Difficulty: Intermediate | Prerequisites: Chapter 2 functional groups, basic understanding of bond rotation and molecular geometry
Conformational analysis is about what happens when you rotate around a single bond. Unlike constitutional isomers (different connectivity) or stereoisomers (different spatial arrangement that requires bond breaking to interconvert), conformations interconvert freely by rotation. This matters because some conformations are more stable than others, and that stability difference shows up in physical properties and reactivity. Newman projections are the tool you use to visualise and compare these conformations. This is where organic chemistry starts to become three-dimensional.
Rotation around C–C single bonds produces different conformations. Staggered conformations are more stable than eclipsed ones. Newman projections let you see and rank these conformations by looking straight down the bond axis. The anti conformation (large groups opposite each other) is the most stable; gauche (large groups 60° apart) is less stable but still staggered.
Conformation
A specific three-dimensional arrangement of atoms that results from rotation around a single bond. Conformations interconvert rapidly at room temperature. Think of it as the molecule's posture at any given instant.
Dihedral angle (torsion angle)
The angle between two groups on adjacent carbons, measured by looking down the C–C bond axis. Ranges from 0° to 360°. This is the variable on the x-axis of an energy diagram for conformational analysis.
Newman projection
A way of drawing a molecule by looking straight down a C–C bond. The front carbon is shown as a dot (intersection of three bonds), and the back carbon as a circle. This view makes dihedral angles easy to read.
Eclipsed conformation
A conformation where the bonds on the front carbon line up directly with the bonds on the back carbon (dihedral angle = 0°, 120°, 240°). Higher in energy due to torsional strain.
Staggered conformation
A conformation where the bonds on the front carbon fall between the bonds on the back carbon (dihedral angle = 60°, 180°, 300°). Lower in energy; more stable.
Anti conformation
The staggered conformation where the two largest substituents are 180° apart. The most stable conformation for simple cases.
Gauche conformation
A staggered conformation where two large substituents are 60° apart. More stable than any eclipsed form, but less stable than anti due to steric strain between the nearby groups.
Torsional strain
The increase in energy that occurs when bonds on adjacent carbons are eclipsed. Caused by repulsion between bonding electrons.
Steric strain
The increase in energy from large groups being forced too close together in space. Present in gauche and certain eclipsed conformations.
Rotational barrier
The energy difference between the lowest-energy (most stable) and highest-energy (least stable) conformations. For ethane, this barrier is approximately 3.0 kcal/mol.
How to draw a Newman projection
Pick the C–C bond you want to analyse
Look straight down the bond from one end
The front carbon is drawn as a point (where three lines meet)
The back carbon is drawn as a circle
Attach the remaining substituents to each carbon at 120° intervals
Reading the dihedral angle
The dihedral angle is measured between the same two substituents (usually the largest ones) on the front and back carbons. At 0° they are eclipsed; at 60° they are gauche; at 180° they are anti.
Ethane as the simplest case
Ethane (CH₃–CH₃) has a rotational barrier of about 3.0 kcal/mol. The energy maximum occurs at each eclipsed conformation (0°, 120°, 240°), and the energy minimum at each staggered conformation (60°, 180°, 300°). The energy profile is a smooth, repeating wave with three peaks and three troughs per full 360° rotation.
Butane and larger molecules
When substituents differ in size, the eclipsed and staggered conformations are no longer equivalent. For butane (looking down the C2–C3 bond):
The anti conformation (CH₃ groups 180° apart) is the global energy minimum
The gauche conformations (CH₃ groups 60° apart) are local energy minima, higher than anti
The highest-energy eclipsed conformation has the two CH₃ groups directly eclipsing each other (0°)
The other eclipsed conformations (CH₃ eclipsing H) are high in energy but not as high as CH₃/CH₃ eclipsed
Energy values for butane
Anti is the reference (lowest energy)
Gauche is approximately 3.8 kJ/mol above anti
CH₃/H eclipsed is approximately 16 kJ/mol above anti
CH₃/CH₃ eclipsed is the highest, approximately 19 kJ/mol above anti
2,3-Dimethylpentane example (from lecture)
For more substituted molecules, the same principles apply but with more steric interactions. The stability ranking from the lecture notes for the six labelled conformations of 2,3-dimethylpentane is:
E > C > A > D > F > B
(where E is the least stable and B is the most stable)
The key rule: place the largest groups anti to each other for the most stable conformation. The most unstable conformation has the largest groups eclipsed.
Thermodynamic reasoning
The relationship ΔG = −RT ln K can be used to calculate the equilibrium ratio between conformations. At room temperature (298 K), a free-energy difference of about 20 kJ/mol corresponds to a ratio of roughly 1:10¹⁴, meaning the less stable conformation is effectively unpopulated. Smaller differences (like anti vs. gauche at 3.8 kJ/mol) mean both conformations are populated, with anti predominating.
Students often think that eclipsed conformations cannot exist. They can and do exist momentarily during rotation, but molecules do not rest in eclipsed conformations because they are energy maxima, not minima.
Confusing conformations with isomers. Conformations interconvert by bond rotation without breaking any bonds. Isomers require bond breaking to interconvert. You cannot "trap" a conformation the way you can isolate an isomer (at room temperature).
Assuming that anti is always 180° and gauche is always 60°. These angles refer to the dihedral between the two largest groups specifically. If you are measuring between different pairs of substituents, the numbers still apply, but make sure you are tracking the same two groups.
Forgetting that the energy diagram is periodic. The pattern of peaks and troughs repeats every 360°. If you can draw 0° to 180°, you know the rest by symmetry.
⚠️ "Draw the Newman projection of X looking down the C2–C3 bond and identify the most stable conformation" is a near-certain exam question.
⚠️ You will be asked to rank conformations by stability. The rule: staggered beats eclipsed, anti beats gauche, and the largest groups eclipsing each other is the worst case.
⚠️ Know the rotational barrier for ethane (approximately 3.0 kcal/mol). It is a reference value that comes up in problem sets.
⚠️ Be able to sketch an energy vs. dihedral angle diagram for a simple molecule, labelling each peak and trough with the correct conformation type.
True or false: The anti conformation of butane has the two methyl groups at a dihedral angle of 60°.
Fill in the blank: In a Newman projection, the front carbon is represented as a ______ and the back carbon as a ______.
True or false: Staggered conformations are higher in energy than eclipsed conformations.
Fill in the blank: The rotational barrier in ethane is approximately ______ kcal/mol.
True or false: Conformations require bond breaking to interconvert.
Answers: 1. False (anti = 180°). 2. Point (or dot); circle. 3. False (staggered is lower in energy). 4. 3.0. 5. False (only bond rotation is needed).
Q: Draw the most stable Newman projection for butane looking down the C2–C3 bond.
A: Place the two methyl groups anti to each other (180° apart). All other substituents (hydrogens) fall into staggered positions naturally.
Q: Why is the gauche conformation of butane less stable than the anti conformation?
A: In the gauche conformation, the two methyl groups are only 60° apart, leading to steric strain from their proximity. In anti, they are 180° apart, minimising steric interactions.
Q: Sketch the energy diagram for rotation about the C2–C3 bond of butane from 0° to 360°. Label the highest and lowest energy points.
A: The diagram has three peaks and three troughs. The highest peak occurs at 0° (and 360°), where the two methyl groups are fully eclipsed. The lowest trough is at 180° (the anti conformation). The two gauche troughs at 60° and 300° sit slightly above the anti minimum. The two other eclipsed peaks at 120° and 240° (where CH₃ eclipses H) are lower than the 0° peak.
Q: Using ΔG = −RT ln K, explain why a 20 kJ/mol energy difference effectively eliminates the higher-energy conformation at room temperature.
A: At 298 K, RT is about 2.5 kJ/mol. Plugging in ΔG = 20 kJ/mol gives ln K ≈ 8, so K ≈ 3000. The lower-energy conformation outnumbers the higher-energy one by roughly 3000 to 1, meaning the latter is negligibly populated.
Conformational analysis reappears in cyclohexane chemistry (Chapter 4), where axial vs. equatorial positions are a direct extension of the gauche and anti concepts covered here. Ring flips in cyclohexane are conformational changes, not isomerisations. Understanding Newman projections now makes the transition to chair conformations much smoother.
This also connects to stereochemistry (Chapter 5), where the spatial arrangement of groups determines optical activity and reactivity.
Newman projection, conformational analysis, dihedral angle, torsion angle, eclipsed, staggered, anti, gauche, torsional strain, steric strain, rotational barrier, ethane rotation, butane conformations, 2,3-dimethylpentane, energy diagram, ΔG, conformational energy, organic chemistry chapter 2, UW organic chemistry, Prof. Forrest Michael