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Alkanes consist entirely of C–C and C–H single bonds with tetrahedral (sp3) geometry at each carbon. Rotation around C–C bonds is nearly free, giving rise to an infinite set of conformations — spatial arrangements that interconvert without breaking bonds. Newman projections visualize conformations along a C–C bond axis; staggered arrangements (anti and gauche) are more stable than eclipsed due to torsional and steric strain. Butane's conformational energy diagram introduces the concept of preferred molecular geometry arising from non-bonded interactions.
Build Newman projections of ethane and butane by hand, rotating the front carbon in 60° increments. Sketch the rotational potential energy diagram for butane, labeling anti, gauche, and eclipsed conformations at each energy minimum and maximum.
Alkanes are the simplest organic molecules — chains and branches of carbon atoms connected exclusively by single bonds, with hydrogen filling every remaining bonding position. From your work on IUPAC nomenclature, you already know how to name and draw these structures. Now the question shifts from "what is this molecule?" to "what shape does it actually take in three dimensions?" The answer is more interesting than it might seem, because single bonds allow free rotation, and this rotation creates a continuous range of three-dimensional arrangements called conformations.
Imagine looking down the axis of a C–C bond. The front carbon and its three attached groups are fixed in your view; the back carbon and its groups can rotate freely. A Newman projection captures this view — the front carbon is a dot, the back carbon is a circle, and the bonds radiate outward from each. As you rotate the back carbon, the groups attached to it sweep through different positions relative to the front carbon's groups. When the front and back groups are aligned directly behind each other, you have an eclipsed conformation. When they are perfectly staggered between each other (offset by 60°), you have a staggered conformation.
These conformations are not equal in energy. In the eclipsed arrangement, electron clouds in adjacent bonds are forced into close proximity, creating torsional strain — a repulsive interaction that raises the energy. In the staggered arrangement, bonds are as far apart as possible, minimizing this repulsion. For ethane, the energy difference between eclipsed and staggered is about 12 kJ/mol — small enough that rotation is essentially free at room temperature, but large enough that the molecule spends most of its time near staggered conformations.
Butane reveals a further subtlety. With a four-carbon chain, there are two distinct types of staggered conformations when viewed along the central C2–C3 bond. In the anti conformation, the two methyl groups are 180° apart — maximally separated and at the lowest energy. In the gauche conformation, the methyls are 60° apart, close enough to experience steric strain (van der Waals repulsion between their electron clouds). The gauche conformation is about 3.8 kJ/mol higher than anti. Plotting energy against the dihedral angle produces the characteristic conformational energy diagram: a repeating pattern of minima (staggered) and maxima (eclipsed), with the anti minimum being the global energy floor. This concept — that molecular shape is governed by minimizing non-bonded interactions — becomes foundational for understanding ring conformations, protein folding, and reactivity throughout organic chemistry.