conformations

conformers (conformations)

different molecular shapes of the same molecule generated by the rotation of σ-bonds

Newman projections

visualize conformations of chemical bonds from a front to back perspective (along the bond)

staggered conformation

groups on front and back carbons are not aligned (Θ=60°, Θ=180°)

eclipsed conformation

groups on front and back carbons are aligned (Θ=0°, Θ=120°)

dihedral angle

perceived projection angle between front and back groups on a Newman projection

gauche Newman conformation

biggest groups are next to each other (staggered)

syn Newman conformation

biggest groups overlap, highest energy (eclipsed)

anti Newman conformation

biggest groups are opposite each other, lowest energy (staggered)

Fischer projections

• horizontal lines represent bonds that are pointing up

• vertical lines represent bonds that are pointing down

Fischer projections manipulations

• bond rotation: rotate three of four groups around a cross

• molecule rotation: 180° rotation works

common cyclohexane shapes

• cyclopropane (triangle)

• cyclobutane (square)

• cyclopentane (pentagon)

• cyclohexane (hexagon)

cyclopropane

• only has one conformation (flat ring)

• three carbons are restricted to 60° bond angles, leading to high angle strain

• all eclipsing interactions, leading to high torsional strain

• high overall ring strain → high energy

cyclobutane

• two conformations (flat ring, puckered ring)

• each carbon is restricted to ~90° bond angles, leading to moderate angle strain

• fewer eclipsing interactions in puckered conformation, leading to moderate torsional strain

• moderate overall ring strain

cyclopentane

• two conformations (flat ring, puckered “envelope”)

• each carbon can achieve almost ideal bond angles, leading to low angle strain

• some staggered interactions in envelope conformation, leading to low torsional strain

• low overall ring strain

cyclohexane

• two main conformations (boat, chair)

• each carbon can achieve ideal bond angles, leading to no angle strain

• all staggered interactions in chair conformation, leading to no torsional strain

• very low overall ring strain

cyclohexane chair flipping

• when both edges of the cyclohexane are up (or down), it is in boat conformation

• when one edge is up and the other is down, it is in chair conformation

cyclohexane energies (increasing)

1. chair

2. twist

3. boat

4. half-chair

axial and equatorial positions

flipping cyclohexane ring between chair conformations interconvert all axial and equatorial positions

monosubstituted cyclohexane

G equatorial position is preferred since it leaves more space between large groups

disubstituted (or more) cyclohexane

lowest energy conformation has as many groups equatorial as possible, or the biggest group equatorial