Can isomers be identical

The group priority is then assigned based on the atomic numbers of the atoms at the first point of difference. Rotate the molecule so the group of lowest priority is away from you. If going from the group of 1 st to 2 nd to 3 rd priority traces a clockwise direction, the stereogenic centre is assigned the R configuration. If the path traced out is an anticlockwise direction, the stereogenic centre is assigned the S configuration.

Identical compounds are the same compound shown with ALL atoms in the same spatial orientation. Conformers are the same compound shown with different rotations about single bonds. In the example below, we can compare two identical structures for ethane with two conformers of ethane. The rigidity of the pi bonds in double bonds can create geometric isomerism. The rigidity of the double bond creates a line of reference for spatial orientation.

The prefixes cis and trans are used to distinguish between geometric isomers. The cis-stereoisomer has both non-hydrogen atoms on the same side of the double bond. Whereas, the trans-stereoisomer has the non-hydrogen atoms across the double bond. In the same way, we cross the ocean an a trans-Atlantic journey. This small difference may seem insignificant, but geometric isomers are different chemical compounds with different physical properties as shown in the example below.

Despite sharing the same molecular formulae, isomers may have very different physical properties, such as boiling point, melting point, and chemical reactivity. Take cyclohexane b. No matter how different their physical properties, or reactivities, their common molecular formula makes them isomers of each other. Likewise, propionic acid and 1-hydroxypropanone share the same molecular formula, C 3 H 6 O 2 , making them isomers of each other but not isomers of cyclohexane or 1-hexene, of course!

This leads us to the next question. What kind of isomer are they? Isomers divide neatly in to two categories: constitutional isomers different connectivity and stereoisomers same connectivity, different arrangement in space. So what does that actually mean?

Constitutional isomers have the same molecular formula, but different connectivities. The same parts, but arranged in different ways. To take this oldie-but-goodie example, switch a tail and a leg and you make isocats:. Yes — from nomenclature. If two molecules with the same molecular formula have different connectivity, it will be obvious either in the locant e. By way of an example, these 5 molecules are all constitutional isomers of each other.

They have the same empirical formula C 6 H 12 but different connectivity. There is only one way to connect C 6 H 12 together to form cyclohexane, and only one way to connect the same atoms together to get 1-hexene. But there are two ways to connect C 6 H 12 to give molecules with the names 2-hexene, and 3-methylpentene! And four ways to connect C 6 H 12 to give 1-ethylmethylcyclopropane!

For example: there are two ways to arrange the hydrogens on the double bond of 2-hexene; when they are on the same side, we refer to it as cis or Z ; on the opposite side, trans E. Since free rotation about the double bond is not possible, these are completely distinct molecules.

They can be separated, put in different flasks, left on the shelf for years, and never interconvert. What kind of isomers are these? We need another name. Since they differ in the arrangement of their groups in space about the double bond we call them stereoisomers. Stereoisomers can also arise from tetrahedral carbon atoms that are attached to four different substituents i. There are 2 and only 2! These molecules may look the same, but they are actually non-superimposable mirror images more on that a few paragraphs below.

This is not unlike the distinction between diastereomers stereoisomers that ARE NOT non-superimposable mirror images and enantiomers stereoisomers that ARE non-superimposable mirror images. In organic chemistry, two molecules that can be superimposed on each other, through rotation of bonds i. A mole, after all, is 6. Later that evening, after a few drinks, things got awkward. One has a scar over his left eye and the other has a scar over his right.

No amount of twisting and turning on the floor in pain can possibly make them superimposable now. Since they are no longer superimposable, in chemistry terms they are no longer the same. So they're not the same molecule. They have a different three-dimensional configuration, although their bond connections are the same, so these are stereoisomers.

Let's see if they're enantiomers. So if we look at it like this, you put a mirror here, you wouldn't get this guy over here. Then you would have a chlorine out front and a hydrogen. So you won't get it if you get a mirror over there. But if we do the same exercise that we did in the last pair, if you put a mirror behind this guy, and I'm just going to focus on the stuff that's just forward and back, because that's what's relevant if the mirror is sitting behind the molecule.

So if the mirror's sitting behind the molecule, this bromine is actually closer to the mirror than that hydrogen. So the bromine will now be out front and then the hydrogen will be in back.

This hydrogen will be in the back. I'm trying to do kind of a mirror image if it's hard to conceptualize. And then that would all look the same. And then this chlorine will now be out front, and this hydrogen will now be in the back in our mirror image, if you can visualize it.

And then we have another one. And this chlorine is closer to the mirror that it's kind of been sitting on top of. So in the mirror image, it would be pointing out, and then this hydrogen would be pointing back.

Now let's see, is our mirror image the same as this? So the mirror image, our bromine is pointing in the front, hydrogen in the back there.

Then we have hydrogen in-- then in our mirror image, we have the hydrogen in back, chlorine in front. Same there. So far, it's looking like a mirror image. And then in this last carbon over here, chlorine in front, hydrogen in back. But here, we have chlorine in the back, hydrogen in front. So this part, you could think of it this way. This is the mirror image of this, this is the mirror image of this part, but this is not the mirror image of that part.

So when you have a stereoisomer that is not a mirror, when you have two stereoisomers that aren't mirror images of each other, we call them diastereomers. I always have trouble saying that. Let me write it. These are diastereomers, which is essentially saying it's a stereoisomer that is not an enantiomer.

That's all it means: a stereoisomer, not an enantiomer. A stereoisomer's either going to be an enantiomer or a diastereomer. Now, let's do this last one. Let's see we have two-- we have this cyclohexane ring, and they have a bromo on the number one and the number two group, depending how you think about it. It looks like they are mirror images of each other. We could put a mirror right there, and they definitely look like mirror images.

And this is a chiral carbon here. It's bonded to one carbon group that is different than this carbon group. This carbon group has a bromine. This carbon group doesn't.

It just has a bunch of hydrogens on it, if you kind of go in that direction. And it's hydrogen and then a bromine, so that is chiral. And then, same argument, that is also chiral. And obviously, this one is chiral and that is chiral. But if you think about it, they are mirror images of each other, and they each have two chiral centers or two chiral carbons.

But if you think about it, all you have to do is flip this guy over and you will get this molecule. These are the same molecules.