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

c. D-ribose?

Accepted Answer

Hello everyone. Let's do this problem. It says determine the absolute configuration of each of the chiral centers in the following structure. And we are given the structure, the fissure projection of D Golos. So since we are determining the absolute configuration of chiral centers, we are going to have to use the K led pre log nomenclature rules. And specifically for fisher projections, we will make some specifications toward fisher projections. So chiral centers have four different groups on the carbon, right. So the first step, we need to assign priorities of those four groups and we do this by atomic mass. So the higher the atomic mass of the first atom attached to that chiro carbon, then the higher priority group we have there. All right. What if that first atom is the same for a few different groups? What if there is a tie? Well, then we will just look at their adjacent atoms and compare those in the same way using atomic mass. What if we have a double bond or a triple bond, then we will simply count those as multiple atoms bonded to that carbon. So for example, if we have ac double bond O group, we would count this as a carbon with two single bonds to two oxygen atoms. All right. So now once we apply or assign our priorities, we are almost ready to assign R or S configuration. But this depends on our lowest priority group. Group number four. And it depends if group number four is horizontal in a fisher projection, which is just wedged, right? Or it depends if group number four is vertical, there are separate criteria for these conditions. And when a group is vertical, that is dashed orientation in a fissure projection. So to determine R or S we connect groups 12 and three in our, the groups that we assign priorities. So the groups that we assign 12 and three, we connect those in a circle. And if that circle goes counterclockwise, oops that will be one configuration. If that circle goes clockwise, that will be a different orientation. But again, it depends on our lowest priority group. So if our lowest priority group is horizontal, if we connect groups 12 and three, and that goes counterclockwise, we will have our orientation. If we connect groups 12 and three and that goes clockwise, we have S orientation and the opposite is true for when group number four is vertical, counterclockwise will be s and clockwise will be R in a fisher projection. It's more common to see group number four in the horizontal position because the vertical groups are usually carbon groups. And usually if we have a hydrogen, that will be our lowest priority group and that will be horizontal. So I'll highlight that in yellow because that's probably what we will use for this compound. So let's look at our D Golos and see which carbons are chiral, which carbons have four different groups. So we actually have four carl centers. All of the intersections of carbons here have four different groups. I'm going to alternate colors for each carbon just to differentiate them a little bit. So starting with the top most carbon, let's assign the priority of the groups attached to that carbon. So the highest priority is going to be this alcohol group on the right, lowest priority will be the hydrogen on the left. And let's see, we have two carbon groups. So the top carbon group has a double bond to oxygen and a hydrogen, right? That's an aldehyde group. And the other carbon group below has a carbon oxygen and hydrogen atom. So which one has higher priority c attached to ooh or C attached to och that ooh right, the oxygen has a higher atomic mass than carbon. So the aldehyde group will be second highest priority and the rest of our carbon chain will be third priority. So connecting groups 12 and three that goes counterclockwise. And since group number four is in the horizontal position, this means we have our configuration on that first chiral carbon. All right, So moving on to our second carro carbon, I noticed that the hydrogen and alcohol group are in the same position as the first croc carbon. So I think we're going to have the same orientation. Let's see, the alcohol group has highest priority. Hydrogen has lowest priority. And even if we move further down the chain, that top group is still going to have the highest priority because of that aldehyde. So let's look if we have, if we go to the carbon group, on top, that carbon is attached to carbon oxygen hydrogen, if we go to the carbon group below it is attached to carbon oxygen hydrogen. So then we would look at the other atoms that that carbon is attached to. So for the top, it's the aldehyde group, which we know is high priority. And if we go below, we have just another carbon oxygen hydrogen. So it's always going to be highest priority for the top part of the chain towards the aldehyde. All right. So connecting groups 12 and three, again, that's going counterclockwise group four is horizontal. So this is our configuration. Now, on the third chiral carbon, the alcohol group and the hydrogen are switched. So we're going to have s orientation, the orientation is going to change because those two groups flip flopped. So let's see, alcohol is highest priority. Hydrogen's lowest priority. We said the top of the chain will be second highest priority. Bottom part of the chain is third priority connecting groups 12 and three, that's going clockwise, which confirms that we have s orientation since group four is horizontal still. And lastly, we have reverted back to hydrogen on the left and alcohol on the right, for the fourth chiro carbon assigning our priorities. One for the alcohol four, for the hydrogen two, for the top part of the chain three for the C H2O H group connecting groups 12 and three that goes counter clockwise. So we again have our configuration. So let me redraw this structure so we can neatly summarize or answer. So I'm just redrawing D Golos and writing our orientation for our croc carbons, which I'll give those chiro carbons a an asterisk because that's how we denote a chiro carbon. And the orientation from top to bottom of croc carbons is RRSR which makes d the correct answer for this problem. That's it for this one. I hope this video was helpful and thank you for watching.

What is the configuration of each of the asymmetric centers in the Fischer projection of
c. D-ribose?

Key Concepts

Fischer Projections

Chirality and Asymmetric Centers

D and L Configuration

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