Source: CHEM 237 Practice Exam 1, University of Washington | Chapter 7 (first half)
Tags: conformational analysis, Newman projection, anti, gauche, eclipsed, steric strain, torsional strain, chair conformation, axial, equatorial, ring flip, cis-trans isomers, cyclohexane, 1,3-diaxial interaction, CHEM 237
Difficulty: Intermediate
Prerequisites: IUPAC nomenclature, line-angle structures, Lewis structures, hybridization and geometry (see Parts 1 and 2).
Organic molecules are not rigid. Single bonds rotate freely, and the different arrangements produced by that rotation are called conformations. Some conformations are lower in energy (more stable) than others, and understanding why is central to predicting molecular behaviour.
Cycloalkanes add another layer: rings constrain rotation, and six-membered rings (cyclohexanes) adopt a chair shape with axial and equatorial positions. Whether a substituent sits axial or equatorial has real energetic consequences, and this determines which cis/trans isomer is more stable. This material is heavily tested in CHEM 237 and forms the basis for understanding stereochemistry and reaction selectivity later on.
You should be comfortable with sp3 geometry and line-angle structures before working through this topic.
Rotation around single bonds creates different conformations (staggered, eclipsed, anti, gauche), and the anti conformation is the most stable because groups are as far apart as possible. In cyclohexane, substituents prefer the equatorial position to avoid 1,3-diaxial interactions. Whether cis or trans is more stable on a cyclohexane depends on the substitution pattern (1,2 vs 1,3 vs 1,4), because this determines whether the trans or cis isomer can place both substituents equatorial.
Conformer (conformation)
An arrangement of atoms in a molecule that results from rotation about a single bond. Conformers are not different molecules; they interconvert rapidly at room temperature.
Think of it as the same molecule caught in different poses.
Staggered conformation
A conformation in which bonds on adjacent carbons are offset by 60° when viewed in a Newman projection. Lower energy than eclipsed.
Eclipsed conformation
A conformation in which bonds on adjacent carbons line up directly behind one another in a Newman projection. Higher energy due to torsional strain.
Anti conformation
The staggered conformation in which the two largest substituents on adjacent carbons are 180° apart (directly opposite). The most stable conformation of butane.
Gauche conformation
The staggered conformation in which the two largest substituents are 60° apart. More stable than eclipsed, but less stable than anti, due to steric strain between the nearby groups.
Torsional strain
The increase in energy that results when bonds on adjacent atoms are eclipsed. Caused by repulsion between bonding electron pairs that are forced into close proximity.
In simple terms, electrons in neighbouring bonds do not like being lined up directly behind each other.
Steric strain
The increase in energy caused by repulsion between electron clouds of atoms or groups that are too close in space. Larger groups produce more steric strain.
Newman projection
A way of drawing a molecule looking straight down a carbon-carbon bond. The front carbon is shown as a dot, the back carbon as a circle. Bonds radiate outward from each.
Chair conformation
The most stable conformation of cyclohexane, in which all bond angles are near 109.5° and all adjacent C-H bonds are staggered. It has two types of positions: axial and equatorial.
Axial position
Bonds that point straight up or straight down from the ring in a chair conformation, parallel to the vertical axis of the ring.
Equatorial position
Bonds that point outward from the ring, roughly in the plane of the ring's "equator". Substituents in equatorial positions experience less steric strain than in axial positions.
1,3-diaxial interaction
The steric strain between an axial substituent and the axial hydrogens (or groups) on the same side of the ring, two carbons away. This is the main reason substituents prefer equatorial positions.
Think of it as the axial group bumping into the other axial groups above (or below) the ring.
Ring flip
The interconversion between the two chair conformations of cyclohexane. Every axial substituent becomes equatorial, and every equatorial substituent becomes axial.
Cis and trans isomers (on rings)
Cis: two substituents on the same side of the ring plane. Trans: on opposite sides. Unlike conformers, cis and trans isomers cannot interconvert by rotation; they are different compounds (specifically, stereoisomers).
Butane as the model compound
Looking down the C2-C3 bond of butane, the two methyl groups (on C1 and C4) can adopt several conformations as the bond rotates:
Anti (180° dihedral): methyl groups are as far apart as possible. Lowest energy, most stable.
Gauche (60° dihedral): methyl groups are 60° apart. Higher energy than anti due to steric strain between the nearby methyls, but still staggered (no torsional strain from eclipsing).
Eclipsed (0° or 120°): bonds on C2 and C3 line up. Highest energy due to both torsional strain and (in the fully eclipsed case with methyls aligned) steric strain.
Stability order: anti > gauche > eclipsed (with the methyl-methyl eclipsed being worst of all).
Why 2,5-dimethylhexane has a larger anti-eclipsed energy difference than butane
In 2,5-dimethylhexane, the groups flanking the central C-C bond are isopropyl groups rather than methyl groups. Isopropyl is bulkier than methyl, so the steric strain in the eclipsed conformation is significantly worse. The anti conformation remains roughly similar in energy (substituents are far apart either way), but the eclipsed conformation is much higher in energy due to the larger groups being forced together. The result is a bigger energy gap.
When given two structures and asked to classify the relationship:
Identical compounds: same connectivity and same three-dimensional arrangement. You can superimpose one on the other.
Conformers: same connectivity but different arrangements due to rotation about single bonds. A ring flip of cyclohexane produces conformers.
Skeletal (constitutional) isomers: same molecular formula but different connectivity.
To work these problems, first check molecular formula. If the formulas differ, they are not isomers at all. If the formulas match, trace the connectivity: if the atoms are connected in the same order, look at the 3D arrangement; if you can interconvert them by bond rotation (or a ring flip), they are conformers; if they are already superimposable, they are identical.
Cyclohexane adopts a chair shape where all C-C-C angles are close to 109.5° and all adjacent bonds are staggered. Each carbon has one axial and one equatorial bond. A ring flip swaps all axial and equatorial positions.
Substituent preferences
Larger substituents strongly prefer the equatorial position to avoid 1,3-diaxial interactions. A methyl group in the axial position experiences gauche-like interactions with the axial hydrogens on carbons 3 and 5 of the ring.
trans-1,2-dimethylcyclopropane vs cis-1,2-dimethylcyclopropane
In cyclopropane (a planar ring), the trans isomer places the two methyl groups on opposite sides of the ring, reducing steric strain. The cis isomer forces them onto the same face, so they are closer together and repel each other. Trans is more stable.
trans-1,2-dimethylcyclohexane vs cis-1,2-dimethylcyclohexane
For 1,2-disubstituted cyclohexane, the trans isomer can place both methyl groups equatorial (one up-equatorial and one down-equatorial on adjacent carbons). The cis isomer forces one methyl axial and one equatorial. Because diequatorial is lower in energy than axial-equatorial, trans-1,2 is more stable.
trans-1,3-dimethylcyclohexane vs cis-1,3-dimethylcyclohexane
The pattern reverses for 1,3 substitution. Here, the cis isomer is the one that can place both methyls equatorial (both on the same side of the ring corresponds to both equatorial in one chair). The trans isomer forces one methyl axial. So cis-1,3 is more stable.
The key insight: whether cis or trans is more stable depends on whether "same side" or "opposite side" corresponds to "both equatorial" for that particular substitution pattern. Draw both chair conformations and check.
Students often assume that trans is always more stable than cis on cyclohexane. This is only true for 1,2 and 1,4 disubstitution. For 1,3 disubstitution, cis is more stable because cis-1,3 is the arrangement that allows both groups to be equatorial.
Conformers are frequently confused with constitutional isomers. Conformers have the same connectivity and can interconvert by bond rotation. If you had to break and re-form a bond, you have an isomer, not a conformer.
Students sometimes think that equatorial is always "in the plane of the ring". Equatorial bonds point slightly up or slightly down, at about 109.5° from the axial bonds. They are roughly in the plane, not exactly in it.
A common error on ring flip problems is forgetting that a ring flip swaps all axial and equatorial positions simultaneously. You cannot flip just one substituent.
⚠️ Expect at least one question asking you to compare the stability of two conformations (anti vs gauche vs eclipsed) and explain why using torsional and steric strain.
⚠️ Chair conformation problems are a CHEM 237 staple. You will need to draw both chairs for a substituted cyclohexane, place substituents correctly as axial or equatorial, and identify the more stable chair.
⚠️ The cis/trans stability question is almost guaranteed. Memorising "trans is more stable" without understanding will fail you on 1,3-disubstituted cases. Draw both chairs for each isomer and compare.
⚠️ "Conformers, skeletal isomers, or identical?" is a classic multiple-choice format. Practise tracing connectivity methodically rather than eyeballing it.
True or false: the gauche conformation of butane is higher in energy than the anti conformation because of torsional strain. (False. The gauche conformation is staggered, so it has no torsional strain. It is higher in energy than anti because of steric strain between the two methyl groups.)
Fill in the blank: in a chair conformation of cyclohexane, a ring flip converts all axial bonds to ______ and vice versa. (Equatorial.)
True or false: cis-1,2-dimethylcyclohexane can place both methyl groups in equatorial positions. (False. In cis-1,2, one methyl must be axial and one equatorial.)
Fill in the blank: the steric interaction between an axial substituent and axial hydrogens on carbons two positions away is called a ______ interaction. (1,3-diaxial.)
True or false: conformers are different compounds that cannot interconvert. (False. Conformers interconvert by rotation about single bonds.)
Q: Explain why the anti conformer of butane is more stable than the gauche conformer, which is more stable than the eclipsed conformer.
A: In the anti conformer, the two methyl groups are 180° apart, minimising steric strain. In the gauche conformer, they are 60° apart, close enough for their electron clouds to repel (steric strain), but bonds are still staggered (no torsional strain). In the eclipsed conformer, bonds on adjacent carbons are aligned, adding torsional strain on top of steric strain. Result: anti < gauche < eclipsed in energy.
Q: Why is the anti-eclipsed energy difference larger for 2,5-dimethylhexane than for butane?
A: The groups that eclipse each other in 2,5-dimethylhexane are isopropyl groups, which are bulkier than the methyl groups in butane. Larger groups produce more severe steric strain when forced into an eclipsed arrangement, increasing the energy penalty.
Q: Is trans-1,3-dimethylcyclohexane more or less stable than cis-1,3-dimethylcyclohexane? Explain using chair conformations.
A: Less stable. In cis-1,3-dimethylcyclohexane, both methyl groups can sit equatorial in one chair conformation (both point the same direction, up or down, and on a 1,3 pattern that maps to equatorial-equatorial). In trans-1,3, one methyl is forced axial in both chairs. Axial methyls create 1,3-diaxial interactions, raising the energy. So cis-1,3 is more stable.
Q: Two structures are drawn. One shows methylcyclohexane with the methyl group axial, the other with methyl equatorial. Are these conformers, constitutional isomers, or identical?
A: Conformers. They have the same connectivity and differ only by a ring flip (rotation about C-C bonds in the ring). They interconvert at room temperature.
Q: For 1,2-dimethylcyclohexane, which isomer (cis or trans) can adopt a diequatorial chair conformation?
A: Trans. In trans-1,2-dimethylcyclohexane, the two methyls are on opposite sides of the ring, which on adjacent carbons in a chair means both can be equatorial. Cis-1,2 forces one axial and one equatorial.
Conformational analysis connects to stereochemistry (Chapter 5 in most textbooks), because the three-dimensional arrangement of groups, especially on cyclohexane chairs, determines whether molecules are chiral or achiral. It also connects to reaction selectivity: many organic reactions proceed through chair-like transition states, and understanding axial vs equatorial preferences helps predict which product forms. The steric strain concepts here reappear in substitution and elimination reactions (SN1/SN2/E1/E2), where bulky groups influence reaction rates and pathways.
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