Organic Chemistry

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4. Alkanes and Cycloalkanes

Conformational Isomers

As we’ve learned before, single bonds have the ability to freely rotate, meaning that we have to get used to seeing multiple arrangements of single bonds and understanding that they are all the same thing. 

These multiple arrangements are known as conformers.


Understanding what a conformer is.

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Alright, guys, Now we're gonna lead off this section. Talking about conformers and conformers are kind of the big umbrella under which a lot of different topics fall under. For example, if you've heard of Newman projections or chair flips or anything like that, all of that has to do with confirmation. All changes. So in order for us to understand those really important topics, we're gonna need to understand what a conform ER is. First, let's go ahead and do that. So most organic molecules have the ability to exist in multiple arrangements without experiencing any chemical changes. Okay? And the reason for this is because many of these, um, single bonds or Sigma bonds are able to rotate. Okay, So basically, Sigma Bonds, if you remember, are those single bonds that have one region of overlap. So it's very easy for one of the atoms to rotate over and over without actually changing the strength or the identity of that bond. Okay, so what that means is that as that Adam rotates, that is not gonna be an ice. Um, er, it's because of the fact that it's not actually changing the actual connectivity of the atom. It's still connected the same exact way. It's just rotate a little bit, okay? And structurally, the molecule is never gonna change. Okay, so let me show you an example of hexane now, so hexane would be a six carbon chain. And as you can see, what I've done here is I've made a little dotted bond with a Sigma sign. Okay, Now, this is just a regular signal bond that I want to show you how it can rotate. So if I were to take this bond and analyze Okay, if this was a double bond, would it be CIS or Trans? What we do is we draw our fence and we would say that the big groups are on different sides, so this would be trans okay. And the thing is that single bonds don't have the ability to stay locked in place because remember, a double bond. Once it's Trans, it's going to stay trans forever. Or once it's this is going to Stasis forever. But single bonds are able to freely rotate from these trans positions to the six positions easily. Okay, So instead of calling this Trans insists it's going to get its own name. It's called S Trans s Trans just stands for Sigma Trans. It just means that if the trans confirmation off a Sigma bond does that make sense? What? Turns out that if I want to, I can easily rotate this bond because, remember, it's not locked in place with P Orbital's. It's just a s orbital, so it's easy to twist it, and once it rotates, it's gonna look different. Now, if I draw my fence again, I would say that my big groups are on the same side. So this is what's called the S s confirmation. And it turns out that molecules are constantly moving back and forth between these types different types of confirmations. So even though we draw Hexen like this, it doesn't always look like that. Many times it's gonna look like that. And all the other single bonds are able to rotate, too. So you can imagine hexane actually doesn't always look like the zigzag. Sometimes it's a little bit more crumpled up. Whatever. Okay, so these alternate arrangements, the fact that I have two different positions that my Hexen could be in our called conformers does that make sense? So far? That's the idea behind a performer

Now that we understand what a conformer is, let's see if we can distinguish them from regular isomers. 


Determine if the following pair of molecules is isomers or conformers?

Notice that the only difference between these was one rotation.

This next one is a bit more tricky because it has two rotations in it. See if you can identify them.


Determine if the following pair of molecules is isomers or conformers?

Remember: If double bonds are switching configuration, that’s an isomer. If single bonds are rotating, that a conformer


Determine if the following pair of molecules is isomers or conformers?

One more, hopefully these are getting a little easier!


Determine if the following pair of molecules is isomers or conformers?

We’ll be rotating single bonds all the time in this course, so I’m hoping now you are more comfortable recognizing that multiple rotations really equal the same thing.