Conformational Isomers

Stand at one end of a narrow hallway and look straight down toward the other end. The person standing right in front of you (the front carbon) blocks most of your view, but you can see the arms and legs of someone standing behind them (the rear carbon) poking out from the sides. That is exactly what a Newman projection shows: you are looking straight down the carbon-carbon bond, and the two carbons are stacked on top of each other from your point of view.

Conformational isomers (conformers) are different spatial arrangements of the same molecule created by rotation around a single bond. Unlike constitutional isomers or stereoisomers, conformers do NOT require bond breaking to interconvert. They are the same molecule in different poses, like a person standing with their arms at their sides versus their arms outstretched.

Drawing Newman Projections

A Newman projection uses a specific visual convention:

• Front carbon: Represented by the center point of a circle. Its three substituents radiate outward from the center like spokes.
• Rear carbon: Represented by the circle itself. Its three substituents radiate outward from the edge of the circle.
• Dihedral angle: The angle between substituents on the front and rear carbons, measured as you look down the bond.

The Conformations of Ethane

Ethane (CH3-CH3) is the simplest molecule for understanding conformational analysis. As you rotate around the C-C bond, the molecule passes through an infinite number of conformations, but two extremes matter:

Staggered conformation (dihedral angle = 60 degrees): Each hydrogen on the front carbon is positioned between two hydrogens on the rear carbon. Maximum separation. Lowest energy. Most stable.

Eclipsed conformation (dihedral angle = 0 degrees): Each hydrogen on the front carbon lines up directly with a hydrogen on the rear carbon. Minimum separation. Highest energy. Least stable.

The energy difference between staggered and eclipsed ethane is about 12 kJ/mol. This energy penalty comes from torsional strain - the resistance to having bonds on adjacent carbons line up directly with each other. Torsional strain arises from electron-electron repulsion between the bonding electrons of eclipsed substituents.

The Conformations of Butane

Butane (CH3CH2CH2CH3) is where conformational analysis gets interesting, because the methyl groups on C2 and C3 create steric interactions. Looking down the C2-C3 bond, there are four named conformations:

Anti (dihedral angle = 180 degrees):
The two methyl groups are on opposite sides, as far apart as possible. This is the global energy minimum - the most stable conformation. No torsional strain, no steric strain.

Gauche (dihedral angle = 60 degrees):
The two methyl groups are 60 degrees apart. This is a local energy minimum - more stable than eclipsed, but less stable than anti. There is minor steric strain because the methyl groups are close enough to bump into each other (gauche interaction, about 3.8 kJ/mol above anti).

Eclipsed (dihedral angle = 120 degrees):
A methyl group on the front carbon lines up with a hydrogen on the rear carbon. This is a local energy maximum. Both torsional and steric strain are present.

Totally eclipsed (syn-periplanar, dihedral angle = 0 degrees):
The two methyl groups are directly behind each other. This is the global energy maximum - the least stable conformation. Maximum torsional strain and maximum steric strain. About 19 kJ/mol above anti.

The Energy Diagram

As you rotate 360 degrees around the C2-C3 bond of butane, the energy traces a wave pattern:

The diagram repeats every 360 degrees. There are two gauche minima (at 60 degrees and 300 degrees), two eclipsed maxima (at 120 degrees and 240 degrees), and one totally eclipsed maximum (at $\frac{0}{360}$ degrees).

Types of Strain

Three types of strain destabilize certain conformations:

Torsional strain: Resistance to having bonds on adjacent atoms aligned in the same plane (eclipsed). Arises from electron-electron repulsion in eclipsed bonds. Present in ALL eclipsed conformations, even in ethane where there is no steric issue.

Steric strain: Repulsion between electron clouds of bulky groups that are too close together. This is why gauche butane is higher energy than anti butane - the methyl groups are close enough to push against each other. Steric strain increases with the size of the groups.

Angle strain: Deviation from ideal bond angles. Not a factor in acyclic conformational analysis, but critically important in ring systems (Section 2.11).

Conformational Analysis Strategy for the MCAT

When given a Newman projection question:

1. Identify the two key substituents (usually the largest groups on the front and rear carbons).
2. Measure their dihedral angle (the angle between them as you look down the bond).
3. Classify the conformation: 0 degrees = totally eclipsed, 60 degrees = gauche, 120 degrees = eclipsed, 180 degrees = anti.
4. Rank the energy: Anti < gauche < eclipsed < totally eclipsed.
5. Identify strain types: Eclipsed = torsional strain. Large groups close together = steric strain.

Free Rotation at Room Temperature

An important concept: at room temperature, rotation around single bonds is essentially free. The energy barriers between conformations (12-19 kJ/mol for butane) are small enough that molecules constantly rotate through all conformations. A sample of butane is not “stuck” in the anti conformation; it is rapidly interconverting among all conformations but spending the most time in the lowest-energy ones (anti and gauche).

The Boltzmann distribution determines the population of each conformation. At room temperature, butane spends roughly 70-80% of its time in the anti conformation and the remainder in gauche conformations. It passes through eclipsed conformations but does not linger there.