Q.The order of stabilities of various conformations of Ethane are :

Physical Properties and Conformations of Alkanes

Physical properties track chain length and shape. State at room temperature: C1-C4 gases, C5-C17 liquids, C18+ solids. Alkanes are non-polar, so they are insoluble in water but soluble in non-polar organic solvents, and, being less dense than water, float on it. Boiling point rises roughly 30 degC per added carbon along a straight chain (larger surface area strengthens the weak Van der Waals forces holding molecules together), but FALLS with increased branching at a fixed carbon count, since a more compact, branched shape reduces the area of intermolecular contact.

Conformations describe the different three-dimensional shapes a molecule can adopt purely by rotating around its C-C single bonds (sp3 carbons, tetrahedral geometry), without breaking any bond. For ethane, rotating one methyl group relative to the other sweeps through an infinite series of conformers between two named extremes: the ECLIPSED conformation (H's on the front carbon sit directly in front of H's on the back carbon; maximum torsional strain/repulsion; LEAST stable) and the STAGGERED conformation (H's on the two carbons are as far apart as possible; no torsional strain; MOST stable), with intermediate SKEW conformations in between (stability order staggered > skew > eclipsed; energy gap approx. 12.5 kJ/mol). These are drawn as Newman projections (front atom = a dot, back atom = a cir …