H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain conformational isomerism and rotational barriers
H3 Chemistry: Explain conformational isomerism and rotational barriers
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Explain conformational isomerism and rotational barriers
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Follow rotation without breaking bonds
Ethane and butane adopt different spatial arrangements as a carbon–carbon single bond rotates.
A cyclohexane ring changes chair conformation by coordinated bond rotations while every carbon–carbon bond remains intact.
Conformers and rotational barriers
Conformers are stereoisomers interconverted by rotation about single bonds; a dihedral angle measures the angle between two bonds viewed along the rotating bond.
A rotational energy barrier is the energy needed to pass from a minimum-energy conformer through a higher-energy arrangement such as an eclipsed form.
For butane, the staggered anti conformer is the global minimum, the two staggered gauche conformers are local minima, eclipsed CH3–H arrangements are maxima and the CH3–CH3 eclipsed arrangement is the highest maximum.
A cyclohexane chair avoids angle and eclipsing strain; a ring flip exchanges axial and equatorial positions but preserves whether each substituent is up or down.
Text alternative: The text supplies every angle, relative energy and axial/equatorial-up change, so neither colour nor an unlabelled shape carries the conclusion.
Connect geometry to energy
For butane, the staggered anti conformer is the global minimum, the two staggered gauche conformers are local minima, eclipsed CH3–H arrangements are maxima and the CH3–CH3 eclipsed arrangement is the highest maximum.
A cyclohexane chair avoids angle and eclipsing strain; a ring flip exchanges axial and equatorial positions but preserves whether each substituent is up or down.
Rank the butane rotation
At a 180° CH3–C–C–CH3 dihedral, the methyl groups are anti and maximally separated.
Rotating to 60° gives gauche; continuing to 0° aligns the methyl groups and produces the largest steric and torsional penalty.
- Rank anti, gauche and methyl–methyl eclipsed butane from lowest to highest energy.
Open the feedback checkpoint after attempting
- Award anti < gauche < methyl–methyl eclipsed, with methyl separation and eclipsing used as the reasons.
Trace ethane through 60°
Staggered ethane is an energy minimum because adjacent C–H bonds are offset.
A 60° rotation reaches eclipsed ethane, an energy maximum; another 60° reaches an equivalent staggered minimum.
- Describe energy and geometry at 0°, 60° and 120° during rotation of ethane from a staggered reference.
Open the feedback checkpoint after attempting
- Accept staggered minimum, eclipsed maximum, staggered minimum in that order, with periodic repetition every 120°.
Choose a methylcyclohexane chair
A ring flip converts an axial methyl group into an equatorial methyl group without changing its up/down orientation.
Equatorial methylcyclohexane is lower in energy because it avoids two unfavourable 1,3-diaxial interactions.
- Compare the two chair conformers of up-methylcyclohexane and identify the major conformer.
Open the feedback checkpoint after attempting
- Choose the chair with the up methyl group equatorial and cite reduced 1,3-diaxial repulsion.
Do not confuse conformation with configuration
Rotation or ring flipping changes a conformation, not atom connectivity or an R/S descriptor at an unchanged stereogenic centre.
An energy maximum can be crossed thermally; it is not a second covalent compound that requires a bond-breaking reaction.
- Repair: ‘A cyclohexane ring flip changes an up substituent into a down substituent.’
Open the feedback checkpoint after attempting
- Reject the claim: axial/equatorial changes, whereas up/down is retained.
Use a complete energy argument
Name the fixed bond or ring, identify the conformers, state the interaction responsible for each energy difference and describe the interconversion path.
After conformational control, distinguish configurations locked by a double bond or saturated ring.
- State the evidence needed to explain why anti-butane predominates over gauche-butane.
Open the feedback checkpoint after attempting
- Require a 180° methyl dihedral, greater methyl separation, lower steric repulsion and interconversion by C2–C3 rotation.