Absolute Configuration for Stereochemistry | MCAT Organic Chemistry Prep
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Need help preparing for the Organic Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about absolute configuration for stereochemistry. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Absolute Configuration
Absolute configuration is an unambiguous and consistent method of describing the spatial arrangement of atoms at a chiral center. Chiral centers are atoms or molecules that are bound to four different substituents. Absolute configurations are either denoted as R or S.
Part 1: Assigning Priorities
In order to determine whether a chiral center is in the R or S configuration, we need to first assign priorities according to Cahn-Ingold-Prelog rules. The first step in doing so is to simply rank substituents by atomic number. Since you will have a periodic table available on the MCAT, it is always acceptable to simply reference this to figure out that oxygen will be priority 1, followed by nitrogen as priority 2, the methyl group as priority 3 and hydrogen finally as priority 4. You do want to generally have a good sense of the atomic numbers of common atoms however, to make this process more fluid.
In some instances, ranking by atomic number won’t suffice, as we may encounter a tie. Both deuterium and hydrogen have an atomic number of 1. In instances where atomic number is the same, the highest atomic mass will break the tie. Since deuterium has twice the atomic mass of hydrogen, it is assigned the higher priority in this case.
So we want to look at the atomic mass of deuterium and hydrogen. And we know that deuterium has a greater atomic mass than hydrogen, because deuterium has an extra neutron than hydrogen alone, so that would allow us to rank deuterium as priority number three and hydrogen as priority number four.
Ties for both atomic mass and atomic number are also possible, particularly in cases with polyatomic substituents. In one such case, two different carbon substituents are present. here, the tie is broken by finding the first point of difference, meaning the first instance where different atoms or a different number of connected atoms are present. Frequently in such cases the first point of difference is characterized by a double bond (like in this case, to a heteroatom). Double-bonded substituents count twice, and triple bonded substituents count 3 times. We can see that this allows us to assign priorities to the substituents attached at the chiral center, since the double bond to oxygen on the carbonyl carbon will rank higher than the single bond to the alcohol functional group.
As an exercise for the reader to verify their knowledge of this system, we have provided cysteine with its substituents to the chiral center labeled (and hydrogen hidden). Use this to check that your current knowledge of CIP rules is correct!
Part 2: Determining Configurations
Now that we know how to assign priorities to the different substituents on the chiral center, we now need to use this information to determine the absolute configuration of our molecule of interest. We recommend in this instance a simple right-hand rule: Take your right hand and align the thumb with the lowest priority substituent. Now point your fingers towards priority number one. Finally curl your hand and see which substituents your fingers connect with first. If they go sequentially, meaning 1-2-3, your chiral center is R. If they go out of order, meaning 1-3-2, your chiral center is S.
(Note: If you lack a right hand, a variation on this system this is also possible with your left hand – although in that instance, sequential ordering will indicate S and an out of order sequence will indicate R.)
Applying this approach to a molecule should yield R and doing the same to a differeing molecule should yield S. Solving molecule number 3 requires us to recall the conventions of Fischer projections – vertical lines go into the page, horizontal lines come out of the page. Once this is applied, it however should rapidly become clear that this molecule is in the S configuration. For molecule number 4, we need to reason by process of elimination that our lowest priority substituent, hydrogen, must be going into the page (since the R-group is coming out of the page). This leads us to determine that this must be an R configuration.
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Absolute Configuration
Absolute configuration is an unambiguous and consistent method of describing the spatial arrangement of atoms at a chiral center. Chiral centers are atoms or molecules that are bound to four different substituents. Absolute configurations are either denoted as R or S.
Part 1: Assigning Priorities
In order to determine whether a chiral center is in the R or S configuration, we need to first assign priorities according to Cahn-Ingold-Prelog rules. The first step in doing so is to simply rank substituents by atomic number. Since you will have a periodic table available on the MCAT, it is always acceptable to simply reference this to figure out that oxygen will be priority 1, followed by nitrogen as priority 2, the methyl group as priority 3 and hydrogen finally as priority 4. You do want to generally have a good sense of the atomic numbers of common atoms however, to make this process more fluid.
In some instances, ranking by atomic number won’t suffice, as we may encounter a tie. Both deuterium and hydrogen have an atomic number of 1. In instances where atomic number is the same, the highest atomic mass will break the tie. Since deuterium has twice the atomic mass of hydrogen, it is assigned the higher priority in this case.
So we want to look at the atomic mass of deuterium and hydrogen. And we know that deuterium has a greater atomic mass than hydrogen, because deuterium has an extra neutron than hydrogen alone, so that would allow us to rank deuterium as priority number three and hydrogen as priority number four.
Ties for both atomic mass and atomic number are also possible, particularly in cases with polyatomic substituents. In one such case, two different carbon substituents are present. here, the tie is broken by finding the first point of difference, meaning the first instance where different atoms or a different number of connected atoms are present. Frequently in such cases the first point of difference is characterized by a double bond (like in this case, to a heteroatom). Double-bonded substituents count twice, and triple bonded substituents count 3 times. We can see that this allows us to assign priorities to the substituents attached at the chiral center, since the double bond to oxygen on the carbonyl carbon will rank higher than the single bond to the alcohol functional group.
As an exercise for the reader to verify their knowledge of this system, we have provided cysteine with its substituents to the chiral center labeled (and hydrogen hidden). Use this to check that your current knowledge of CIP rules is correct!
Part 2: Determining Configurations
Now that we know how to assign priorities to the different substituents on the chiral center, we now need to use this information to determine the absolute configuration of our molecule of interest. We recommend in this instance a simple right-hand rule: Take your right hand and align the thumb with the lowest priority substituent. Now point your fingers towards priority number one. Finally curl your hand and see which substituents your fingers connect with first. If they go sequentially, meaning 1-2-3, your chiral center is R. If they go out of order, meaning 1-3-2, your chiral center is S.
(Note: If you lack a right hand, a variation on this system this is also possible with your left hand – although in that instance, sequential ordering will indicate S and an out of order sequence will indicate R.)
Applying this approach to a molecule should yield R and doing the same to a differeing molecule should yield S. Solving molecule number 3 requires us to recall the conventions of Fischer projections – vertical lines go into the page, horizontal lines come out of the page. Once this is applied, it however should rapidly become clear that this molecule is in the S configuration. For molecule number 4, we need to reason by process of elimination that our lowest priority substituent, hydrogen, must be going into the page (since the R-group is coming out of the page). This leads us to determine that this must be an R configuration.
MEDSCHOOLCOACH
To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach
IOS Link: https://play.google.com/store/apps/de...
Apple Link: https://apps.apple.com/us/app/mcat-pr...
#medschoolcoach #MCATprep #MCATstudytools
4 سال پیش
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