Stereochemistry and Isomerism (5B) - Chem & Physics

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

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

2. What is the definition of constitutional isomers in organic chemistry?

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

3. Using E/Z notation, what does E mean for the highest-priority groups on an alkene?

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

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

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

5. What is the definition of geometric (cis/trans or E/Z) isomerism?

   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.

6. What is the definition of enantiomers?

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

7. In CIP rules, how are double bonds treated when ranking substituent priority?

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

8. What does it mean for a compound to be meso?

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

9. Which option correctly gives the maximum stereoisomer count for $n=3$ stereocenters with no symmetry?

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

10. What is a chiral center (stereogenic center) in typical MCAT organic molecules?

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

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

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

12. Which property distinguishes enantiomers in an achiral environment: melting point or optical rotation?

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

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

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

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

    Clockwise $1 \rightarrow 2 \rightarrow 3$ gives R. With lowest priority away, clockwise priority sequence assigns R configuration.

15. What is the relationship between R/S configuration and (+)/(−) optical rotation?

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

16. What is the definition of diastereomers?

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

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

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

18. What is the definition of a conformational isomer (conformer)?

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

19. Which conformation is most stable for substituted ethane: staggered or eclipsed?

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

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

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

21. What is the definition of a racemic mixture?

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

22. What is the definition of stereoisomers?

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

23. What is the correct first step of CIP priority assignment at a 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.

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

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

25. Using E/Z notation, what does Z mean for the highest-priority groups on an alkene?

    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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