Influence of Steric Effects on Structures

With all the substituents as hydrogen, consider the staggered and eclipsed conformations of ethane 1 as shown below. The staggered conformation is more stable than the eclipsed conformer by 3.0 kcal mol-1. The electron pairs of the eclipsed bonds repel each other to raise the energy of the system by 1.0 kcal mol-1. Three such interactions make up to 3.0 kcal mol-1. 

Steric and Stereoelectronic Effects in Organic Chemistry, 

Consider the structure 4a for cyclohexane. The axial bonds on any two adjacent ring positions, such as Cl and C2, are parallel and also anti to each other. The three bonds involved in this relationship are a, b, and c and they could also be viewed to be in the same plane geometrically. The anti , the parallel , and the same plane are put together is termed antiperiplanar . So, the axial bonds on two adjacent cyclohexane ring positions are antiperiplanar. 

Consider tmns-1,2-di methylcyclohexane 6. In the conformer 6a, the two equatorial methyl groups are gauche to each other, which will raise the energy by 0.9 kcal mol-1. In the conformer 6b, the product of chair inversion of 6a, each axial methyl group is engaged in two butane gauche interactions. This will raise the energy by 2 x (2 x 0.9) = 3.6 kcal mol-1. The conformer 6a, therefore, is more stable than 6b by 3.6 - 0.9 = 2.7 kcal mol-1. Thus, trans- 1,2-disubstituted cyclohexane must prefer the conformer in which both the substituents occupy equatorial positions. 

Consider cis-, 2-dimethyl cyclohexane. In either of the two conformers 7a and 7b, one methyl is axial and the other equatorial. The two methyl groups are mutually gauche to each other and the ax-methyl is further gauche to two axial H atoms as shown. Both the conformers are one and the same. In the event that one substituent is different from the other, the molecule will largely adopt the conformer in which the larger substituent occupies the equatorial position. 

---