Conformational Analysis: The Fundamentals conformational analysis: study of the energetics of different conformations conformations: structures related by bond rotations; usually interconvertible at RT conformer: conformation at potential energy minima Newman Projections H H H H H H H H H H H H back carbon (circle) front carbon (point) q = dihedral angle Barrier to Rotation: torsional energy; energy required to twist a bond into a specific conformation; Barrier to Rotation = Erel (highest energy conformer) – Erel (lowest energy conformer) Ethane H H H HH H H H Propane H H staggered q = 60° Erel = 0 kcal/mol H H H H H Me H staggered q = 60° Erel = 0 kcal/mol eclipsed q = 0° Erel = 3.0 kcal/mol Barrier to Rotation = 3.0 – 0 = 3.0 kcal/mol HH H Me H H H eclipsed q = 0° Erel = 1.4 kcal/mol Barrier to Rotation = 1.4 – 0 = 1.4 kcal/mol H/Me-eclipsing = 1.4 kcal/mol H/H-eclipsing = 1.0 kcal/mol • One explanation for the lower energy of staggered ethane is that the staggered conformer is stabilized by hyperconjugation (stabilizing overlap between sCH and s*CH orbitals that does not occur in the eclipsed conformer). H H H H H H H sCH H2C H H CH2 H Hyperconjugation (staggered) H sCH H H s*CH H (good overlap) H H H H s*CH H H eclipsed (poor overlap) • The barrier to rotation for propane is slightly higher than ethane because there is electron repulsion between the methyl group and the eclipsed hydrogen atom in the eclipsed conformer. Butane MeMe H H H H H fully eclipsed (eclipsed) qMe/Me = 0° Erel = 4.6 kcal/mol H Me H HMe Me H H H gauche (staggered) qMe/Me = 60° Erel = 0.9 kcal/mol H Me H H eclipsed (eclipsed) qMe/Me = 120° Erel = 3.8 kcal/mol Me Me H H anti (staggered) qMe/Me = 180° Erel = 0 kcal/mol Barrier to Rotation = 4.6 – 0 = 4.6 kcal/mol Me/Me-eclipsing = 2.6 kcal/mol* Gauche Butane Interaction (gbi) = 0.9 kcal/mol Numbers to Remember H/H-eclipsing = 1.0 kcal/mol The Me/Me-eclipsing interaction is said to be worth various values in other sources. A Me/Me-eclipsing interaction is significantly higher than an H/H- or H/Me-eclipsing interaction because the hydrogens on the two methyl groups can actually bump into each other (steric strain). H/Me-eclipsing = 1.4 kcal/mol H Me/Me-eclipsing = 2.6 kcal/mol* Gauche Butane (gbi) = 0.9 kcal/mol H H H/H H H H H H H/Me H H H H Me/Me