lecture recording on 07 March 2025 at 13.51.16 PM

Stability of Ring Structures

• Ring Size and Stability

  • Three-member rings are the least stable due to high angle strain (60°) compared to the ideal bond angle for sp³ hybridized carbons (109.5°).

  • Four-member rings are slightly more stable (90° angle), but still not common in structures.

  • Five-member and six-member rings are prevalent and more stable, approaching the ideal bond angle.

Chair Conformers

• Six-Membered Rings:

  • Bending and Structure: Six-membered rings do not lie flat on a page, as this configuration does not accommodate the ideal bond angles. Rather, they adopt a "chair" conformation to minimize strain.

  • Chair Flip: 1 side of the ring is lifted up while the other side is pushed down. This movement is accompanied by bond angles approaching 109.5°.

Drawing Chair Conformers

• How to Draw a Chair Conformer:

  1. Start with two parallel lines to represent the sides of the chair.

  2. Connect them with more parallel lines, ensuring they look like a chair.

  3. Avoid shapes resembling bow ties, which obscure structural clarity.

• Identifying Carbons:

  • Numbering carbons is arbitrary: always start sequentially from any carbon and maintain the same numbering through any conformers to keep track of substituents.

Axial and Equatorial Groups

• Positioning of Substituents:

  • Axial Substituents: alternate up/down around the ring, aligned with the vertical bonds.

  • Equatorial Substituents: oriented at an angle up or down but always parallel to the structure, they have more room and less steric hindrance.

• Switching Positions:

  • When switching between chair conformers, axial becomes equatorial and vice versa. However, their relative up/down positions remain the same (up stays up, down stays down).

Stability Considerations for Substituents

• Stability is influenced by the size of substituents:

  • Equatorial position is preferred for larger groups due to decreased steric strain.

  • Axial position is less stable, causing interactions (like gauche interactions) that increase potential energy of the structure.

• Influence of Larger Groups:

  • Larger substituents have a significant impact on the stability of the conformations and their preferred placements.

Methyl Groups Interaction

• Example of Methyl Groups:

  • When 2 methyl groups are present, drawing structures in different conformers (e.g., both groups up, one up and one down) affects the ring's overall stability.

  • Conformers with groups in equatorial positions are usually more stable than those with axial positions.

• Anti and Syn Configurations:

  • Anti: groups opposing each other in terms of axial and equatorial placement. Syn: groups that are on the same side (cis configuration).

Equilibrium and Group Interaction

• Identifying Stability in Equilibria:

  • Identical substituents (like 2 methyl groups) don’t always lead to stability; the arrangement influences steric interactions.

  • Comparing different conformers provides insights into which forms are more stable, accounting for both axial/equatorial positions and group sizes.