Q.Define isomerism and explain conformational isomerism in ethane.

Isomers share a molecular formula but differ in structure; ethane's conformers arise purely from rotation about its single C-C bond, ranging from the stable staggered form to the less stable eclipsed form.

Isomerism. Isomerism is the phenomenon in which two or more compounds have the same molecular formula but differ in their structural arrangement of atoms (structural isomerism) or in the spatial arrangement of atoms in space (stereoisomerism), and consequently often differ in physical/chemical properties. E.g., C4H10 exists as n-butane and isobutane (structural isomers).

Conformational isomerism in ethane (CH3-CH3).

The two carbon atoms of ethane are joined by a sigma (C-C) bond, about which free rotation is possible (sigma bonds are cylindrically symmetric, so rotation does not break orbital overlap). As one -CH3 group rotates relative to the other, an infinite number of momentary spatial arrangements are possible — these are called conformations (or conformers), and they are represented using Newman projections (viewing down the C-C bond).

• Staggered conformation: the hydrogens on the front carbon are positioned exactly between (60° dihedral angle from) the hydrogens on the back carbon. This minimises repulsion between the C-H bonding electron pairs (minimum torsional strain), making it the most stable conformation.
• Eclipsed conformation: the hydrogens on the front carbon are directly aligned (0° dihedral angle) with those on the back carbon. Electron-electron repulsion between the eclipsing C-H bonds is maximum here (maximum torsional strain), making it the least stable conformation.
• Between these two extremes lie an infinite number of skew conformations of intermediate energy. …