1.2- Difluoroethane, gauche effect

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

Figure 3.63 Rotation-barrier profiles illustrating the gauche effect for (a) lone pairs in hydrazine and (b) polar bonds in 1,2-difluoroethane. (The angle 4> = 0°, as

Figure 2.1 Preferred conformations of butane and 1,2-difluoroethane, illustrating the gauche effect. In the gauche conformation each C-F a orbital can overlap with a C-H a orbital on the vicinal carbon, corresponding to no-bond resonance as shown. Such no-bond resonance would be disfavoured if the C-F bonds were trans, since it would remove electron density from an electronegative element. The effect is strong enough in this case to overcome the electrostatic repulsion between the two C-F dipoles, which favours the trans form.

Figure 6.23 (a) The gauche effect for 1,2-difluoroethane. (b) Anti and cis overlaps ofa and bonds, show... 

The diaxial preference is further strengthened in 1,4-difluorocyclohexane whereas the ee-isomer is more stable in the respective 1,3-dihalocyclohexanes. This difference suggests that the unusual conformational preference of 1,4-isomer is likely to be stereoelectronic (note the unfavorable double hyperconjugation pattern where two o CF acceptors compete for the same donor - more details in Chapter 9). The conformational preferences in 1,2-difluorocyclohexane can be understood as an extension of the conformational preferences of 1,2-difluoroethane (the gauche effect). 

Fig. 1 Gauche effect in 1,2-difluoroethane (left) and the anomeric effect in substituted tetrahydropyrans (right)...

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. 

A conformational effect related to the anomeric effect takes place in X-C-C-Y segments, where X and Y are electronegative groups. In particular, it has been found that molecules such as 1,2-difluoroethane, hydrogen peroxide, and others exhibit a preference for the gauche arrangement over the anti (Scheme 18). 

Effect of curved bond path on C-C bonding in anti (left) and gauche (right) conformers of 1,2-difluoroethane. (Reproduced from reference 138.)... 

The orbitals are bent in about the same direction with the gauche form, giving it the lower energy in this case. The effect of diffuse functions on the electron density distribution is in agreement with this proposal. It should be related to the electronegativity, and so it is not surprising that the gauche preference is smaller for 1,2-dichloroethane than for 1,2-difluoroethane. 

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