Explain conformational isomerism and rotational barriers

Ethane and butane adopt different spatial arrangements as a carbon–carbon single bond rotates.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Explain conformational isomerism and rotational barriers

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.

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.

Think of a conformational energy diagram as a record of changing repulsions during rotation. Staggering lowers torsional strain, separating large groups lowers steric strain, and a ring flip changes axial/equatorial positions without changing which face 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.

Try this

Rank anti, gauche and methyl–methyl eclipsed butane from lowest to highest energy.

Check your answer

A strong answer gives anti < gauche < methyl–methyl eclipsed, with methyl separation and eclipsing used as the reasons.

Trace ethane through 60°

Start from staggered ethane at 0° and rotate the rear carbon in 60° steps.

At each angle, decide whether the front and rear C–H bonds are offset or aligned, then decide whether that geometry is an energy minimum or maximum.

Try this

Describe energy and geometry at 0°, 60° and 120° during rotation of ethane from a staggered reference.

Check your answer

0°: staggered, C–H bonds offset, energy minimum. 60°: eclipsed, C–H bonds aligned, energy maximum. 120°: staggered again, an equivalent minimum. The pattern repeats every 120°.

Choose a methylcyclohexane chair

Supplied structure: methylcyclohexane with its methyl group on the upper face of the ring.

Draw both chairs related by a ring flip, and for each say whether the methyl is axial or equatorial and whether it is still up. Then compare the 1,3-diaxial interactions.

Try this

Compare the two chair conformers of up-methylcyclohexane and identify the major conformer.

Check your answer

A ring flip swaps axial and equatorial but keeps the methyl up. The axial-methyl chair has two 1,3-diaxial interactions with axial H atoms; the equatorial-methyl chair avoids them, so it is lower in energy and is the major conformer.

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.

Try this

Better reasoning: ‘A cyclohexane ring flip changes an up substituent into a down substituent.’

Check your answer

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.

An exam comparison should name the conformers, identify the strain that differs and connect that strain to the energy order. Do not stop at ‘anti is more stable’ without explaining why.

Try this

State the evidence needed to explain why anti-butane predominates over gauche-butane.

Check your answer

Your answer should include a 180° methyl dihedral, greater methyl separation, lower steric repulsion and interconversion by C2–C3 rotation.