Effect of bond orbital interactions on the conformational stabilities of 1,2-difluoroethane and 2,3-difluorobutanes

The Emergence and Properties of Selectively Fluorinated 'Janus' Cyclohexanes

This account describes the evolution of a research programme that started by linking fluoromethylene (-CHF-) groups along aliphatic chains and then progressing to alicyclic rings with contiguous fluorine atoms. Different stereoisomers of aliphatic chains tend to adopt low polarity conformations. In order to force polar conformations, the programme began to address ring systems and in particular cyclohexanes, to restrain conformational freedom and co-aligned C-F bonds. The flagship molecule, all-cis-1,2,3,4,5,6-hexafluorocyclohexane 7, emerged to be the most polar aliphatic compound recorded. The polarity arises because there are three co-aligned triaxial C-F bonds and the six fluorines occupy one face of the ring. Conversely the electropositive hydrogens occupy the other face. These have been termed Janus face cyclohexanes after the Roman god with two faces. The review outlines progress by our group and others in preparing derivatives of the parent cyclohexane 7, in order to explore properties and potential applications of these Janus cyclohexanes.

Can 2-X-Ethanols Form Intramolecular Hydrogen Bonds?

For 2-X-ethanols, where X = F, OH, or NH₂, the gauche conformer is favored over the trans conformer by at least 2 kcal/mol. Initially, this preference, ΔE, was attributed to an intramolecular hydrogen bond, IMHB, between the OH and X groups. Over the years, this conclusion has been challenged by two major arguments. One claim is that the entirety of ΔE can be accounted for by the gauche effect. Against this, calculations using five different methods show that the maximum contribution of the gauche effect to ΔE is less than 1 kcal/mol. A second argument employs the quantum theory of atoms in molecules to contend that the absence of a bond critical point (BCP) between the OH and X groups in 2-X-ethanols denotes the lack of an IMHB. By looking at the 2-X-ethanols at fixed XCCO torsional angles ranging from 0° to 60°, it is shown that the BCP criterion is inconsistent with other properties such as energy, bond lengths, and stretching frequencies. These inconsistencies are removed when the theory of noncovalent interactions is used. The IMHBs in 2-X-ethanols are found to be similar in form but smaller in magnitude than their intermolecular counterparts. This work concludes that 2-X-ethanols form IMHBs.