How to draw one
- Choose the bond to look along, for example C2–C3 of butane.
- Front carbon: a point, with its three bonds meeting at the centre, 120° apart.
- Back carbon: a circle, with its three bonds coming from the edge of the circle.
- Add the groups in the right positions, keeping the same 3D arrangement as your original drawing.
Conformations of butane
Rotating about the C2–C3 bond gives an infinite number of conformations. Four matter:
The energy diagram
Two effects raise the energy:
- Torsional strain: eclipsed bonds repel each other. This is the whole 12 kJ/mol barrier in ethane.
- Steric strain: big groups (CH₃) are pushed into each other in gauche and eclipsed forms.
At room temperature the molecule rotates millions of times a second, so the conformations can't be separated. They're not isomers. Most butane molecules are anti at any moment, with a smaller share gauche.
Why it matters
E2 eliminations need the H and the leaving group anti to each other (antiperiplanar), which you can check with a Newman projection. The same ideas explain cyclohexane: see chair conformations.
Quick answers
What is a Newman projection?
A Newman projection is a drawing that looks straight along one C–C bond. The front carbon is a point, the back carbon is a circle, and the groups on each carbon are shown 120° apart.
What is the difference between gauche and anti?
Both are staggered conformations. In anti, the two largest groups are 180° apart. In gauche, they are 60° apart, which is slightly higher in energy because of steric strain.
Which conformation of butane is most stable?
The anti conformation, with the two methyl groups 180° apart. The fully eclipsed conformation, with the methyls lined up, is the least stable.
Spin butane in Bondline's Newman explorerFree, works offline, no account needed.
Open in Bondline