Organic Chemistry Stereochemistry and Conformational Analysis Syllabus

Stereochemistry and Conformational Analysis syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Stereochemistry and Conformational Analysis in Organic Chemistry, not a summary of it.

1. 1

   Conformational Analysis of Alkanes

   3 topics

   • Newman Projections
   • Torsional and Steric Strain
   • Ethane and Butane Conformers
2. 2

   Conformations of Cyclic Compounds

   3 topics

   • Ring Strain in Cycloalkanes
   • Cyclohexane Chair Conformations
   • Mono- and Disubstituted Cyclohexanes
3. 3

   Chirality and Stereoisomers

   5 topics

   • Chirality Centers and Stereocenters
   • Enantiomers and Optical Activity
   • R and S Configuration
   • Diastereomers and Meso Compounds
   • Fischer Projections
4. 4

   Stereochemistry of Reactions

   3 topics

   • Stereospecific vs Stereoselective Reactions
   • Racemization and Inversion
   • Resolution of Enantiomers

Stereochemistry and Conformational Analysis flashcards for Organic Chemistry

21 of 52 cards from the Stereochemistry and Conformational Analysis deck — real questions with worked answers.

1. What is a Newman projection and what does it depict?

   A Newman projection is a way of viewing a molecule down a specific carbon–carbon bond. The front atom is drawn as a dot with three bonds radiating from it, and the back atom is drawn as a circle with three bonds emerging from its edge. It shows the relative rotational orientation (dihedral/torsion angle) of the substituents on the two carbons.

2. Define the dihedral (torsion) angle in a Newman projection.

   The dihedral (torsion) angle is the angle between a bond on the front carbon and a bond on the back carbon as viewed down the C–C axis. It is $0^{\circ}$ for eclipsed and $60^{\circ}$ for staggered arrangements.

3. What is the difference between a staggered and an eclipsed conformation?

   In a staggered conformation the front and back bonds are offset with dihedral angles of $60^{\circ}$ (bonds bisect the gaps), minimizing repulsion. In an eclipsed conformation the front and back bonds overlap at a dihedral angle of $0^{\circ}$, maximizing repulsion and energy.

4. What is torsional strain and what causes it?

   Torsional strain is the increase in energy of a molecule caused by eclipsing interactions between bonds (electron pairs) on adjacent atoms. It arises when bonds are forced into an eclipsed arrangement, and is highest at dihedral angles of $0^{\circ}$.

5. What is steric strain?

   Steric strain is the repulsive energy that results when two atoms or groups are forced closer together than their van der Waals radii allow. It is most significant when large groups are near each other, such as in the gauche or eclipsed conformations of butane.

6. What is the total torsional strain of eclipsed ethane, and the strain per H–H eclipsing interaction?

   Eclipsed ethane has a total torsional strain of about $12\ \text{kJ/mol}$ ($2.9\ \text{kcal/mol}$), distributed over three H–H eclipsing interactions, giving roughly $4\ \text{kJ/mol}$ ($1.0\ \text{kcal/mol}$) per interaction.

7. Describe the potential energy diagram for rotation about the C–C bond of ethane.

   Ethane's energy varies sinusoidally with dihedral angle: staggered conformations are energy minima and eclipsed conformations are maxima (about $12\ \text{kJ/mol}$ higher). Because all H's are identical, all staggered forms are equivalent and all eclipsed forms are equivalent, giving three identical peaks and three identical valleys per $360^{\circ}$.

8. Name the four key conformations of butane about the C2–C3 bond in order of increasing energy.

   From lowest to highest energy: anti (staggered, $\ce{CH3}$ groups $180^{\circ}$ apart) < gauche (staggered, $\ce{CH3}$ groups $60^{\circ}$ apart) < eclipsed (methyl/H eclipsed) < totally eclipsed/syn (methyl groups eclipsing, $0^{\circ}$).

9. What is the energy cost of the gauche interaction in butane, and why does it occur?

   The gauche conformation of butane is about $3.8\ \text{kJ/mol}$ ($0.9\ \text{kcal/mol}$) higher in energy than the anti conformation. This is due to steric strain between the two methyl groups that are only $60^{\circ}$ apart.

10. What is the energy of the fully eclipsed (syn) conformation of butane relative to anti?

    The fully eclipsed (syn) conformation, in which the two methyl groups eclipse each other at $0^{\circ}$, is about $19\ \text{kJ/mol}$ ($4.5\ \text{kcal/mol}$) higher than anti. It represents the global energy maximum for rotation about C2–C3.

11. According to Baeyer strain theory, what is ring strain?

    Ring strain is the extra energy in a cyclic molecule due to deviation of bond angles from the ideal tetrahedral $109.5^{\circ}$ (angle strain), combined with torsional strain from eclipsing and steric strain across the ring.

12. Rank cyclopropane, cyclobutane, cyclopentane, and cyclohexane by total ring strain.

    Cyclopropane (most strained, $\approx 115\ \text{kJ/mol}$) > cyclobutane ($\approx 110\ \text{kJ/mol}$) > cyclopentane ($\approx 26\ \text{kJ/mol}$) > cyclohexane ($\approx 0\ \text{kJ/mol}$, essentially strain-free).

13. Why is cyclopropane so strained?

    Cyclopropane has internal bond angles of $60^{\circ}$, far from the ideal $109.5^{\circ}$, causing severe angle strain. Its bonds are bent ('banana bonds' with poor orbital overlap), and all C–H bonds are eclipsed, adding torsional strain.

14. How does cyclohexane avoid ring strain?

    Cyclohexane adopts a puckered chair conformation in which all C–C–C bond angles are $\approx 111^{\circ}$ (near ideal tetrahedral) and all adjacent C–H bonds are perfectly staggered, eliminating both angle strain and torsional strain.

15. What are axial and equatorial positions in the cyclohexane chair?

    Each carbon in a chair has one axial bond (parallel to the ring's vertical axis, pointing straight up or down) and one equatorial bond (pointing outward, roughly along the ring's 'equator'). Axial and equatorial positions alternate up/down around the ring.

16. What happens to axial and equatorial substituents during a chair–chair ring flip?

    A ring flip interconverts the two chair forms. All groups that were axial become equatorial, and all groups that were equatorial become axial. The up/down (cis/trans) relationship of each group is preserved.

17. In monosubstituted cyclohexanes, why is the equatorial conformer preferred?

    An axial substituent experiences 1,3-diaxial interactions (steric strain with the two axial H's three carbons away). Placing the group equatorial avoids these repulsions, so the equatorial conformer is lower in energy and predominates at equilibrium.

18. What is a 1,3-diaxial interaction?

    A 1,3-diaxial interaction is the steric strain between an axial substituent and the two other axial groups (usually H atoms) located on the same face of the ring at the 3 and 5 positions relative to it. Larger axial groups give greater 1,3-diaxial strain.

19. What is the A-value of a substituent?

    The A-value is the energy difference (preference for equatorial over axial) for a monosubstituted cyclohexane, equal to the free energy penalty of placing that group axial. Larger A-values mean stronger equatorial preference; e.g., $\ce{CH3}$ is $\approx 7.6\ \text{kJ/mol}$ and $t\text{-Bu}$ is $\approx 21\ \text{kJ/mol}$.

20. For a disubstituted cyclohexane, how do you decide the most stable chair?

    Choose the chair that places the greater number of substituents—especially the largest group—in equatorial positions. A very bulky group such as tert-butyl will 'lock' the ring by forcing itself equatorial.

21. For cis- vs trans-1,2-dimethylcyclohexane, which is more stable and why?

    Trans-1,2-dimethylcyclohexane is more stable. Its diequatorial (e,e) chair places both methyls equatorial, whereas the cis isomer must always be one axial/one equatorial (a,e), giving one methyl unavoidable 1,3-diaxial strain.

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