Stereoisomers | 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 stereoisomers for isomers. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Overview of Stereoisomers
Two main types of configurational, non-geometric stereoisomers, enantiomers (also called optical isomers), and diastereomers.
Enantiomers
Enantiomers are chiral molecules that have non-superimposable mirror images. They have no internal plane of symmetry, which is why they're non-superimposable on their mirror images. They however have identical physical properties to each other and differ only in their interactions with plane polarized light and other chiral molecules, whether stereoselective chemical agents or biological ones.
The optical activity of any given pair of enantiomers will be opposite in its direction of rotation of plane polarized light, but equal in magnitude. Meaning if the R-enantiomer rotates plane polarized 23 degrees clockwise, the S-enantiomer will rotate plane polarized light 23 degrees counter-clockwise. On ways of classifying chiral centers, recall however that there is no predictable interaction between the configuration of a stereocenter or the structure of a compound and the magnitude or direction in which it may rotate plane polarized light. The only predictable property of such compounds is that if they can exist in two enantiomeric forms, meaning they are chiral and have no internal plane of symmetry, they will rotate plane polarized light incident on them in some fashion. Conversely, compounds with multiple chiral centers but an internal plane of symmetry are NOT optically active!
Enantiomers also need to be different at every chiral center. For instance, a compound with 3 chiral centers may be 2R, 3S, 4R – the corresponding enantiomer must be 2S, 3R, 4S. However, beware that if any arrangement of multiple chiral centers results in an internally symmetrical molecule, you are very likely dealing with a meso compound.
Finally, racemic mixtures are those in which both enantiomers of a single compound (or all compounds within) are present in equal proportion. Therefore, while several optically active compounds may be present in a racemic mixture, the mixture itself will be optically inactive and not rotate plane polarized light – rather it will transmit it unrotated.
A set of 4 isomeric molecules provides a good illustration of the difference between enantiomers and diastereomers. Notice that between the left-hand and right-hand molecules on top, every single chiral center is inverted. This also holds for the left-hand and right-hand molecules on the bottom. However, between the top-left and bottom-left molecule, only 2 out of 3 chiral centers differ – therefore they are diastereomers. The same relationship holds for the top-right and bottom-right molecules. Between the two diagonal pairs however, we observe a sub-type of diastereomers: These are epimers, a type of compound with multiple stereocenters where only a single such center differs!
Diastereomers often have different chemical and physical properties from each other. Their optical activity is also largely unpredictable, and not predictably related to the number of chiral centers that differ. Meaning one diastereomer may have an optical rotation of +47˚, while another may have +12 ˚. This means that the convenient relationship optical activity between enantiomers does not hold for diastereomers.
Finally, there are a lot of different types of diastereomers, some with chiral centers, some without. And these include epimers, anomers, meso compounds, and geometric isomers. Keep a look out for these in future sections.
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Overview of Stereoisomers
Two main types of configurational, non-geometric stereoisomers, enantiomers (also called optical isomers), and diastereomers.
Enantiomers
Enantiomers are chiral molecules that have non-superimposable mirror images. They have no internal plane of symmetry, which is why they're non-superimposable on their mirror images. They however have identical physical properties to each other and differ only in their interactions with plane polarized light and other chiral molecules, whether stereoselective chemical agents or biological ones.
The optical activity of any given pair of enantiomers will be opposite in its direction of rotation of plane polarized light, but equal in magnitude. Meaning if the R-enantiomer rotates plane polarized 23 degrees clockwise, the S-enantiomer will rotate plane polarized light 23 degrees counter-clockwise. On ways of classifying chiral centers, recall however that there is no predictable interaction between the configuration of a stereocenter or the structure of a compound and the magnitude or direction in which it may rotate plane polarized light. The only predictable property of such compounds is that if they can exist in two enantiomeric forms, meaning they are chiral and have no internal plane of symmetry, they will rotate plane polarized light incident on them in some fashion. Conversely, compounds with multiple chiral centers but an internal plane of symmetry are NOT optically active!
Enantiomers also need to be different at every chiral center. For instance, a compound with 3 chiral centers may be 2R, 3S, 4R – the corresponding enantiomer must be 2S, 3R, 4S. However, beware that if any arrangement of multiple chiral centers results in an internally symmetrical molecule, you are very likely dealing with a meso compound.
Finally, racemic mixtures are those in which both enantiomers of a single compound (or all compounds within) are present in equal proportion. Therefore, while several optically active compounds may be present in a racemic mixture, the mixture itself will be optically inactive and not rotate plane polarized light – rather it will transmit it unrotated.
A set of 4 isomeric molecules provides a good illustration of the difference between enantiomers and diastereomers. Notice that between the left-hand and right-hand molecules on top, every single chiral center is inverted. This also holds for the left-hand and right-hand molecules on the bottom. However, between the top-left and bottom-left molecule, only 2 out of 3 chiral centers differ – therefore they are diastereomers. The same relationship holds for the top-right and bottom-right molecules. Between the two diagonal pairs however, we observe a sub-type of diastereomers: These are epimers, a type of compound with multiple stereocenters where only a single such center differs!
Diastereomers often have different chemical and physical properties from each other. Their optical activity is also largely unpredictable, and not predictably related to the number of chiral centers that differ. Meaning one diastereomer may have an optical rotation of +47˚, while another may have +12 ˚. This means that the convenient relationship optical activity between enantiomers does not hold for diastereomers.
Finally, there are a lot of different types of diastereomers, some with chiral centers, some without. And these include epimers, anomers, meso compounds, and geometric isomers. Keep a look out for these in future sections.
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
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