What is the configuration of each of the asymmetric centers in...

Question

What is the configuration of each of the asymmetric centers in the Fischer projection of

b. D-galactose?

Accepted Answer

Hey, everyone, let's do this problem and says, determine the absolute configuration of each of the Cairo centers in the following structure and were given the Fischer projection of D Aloes. So since we're determining the configuration of the Cairo centers in our structure, that means we're going to need to use the con mingled pre log nomenclature rules. And in this case, specifically for a Fischer projection, which doesn't change anything much. So first archival carbon is going to have four different groups, right? That's part of the definition of chirality. So the first step we assign priorities to those four different groups. So we number them one through four and we do this by atomic mass. So the higher the atomic mass, the higher the priority. So highest atomic mass will get priority. No one. What if the first atom connected to our caramel center is a tie? What if there are two atoms that are the same? Well, then we simply apply the rules to the adjacent atoms. So look at the other atoms connected to those atoms in question. And another important thing is that double bonds will count almost like two single bonds. So you basically count, say if we have a carbon double bonded to an oxygen, you look at that as if the carbon has two oxygen atoms single bonded to it. And the same applies for a triple bond except times three. So now we have our priorities assigned. So we're going to see if we have our Torres configuration, but this depends on the orientation of our fourth and lowest priority group. So if our group number four is horizontal in a Fischer projection, which just means it's wedge, right? Even if it's not drawn as wedge in a Fischer projection, horizontal means wedge That will be different than if our group # is vertical or dashed. So when we assign our priorities, we want to connect a circle after we do that. And if we connect groups 12 and three in a circle and that circle goes counterclockwise. If group number four is horizontal, that means we have our configuration. If group number four is vertical, that means s configuration. If we connect groups 12 and three and they go clockwise, then we will have s configuration for our lowest priority group being horizontal. Our configuration if group number four is vertical, An official projection group #4 is usually horizontal because the vertical groups are usually carbon groups. So I'm going to highlight The first set of criteria. Group # four being horizontal because that's probably what we'll use in this case. So let's look at our structure and identify the Cairo carbons. So we actually have four carbons, carbons. All of these intersections Of carbons have four different groups on them. So I'm going to alternate colors for each carbon just to make it a little a little more clear. So let's assign priorities of the four groups on our first Cairo carbon. So hydrogen will always have lowest priority are alcohol group will have highest priority here because the other two groups are carbon groups. So the top carbon is an alga hide group. So that's bonded to oxygen, oxygen hydrogen, whereas the other carbon group is bonded to oxygen, carbon hydrogen. So those two oxygen atoms are going to make the alga hide group. Highest next highest priority. Second, highest priority. The rest of the carbon chain will be priority number three. And that's actually going to be the case for every single carbon carbon, the chain, the part of the chain that leads to the al to hide or the top part of the chain is always going to have higher priority than the part of the chain leading to the bottom of the structure. So we will apply this to all the carbo carbons. But to finish up this carbon connecting groups, 12 and three, that is going counterclockwise. And since group number four is horizontal, that means this carbon is our configuration. Okay. The second Cairo carbon, I think we're going to have the same configuration for each carbon right, because the hydrogen czar all on the left side, the alcohol groups are all on the right. And like we said, the top part is going to be the same because of that to hide, the bottom part is going to be the same because it's the methanol, right? It's not changing. So let's just do it for each one though and see if our hypothesis is correct. So we have fourth priority for the hydrogen. First priority for the alcohol. Second priority for the top part with the al to hide the top part of the chain. Third priority for the lower part of the chain connecting 12 and three, we go counterclockwise. That is our configuration. Cairo carbon 3/4 priority hydrogen, first priority alcohol, second priority carbon chain leading to the alga hide third priority carbon chain leading to that other alcohol group connecting groups 12 and three. That's counterclockwise which is our configuration in our last carbon. 4th priority hydrogen, first priority alcohol. Now let's see how this works since you might be thinking. Oh, well, that al Jihad group is so far away. How is that still highest priority than this other alcohol group down here? Well, let's take a look at it. So we have carbon that is attached to hydrogen, oxygen, carbon, okay, carbon attached to oxygen, hydrogen, hydrogen. Well, that carbon is going to take priority over that hydrogen. So carbon oxygen, hydrogen is higher priority than oxygen. Hydrogen hydrogen. So our priorities are still the same being too for the north part of the chain three, for the south part of the chain connecting 12 and three, we get our configuration again. So let's draw this all out as a quick summary to more clearly write our answer. So we have hydrogen on the left side of the Fischer projection alcohols all on the right. And these, all, all of these hydrocarbons have our configuration are R R R. So that makes C the correct answer choice for this problem. That's it for this one. I hope this video is helpful and thanks for watching.

What is the configuration of each of the asymmetric centers in the Fischer projection of
b. D-galactose?

Key Concepts

Fischer Projections

Chirality and Asymmetric Centers

D and L Notation

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