Chemistry - STEREO ISOMERISM Question with Solution | TestHub
Contrary to ClCH₂-CH₂Cl, in FCH₂-CH₂F the gauche conformer is the more stable conformer. Explain this fact.
Options:
Answer:
Solution:
The situation in FCH₂CH₂F is quite different from either ClCH₂CH₂Cl or BrCH₂CH₂Br : the gauche conformer is found to be more stable than the anti conformer, even in the gaseous state.
gauche form (more stable) anti form (less stable)
Allinger explained the stability of the gauche form on the basis of hyperconjugative interaction, initiated by the high electronegativity of fluorine : the σ(C-H) bonds donate into the σ*(C-F) orbitals, and to involve both fluorine atoms simultaneously the two C-F bonds must be orthogonal to the best donor C-H bonds - exactly the gauche arrangement. The small size of fluorine also minimizes the van der Waals repulsive interaction in the gauche conformer. This preference is an example of the gauche effect.
