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Conformations (Rotamers)

Atoms joined by a single bond rotate relative to each other at a very low energy cost.

Video: Conformations: single bonds rotate▶ Watch the video on YouTube

In ETHANE the C-C bond is the consequence of the head-on overlap of two sp3 hybrid orbitals. Let us take as an axis the imaginary line through the two bonded atoms. It is quite intuitive that if one hybrid orbital is rotated relative to the other about this axis, keeping the carbons at the same distance, their overlap does not change and, in principle, the bond is not weakened at all.

The C–C bond of ethane: head-on overlap of two sp3 orbitals that can rotate
Space-filling model of ethane rotating about its C–C bond

The number of possible conformations (rotamers) around a single bond is infinite. However, we can distinguish two significant ones:

Staggered and eclipsed conformations of ethane in side and front view

The most practical and visual way of representing conformations is the so-called NEWMAN PROJECTION

Melvin S. Newman (1908-1993): Professor at The Ohio State University (USA). His research dealt with steric effects in congested molecules. He made important contributions to reaction mechanisms and to the chiral properties of numerous molecules. His teaching abilities were outstanding and, among many other things, he proposed the popular projection, named after him, used to represent the conformations of organic compounds.

Melvin S. Newman
Staggered and eclipsed conformations of ethane as lines and wedges, sawhorse and Newman projections
Animation of the staggered conformation in Newman projection
STAGGERED
Animation of the eclipsed conformation in Newman projection
ECLIPSED

Even though the overlapping of orbitals is not affected by their twisting, the rotation of a single bond does have a small energy cost that depends on the size of the groups attached to the rotating atoms.

Animation of the rotation about the C–C bond of ethane and its energy cost

Energy difference between the staggered and eclipsed conformations of ethane
(3 kcal/mol)

Energy of ethane versus dihedral angle: eclipsed maxima at 0° and 120° and staggered minima at 60° and 180°
Steric interactions in ethane, propane and butane (distances in Å)

What would the energy profile for rotation about a C-C bond of PROPANE look like?

And for the central C-C bond in BUTANE?

One of the factors creating the barrier to rotation of a single bond is the STERIC INTERACTION of the flanking hydrogens in the eclipsed conformation. A methyl group is larger than a hydrogen and its steric interaction is logically larger, making the energy barrier higher.

Propane in Newman projection
PROPANE
Energy profile of propane versus dihedral angle: 3.4 kcal/mol barrier
Animated space-filling model of propane
Butane in Newman projection along the C2–C3 bond
BUTANE
Energy profile of butane versus dihedral angle: 3.6 and 3.8 kcal/mol barriers and 0.9 kcal/mol between gauche and anti
Animated space-filling model of butane
Animation of the rotation about the central bond of butane and its energy cost

In BUTANE, the energy difference between the lowest energy minimum (rotamer with the methyl groups antiperiplanar, at 180º) and the other local minima (rotamers with the methyl groups gauche, at 60º and 300º) is 0.9 kcal/mol. This is the energy (0.9 kcal/mol) that measures the destabilizing interaction between two methyl groups in gauche arrangement.

Conformations of butane every 60°: synperiplanar, synclinal (gauche), anticlinal and antiperiplanar