makes sense the way it naturally is. You would just write it out according to the connectivity. The left side is the part that's a little bit tricky.
Now, some of you guys might be asking this question, “Johnny, but what if it's at the top or the bottom because that's not really – how about if it's in between.” So how about if I had a line right here and I wanted to put text there? Well, if it's right in the middle, then obviously go to the right because I can just do CH3, O. Or whatever. I could do, in this case, it would be O, CH3 if it had been there.
So obviously, what I'm trying to say is that you would only go to the left, you'd only reverse it if you absolutely have to because it's on the left-hand side. Does that make sense? But if it's at the top or the bottom, just go to the right like normal because there's plenty of room to draw it.
So let's go to the last example which is this one down here that has a double bond to carbon and to oxygen. That's this thing right there. This is a situation where if you have a double bond, many times your professor is going to put an equal sign there. An equal sign represents double bond, which is pretty easy. If it was a triple bond, then they would put on of the equal signs that has three bars.
But sometimes, your professors will be really tricky and they will just write this like C, O, CH3 and they will skip the equal's sign. That's tricky, right? How are you supposed to know that that is a double bond without being told that it's a double bond? Can you guys think about it? Is there any way to know? The answer is it goes back to carbon having four bonds.
If this is a carbon and an oxygen and if that carbon needs four bonds, then what that means is that this carbon must be attached to that oxygen with a double bond. The reason is because I have a bond on the left, I have a bond on the right and then I must have two bonds that are missing. Those two bonds that are missing are the ones that are attached to the O that make it a double bond right there. Does that kind of make sense?
Now, like I was saying, most professors are going to be nice and they're going to give you that double bond, but I'm just trying to show you just in case your professor decides to be a tool and not put that in there so that you guys will know that you can actually calculate it based on carbon having four bonds.
So I'm going to give you plenty of practice. Don't worry. 

This is similar to normal condensed structure, except there is a bondline ringed component. Always draw your condensed letters in terms of connectivity!

  • If to the right of the ring: Draw branch normally.
  • If to the left of the ring: Draw branch in reverse.  

Problem: Convert the condensed structure into a bondline structure 

5m

Remember, the exact direction of your zig-zag pattern doesn’t matter as long as everything is connected correctly. Single bonds can rotate freely, so let’s not spend lots of time worrying about the exact angles you drew.