Stereochemistry and Isomerism (5B) - MCAT Chemical and Physical Foundations of Biological Systems
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What is the definition of constitutional isomers in organic chemistry?
What is the definition of constitutional isomers in organic chemistry?
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Same formula, different atom connectivity (bonding sequence). Constitutional isomers share the same molecular formula but differ in the sequence of atom connections.
Same formula, different atom connectivity (bonding sequence). Constitutional isomers share the same molecular formula but differ in the sequence of atom connections.
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What is the definition of stereoisomers?
What is the definition of stereoisomers?
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Same connectivity, different 3D arrangement of atoms. Stereoisomers have identical bonding sequences but vary in spatial atom arrangements.
Same connectivity, different 3D arrangement of atoms. Stereoisomers have identical bonding sequences but vary in spatial atom arrangements.
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What is the definition of enantiomers?
What is the definition of enantiomers?
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Non-superimposable mirror-image stereoisomers. Enantiomers are chiral stereoisomers that are mirror images but cannot be superimposed.
Non-superimposable mirror-image stereoisomers. Enantiomers are chiral stereoisomers that are mirror images but cannot be superimposed.
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Identify the R/S rule: with the lowest priority group pointing away, what rotation gives R?
Identify the R/S rule: with the lowest priority group pointing away, what rotation gives R?
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Clockwise $1
ightarrow 2
ightarrow 3$ gives R. With lowest priority away, clockwise priority sequence assigns R configuration.
Clockwise $1 ightarrow 2 ightarrow 3$ gives R. With lowest priority away, clockwise priority sequence assigns R configuration.
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Identify the correction rule: if the lowest priority group points toward you, how do you adjust R/S?
Identify the correction rule: if the lowest priority group points toward you, how do you adjust R/S?
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Assign as usual, then invert the result (R $
ightleftarrows$ S). Reversing the apparent configuration corrects for lowest priority facing the viewer.
Assign as usual, then invert the result (R $ ightleftarrows$ S). Reversing the apparent configuration corrects for lowest priority facing the viewer.
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In CIP rules, how are double bonds treated when ranking substituent priority?
In CIP rules, how are double bonds treated when ranking substituent priority?
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As if bonded to duplicate atoms (double bond counts twice). CIP treats multiple bonds as replicated single bonds for priority ranking.
As if bonded to duplicate atoms (double bond counts twice). CIP treats multiple bonds as replicated single bonds for priority ranking.
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How does the CIP rule treat isotopes when assigning priority (for example, $^2H$ vs $^1H$)?
How does the CIP rule treat isotopes when assigning priority (for example, $^2H$ vs $^1H$)?
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Higher mass number gets higher priority (e.g., $^2H > ^1H$). CIP rules prioritize isotopes by higher atomic mass when atomic numbers match.
Higher mass number gets higher priority (e.g., $^2H > ^1H$). CIP rules prioritize isotopes by higher atomic mass when atomic numbers match.
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What is the correct first step of CIP priority assignment at a stereocenter?
What is the correct first step of CIP priority assignment at a stereocenter?
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Rank substituents by atomic number of the directly attached atom. CIP rules begin by comparing atomic numbers of atoms directly bonded to the stereocenter.
Rank substituents by atomic number of the directly attached atom. CIP rules begin by comparing atomic numbers of atoms directly bonded to the stereocenter.
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What is the relationship between R/S configuration and (+)/(−) optical rotation?
What is the relationship between R/S configuration and (+)/(−) optical rotation?
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No general correlation; R/S does not predict (+) or (−). R/S assignment follows priority rules independently of optical rotation direction.
No general correlation; R/S does not predict (+) or (−). R/S assignment follows priority rules independently of optical rotation direction.
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What is the maximum number of stereoisomers possible for a molecule with $n$ stereocenters (no symmetry)?
What is the maximum number of stereoisomers possible for a molecule with $n$ stereocenters (no symmetry)?
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$2^n$ stereoisomers. Without symmetry, $n$ stereocenters yield $2^n$ configurations from independent chirality combinations.
$2^n$ stereoisomers. Without symmetry, $n$ stereocenters yield $2^n$ configurations from independent chirality combinations.
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What is the definition of diastereomers?
What is the definition of diastereomers?
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Stereoisomers that are not mirror images of each other. Diastereomers are stereoisomers differing in configuration without being mirror images.
Stereoisomers that are not mirror images of each other. Diastereomers are stereoisomers differing in configuration without being mirror images.
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What is a chiral center (stereogenic center) in typical MCAT organic molecules?
What is a chiral center (stereogenic center) in typical MCAT organic molecules?
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An $sp^3$ atom bonded to four different substituents. A chiral center features a tetrahedral $sp^3$ carbon with four unique substituents, enabling enantiomerism.
An $sp^3$ atom bonded to four different substituents. A chiral center features a tetrahedral $sp^3$ carbon with four unique substituents, enabling enantiomerism.
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Which property distinguishes enantiomers in an achiral environment: melting point or optical rotation?
Which property distinguishes enantiomers in an achiral environment: melting point or optical rotation?
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Optical rotation (melting points are identical). Enantiomers exhibit identical physical properties except for opposite rotations of plane-polarized light.
Optical rotation (melting points are identical). Enantiomers exhibit identical physical properties except for opposite rotations of plane-polarized light.
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What is the definition of a racemic mixture?
What is the definition of a racemic mixture?
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A $50:50$ mixture of two enantiomers; net rotation $=0$. Equal enantiomer proportions in a racemic mixture cancel out optical activity.
A $50:50$ mixture of two enantiomers; net rotation $=0$. Equal enantiomer proportions in a racemic mixture cancel out optical activity.
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What does it mean for a compound to be meso?
What does it mean for a compound to be meso?
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Achiral despite stereocenters; has an internal plane of symmetry. Meso compounds are achiral due to a symmetry plane despite having stereocenters.
Achiral despite stereocenters; has an internal plane of symmetry. Meso compounds are achiral due to a symmetry plane despite having stereocenters.
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Which option correctly gives the maximum stereoisomer count for $n=3$ stereocenters with no symmetry?
Which option correctly gives the maximum stereoisomer count for $n=3$ stereocenters with no symmetry?
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$2^3 = 8$ stereoisomers. Three stereocenters without symmetry produce $2^3$ unique stereoisomer combinations.
$2^3 = 8$ stereoisomers. Three stereocenters without symmetry produce $2^3$ unique stereoisomer combinations.
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Identify the relationship: same connectivity, not mirror images, and different configurations at some stereocenters; what are they?
Identify the relationship: same connectivity, not mirror images, and different configurations at some stereocenters; what are they?
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Diastereomers. Diastereomers share connectivity but differ in some stereocenter configurations without being enantiomers.
Diastereomers. Diastereomers share connectivity but differ in some stereocenter configurations without being enantiomers.
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Identify the relationship: two molecules are mirror images and non-superimposable; what are they called?
Identify the relationship: two molecules are mirror images and non-superimposable; what are they called?
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Enantiomers. Non-superimposable mirror images characterize enantiomers due to molecular chirality.
Enantiomers. Non-superimposable mirror images characterize enantiomers due to molecular chirality.
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Using E/Z notation, what does E mean for the highest-priority groups on an alkene?
Using E/Z notation, what does E mean for the highest-priority groups on an alkene?
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Highest-priority groups are on opposite sides (E, entgegen). E indicates opposite-side arrangement of highest-priority groups on the alkene.
Highest-priority groups are on opposite sides (E, entgegen). E indicates opposite-side arrangement of highest-priority groups on the alkene.
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Using E/Z notation, what does Z mean for the highest-priority groups on an alkene?
Using E/Z notation, what does Z mean for the highest-priority groups on an alkene?
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Highest-priority groups are on the same side (Z, zusammen). Z denotes same-side placement of highest-priority groups across the double bond.
Highest-priority groups are on the same side (Z, zusammen). Z denotes same-side placement of highest-priority groups across the double bond.
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What condition must be met for an alkene to show cis/trans (or E/Z) isomerism?
What condition must be met for an alkene to show cis/trans (or E/Z) isomerism?
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Each alkene carbon must have two different substituents. Distinct substituents on each double-bond carbon enable stable cis-trans isomers.
Each alkene carbon must have two different substituents. Distinct substituents on each double-bond carbon enable stable cis-trans isomers.
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What is the definition of geometric (cis/trans or E/Z) isomerism?
What is the definition of geometric (cis/trans or E/Z) isomerism?
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Stereoisomerism from restricted rotation (often at C=C or rings). Geometric isomerism occurs due to rotation barriers in double bonds or rings, creating distinct configurations.
Stereoisomerism from restricted rotation (often at C=C or rings). Geometric isomerism occurs due to rotation barriers in double bonds or rings, creating distinct configurations.
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In cyclohexane, which chair substituent position is generally more stable for bulky groups: axial or equatorial?
In cyclohexane, which chair substituent position is generally more stable for bulky groups: axial or equatorial?
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Equatorial (minimizes $1,3$-diaxial interactions). Equatorial positions reduce steric hindrance from 1,3-diaxial interactions in chair forms.
Equatorial (minimizes $1,3$-diaxial interactions). Equatorial positions reduce steric hindrance from 1,3-diaxial interactions in chair forms.
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Which conformation is most stable for substituted ethane: staggered or eclipsed?
Which conformation is most stable for substituted ethane: staggered or eclipsed?
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Staggered. Staggered conformations minimize torsional strain in ethane-like molecules.
Staggered. Staggered conformations minimize torsional strain in ethane-like molecules.
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What is the definition of a conformational isomer (conformer)?
What is the definition of a conformational isomer (conformer)?
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Isomers interconverted by rotation about a single $sigma$ bond. Conformers arise from free rotation around sigma bonds, yielding different spatial forms.
Isomers interconverted by rotation about a single $sigma$ bond. Conformers arise from free rotation around sigma bonds, yielding different spatial forms.
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