A student decided that the configuration of the asymmetric cen...

Question

A student decided that the configuration of the asymmetric centers in a sugar such as D-glucose could be determined rapidly by simply assigning the R configuration to an symmetric center with an OH group on the right and the S configuration to an symmetric center with an OH group on the left. Is he correct?

Accepted Answer

Welcome back. Everyone consider a student's proposal for determining the stereo chemical configuration at asymmetric centers in al fructose using the Fisher projection. The student suggests that an asymmetric center can be assigned an R configuration if the O group is on the left and an S configuration, if the A age group is on the right is this method correct. So essentially what we want to do is assign the configuration for each chiral position that we have, we can begin with the first one that has four different substitutions. We have to recall the K Ingle Prilo rules. We have a central carbon that is bonded to hydrogen. Let's recall that in the facial projection, those bonds that are horizontal represent wedges. So we have the carbon hydrogen wedged bond, we have carbon oxygen wedged bond for the hydroxyl substituent. If we move up, we can essentially represent those as solid lines. The last ones are dashed bonds, right. So essentially we can just throw those dashed bonds. Now, we have a solid line and then this solid line represents carbon bonded to oxygen hydrogen and another carbon. If we look down, we have a dashed band and there's carbon von see hydrogen, another hydrogen and oxygen. Let's assign priorities. So oxygen has the highest atomic number. It gets priority number one. Hydrogen has the lowest atomic number. It gets priority number four, between the two carbons. We can just neglect the same atoms specifically, we can eliminate oxygens, we can eliminate hydrogens and we are comparing carbon against hydrogen. So that means the top substituent is higher in priority than the bottom one. And now, if we trace our path 123, this is going clockwise, but priority number four is pointing up. Meaning we have to essentially swap it to the counter clockwise rotation, which is actually s so we can essentially state that the configuration of the first car position is s and now we are going to continue this process moving on. We have our carbon hygen wedged bond, we have carbon oxygen or the hydroxyl substituent. On top, we have carbon bond to oxygen, carbon and hydrogen. If we look down, we have carbon bonded to oxygen, hydrogen and carbon. Now let's consider our priorities once gain oxygen gets priority number one and hydrogen gets priority number four between the two carbon atoms, we can eliminate carbons, oxygens and hydrogens. So essentially, so far we have the same substituent, right chtoh. Now let's take a look at the carbon atom that we're interested in, right? Let's circle it. So on the bottom, we have ch oh which is bonded to another carbon right. And on top, we have cho h bonded to another carbon. So that means we need to consider the next substitution because the two substitutions are equivalent. So if we expand those bonds, we notice that we once again have carbon atoms and the one on the bottom has hygen hygen oxygen. The one on top essentially has double bonded oxygen. So we can say that those are two oxygens, we can just consider a double bonded oxygen as two oxygen atoms with single bonds and carbon. So now if we eliminate one oxygen, we are essentially comparing oxygen versus hydrogen. So the substituent on top has a higher priority and the substitute on the bottom has a lower priority. 123, we are going clockwise but hygiene is pointing up. So we have to essentially swap it to the opposite configuration of S and finally, if we look at the final chiral position, we have carbon hydrogen on a wedge, carbon oxygen bond on a wedge. On top, we have carbon oxygen, oxygen, carbon and on the bottom, we have carbon boity hydrogen oxygen and another carbon priority number four for hydrogen. Priority number one, for oxygen. If we eliminate oxygens in carbons, the top substituent gets a high priority because we're comparing oxygen versus hydrogen. So we have our priority number two and priority number three. In this case, if we go from 1 to 3, 12 and three, this gives us counterclockwise rotation. But once again, hydrogen is pointing up. So we have to swap it to the clockwise rotation, which is R. Now, if we look at the student's proposal, that student says that R configuration is assigned. If the age group is on the left, it's actually opposite. A wage is on the right. We get R now, if our wages on the right, we should get an S configuration which is once again opposite. If our wages on the left, we get an S configuration. So the students suggestion is incorrect, it's false, it's actually opposite. And we have to keep in mind that this is only applicable to a fructose. This is definitely not the case for any facial projection, right? This is only applicable to this specific compound. Otherwise we have to consider all of the possible subscriptions present in the fissure projection. Thank you for watching.

Key Concepts

Asymmetric Centers

Cahn-Ingold-Prelog Priority Rules

D- and L- Configuration in Sugars

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