Difficulty: Intermediate | Prerequisites: Bonding, 3D structure, basic stereochemistry.
Conformational analysis deals with the different shapes a molecule can adopt by rotation around single bonds, without breaking any bonds. Unlike constitutional isomers or stereoisomers, conformations interconvert freely at room temperature. The energy differences between conformations explain molecular behaviour, reactivity and even physical properties. This topic builds directly on your understanding of 3D structure and steric effects.
Rotation around C-C single bonds produces different conformations with different energies. Staggered conformations (60° apart) are lower in energy than eclipsed conformations (0° apart). For butane, the energy order is: anti (lowest) < gauche < eclipsed < fully eclipsed (highest). In cyclohexane, equatorial substituents are more stable than axial ones because they avoid 1,3-diaxial interactions.
Conformation
A specific three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond. Conformations interconvert freely and are not isolable at room temperature.
Newman projection
A way of viewing a molecule by looking directly down a C-C bond. The front carbon is a dot; the back carbon is a circle. Substituents on each carbon radiate outward.
Staggered conformation
A conformation where the bonds on the front carbon bisect the angles between bonds on the back carbon (60° apart in a Newman projection). Lower energy than eclipsed.
Eclipsed conformation
A conformation where the bonds on the front carbon directly overlap with bonds on the back carbon (0° dihedral angle). Higher energy due to torsional strain.
Anti conformation
The staggered conformation where the two largest substituents are 180° apart. The global energy minimum for butane.
Gauche conformation
A staggered conformation where two large substituents are 60° apart. Higher in energy than anti due to steric repulsion between the groups (gauche interaction).
Think of it as: the groups are staggered but still uncomfortably close.
Torsional strain
The energy cost of eclipsing bonds. Arises from repulsion between bonding electrons on adjacent atoms when their bonds align. The ~3 kcal/mol barrier in ethane is primarily torsional strain.
Steric strain
The energy cost of two bulky groups being forced too close together in space. Distinct from torsional strain; steric strain is about the size of the groups, not just bond alignment.
Gauche interaction
A specific type of steric strain that occurs when two large substituents are 60° apart in a staggered conformation.
Chair conformation
The lowest-energy shape of cyclohexane, where all bond angles are close to the ideal tetrahedral 109.5° and all adjacent C-H bonds are staggered.
Axial position
Bonds that point straight up or straight down from the cyclohexane ring, parallel to the ring's axis of symmetry.
Equatorial position
Bonds that point roughly outward from the ring, along the "equator." Equatorial substituents experience less steric strain than axial ones.
1,3-diaxial strain
The steric interaction between an axial substituent and the axial hydrogens (or other groups) on the same side of the ring, two carbons away. This is why large groups prefer the equatorial position.
The energy barrier between staggered and eclipsed ethane is approximately 3 kcal/mol. This is almost entirely torsional strain: the C-H bonds on adjacent carbons repel each other when they align. There is negligible steric strain because hydrogen atoms are small.
Looking down the C2-C3 bond of butane, the two methyl groups can occupy different relative positions. The energy order from most stable to least stable:
Anti (180°): methyl groups are as far apart as possible. Lowest energy. No steric strain, minimal torsional strain.
Gauche (60° and 300°): methyl groups are 60° apart in a staggered arrangement. Higher energy than anti by about 0.9 kcal/mol due to the gauche interaction (steric repulsion between the methyl groups).
Eclipsed (120° and 240°): H-CH3 eclipsing. High energy due to torsional strain plus some steric strain.
Fully eclipsed (0°/360°): CH3-CH3 eclipsing. Highest energy. Maximum torsional strain combined with maximum steric repulsion between the two methyl groups.
Energy order: Anti < Gauche < Eclipsed < Fully Eclipsed.
For n-hexane viewed through the C3-C4 bond, the same principles apply. In a conformational energy plot, the peaks (highest energy) occur at eclipsed conformations. The energy minima at 60° and 300° are gauche conformations, and the global minimum at 180° is the anti conformation.
Cyclohexane adopts a chair shape that eliminates angle strain (all C-C-C angles are close to 109.5°) and torsional strain (all adjacent bonds are staggered). Each carbon has one axial and one equatorial bond.
A ring flip interconverts the two chair conformations: every axial bond becomes equatorial and vice versa. For unsubstituted cyclohexane, both chairs are identical.
Larger substituents prefer the equatorial position because axial substituents experience 1,3-diaxial strain with other axial groups on the same face of the ring.
For cis-1,3-dimethylcyclohexane, both methyl groups can be equatorial simultaneously (the most stable chair). This is because on a 1,3-cis arrangement, the two groups are on the same face but can both point equatorially.
For trans-1,2-dimethylcyclohexane, one methyl must be axial and one equatorial in each chair conformation; a ring flip simply swaps which is which.
Students often confuse torsional strain with steric strain. Torsional strain is about eclipsing of bonds (electron repulsion when bonds align). Steric strain is about the physical size of groups being forced too close together. The eclipsed ethane barrier is primarily torsional. The extra energy cost of gauche butane (relative to anti) is primarily steric.
Thinking that gauche is an eclipsed conformation. Gauche is staggered, just not the most favourable staggered arrangement. The groups are 60° apart, not directly overlapping.
Assuming that cis substituents on cyclohexane must always be one axial and one equatorial. Whether cis substituents can both be equatorial depends on their positions (1,3-cis can both be equatorial; 1,2-cis requires one axial and one equatorial).
Confusing energy minima with energy maxima on conformational energy plots. Minima are staggered conformations (gauche and anti). Maxima are eclipsed conformations.
⚠️ Be ready to rank butane conformations from lowest to highest energy and explain the type of strain responsible for each.
⚠️ Questions about conformational energy plots of hexane (or similar) will ask you to identify which conformations correspond to peaks and troughs. Peaks = eclipsed, troughs = staggered (gauche at 60°/300°, anti at 180°).
⚠️ Chair conformation questions often ask which chair is more stable for a given disubstituted cyclohexane. The answer always comes down to putting the larger (or more) substituents equatorial.
⚠️ The ~3 kcal/mol barrier in ethane is specifically tested. Know that it is torsional strain, not steric strain.
The energy difference between staggered and eclipsed ethane is approximately ____. (3 kcal/mol.)
True or false: steric strain is the primary cause of the ethane rotational barrier. (False. Torsional strain.)
In butane, the anti conformation has the methyl groups ____ degrees apart. (180°.)
A gauche interaction is a type of ____ strain. (Steric.)
True or false: in cis-1,3-dimethylcyclohexane, both methyls can be equatorial simultaneously. (True.)
Q: The energy minima at 60° and 300° in a conformational energy plot of n-hexane (C3-C4 bond) correspond to which conformations?
A: Gauche conformations. These are staggered arrangements where the ethyl groups on C3 and C4 are 60° apart.
Q: The interaction between two methyl groups in a Newman projection at a 60° dihedral angle is called what?
A: A gauche interaction. It is a type of steric strain that occurs between two large groups in a staggered conformation when they are 60° apart.
Q: Which chair conformation of cis-1,3-dimethylcyclohexane is most stable?
A: Both methyl groups in equatorial positions. In the cis-1,3 arrangement, this is achievable and avoids 1,3-diaxial strain.
Q: The ~3 kcal/mol energy difference between staggered and eclipsed ethane is primarily due to what?
A: Torsional strain, caused by the repulsion between bonding electrons on adjacent carbons when their C-H bonds are eclipsed.
Q: Analyse the stability of butane conformations and rank them from lowest to highest energy.
A: Anti (180°, lowest energy, no steric strain) < Gauche (60°, steric repulsion between methyls) < Eclipsed (120°, torsional strain from H-Me eclipsing) < Fully eclipsed (0°, maximum torsional strain plus maximum steric repulsion between two eclipsed methyl groups).
Conformational analysis feeds into reaction mechanisms: the geometry of the transition state in E2 eliminations requires an anti-periplanar arrangement of the leaving group and the proton being removed, which is a conformational consideration. It also matters in carbohydrate chemistry, where the preferred chair conformation of sugars determines their reactivity and biological recognition.
Conformational analysis, Newman projection, staggered, eclipsed, anti, gauche, torsional strain, steric strain, gauche interaction, butane conformations, hexane conformational analysis, dihedral angle, cyclohexane chair, ring flip, axial, equatorial, 1,3-diaxial strain, cis-1,3-dimethylcyclohexane, ethane rotational barrier, 3 kcal/mol, CH 2420, University of Michigan organic chemistry