Q.Name two conformations in which ethane exists. Which of the two is more stable and why?

Conformations are spatial arrangements interconvertible by rotation about a C-C single bond; ethane's staggered conformation (hydrogens maximally apart) is more stable than its eclipsed conformation (hydrogens directly opposite).

Conformations (conformational isomers or conformers) are the different momentary arrangements of atoms in space that a molecule can adopt as a result of free rotation around one or more carbon-carbon single bonds. Unlike configurational isomers, conformers are not truly separable/isolable species -- they interconvert rapidly at room temperature without any bond breaking, simply by bond rotation, and represent different points along a continuous rotational energy profile.

Sawhorse projection of ethane (CH3-CH3): in a sawhorse projection, the C-C bond is drawn as a diagonal line going from the lower-left to the upper-right of the page. The front (lower-left) carbon has its three C-H bonds drawn splayed out to the left, up, and down from it; the back (upper-right) carbon similarly has its three C-H bonds splayed out to the right, up, and down from it.

• In the staggered conformation, the three hydrogens on the back carbon are drawn rotated 60 degrees relative to the front carbon's hydrogens, so each back C-H bond bisects the angle between two front C-H bonds when viewed end-on (i.e., all six hydrogens are maximally spread apart from each other). This is the most stable conformation of ethane because it minimizes repulsion (torsional strain) between the electron clouds of the front and back C-H bonds. …

Free rotation about the C-C single bond of ethane produces infinite conformations, the two extremes being the eclipsed (least stable) and staggered (most stable) forms.

Ethane, CH3-CH3, has a C-C single (sigma) bond about which the two -CH3 groups can rotate almost freely. Different spatial arrangements obtained by this rotation are called conformations (or conformers) — they are not different compounds, just different rotational orientations, and are represented using Sawhorse or Newman projections.

1. Staggered conformation: The hydrogen atoms on the front carbon are positioned exactly between (staggered relative to) the hydrogens on the back carbon, i.e. a dihedral (torsional) angle of 60°. This arrangement keeps the C-H bonds as far apart as possible, minimising electron-electron repulsion (torsional strain). It is the most stable conformation, of lowest potential energy.

2. Eclipsed conformation: The hydrogen atoms on the front carbon lie directly in front of (eclipse) the hydrogens on the back carbon, i.e. a dihedral angle of 0°. Torsional strain is maximum here, making it the least stable, highest-energy conformation.

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