Organic Chemistry · Organic Compounds: Alkanes and Their Stereochemistry
Conformations of Ethane
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Single bonds allow free rotation: the two halves of ethane (CH3-CH3) can spin relative to each other around the C–C σ bond, and each rotational position is a different Conformation A molecular shape reached by rotation about single bonds Full entry → (also called a conformer or Rotamer Another name for a conformer Full entry →). Chemists visualize these positions with Newman projections — views straight down the C–C bond axis, with the front carbon drawn as a dot and the back carbon as a circle. Rotating the back carbon 360° sweeps through two special arrangements that repeat every 60°: the staggered conformation, where the C–H bonds of the front carbon sit midway between those of the back carbon, and the eclipsed conformation, where front and back C–H bonds line up directly behind one another. The staggered form is the more stable: its hydrogens are maximally separated, so there is no Torsional strain Destabilization from eclipsed bond–bond crowding Full entry →. The eclipsed form is higher in energy by about 12 kJ/mol (≈ 2.9 kcal/mol) because the front and back C–H bonds are forced close together. That barrier is small — ethane rotates essentially freely at room temperature, interconverting among conformations millions of times per second — so no single conformer can ever be isolated. Conformational analysis is the first step toward understanding why molecular shape matters: it reappears with butane (gauche vs. anti), cyclohexane (chair vs. boat), and the three-dimensional fit of drugs to their targets.
Why this matters
- Shape determines function: Biological molecules bind, enzymes catalyze, and drugs act only when shapes fit — and single-bond rotation constantly reshapes molecules. Conformations are the "breathing room" of organic structure.
- Energy logic: The staggered/eclipsed energy difference is the simplest energy diagram in organic chemistry — the template for every later reaction-coordinate and conformational diagram.
- Stereochemistry foundation: Conformational analysis is a prerequisite for cis/trans and enantiomer topics later in the book; "is the molecule flat?" and "can these groups get near each other?" are conformational questions.
- Real molecules: Polymer flexibility, lipid bilayer fluidity, and protein folding all depend on single-bond rotation energetics.
- Exams: Drawing Newman projections, identifying staggered vs. eclipsed, and converting energy units are classic test items.
The college version
Core Concepts
Free rotation about single bonds
A σ bond is cylindrically symmetric: rotating one carbon relative to the other does not break the bond, so ethane interconverts among a continuous family of conformations. Only rotationally related shapes are conformers; isomers that require bond breaking (constitutional or configurational) are a different category.
Newman projections: how to draw and read them
To draw a Newman projection Drawing viewed down a C–C bond axis (front = dot, back = circle) Full entry → of ethane: (1) look straight down the C1–C2 bond axis; (2) draw the front carbon as a dot with its three C–H bonds radiating at 120°; (3) draw the back carbon as a circle with its three C–H bonds at 120°, offset relative to the front bonds. When the back bonds appear exactly behind the front bonds (Dihedral angle Angle between a front-carbon bond and a back-carbon bond in the projection Full entry → 0°), the conformation is eclipsed; when the back bonds are rotated 60° so they sit between the front bonds, it is staggered. The dihedral angle is the angle between a bond on the front carbon and a bond on the back carbon measured in this view.
Staggered vs. eclipsed: the energy difference
The Staggered conformation Back bonds centered between front bonds Full entry → minimizes electron–electron repulsion between the C–H bonds on the two carbons; the eclipsed form forces each front C–H bond directly against a back C–H bond. The resulting torsional strain raises the eclipsed energy by about 12 kJ/mol (≈ 2.9 kcal/mol, since 1 kcal = 4.184 kJ). The 60° rotation from staggered to eclipsed is the energy maximum; another 60° returns to staggered. Because the barrier is tiny compared with room-temperature thermal energy (RT ≈ 2.5 kJ/mol at 25 °C), ethane interconverts freely — samples are dynamic mixtures dominated by staggered conformations.
Why the barrier is small
Torsional strain in ethane is not steric (through-space) repulsion — the hydrogens never actually touch. It is the small destabilization of bonding electrons in eclipsed C–H bonds pushed together. The lesson for later: the size of the rotating groups controls the barrier — replace H with CH₃ (butane) and the eclipsed forms become far more strained, exactly the next topic.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Conformer | Constitutional isomer | Conformers interconvert by single-bond rotation (no bonds broken); isomers differ in connectivity and need bond breaking to interconvert |
| Staggered = "more stable" | Staggered = "locked in place" | Staggered is merely the energy minimum; ethane keeps rotating through eclipsed forms constantly |
| Eclipsed hydrogens "touching" | Steric (through-space) repulsion | Ethane's barrier is torsional strain of bonding electrons, not atoms colliding; true steric strain needs bulky groups |
| Eclipsed = cis | Staggered = trans | cis/trans describes double bonds or rings; conformations use dihedral angle (0° vs 60°) |
| One staggered conformer | Distinct staggered forms | All three staggered (and all three eclipsed) forms of ethane are identical — rotation just relabels equivalent hydrogens |
| Barrier magnitude "large" | Barrier magnitude in butane | 12 kJ/mol is tiny — far smaller than the tens of kJ/mol barriers that appear once methyls replace hydrogens |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Ethane is like two triangle-shaped fans joined at their centers, able to spin around the rod between them. When the blades of the two fans are spread apart so they never overlap, that's the staggered position — relaxed and happy, the lowest-energy shape. When you spin one fan so its blades line up directly behind the other fan's blades, that's the eclipsed position — the blades crowd each other and the molecule is slightly tense. The spin is so easy that ethane never holds still; it is always twirling, mostly staying in the comfortable staggered spots.
Worked example
Example 1: Converting the rotation barrier between energy units
The eclipsed conformation of ethane is about 12 kJ/mol higher in energy than staggered. Express this barrier in kcal/mol. (Conversion: 1 kcal = 4.184 kJ.)
Write the conversion factor and set up the calculation:
ΔE = 12 kJ/mol × 1 kcal4.184 kJ
Compute:
ΔE = 124.184 kcal/mol ≈ 2.9 kcal/mol
Answer: the barrier is about 2.9 kcal/mol. The same dimensional-analysis setup converts any energy quantity between the two unit systems — a routine operation when comparing textbook values.
Example 2: Comparing the barrier with thermal energy
At 25 °C, RT (the product of the gas constant and temperature) equals about 2.5 kJ/mol. How many times larger is the ethane rotation barrier than RT?
Set up the ratio:
ΔERT = 12 kJ/mol2.5 kJ/mol ≈ 4.8
Answer: the barrier is roughly 5 times RT. A barrier only a few times RT is crossed constantly at room temperature — exactly why ethane rotates freely and staggered and eclipsed forms cannot be separated. (Boltzmann statistics still favor staggered: at any instant most molecules sit near a staggered minimum.)
Example 3: Identifying conformations from a Newman projection description
A Newman projection of ethane shows the three bonds of the back carbon drawn exactly behind the three bonds of the front carbon. Is this staggered or eclipsed? What is the dihedral angle?
Rule: back bonds aligned with front bonds (0°) = eclipsed; back bonds between front bonds (60°) = staggered.
Answer: eclipsed, with a dihedral angle of 0°. Rotating the back carbon 60° converts it to the staggered form — the two are always separated by 60° of rotation.
Key takeaways
- Conformation = any arrangement reached by rotation about single bonds; conformers interconvert without breaking bonds.
- Newman projection: view down the C–C axis; front carbon = dot, back carbon = circle; bonds at 120°.
- Staggered: back C–H bonds between front C–H bonds (dihedral angle 60°); eclipsed: back bonds directly behind front bonds (0°).
- Staggered is more stable; eclipsed has torsional strain — higher energy by ≈ 12 kJ/mol (≈ 2.9 kcal/mol) in ethane.
- Conversion factor: 1 kcal = 4.184 kJ (exact). The barrier is small relative to room-temperature thermal energy → free rotation; conformers cannot be isolated.
- Every 60° of rotation alternates staggered ↔ eclipsed; there are three staggered and three eclipsed forms of ethane, all equivalent.
- Torsional strain ≠ steric strain: hydrogens do not collide; the effect grows when rotating groups are larger (butane, next topic).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the dihedral angle in the fully Eclipsed conformation Back bonds directly aligned with front bonds Full entry → of ethane? In the staggered?
Show answer
Eclipsed: 0° (back C–H bonds directly behind front C–H bonds). Staggered: 60° (back bonds centered between front bonds).
Which conformation of ethane is lower in energy, and by how much?
Show answer
Staggered is lower in energy; the eclipsed form is about 12 kJ/mol (≈ 2.9 kcal/mol) higher due to torsional strain.
Convert 12 kJ/mol to kcal/mol.
Show answer
12 kJ/mol × 1 kcal4.184 kJ ≈ 2.9 kcal/mol.
Why can't you isolate a pure sample of "staggered ethane"?
Show answer
The rotation barrier (~12 kJ/mol) is only a few times RT at room temperature, so molecules rotate through all conformations billions of times per second; no single conformation persists.
In a Newman projection, which atom is drawn as the dot and which as the circle?
Show answer
The front carbon is the dot (closest to you); the back carbon is the circle (farther away).
Rotation by what angle converts a staggered conformation of ethane into an eclipsed one?
Show answer
60° — alternating staggered and eclipsed forms repeat every 60° of rotation.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Conformation
- A molecular shape reached by rotation about single bonds
- Newman projection
- Drawing viewed down a C–C bond axis (front = dot, back = circle)
- Dihedral angle
- Angle between a front-carbon bond and a back-carbon bond in the projection
- Staggered conformation
- Back bonds centered between front bonds
- Eclipsed conformation
- Back bonds directly aligned with front bonds
- Torsional strain
- Destabilization from eclipsed bond–bond crowding
- Rotamer
- Another name for a conformer
Sources & references
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