Organic Chemistry I · Lesson 05
Alkanes and Conformations
Carbon-carbon single bonds rotate freely. That single fact creates an entire world of three-dimensional shapes for the same molecule. Learning to draw, compare, and rank these shapes is the foundation for everything spatial in organic chemistry.
Learning Goals
- Draw alkanes as line-angle structures, expanded structures, Newman projections, and chair cyclohexanes.
- Explain why rotation around C-C single bonds creates different conformations, not different molecules.
- Rank conformations by stability using torsional strain and steric strain.
- Draw and interpret Newman projections by looking down a specific C-C bond.
- Identify axial and equatorial positions on a cyclohexane chair and predict ring flip outcomes.
- Determine the most stable chair conformation for a substituted cyclohexane.
Key Terms
Tap any term to see its definition.
Section 1
Rotation Around Single Bonds
A σ (sigma) bond is formed by direct, head-on overlap of orbitals along the bond axis. Because the overlap is symmetric around that axis, the two atoms connected by a σ bond can rotate relative to each other without breaking the bond.
Each different rotational position is called a conformation, and a molecule that exists in a particular conformation is called a conformer. Here is the part I really want you to hold onto: different conformations of the same molecule are not different molecules. They are the same molecule captured at different points of rotation. Rotation only becomes restricted, and conformations become locked, when a double bond or ring prevents free spinning.
Main idea
Here is the loop I want you running in your head the whole lesson: draw, rotate, compare strain, rank stability. Every section below is just me repeating that same four-step move on a new molecule.
Section 2
Ethane: The Simplest Example
Ethane (CH₃-CH₃) is the smallest molecule that shows conformational behavior. Rotating around the C-C bond produces two extreme conformations: staggered and eclipsed.
Ethane
Staggered
All six C-H bonds are as far apart as possible. This is the lowest energy conformation.
Eclipsed
Front and back C-H bonds line up directly. This is the highest energy conformation.
The energy difference between staggered and eclipsed ethane comes from torsional strain, which is repulsion between the electrons in the C-H bonds when they line up directly across the bond axis. Notice there is no actual atom-to-atom steric clash in ethane. The strain here is purely electronic, and I want you to keep that distinction in mind because butane is about to add a second kind of strain on top.
Energy vs. rotation angle for ethane
How to read a Newman projection
- 01Look down the C-C bond connecting the two carbons.
- 02The front carbon's three C-H bonds are drawn from a dot.
- 03The back carbon's three C-H bonds are drawn from a circle.
- 04Rotating the back carbon 60° converts staggered into eclipsed, and vice versa.
Section 3
Butane: Adding Steric Strain
Butane (CH₃-CH₂-CH₂-CH₃) introduces a new variable. Looking down the C2-C3 bond, each carbon now carries a bulky methyl group instead of just hydrogens. That means rotation creates not just torsional strain, but also steric strain, the physical crowding between the methyl groups.
Butane, looking down C2–C3
This is the most important stability ranking in the whole lesson. Sure, memorize the order, but what I really want is for you to understand why each one ranks where it does, because once the why clicks you will never have to memorize it again.
Anti
The two methyl groups are 180° apart. Maximum distance, minimum strain. Most stable conformation.
Gauche
The two methyl groups are 60° apart, staggered but close together. Some steric strain from the nearby methyls, but still staggered overall.
Eclipsed CH₃/H
A methyl group lines up directly with a hydrogen. Torsional strain from the eclipsing, plus some steric interaction.
Fully eclipsed CH₃/CH₃
Both methyl groups line up directly with each other. Maximum torsional strain and maximum steric strain. Least stable conformation.
Anti, most stable
Gauche
Eclipsed CH₃/H
Fully eclipsed CH₃/CH₃, least stable
Energy vs. rotation angle for butane
Memory shortcut
The way I anchor this: anti is the best, fully eclipsed methyl-methyl is the worst, and everything else just falls somewhere between those two extremes.
Section 4
Practicing Newman Projections
Reading a finished Newman projection is easy. Drawing one yourself, starting from a line-angle structure, is the actual skill you need. Practice this conversion until it feels automatic: line-angle, then Newman projection, then stability ranking. I really do recommend heading to the practice section to drill the drawing part.
For each one, identify which bond you are looking down, draw the front carbon's three substituents from a dot, draw the back carbon's three substituents from a circle, then rotate the back carbon to find the most and least stable conformations. For this lesson I am going to stick to one consistent drawing convention: groups shown as wedges in the line-angle structure go on the right side of the Newman projection, and groups shown as dashed bonds go on the left. Just so you know, that is my convention to keep things tidy, not a universal rule. The exact Newman projection always depends on which bond you are looking down and from which direction you are viewing it.
Section 5
Cyclohexane Conformations
A ring restricts rotation compared to an open chain, but cyclohexane can still pucker into several distinct three-dimensional shapes. Three matter most.
Chair
All bond angles near 109.5°, all substituents staggered. This is the most stable conformation of cyclohexane by a wide margin.
Boat
Two carbons point upward together. Eclipsing interactions along the sides plus a steric clash between the two 'flagpole' hydrogens make this much less stable than the chair.
Twist-boat
A slightly twisted version of the boat that relieves some strain. Still less stable than the chair, but more stable than the plain boat.
Stability order
Chair > twist-boat > boat. The chair is so much more stable that, at room temperature, the vast majority of cyclohexane molecules exist in a chair conformation at any given moment.
Section 6
Axial, Equatorial, and the Ring Flip
On a chair, every carbon has one bond pointing roughly parallel to the ring's axis (axial) and one pointing roughly outward along the ring's equator (equatorial).
Axial (up/down) vs. equatorial (outward) positions
A ring flip converts the chair into its mirror-image chair. Every axial position becomes equatorial, and every equatorial position becomes axial, but here is the catch that trips everyone up: a substituent's up/down face never changes.
Before and after ring flip
Ring flip facts
- Every axial position becomes equatorial after a ring flip, and every equatorial position becomes axial.
- A substituent that points 'up' stays pointing 'up' after the flip. Only its axial/equatorial character changes, not its up/down face.
- Ring flipping happens rapidly at room temperature, so cyclohexane constantly interconverts between two chair forms.
- For a monosubstituted cyclohexane, the chair with the substituent equatorial is almost always more stable and therefore more populated.
An axial substituent sits close to the axial hydrogens (or groups) on the carbons two positions away on either side. This produces 1,3-diaxial interactions, a form of steric strain.
1,3-diaxial interaction
Main rule
My one rule for this whole section: bulky groups want to be equatorial, because sitting axial forces those 1,3-diaxial clashes. And the bigger the group, the harder it fights to get equatorial.
Section 7
Substituted Cyclohexanes
The core skill here is finding the most stable chair, and my rule of thumb is simple: put the largest group (or groups) equatorial whenever you possibly can.
Methylcyclohexane
Equatorial methyl is more stable
The chair with methyl equatorial avoids 1,3-diaxial strain and is significantly more populated, so about 95% of molecules exist in this form at equilibrium.
tert-Butylcyclohexane
Equatorial is overwhelmingly preferred
The tert-butyl group is so bulky that the axial conformation is essentially never observed. Equatorial tert-butyl accounts for over 99.9% of molecules.
cis- and trans-Dimethylcyclohexane
cis-1,2-dimethylcyclohexane
trans-1,2-dimethylcyclohexane
With two substituents you cannot always get both equatorial. It depends on their relative positions (1,2 / 1,3 / 1,4) and whether they are cis or trans to each other. Work out each substituent's position separately, then compare the total strain of each possible chair. Here is how I actually run it in my head: first I ask, is the important group axial or equatorial? Then I check cis or trans, so I do not accidentally turn it into a different molecule.
Section 8
Common Mistakes
Practice Set
Try each question before opening the answer.
1. Why are staggered conformations of ethane lower in energy than eclipsed conformations?
Answer: In the staggered conformation, the C-H bonds on the front and back carbons are as far apart as possible, minimizing torsional strain. In the eclipsed conformation, the bonds line up directly, increasing electron-electron repulsion between the bonding pairs.
2. Rank the four key butane conformations from most to least stable.
Answer: Anti > gauche > eclipsed CH₃/H > fully eclipsed CH₃/CH₃.
3. In a Newman projection, what does the dot represent? What does the circle represent?
Answer: The dot represents the front carbon, the carbon closer to the viewer. The circle represents the back carbon, the carbon farther from the viewer.
4. Are gauche and anti conformations of butane different molecules?
Answer: No. They are different conformations of the same molecule, related by rotation around a single bond. Conformations are not different molecules unless rotation is somehow restricted, such as in a ring or by a double bond.
5. Why is the chair conformation of cyclohexane more stable than the boat conformation?
Answer: The chair has all staggered substituents and bond angles close to the ideal 109.5°, minimizing both torsional and steric strain. The boat has eclipsing interactions along its sides and a steric clash between the two flagpole hydrogens at the bow and stern.
6. What happens to an axial substituent after a ring flip?
Answer: It becomes equatorial. Every axial position converts to equatorial and every equatorial position converts to axial during a ring flip, while the substituent's up/down face stays the same.
7. Why do bulky substituents prefer the equatorial position on a cyclohexane chair?
Answer: An axial substituent experiences 1,3-diaxial interactions, which is steric strain with the axial hydrogens (or groups) on the carbons two positions away on each side. The equatorial position points outward, away from the ring, avoiding this strain entirely.
8. For tert-butylcyclohexane, why is the equatorial tert-butyl conformation overwhelmingly preferred?
Answer: The tert-butyl group is extremely bulky. Forcing it into the axial position would create severe 1,3-diaxial strain. The energy penalty is so large that essentially all molecules adopt the chair with tert-butyl equatorial.
Ready for a bigger set?
Work through the full Alkanes and Conformations practice page: drawing Newman projections from scratch, ranking conformer stability, chair drawing, and finding the most stable chair for substituted cyclohexanes.
Do Alkanes and Conformations PracticeLesson Summary
Single bonds rotate freely, creating different conformations of the same molecule. Staggered conformations beat eclipsed ones due to torsional strain, and anti beats gauche due to steric strain, which together give the full butane stability order: anti > gauche > eclipsed CH₃/H > fully eclipsed CH₃/CH₃. Cyclohexane strongly prefers the chair conformation, where axial substituents suffer 1,3-diaxial strain that equatorial substituents avoid entirely. If you remember nothing else, remember the loop: draw, rotate, compare strain, rank stability.