Conformational Isomerism in Ethane | steroisomerism | BSc 1st semester Stereochemistry

Main ideas / lessons

• The lecture introduces stereochemistry, focusing specifically on stereoisomerism.
• Stereoisomerism is split into:
  • Conformation: how a molecule is arranged in 3D that changes by rotation
  • Configuration: arrangement that does not come from rotation; includes geometry and optical isomerism
• The main topic for today is conformational isomerism in ethane.

Conformation and conformers

• Conformation isomers (conformers):
  • Definition: different 3D arrangements that differ only by rotation around a single bond (a single σ bond), without breaking or forming bonds.
  • A molecule can have infinite possible conformations due to continuous rotation.
  • In practice, only the stable conformers (those at energy minima) are considered.
• Terms used:
  • Conformers
  • Rotational isomers / rotamers (used as synonymous naming in the lecture context)

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Methodology / procedure taught (ethane conformation using Newman projection)

1) Use the Newman projection concept

• Start from an ethane molecule and consider rotation about the C–C single bond.
• In the Newman projection:
  • Consider the front carbon and the back carbon.
  • Track rotation using the relative positions of substituents (the CH₃ groups and their attached H atoms).

2) Identify the key conformations by rotating in steps

• Start with a reference orientation at 0°.
• Rotate in 60° increments to cover the repeating set across 360°.
• For each step, assess closeness of atoms—especially H–H proximity—since that strongly affects energy through steric repulsion.

3) Dihedral angle / atom proximity idea (energy driving factor)

• The lecture emphasizes:
  • When hydrogens are closest (e.g., eclipsed positions), repulsion is highest → energy is highest → least stable.
  • When hydrogens are farthest apart (e.g., staggered positions), repulsion is minimized → energy is lowest → most stable.

4) Construct the energy profile diagram (qualitative)

• Plot energy vs. dihedral angle as rotation proceeds.
• Key points:
  • At 0° (eclipsed): hydrogens are maximally close → maximum energy (lowest stability)
  • At 60° (staggered): hydrogens are farthest apart → minimum energy (highest stability)
  • Continue for 120°, 180°, 240°, 300°, 360° similarly.
• The energy diagram repeats every 120°.
  • Because rotations proceed in 60° steps, energy maxima and minima alternate.
• At 360°, the molecule returns to the original conformation, so the cycle repeats.

5) Names of conformations mentioned for ethane

• The lecture explicitly refers to:
  • Eclipsed and staggered forms
• The most stable conformer is the staggered form (hydrogens farthest apart).

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Forward-looking note (next class)

• The instructor says that in the next lecture, ethane’s “button”/“board” concept will be modified slightly (wording unclear due to subtitle errors).
• Additional conformations will be discussed for the “next case.”
• Specifically mentioned: two extra forms, including one labeled like “gauche form” and another described as “partially eclipsed” (names inferred as best as possible from subtitles).

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Speakers / sources featured

• Course instructor / teacher (spoken throughout; no name given)
• “Students” (audience referenced by the instructor, not a distinct speaker)