Final answer:
The term 'syn' refers to a type of staggered conformation, in contrast to the higher energy eclipsed conformation. In the staggered conformation of ethane, electron repulsion is minimized, making it more stable by 12.5 kJ/mol compared to the eclipsed form.
Step-by-step explanation:
The terms syn, anti, eclipsed, and staggered refer to the spatial arrangements of atoms in molecules, particularly those with single bonds that allow for rotation such as the C-C bond in ethane. The syn conformation is a type of staggered conformation where two specific substituents are closely aligned with each other. This contrasts with the anti conformation, also staggered, where substituents are opposite to each other.
The eclipsed conformation is a higher energy, unstable state where substituents on adjacent atoms are aligned with each other, resulting in increased electron repulsion. Figure 3.2.3 illustrates these different conformations in both sawhorse and Newman projections. The staggered conformation of ethane is more stable than the eclipsed by 12.5 kJ/mol because it minimizes this electron repulsion. Rotating the molecule around the C-C bond by 60 degrees can interconvert these conformations. In cyclic compounds like cyclobutane, the molecule adopts a puckered conformation to reduce the strain from eclipsing interactions, which significantly lowers the energy despite slightly increased angle strain.