MCAT content library / Chemistry & physics / Conformation and restricted rotation
Distinguish rotation about a permitted single bond from a true change in configuration, then use Newman projections and energy comparisons to rank conformers.
About 16 minutes, including a butane conformational analysis and checks.
On this page
- Conformations interconvert without changing connectivity
- Concept: Newman projections show dihedral angle
- Worked application: rank butane conformers
- Pattern: restricted rotation can create configurational stability
- Rule: rotation does not automatically change R or S
- Check your reasoning
- Continue learning
- Sources
Conformations interconvert without changing connectivity
Conformations are different spatial arrangements produced by rotation about bonds that can rotate without changing molecular connectivity. A molecule can move between conformers without breaking and reforming the sigma-bond framework.
Configuration is different. Converting one alkene E/Z isomer into the other or one isolated stereocenter configuration into its mirror-image configuration generally requires breaking bonds or passing through a process that changes configurational identity. Do not label every 3D drawing difference as a new stereoisomer.
Concept: Newman projections show dihedral angle
A Newman projection looks down a carbon–carbon bond. The front carbon is represented by a point and the rear carbon by a circle. The relative angle between substituents on the two carbons is the dihedral angle.
Staggered conformations place rear bonds between front bonds and reduce torsional strain. Eclipsed conformations align bonds and raise torsional strain. When large groups are present, their relative positions also create steric differences among staggered conformers.
Worked application: rank butane conformers
Look down the central C2–C3 bond of butane. The two methyl groups can adopt several relative positions:
| Methyl–methyl dihedral angle | Conformation | Relative stability |
|---|---|---|
| 180° | Anti, staggered | Most stable of the listed conformers |
| 60° or 300° | Gauche, staggered | Higher than anti |
| 120° or 240° | Eclipsed CH₃/H interactions | Higher still |
| 0° | Fully eclipsed CH₃/CH₃ | Highest of the listed conformers |
The anti conformer minimizes both torsional strain and direct methyl–methyl steric interaction. Gauche is still staggered but brings the methyl groups closer. Eclipsed conformations add torsional strain, with the methyl–methyl eclipsed arrangement especially unfavorable.
This ranking is about relative conformational energy, not a claim that only the anti conformer exists. At ordinary temperatures, molecules sample multiple accessible conformations.
Pattern: restricted rotation can create configurational stability
A carbon–carbon double bond contains a pi bond that prevents free rotation without disrupting pi overlap. This is why appropriately substituted alkenes can exist as distinct E and Z configurations.
Amide C–N bonds also show restricted rotation because resonance gives the bond partial double-bond character. That restriction helps make peptide bonds relatively planar. The barrier is not identical to a carbon–carbon double bond, but the same reasoning principle applies: delocalization can make a formally single bond less freely rotatable.
Rule: rotation does not automatically change R or S
Rotating a whole molecule or rotating around an ordinary single bond changes the drawing but not the connectivity at a stereocenter. R/S assignment depends on the spatial ordering of substituents according to priority rules, not on whether the wedge happens to point left or right on the page.
If two drawings can be made identical by allowed rotations without breaking bonds, they represent the same configuration. If superposition requires reflection or a configurational change, they may represent different stereoisomers.
ACTIVE PRACTICE · MOLECULAR CONFORMATION
Rank Butane Conformations by Geometry
The original example looks down the C2–C3 bond of butane. Anti (180°) is the most stable listed conformation, gauche (60° or 300°) is higher, CH₃/H-eclipsed (120° or 240°) is higher still, and the fully CH₃/CH₃-eclipsed form (0°) is highest. This is a qualitative ordering, not a measured energy profile.
Anti staggered — lowest relative conformational energy of those listed.
Check your reasoning
Choose an answer before opening its feedback.
Check 1
Which butane conformer is lowest in energy among the standard Newman projections?
A. Anti staggered
Correct. It minimizes torsional strain and methyl–methyl steric interaction.
B. Fully eclipsed methyl–methyl
Review this choice. That arrangement is especially high in energy.
C. Any eclipsed conformer
Review this choice. Eclipsing raises torsional strain.
D. All conformers are identical in energy.
Review this choice. Steric and torsional interactions differ with dihedral angle.
Check 2
Why can an alkene display stable E/Z configurations?
A. Rotation around the C=C bond requires disrupting pi overlap.
Correct. The pi bond restricts free rotation.
B. Carbon–carbon single bonds cannot rotate.
Review this choice. Ordinary single bonds generally can rotate.
C. Hydrogen atoms are magnetic.
Review this choice. Magnetism is not the reason for alkene configurational stability.
D. E and Z are different molecular formulas.
Review this choice. They have the same formula and connectivity.
Check 3
A molecule is redrawn after rotating around a C–C single bond. The stereocenter connectivity is unchanged. What is the safest conclusion?
A. The conformation may have changed without changing configuration.
Correct. Single-bond rotation can alter shape while preserving stereochemical identity.
B. R must become S.
Review this choice. Simple rotation does not invert a stereocenter.
C. A new constitutional isomer must form.
Review this choice. Connectivity is unchanged.
D. A covalent bond must have broken.
Review this choice. Conformational rotation does not require bond cleavage.