(i) Which of the following pairs of compounds would you expect...

Question

(i) Which of the following pairs of compounds would you expect to have different physical properties?

(ii) What is the relationship between each of the pairs?

(iii) Assign the absolute configuration of each stereocenter to confirm your answer.

(b) Structural comparison of two isomer compounds with hydroxyl and aldehyde groups, highlighting differences in configuration.

Accepted Answer

Hi, everyone. Let's look at our next question. It says, and we have three different parts. This question, number one, identify the relationship between the pair of molecules. Number two are the following compounds expected to have different physical properties. And number three identify the absolute configuration of each stereo center and we have two compounds and then four different multiple choice possibilities. With answers to each part, we'll talk about them in more detail as we look more closely at this. So on the left, our compound is a cyclops ring and it has an alcohol and two chlorine groups, the chlorine groups are on adjacent carbons. If we call one of them carbon one and one of them carbon two, then the alcohol group is on carbon four. So the two chlorine groups both are on dashed wedges, meaning uh pointing down out of the behind the plane of the screen, we should say, and then our alcohol group is on a solid wedge, meaning it's coming up towards us out of the plane of the screen. And then it says that's our first molecule. And then the other molecule on the right that we're supposed to compare it to we have it same uh substituents in the same locations oh on a solid wedge, chlorine on dashed wedges but facing the opposite direction. So molecule on the left, the oh is on the right chlorine, on the left molecule, on the right o on the left chlorine's on the right. So let's look at question one. What is the relationship between the pair of molecules? When we look at our second molecule, all we need to do is essentially rotate it 180 degrees to the right and it matches the other. So relationship is that these are identical because you can rotate them around and they're exactly the same. So we have identical molecules. And so then we look at question two, are they, do they have different physical properties? The answer is no, because they're the same compound. So let's look through our answer choices to see if we can eliminate any of them. In choice. A, the answer to number one is identical. So that's correct. The answer to number two is no, they are not expected to have different properties. So those are both, all right. Choice B scroll down a bit to make it show, it says for number one ditherers. So this is incorrect since they're identical. So choice B, we'll cross that out. Choice C for the number one says constitutional isomers, so that's incorrect. So cross out choice C and then choice D says for number one and an tumors. But again, we have identical molecule. So to d not a correct answer, recall that dimers are molecules that have a different arrangement in space but are not mirror images and anti Immers, non superimposable mirror images. And then constitutional isomers having different connections or different bonds. But again, we have identical molecules here. Well, by process of elimination, we've already got our answer choice. A which you would pick if you were on a test and then be done. But we want to be a little more thorough. So we will look at question three, identify the ab absolute configuration of each stereo center. So where are our stereo centers here? Well, our alcohol group, the carbon it's on is not a stereo center because this is symmetrical, there's a plaintiff symmetry passing through that alcohol group. So it has, does not have four different groups attached to it. The two attached part of the rings are identical. However, when we look at our carbons where the chlorine are attached, because each has an adjacent chlorine, they have four different groups. So they have a hydrogen, a chlorine and the two parts of the ring, which are not symmetrical. So the two carbons of chlorine attached are both stereo centers. So we need to label them as being R or S. Well, first, we need to find the lowest priority substitute and where it is. So the one that is not drawn here is hydrogen and in both cases, the hydrogen is in the back. So we'll note over here, hydrogen and back or excuse me, the hydrogen is in the front, hydrogen is in the front, the chlorine is in the back, hydrogen in the front. So recall that when we think about the arrangement of our different groups and are looking to see whether groups 1 to 3 are in clockwise or counterclockwise, then hydrogen in front means we need to do everything opposite. Think hydrogen's lowest priority group, if it's in the front, that makes that's sort of the opposite of what you'd expect. You think, lowest priority, least important in the back. So that means you need to switch, if you're clockwise, you are s if you're counterclockwise, you're R. So let's label our other groups on the carbon molecule on the top. Our chlorine would be priority one going to label this a number in red to keep it straight from the other carbon. And then we have the two carbons in the ring, one of which has a chlorine attached and one of which just has two hydrogens. So the adjacent carbon with the chlorine on it is priority two. And then the carbon without a chlorine is number three. So to go from 1 to 2 to 3, we go counterclockwise. So our top carbon, both cases hydrogen in front, counterclockwise. All right. CCW. So that would be the, since we have to flip it around the R arrangement. And our bottom carbon hydrogen in front, let's label our other priority groups, which I'll do in green. Priority one is obviously the chlorine priority two is the adjacent carbon with a chlorine and priority three, the carbon in the ring with two hydrogens. So in this case, we need to go clockwise to go from 1 to 2 to 3. So we've got clockwise and since our hydrogen is in front, that would be the s arrangement or S configuration. So our top carbon has the R configuration, our bottom has the S that matches what's on choice A. So we have for certain, our correct answer, the relationship between the pair of molecules is choice a identical. And for number two, no, the comps are not to expect not expected to have different physical properties. And for part three, we've labeled the stereo centers, the carbon on top is R and the carbon below is S see you in the next video.

(i) Which of the following pairs of compounds would you expect to have different physical properties?
(ii) What is the relationship between each of the pairs?
(iii) Assign the absolute configuration of each stereocenter to confirm your answer.
(b)

Key Concepts

Stereochemistry

Physical Properties of Compounds

Absolute Configuration

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