MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 5b Stereochemistry Isomerism

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This deck focuses on 5b Stereochemistry Isomerism, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the definition of constitutional isomers in organic chemistry?

Answer: Same formula, different atom connectivity (bonding sequence). Constitutional isomers share the same molecular formula but differ in the sequence of atom connections.

Flashcard 2: What is the definition of stereoisomers?

Answer: Same connectivity, different 3D arrangement of atoms. Stereoisomers have identical bonding sequences but vary in spatial atom arrangements.

Flashcard 3: What is the definition of enantiomers?

Answer: Non-superimposable mirror-image stereoisomers. Enantiomers are chiral stereoisomers that are mirror images but cannot be superimposed.

Flashcard 4: Identify the R/S rule: with the lowest priority group pointing away, what rotation gives R?

Answer: Clockwise $1 ightarrow 2 ightarrow 3$ gives R. With lowest priority away, clockwise priority sequence assigns R configuration.

Flashcard 5: Identify the correction rule: if the lowest priority group points toward you, how do you adjust R/S?

Answer: Assign as usual, then invert the result (R $ightleftarrows$ S). Reversing the apparent configuration corrects for lowest priority facing the viewer.

Flashcard 6: In CIP rules, how are double bonds treated when ranking substituent priority?

Answer: As if bonded to duplicate atoms (double bond counts twice). CIP treats multiple bonds as replicated single bonds for priority ranking.

Flashcard 7: How does the CIP rule treat isotopes when assigning priority (for example, $^2H$ vs $^1H$)?

Answer: Higher mass number gets higher priority (e.g., $^2H > ^1H$). CIP rules prioritize isotopes by higher atomic mass when atomic numbers match.

Flashcard 8: What is the correct first step of CIP priority assignment at a stereocenter?

Answer: Rank substituents by atomic number of the directly attached atom. CIP rules begin by comparing atomic numbers of atoms directly bonded to the stereocenter.

Flashcard 9: What is the relationship between R/S configuration and (+)/(−) optical rotation?

Answer: No general correlation; R/S does not predict (+) or (−). R/S assignment follows priority rules independently of optical rotation direction.

Flashcard 10: What is the maximum number of stereoisomers possible for a molecule with $n$ stereocenters (no symmetry)?

Answer: $2^n$ stereoisomers. Without symmetry, $n$ stereocenters yield $2^n$ configurations from independent chirality combinations.

Flashcard 11: What is the definition of diastereomers?

Answer: Stereoisomers that are not mirror images of each other. Diastereomers are stereoisomers differing in configuration without being mirror images.

Flashcard 12: What is a chiral center (stereogenic center) in typical MCAT organic molecules?

Answer: An $sp^3$ atom bonded to four different substituents. A chiral center features a tetrahedral $sp^3$ carbon with four unique substituents, enabling enantiomerism.

Flashcard 13: Which property distinguishes enantiomers in an achiral environment: melting point or optical rotation?

Answer: Optical rotation (melting points are identical). Enantiomers exhibit identical physical properties except for opposite rotations of plane-polarized light.

Flashcard 14: What is the definition of a racemic mixture?

Answer: A $50:50$ mixture of two enantiomers; net rotation $=0$. Equal enantiomer proportions in a racemic mixture cancel out optical activity.

Flashcard 15: What does it mean for a compound to be meso?

Answer: Achiral despite stereocenters; has an internal plane of symmetry. Meso compounds are achiral due to a symmetry plane despite having stereocenters.

Flashcard 16: Which option correctly gives the maximum stereoisomer count for $n=3$ stereocenters with no symmetry?

Answer: $2^3 = 8$ stereoisomers. Three stereocenters without symmetry produce $2^3$ unique stereoisomer combinations.

Flashcard 17: Identify the relationship: same connectivity, not mirror images, and different configurations at some stereocenters; what are they?

Answer: Diastereomers. Diastereomers share connectivity but differ in some stereocenter configurations without being enantiomers.

Flashcard 18: Identify the relationship: two molecules are mirror images and non-superimposable; what are they called?

Answer: Enantiomers. Non-superimposable mirror images characterize enantiomers due to molecular chirality.

Flashcard 19: Using E/Z notation, what does E mean for the highest-priority groups on an alkene?

Answer: Highest-priority groups are on opposite sides (E, entgegen). E indicates opposite-side arrangement of highest-priority groups on the alkene.

Flashcard 20: Using E/Z notation, what does Z mean for the highest-priority groups on an alkene?

Answer: Highest-priority groups are on the same side (Z, zusammen). Z denotes same-side placement of highest-priority groups across the double bond.

Flashcard 21: What condition must be met for an alkene to show cis/trans (or E/Z) isomerism?

Answer: Each alkene carbon must have two different substituents. Distinct substituents on each double-bond carbon enable stable cis-trans isomers.

Flashcard 22: What is the definition of geometric (cis/trans or E/Z) isomerism?

Answer: 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.

Flashcard 23: In cyclohexane, which chair substituent position is generally more stable for bulky groups: axial or equatorial?

Answer: Equatorial (minimizes $1,3$-diaxial interactions). Equatorial positions reduce steric hindrance from 1,3-diaxial interactions in chair forms.

Flashcard 24: Which conformation is most stable for substituted ethane: staggered or eclipsed?

Answer: Staggered. Staggered conformations minimize torsional strain in ethane-like molecules.

Flashcard 25: What is the definition of a conformational isomer (conformer)?

Answer: Isomers interconverted by rotation about a single sigma bond. Conformers arise from free rotation around sigma bonds, yielding different spatial forms.