The Fluorine gauche Effect and a Comparison with Other Halogens in 2‐Halofluoroethanes and 2‐Haloethanols

While the gauche effect in 1,2‐difluoroethane is widely known as being due to hyperconjugative interactions between σCH electron‐donating orbitals and σ*CF electron‐accepting orbitals, the corresponding 1,2‐dichloro, 1,2‐dibromo, and 1,2‐diiodo derivatives are preferentially in the anti conformation. 2‐Halofluoroethanes (F‐CH2‐CH2‐X) combine a small halogen (fluorine) and a vicinal low‐lying energy antibonding orbital (σ*CX) that activates a stabilizing antiperiplanar σCH → σ*CX electron delocalization, which can induce the gauche effect. On the other hand, σCX orbitals are good electron donors to σ*CF, that would favor an “anti effect”, in addition to traditional interpretations based on steric and electrostatic repulsion. Therefore, a balance of steric, dipolar and hyperconjugative effects drive the conformational equilibrium of these compounds – hyperconjugation was found to explain the gauche effect in some cases, whilst Lewis‐type interactions determine the anti preference in others. The gauche effect takes place in a polar solvent, but not for 1‐fluoro‐2‐iodoethane. According to NMR three‐bond spin‐spin coupling constants, the gauche population increases when fluorine is replaced by a hydroxyl group (except for 2‐fluoroethanol relative to 1,2‐difluoroethane), but this is not primarily due to intramolecular hydrogen bond. A balance of steric, dipolar and hyperconjugative effects drives the conformational equilibrium of 2‐halofluoroethanes and 2‐haloethanols – hyperconjugation was found to explain the gauche effect in some cases, whilst Lewis‐type interactions determine the anti preference in others.