Difficulty: Intermediate | Prerequisites: Parts 1 and 2 of these notes, basic understanding of bonding and molecular geometry (VSEPR)
Molecules are not flat drawings on paper. They occupy three-dimensional space, and atoms connected by single bonds can rotate relative to each other. This chapter section introduces the concept of conformational analysis: how different spatial arrangements of the same molecule have different energies, and why that matters. You will also learn how cyclic alkanes adopt non-planar shapes to minimise strain, with cyclohexane and its chair conformation being the most important case. Finally, when substituents are locked on a ring, cis/trans relationships introduce stereoisomerism, a concept that deepens considerably in later chapters.
Rotation around C–C single bonds produces different conformations (staggered vs eclipsed). Staggered conformations are lower in energy because atoms are further apart. For butane, the anti conformation is the most stable. Cyclic alkanes adopt puckered shapes to reduce angle and torsional strain. Cyclohexane prefers the chair conformation with equatorial substituents. Cis and trans arrangements on rings are stereoisomers: same connectivity, different spatial arrangement.
Conformation
A specific three-dimensional arrangement of atoms in a molecule that results from rotation around a single bond. Different conformations are not different molecules; they interconvert rapidly. Think of it as: the same molecule caught in different poses.
Newman projection
A way of drawing a molecule by looking straight down a C–C bond axis. The front carbon is shown as a dot (centre) and the back carbon as a circle. Bonds radiate outward at 120° angles. Think of it as: staring down the barrel of a bond.
Staggered conformation
A conformation where the bonds on adjacent carbons are offset by 60°, as far apart as possible. This is the lowest-energy arrangement for ethane. Think of it as: the atoms are neatly out of each other's way.
Eclipsed conformation
A conformation where the bonds on adjacent carbons line up directly behind one another (0° dihedral angle). This is the highest-energy arrangement. Think of it as: atoms are directly in each other's face.
Dihedral angle (torsion angle)
The angle between two groups on adjacent carbons when viewed along the bond axis in a Newman projection. Staggered = 60°, eclipsed = 0°, anti = 180°.
Anti conformation
A staggered conformation where the two largest groups on adjacent carbons are 180° apart. The most stable conformation for butane. Think of it as: the bulky groups are as far apart as they can get.
Gauche conformation
A staggered conformation where the two largest groups on adjacent carbons are 60° apart. Higher energy than anti because the groups are closer together, but still lower than any eclipsed arrangement.
Steric effects (steric strain)
The increase in energy that occurs when atoms or groups are forced too close together. Electron clouds repel each other. Think of it as: atoms need their personal space.
Torsional strain
The strain that arises from eclipsed bonds. Even when the eclipsing groups are small (like hydrogens), there is an energy cost.
Ring strain (angle strain)
The strain in a cyclic molecule caused by bond angles being forced away from the ideal tetrahedral angle of 109.5°. Smaller rings have more ring strain.
Chair conformation
The most stable conformation of cyclohexane, where all C–C–C bond angles are close to 109.5° and all adjacent C–H bonds are staggered. Think of it as: the shape of a reclining lawn chair.
Boat conformation
A higher-energy conformation of cyclohexane with eclipsed C–H bonds along two edges and "flagpole" hydrogens that point toward each other, causing steric strain.
Twist-boat conformation
A slightly more stable version of the boat, where the ring is twisted to reduce the flagpole interaction and some of the eclipsing strain.
Axial position
Bonds that point straight up or straight down, roughly parallel to the axis running through the centre of the cyclohexane ring. Six hydrogens occupy axial positions (alternating up and down around the ring).
Equatorial position
Bonds that point outward from the ring, roughly along the "equator." Six hydrogens occupy equatorial positions. Equatorial positions have more room, so large substituents prefer them.
Ring flip (chair-chair interconversion)
The process by which cyclohexane flips from one chair conformation to another through a boat intermediate. Every axial position becomes equatorial and vice versa.
1,3-Diaxial interaction
The steric strain between an axial substituent and the axial hydrogens on the same side of the ring, two carbons away. This is why large groups prefer the equatorial position.
Stereoisomers
Molecules with the same molecular formula and the same connectivity but a different arrangement of atoms in three-dimensional space. They cannot be interconverted by rotation around single bonds.
Cis (same side)
Describes two substituents on the same side of a ring. Example: cis-1,2-dimethylcyclohexane.
Trans (opposite side)
Describes two substituents on opposite sides of a ring. Example: trans-1,2-dimethylcyclohexane.
Methane (CH₄) is tetrahedral with a bond angle of 109.5°. Rotation around any C–H bond does not change the 3D shape, so methane has only one spatial arrangement.
Ethane is different. Rotation around the central C–C bond produces an infinite number of conformations, but two extremes matter:
Staggered (dihedral angle = 60°): lowest energy, most stable. Bonds on the front and back carbons are offset.
Eclipsed (dihedral angle = 0°): highest energy, least stable. Bonds on the front and back carbons line up directly.
The energy difference between staggered and eclipsed ethane is about 12.6 kJ/mol (the rotation barrier).
Steric effects drive this: when bonds eclipse, their electron clouds are forced close together, raising the energy.
Wedge-and-dash drawings show 3D structure on a flat page. A solid wedge comes toward you; a dashed wedge goes into the page.
Newman projections are the standard tool for visualising conformations.
You look straight down the C–C bond axis.
The front carbon is drawn as a central dot; the back carbon as a larger circle.
The three bonds on each carbon radiate outward at 120° angles.
Staggered: back bonds appear between front bonds.
Eclipsed: back bonds are hidden directly behind front bonds.
Butane (CH₃CH₂CH₂CH₃) introduces non-equivalent staggered and eclipsed conformations because the groups on the rotating bond (C2–C3) differ in size.
The key conformations, ordered by energy:
Anti (180°): the two methyl groups are as far apart as possible. This is the global energy minimum, the most stable conformation.
Gauche (60°): the two methyl groups are 60° apart. Higher in energy than anti by about 3.8 kJ/mol due to steric interaction between the methyl groups.
Eclipsed (methyl–H) (120°): intermediate-energy eclipsed conformation.
Eclipsed (methyl–methyl) (0° or 360°): the global energy maximum, with the two methyl groups directly overlapping.
The energy diagram for butane rotation is asymmetric: there are two different eclipsed maxima and two different staggered minima (anti and gauche), repeating every 360°.
Cyclic alkanes (cycloalkanes) have the general formula CₙH₂ₙ (two fewer hydrogens than their acyclic counterparts because the ring closure uses two bonding positions).
Named by adding the prefix "cyclo-" to the parent alkane name: cyclopropane (3C), cyclobutane (4C), cyclopentane (5C), cyclohexane (6C).
Drawn as simple polygons in line notation: triangle, square, pentagon, hexagon.
Substituents on cycloalkanes are named and numbered just like acyclic alkanes. If there is only one substituent, no number is needed (e.g., methylcyclohexane). With two or more, number the ring to give the lowest set of locants.
The ideal C–C–C bond angle for sp³ carbon is 109.5°. Rings that force this angle to deviate experience strain.
Cyclopropane (C–C–C angle: 60°): severely strained. The ring is necessarily planar, so all C–H bonds are eclipsed. This molecule has both high ring strain and high torsional strain.
Cyclobutane (C–C–C angle: ~88°): still strained, but puckers slightly out of the plane to reduce eclipsing of C–H bonds.
Cyclopentane (C–C–C angle: ~105°): close to ideal. Adopts a puckered "envelope" conformation to avoid eclipsing.
Cyclohexane (C–C–C angle: ~111°): nearly perfect tetrahedral angles. Essentially free of angle strain.
Cyclohexane is the most important cycloalkane. Its chair conformation achieves near-ideal bond angles and fully staggered C–H bonds.
Drawing the chair: draw two offset parallel lines, connect them with downward and upward V shapes. Then add hydrogens so each carbon is tetrahedral.
Two types of hydrogen positions:
Axial (6 total): point straight up or straight down, alternating around the ring, roughly parallel to the ring's central axis.
Equatorial (6 total): point outward, roughly in the plane of the ring.
Adjacent carbons always have their axial bonds pointing in opposite directions (one up, one down).
Boat: two carbons on opposite ends of the ring fold up. The C–H bonds along the sides are eclipsed, and two "flagpole" hydrogens point inward and clash sterically. Energy: about 27.2 kJ/mol (6.5 kcal/mol) above the chair.
Twist-boat: a slight twist of the boat that relieves some eclipsing and reduces the flagpole interaction. Energy: about 20.9 kJ/mol (5 kcal/mol) above the chair.
Half-chair: the transition state between chair and twist-boat. The highest energy point: about 45.1 kJ/mol (10.8 kcal/mol) above the chair.
Cyclohexane can "flip" between two chair conformations, passing through a boat (or twist-boat) intermediate.
The crucial consequence: every axial position becomes equatorial, and every equatorial position becomes axial. This interconversion is rapid at room temperature.
A substituent in the axial position experiences 1,3-diaxial interactions: steric clashes with the axial hydrogens on the same face of the ring, two carbons away.
A substituent in the equatorial position points away from the ring and avoids these clashes.
The equatorial conformation is therefore more stable. For a methyl group, the equatorial chair is favoured by about 7.28 kJ/mol (1.7 kcal/mol).
Larger substituents have a stronger equatorial preference. A tert-butyl group, for instance, has such a strong preference (about 21.0 kJ/mol) that the ring is effectively locked with the tert-butyl group equatorial.
When two substituents sit on a cyclohexane ring, they can be:
Cis: both on the same side of the ring (both "up" or both "down" relative to the ring plane).
Trans: on opposite sides of the ring (one "up" and one "down").
Cis and trans isomers are stereoisomers: same molecular formula, same connectivity, but different spatial arrangement. They cannot be interconverted by rotation around single bonds; you would need to break and reform bonds.
For disubstituted cyclohexanes, cis/trans relationships determine whether substituents end up axial/equatorial or equatorial/equatorial (or axial/axial) in the chair conformations. This directly affects which chair is more stable.
Students frequently confuse conformational isomers with constitutional isomers. Conformers are the same molecule in different rotational poses; constitutional isomers have different connectivity. Conformers interconvert by bond rotation. Constitutional isomers do not.
A very common mistake is thinking that "equatorial = up" and "axial = down" (or vice versa). Both axial and equatorial positions alternate up and down around the ring. "Axial" means parallel to the ring axis; "equatorial" means roughly in the ring plane.
Students often forget that a ring flip swaps all axial and equatorial positions simultaneously. You cannot flip just one substituent.
Confusing cis/trans with axial/equatorial: cis and trans describe which side of the ring plane the substituents sit on. Axial and equatorial describe the orientation of the bond. A cis pair of substituents can be both equatorial in one chair and both axial in the other.
⚠️ Drawing Newman projections of ethane and butane, identifying which conformation is shown, and ranking conformations by energy are staple exam questions.
⚠️ You will need to draw cyclohexane chair conformations correctly, place substituents in axial or equatorial positions, and determine which chair is more stable.
⚠️ Know the energy values: ethane rotation barrier (~12.6 kJ/mol), gauche butane penalty (~3.8 kJ/mol), methyl equatorial preference (~7.28 kJ/mol).
⚠️ Be ready to identify cis vs trans relationships on cyclohexane and determine what that means for axial/equatorial placement in each chair.
⚠️ Understanding 1,3-diaxial interactions is essential for predicting the more stable chair conformation of substituted cyclohexanes.
True or false: The eclipsed conformation of ethane is more stable than the staggered conformation. (False)
Fill in the blank: In the anti conformation of butane, the two methyl groups are ____° apart. (180)
True or false: In a cyclohexane ring flip, axial hydrogens become equatorial and equatorial hydrogens become axial. (True)
Fill in the blank: Cyclopropane has C–C–C bond angles of ____°, far from the ideal 109.5°. (60)
True or false: Cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane are constitutional isomers. (False, they are stereoisomers.)
Q: Draw the Newman projection of the most stable conformation of butane, looking down the C2–C3 bond.
A: The anti conformation. In the Newman projection, the two methyl groups (CH₃) are at 180° from each other. One methyl is at the top of the front carbon; the other is at the bottom of the back carbon (or equivalent positions 180° apart).
Q: What is the most stable conformation of methylcyclohexane, and why?
A: The chair conformation with the methyl group in the equatorial position. The equatorial methyl avoids 1,3-diaxial interactions with the axial hydrogens on the same face of the ring, making it about 7.28 kJ/mol (1.7 kcal/mol) more stable than the axial-methyl chair.
Q: How many degrees of ring strain exist in cyclopropane, and why is this significant?
A: Cyclopropane has C–C–C bond angles of 60°, which is 49.5° less than the ideal 109.5°. This severe angle strain, combined with the fact that all C–H bonds are eclipsed (torsional strain), makes cyclopropane significantly higher in energy and more reactive than larger cycloalkanes.
Q: Are cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane the same compound? Explain.
A: No. They are stereoisomers. Both have the same molecular formula and the same atomic connectivity (a cyclohexane ring with methyl groups on C1 and C2), but the spatial arrangement differs: in the cis isomer, both methyls are on the same side of the ring; in the trans isomer, they are on opposite sides. You cannot interconvert them by rotating around single bonds.
Q: In trans-1,2-dimethylcyclohexane, can both methyl groups be equatorial simultaneously? What about cis-1,2-dimethylcyclohexane?
A: Yes, trans-1,2-dimethylcyclohexane has a chair conformation where both methyls are equatorial (the more stable chair). In cis-1,2-dimethylcyclohexane, one methyl is always axial and the other equatorial in any chair conformation; a ring flip just swaps which is which.
Conformational analysis comes back in nearly every chapter. The preference for staggered conformations and the equatorial position on cyclohexane rings directly affects how reactions proceed and which products form. Stereoisomerism introduced here with cis/trans on rings expands into chirality and R/S configuration in the next major topic. The energy diagrams (potential energy vs dihedral angle) are a model for the reaction coordinate diagrams you will use throughout organic chemistry.
conformation, conformational analysis, Newman projection, staggered, eclipsed, anti, gauche, dihedral angle, torsion angle, steric strain, torsional strain, ring strain, angle strain, cycloalkane, cyclopropane, cyclobutane, cyclopentane, cyclohexane, chair conformation, boat conformation, twist-boat, half-chair, axial, equatorial, ring flip, chair flip, 1,3-diaxial interaction, diaxial strain, cis, trans, stereoisomer, cis-trans isomerism, substituted cyclohexane, CHEM 25500, organic chemistry chapter 2, Purdue