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

Question

What is the configuration of each of the asymmetric centers in the following compounds?

c. Structural representation of a compound showing asymmetric centers with hydrogen and bromine substituents.

d. Structural representation of a compound with asymmetric centers, showing hydroxyl and hydrogen groups.

Accepted Answer

Hello everyone. Let's do this problem. It says assign the absolute configurations to the chiral centers in the following structures. Structure A, we have two Chloro prop penal with the A L ending and structure B, we have 345 trihydrate penan 20. So if we're assigning absolute configurations to these chiral centers, we're going to use the con in gold pre log nomenclature rules and tailored specifically for fisher projections because both of these structures are shown as fissure projections, right. So the first step with a car center, it's going to have four different groups attached to it. So we need to first assign the priorities of the four different groups and we do this according to atomic mass. So the higher the atomic mass of the first atom connected to that car center, then the higher the priority of the whole group. OK. And what if there is a tie between the initial atoms of two groups or multiple groups? Then we just compare their adjacent atoms still according to atomic mass. And what if we have a double bond or a triple bond? Well, we'll just count that atom multiple times. So a double bond will count that atom as bonded two times and a triple bond will count that atom being bonded three times. So for example, if we have a carbon double bonded to oxygen, we equate that to a carbon single bonded to two oxygen atoms. OK. So now once we assign the priorities, we're just about ready to assign R or S configuration, but it depends on the orientation of our lowest priority group group. number four. So in a fisher projection, group, number four can either be horizontal or it can be vertical. And this is similar to wedge and dash configuration, right? So horizontal groups are wedge and vertical groups are dashed in a Fisher projection. So this matters for when we Connect groups 1 2 and three and the circle goes either counterclockwise or clockwise in this direction will depend on or determine whether our configuration is art or s. So if we connect groups 12 and three, when group four is horizontal and that circle goes counterclockwise, that is our configuration clockwise is S configuration for again, group four being horizontal. And for when group number four is vertical, and we connect that circle counter clockwise. That is S configuration clockwise is our configuration. So let's see which carbons in these structures are chiral and determine their configuration. So starting with structure A, we only have one intersection of a carbon and that happens to be a chiral carbon. So let's assign the priorities of these four different groups, the chlorine atom has the highest priority. Hydrogen always has the lowest priority. And then we have two carbon groups. So we have a methyl group which is a carbon with three hydrogens. And then we have an aldehyde group which is a carbon with two oxygens and a hydrogen, right, counting that double bond oxygen as two oxygens. So the carbon with the oxygens is going to have higher priority than the methyl group. So that aldehyde group will get second priority methyl group priority number three. So in this structure, what is the orientation of our lowest priority group? Group? # four is vertical. And when we connect groups 12 and three, does that circle go clockwise or counterclockwise? 123 that is going clockwise? So this chiral center has our configuration. All right, let's go on to structure B how many chiral centers does this carbon have? Sorry. Does this structure have? Well, a trick for Fisher projections is to look at the intersections of these carbons, right? Those are going to be chiral in this case. So both of them have four different groups. So starting with the top most carb carbon numbering the priorities. Hydrogen is lowest priority. Alcohol is highest priority and then we have two carbon groups. So looking at the top carbon group, we have oxygen, oxygen carbon, looking at the bottom carbon group, we have oxygen carbon hydrogen. So that top group is going to have higher priority and the rest of the carbon chain below will have third priority. So in this case, group # four is verte or sorry. Horizontal group, number four is horizontal connecting groups 12 and three, that circle is going counterclockwise. So this also has our configuration. OK? And looking at the second chiral center, it's actually similar groups, right? So the priorities are going to be the same but just different orientation. So hydrogen is highest priority or sorry, lowest priority alcohol is highest priority. Then we have two carbon groups. So the upper carbon group has carbon oxygen, hydrogen, the lower group has hydrogen, hydrogen, oxygen. So the group above has an oxygen and a carbon. So that is going to be the second highest priority and the C H2O H group will be third priority. OK. So in this group or at this chiral center group, number four is also horizontal But connecting groups 1, 2 and three is going clockwise this time. So we have s orientation. So in summary, for structure A at the one chiral center, we have R configuration and for structure B, we have R and then S configuration. So that is it for this problem. I hope this video is helpful and thanks for watching.

What is the configuration of each of the asymmetric centers in the following compounds?
c.
d.

Key Concepts

Chirality

Fischer Projections

R and S Configuration

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