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The Fluorine gauche Effect and a Comparison with Other Halogens in 2‐Halofluoroethanes and 2‐Haloethanols
Author(s) -
Martins Francisco A.,
Freitas Matheus P.
Publication year - 2019
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201901234
Subject(s) - hyperconjugation , chemistry , antibonding molecular orbital , anomeric effect , steric effects , intramolecular force , fluorine , crystallography , computational chemistry , vicinal , halogen , molecular orbital , atomic orbital , stereochemistry , electron , organic chemistry , molecule , physics , alkyl , quantum mechanics , anomer
While the gauche effect in 1,2‐difluoroethane is widely known as being due to hyperconjugative interactions between σ CH 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‐CH 2 ‐CH 2 ‐X) combine a small halogen (fluorine) and a vicinal low‐lying energy antibonding orbital (σ* CX ) that activates a stabilizing antiperiplanar σ CH → σ* CX electron delocalization, which can induce the gauche effect. On the other hand, σ CX 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.