Ethane Gauche effect

Electrostatic interactions have been revealed as important in influencing the "gauche effect , whereby, in 1,2-disubstituted ethanes (4.9bA), the gauche conformer (4.9bB) is populated to a larger extent than the anti conformer (4.9bC) [31, 31a] (Figure 4.9b). 

Wiberg KB, Murcko MA, Laidig KE, MacDougall PJ (1990) Origin of the gauche effect in substituted ethanes and ethenes. J Phys Chem 94 6956-6959... 

The sigma nonbonded interaction between the two substituents fall into pattern d of Scheme 1. Here, unlike the case of 1,2-difluoroethane, we conclude that there will be a preference for the syn conformation due to the sigma nonbonded interaction of the pi systems of the substituents. This will be counteracted by the inherent preference of any ethane molecule for the staggered geometry and a compromise is expected to be reached in the gauche conformation, barring adverse steric effects. 

On the basis of the above discussion, we are led to the conclusion that sigma nonbonded attractive interaction in the form of a hydrogen bond will tend to favor a syn conformation opposing the inherent preference of ethane molecules for a staggered conformation. A compromise is expected to be reached in the gauche conformation. However, severe steric effects may force an anti conformational preference. 

In monosubstituted ethanes, p-substituents impose a larger effect on the antiperiplanar C-H bonds than on the gauche C-H bonds (Figure 12.14). The observed Facceptor abilities of the respective a C-X orbitals (F[Pg.330]

This implies perhaps that the degree of disorder in these polyamides is not very high, and that the MSA effect is likely to stabilize the tram conformer over the gauche in the ethane joint of DMED, but at present we have no proof on this specific arguments. 

It is of interest that A//° (gas phase) for dibromoethane is greater than that for butane (3.7-4.1 kJ mol ) even though bromine and methyl are usually considered to be of comparable size. This suggests that a factor other than steric repulsion affects the situation the obvious candidate is dipole-dipole repulsion in the gauche conformers of the dibromide. This contributor to the overall repulsion should be diminished in more polar solvents, both because of an increase in the effective dielectric constant and a concomitant decrease in coulom-bic repulsion, and because of more effective solvation of the higher-dipole confotmer in more polar solvents. Indeed, A// for 1,2-dibromoethane diminishes to 3.6 kJ mol in the pure liquid (dielectric constant e = 4.8) and to 2.8 kJ mol in acetonitrile (e = 37.5). (It must be noted, however, that because of differential volatility of the gauche and anti conformers of (nonpolar) butane, the conformational enthalpy difference A//° between these conformers also diminishes in the liquid phase, to 2.3-2.4 kJ mol .) We indicated earlier that ethane has a threefold potential (called V3). In the 1,2-dihaloethane, there is a superposed onefold potential (Vi) since the optimum (anti) orientation of the C-X dipoles is achieved only once in the course of a 360° rotation about the C-C bond. 

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