Free Rotation and Conformations
Alkanes contain carbon–carbon sigma $(\sigma)$ bonds. The electron distribution of the $\sigma$ molecular orbital is symmetrical around the internuclear axis of the C–C bond, and this symmetry is not disturbed when one carbon is rotated about that axis. Consequently the C–C single bond permits rotation of one part of the molecule with respect to the other.
This rotation generates different spatial arrangements of the atoms in space, arrangements that can be converted into one another simply by turning about the bond. Such arrangements are called conformations, conformers or rotamers. Because rotation is continuous, an alkane can in principle adopt an infinite number of conformations.
Rotation, however, is not completely free. It is hindered by a small energy barrier — of the order of $1\text{–}20\ \text{kJ mol}^{-1}$ — arising from weak repulsive interactions between adjacent bonds. This repulsive interaction is called torsional strain, and it is the central idea that distinguishes a stable conformation from an unstable one.
| Term | Meaning (per NCERT §9.2.4) |
|---|---|
| Conformation / conformer / rotamer | A spatial arrangement of atoms produced by rotation about a C–C single bond, interconvertible with others by further rotation. |
| Torsional strain | Weak repulsive interaction between electron clouds of bonds on adjacent carbons that hinders rotation. |
| Dihedral (torsional) angle | The angle of rotation about the C–C bond between corresponding bonds on the two carbons. |
| Skew conformation | Any intermediate conformation lying between the staggered and eclipsed extremes. |
Staggered and Eclipsed Forms
The ethane molecule, $\ce{C2H6}$, contains one carbon–carbon single bond, with each carbon attached to three hydrogen atoms. If one $\ce{-CH3}$ group is held stationary and the other is rotated about the C–C axis, the hydrogens on the front carbon sweep through an endless set of positions relative to those on the rear carbon. Two of these positions are the extreme cases that NEET asks about.
| Conformation | Arrangement of H atoms |
|---|---|
| Eclipsed | Hydrogens on the two carbons are as close together as possible; front and rear C–H bonds overlap. |
| Staggered | Hydrogens on the two carbons are as far apart as possible; each rear C–H bond bisects a pair of front C–H bonds. |
| Skew (gauche) | Any intermediate arrangement between the two extremes. |
A point that NCERT stresses, and one that NEET has tested directly, is that in all conformations of ethane the bond angles and the bond lengths remain unchanged. No bond is broken, stretched or bent during rotation; only the relative orientation of the hydrogens changes. The conformers are therefore not different molecules but different shapes of the same molecule.
Newman and Sawhorse Projections
Two diagrammatic conventions are used to depict these three-dimensional arrangements on paper: the sawhorse projection and the Newman projection.
Sawhorse projection
In the sawhorse projection the molecule is viewed from an oblique angle along the molecular axis. The central C–C bond is drawn as a somewhat longer straight line, with its upper end tilted slightly to the right or left. The front carbon sits at the lower end of the line and the rear carbon at the upper end; each carbon carries three lines for its three hydrogens, inclined at angles of $120^\circ$ to one another.
Newman projection
In the Newman projection the molecule is viewed head-on, straight down the C–C bond axis. The front carbon (nearer the eye) is represented by a point, with its three C–H bonds drawn as lines meeting at that point at $120^\circ$. The rear carbon (away from the eye) is represented by a circle, with its three C–H bonds drawn as shorter lines emerging from behind the circle. Because the relative positions of all atoms are easy to read off, the Newman projection is the preferred tool for studying conformations.
Conformations apply to the whole alkane family. Revise the structure and isomerism of the parent series in Alkanes.
The Dihedral (Torsional) Angle
The angle of rotation about the C–C bond is called the dihedral angle (also the torsional angle). In a Newman projection it is the angle between a chosen front C–H bond and the nearest rear C–H bond. This single quantity fully specifies which conformation the molecule is in.
| Dihedral angle | Conformation | Relative energy |
|---|---|---|
| $0^\circ$ | Eclipsed | Maximum (least stable) |
| $60^\circ$ | Staggered | Minimum (most stable) |
| $120^\circ$ | Eclipsed (next) | Maximum |
| $180^\circ$ | Staggered (next) | Minimum |
As the rear methyl group rotates through a full $360^\circ$, the molecule passes alternately through staggered minima (at $60^\circ, 180^\circ, 300^\circ$) and eclipsed maxima (at $0^\circ, 120^\circ, 240^\circ$). Because the three hydrogens on each carbon are identical, the pattern repeats every $120^\circ$.
Torsional Strain and Stability
The relative stability of the conformations is governed entirely by torsional strain. In the staggered form the electron clouds of the carbon–hydrogen bonds are held as far apart as possible. Repulsive forces are therefore minimum, the energy is minimum and the molecule is most stable.
When the staggered form rotates into the eclipsed form, the electron clouds of the C–H bonds are brought closest together. The electron-cloud repulsions increase; to absorb this increased repulsion the molecule must possess more energy, so it is less stable. The magnitude of this torsional strain depends on the dihedral angle — least in the staggered form, maximum in the eclipsed form.
"Eclipsed has no torsional strain so it is more stable" — false.
Examiners reverse the logic to trip you up. The staggered conformation is the more stable one precisely because it has the least torsional strain (lowest energy). The eclipsed conformation has the maximum torsional strain and is the least stable. Lower torsional strain ⇒ lower energy ⇒ greater stability.
Lock it in: Most stable = staggered = dihedral $60^\circ$ = least torsional strain = lowest energy. Least stable = eclipsed = dihedral $0^\circ$ = maximum torsional strain.
Because the staggered form is more stable, the molecule spends most of its time in it; the staggered conformation is therefore described as the preferred conformation. The existence of any energy barrier at all is what shows that rotation about the C–C bond in ethane is not completely free.
The Energy Profile of Rotation
The energy of ethane plotted against the dihedral angle traces a smooth wave with three identical minima (staggered) and three identical maxima (eclipsed). The vertical separation between a minimum and a maximum — the energy difference between the two extreme forms — is of the order of
$$\Delta E \approx 12.5\ \text{kJ mol}^{-1}$$
This barrier is very small. Even at ordinary temperatures the ethane molecule gains enough thermal (kinetic) energy through intermolecular collisions to overcome it. Rotation about the C–C single bond is therefore almost free for all practical purposes, and it has not been possible to separate and isolate the individual conformational isomers of ethane.
A student claims that because the eclipsed conformation is at an energy maximum, ethane can never adopt it. Evaluate the claim.
The claim is incorrect. The eclipsed form is a maximum on the energy curve, but the barrier separating it from the staggered minima is only ≈ 12.5 kJ mol⁻¹ — small enough that thermal collisions at ordinary temperature supply it readily. The molecule continuously passes through the eclipsed arrangement as it rotates; it simply spends less time there because that arrangement is higher in energy. This is exactly why the conformers interconvert too fast to be isolated.
Staggered vs Eclipsed: At a Glance
The table below consolidates every distinguishing feature that NEET has historically tested. Note especially the bottom two rows, which are the most common single-line questions.
| Feature | Staggered | Eclipsed |
|---|---|---|
| Arrangement of H atoms | As far apart as possible | As close together as possible |
| Dihedral angle | $60^\circ$ | $0^\circ$ |
| Torsional strain | Minimum (least) | Maximum |
| Electron-cloud repulsion | Minimum | Maximum |
| Potential energy | Minimum (lowest) | Maximum (highest) |
| Stability | Most stable / preferred | Least stable |
| Bond angles & bond lengths | Same in both — unchanged by rotation | |
Conformations of ethane in one screen
- Rotation about the C–C $\sigma$ bond produces an infinite number of conformations (conformers / rotamers); the two extremes are staggered and eclipsed.
- Newman projection: front carbon = point, rear carbon = circle. Sawhorse projection: oblique view of the C–C bond.
- Dihedral angle is $60^\circ$ for staggered, $0^\circ$ for eclipsed.
- Staggered = least torsional strain = lowest energy = most stable (preferred). Eclipsed = maximum torsional strain = least stable.
- Bond angles and bond lengths are identical in all conformers.
- Energy barrier $\approx 12.5\ \text{kJ mol}^{-1}$ — small enough that rotation is almost free and the conformers cannot be isolated.