Question Sets: Testing Your Knowledge
In stereochemistry, stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular . "Stereoisomers" in cypenv.info, n.l., , Link; ^ Morrison and Boyd. The latter term means that the difference is in the sequence in which atoms are attached to one another. Examples of. Definitions for vocabulary words can be found in the Illustrated Glossary of various categories of structural relationships, namely isomers, constitutional.
So we are essentially made up of the same things, but we are actually two different molecule, actually, two very different molecules here.
Now let's look at this next guy over here. So if we look at this molecule, it does look like this carbon is chiral. It is an asymmetric carbon. It is bonded to four different groups: And so's this one. And they're both made up of the same things. You have the carbon-- and not only are they made up of the same things, but the bonding is the same.
So carbon to a fluorine, carbon to a fluorine, carbon to a bromine, carbon to a bromine, carbon to hydrogen in both of then carbon to the methyl group in both.
But they don't look quite the same. Are they mirror images? This guy's mirror image would have the fluorine popping out here, the hydrogen going back here, and then would have the bromine pointing out here.
Let's see if I can somehow get from this guy to that guy. Let me flip this guy first.
So let me-- a good thing to do would be to just flip to see the fastest way I could potentially get there. Let me just flip it like this. So I'm going to flip out of the page, you can imagine. I'm going to flip it like this. So I'm going to take this methyl group and then put it on the right-hand side. And you can imagine, I'm going to turn it so it would come out of the page and then go back down. So if I did that, what would it look like? I would have the carbon, this carbon here.
I would have the methyl group on that side now. And then since I flipped it over, the bromine was in the plane of the page.
It'll still be in the plane of the page, but since I flipped it over, the hydrogen, which was in the back, will now be in the front. The hydrogen will now be in the front and the fluorine will now be in back because I flipped it over.
Ch 7 : Isomer types
So the fluorine is now in the back. Now, how does this compare to that? Let's see if I can somehow get there. Well, if I take this fluorine and I rotate it to where the hydrogen is, and I take the hydrogen and rotate it to where-- that's all going to happen at once-- to where the bromine is, and I take the bromine and rotate it to where the fluorine is, I get that.
So I can flip it and then I can rotate it around this bond axis right there, and I would get to that molecule there. So even though they look pretty different, with the flip and a rotation, you actually see that these are the same a molecule. So let's see, what do we have here?
Let me switch colors. So over here, this part of both of these molecules look the same. You have the carbons on both of them. This carbon looks like a chiral center. It's bonded to one, two, three different groups. You might say, oh, it's two carbons, but this is a methyl group, and then this side has all this business over it, so this is definitely a chiral carbon. And over, here same thing. It's a chiral carbon. And this has the same thing. It's bonded to four different things. So each of these molecules has two chiral carbons, and it looks like they're made up of the same things.
And not only are they made up of the same things, but the bonds are made in the same way. So this carbon is bonded to a hydrogen and a fluorine, and the two other carbons, same thing, a hydrogen and a fluorine. Carbon, it looks like it's a hydrogen. It's bonded to a hydrogen and a chlorine, so it's made up of the same constituents and they're bonded in the same way.
So these look like-- but the bonding is a little bit different. Over here on this one on the left, the hydrogen goes in the back, and over here, the hydrogen's in the front. And over here, the chlorine's in back, and over here, the chlorine's in front. So these look like sterioisomers. You saw earlier in this video, you saw structural isomers, made up of the same things but the connections are all different. Stereoisomers, they're made up of the same thing, the connections are the same, but the three-dimensional configuration is a little bit different.
For example, here on this carbon, it's connected to the same things as this carbon, but over here, the fluorine's out front, and over here-- out here, the fluorine's out front. Over here, the fluorine's backwards. They do not differ in connectivity, obviously, or they wouldn't both be called by the same name 2-butanol.
They also don't have a cis or trans prefix, to indicate that they are diastereoisomers. They have a very specific, unique relationship to one another, the same relationship which exists between an object and its mirror image. A key aspect of this difference, as we all know, is that a mirror acts to interchange left and right hands.
This means it resembles a human hand in that the left and right hands are not superimposabile but can be readily distinguished at least by some of us. By the same token, a molecule or any object is said to be achiral if it is identical to superimposable upon its mirror image molecule or object.
Many molecules are achiral, but many are chiral, especially complex molecules such as are found in biological systems. How can we anticipate when a molecule is chiral and therefore has an isomer an enantiomer or when it is achiral and has no enantiomer?
Consider 2-butanol, which is an example of a chiral molecule. The illustration below hopefully shows that the mirror image of one 2-butanol isomer is non-superimposable upon the original molecule.
Stereoisomers, enantiomers, diastereomers, constitutional isomers and meso compounds
Your can verify this by making models, but you can also visualize trying to superimpose the two by sliding one structure over mentally on top of the other. We can, for example, slide B over to A and superimpose the OH, the central C, and its attached H of the B molecule over the corresponding gorups of the A molecule, but the ethyl group on B sits over the methyl group of A, and the methyl group on B superimposes upon the ethyl group of A.
The two molecules have all the same kinds of bonds and are extremely similar, but are mirror image isomers. We will learn how to name the two different enantiomers shortly. Although 2-butanol is a chiral molecule and therefore has two enantiomers, the very similar molecule 2-propanol is achiral and does not exist as an enantiomeric pair.
In the illustration, you can see that B slides over onto A with all corresponding groups superimposing perfectly.
Many simple molecules are of this kind. How can we predict whether a molecule is chiral or achiral? One of the simple ways is to use the concept of a stereogenic center. If a molecule has a single stereogenic center it will necessarily be chiral. The most common kind of stereogenic center is a carbon or other atom which has four different atoms or groups directly attached to it.
You can see that the central carbon of 2-butanol the one marked by an asterisk is a stereogenic center, having H,OH,methyl, and ethyl groups attached. Since it has just a single stereogenic centerit must be chiral. On the other hand, 2-propanol has no stereogenic center and is achiral.
The corresponding carbon atom of 2-propanol has an OH,H, and two methyl groups attached.Explain Configuration and Conformation - Stereochemistry - Organic Chemistry
Of course, no methyl carbon atom or methylene carbon can be chiral since these groups automatically have at least two identical groups H's attached. We will see a little later what happens when we have more than one stereogenic center. The second method, especially useful when there is more than one stereogenic center, is the use of symmetry elements. If the molecule or object has either a plane of symmetry or a center of symmetry it is achiral. The examples shown below refer to cis- and trans-1,2-dimethylcyclobutane, The former of which is achiral and the latter chiral.
They both have two stereogenic centers, viz. This emphasizes the point that a molecule or object is guaranteed to be chiral only if it has a single stereogenic center. If it has more than one stereogenic center, it may be either chiral or achiral. Note that in the cis isomer, the two methyls are on the same side of the ring and are equidistant from the plane of symmtery which runs through the center of the ring perpendicular to the ring.
In the trans isomer, the methyls are on opposite sides of the ring, so that where there is a methyl group on the right there is a H on the left. What is the relationship between the cis and trans isomers of 1,2-dimethylcyclobutane??? They are diastereoisomers, having the same connectivity but obviously not being mirror images of each other.
To sum up, there are three isomers of 2,3-dimethylcyclobutane, a single cis isomer, and two enantiomeric trans isomers. The plane of symmetry is relatively easy to find and is the most common one to look for, but one other element of symmetry also guarantees an achiral molecule, and that is the center of symmetry.
This is a point in the molecule for which any line drawn through the point will encounter identical components of the object at equal distances from the center of symmetry. In the case illustrated, 2,3-dimethylbutane the so-called meso isomerthe center of symmetry is at the center point of the C2-C3 carbon-carbon bond. One of the dotted lines shown connects the equivalent bromines on of the two carbons,another connects equivalent methyl groups, and a third connects equivalent hydrogens not shown.
The meso isomer is just one of the three stereoisomers of this system. Again, there is one enantiomeric pair plus this meso isomer, which is achiral. A center of symmetry will be encountered in any molecule which has two equivalent chiral centers i.
The two carbons of this molecule both have H,methyl,bromine, and 1-bromoethyl substituents. Please note that the stereogenic center need not be carbon. It can be a quaternary nitrogen atom the nitrogen of an ammonium salt, if there are four different groups attached to the nitrogen. The convention which is used is called the R,S system because one enantiomer is assinged as the R enantiomer and the other as the S enantiomer.
What are the rules which govern which is which?? Priorities are assigned to each of the four different groups attached to a given stereogenic center one through four, one being the group of highest priority. It should be understood that each stereogenic center has to be treated separately. Beyond Constitutional Isomerism Stereochemistry The last three C's - constitution, configuration, and conformation - all have to do with isomerism.
As we have seen before, constitutional isomers differ in the nature and sequence of their bonds. But these are not the only kind of isomer. Configurational and conformational isomers differ not in their bonding pattern constitutionbut in the position, or geometric arrangement, of their atoms in space - their stereochemistry. Recall that the position of atoms in space is precisely what older, established chemists had been warning their students not to think about in the second half of the 19th century recall Cannizzaro from Lecture 24 or Kolbe from slide 1 of this lecture.
Isomers with the same constitution are called stereoisomers. They can differ either in configuration or in conformation. Keep in mid that the distinction between configurational and conformational isomers is artificial, i. Configurational Isomers To convert one configurational isomer to another, a bond must be broken and reformed.