Anti-skew-conformation

In the reaction of menthyltrimethylammonium ion with strong bases up to 27% of menthene-3 is obtained (190). Baldwin et al. (1967) believe that an em/i-like elimination from a skew conformation or a syn elimination from a carbanion species can account for this result anti elimination from the other chain isomer, in which all substituents were axial, would be energetically less favorable overall. 

It has been pointed out that there are two distinct stereochemical orientations (referred to here as syn and anti) of the C-H bond undergoing a 1,3-hydrogen elimination from a carbonium ion (797). However, since 1,2-alkyl rearrangement is likely to be more rapid than 1,3-hydrogen elimination, distinguishing between these two modes of elimination becomes difficult. Nevertheless, there does appear to be a stereochemical requirement of co-planarity, i.e. 1,3-elimination is evidently unfavorable from a skew conformation. 

The gauche conformation of butane already shows conformational distortions in that the C—C—C—C dihedral angle is opened to about 70°, while the methyl group hydrogens are calculated to be skewed coherently by about 4.6°. The rotational barrier to anti-gauche interconversion derived spectroscopically is 3.63 kcal mol and as to the methyl eclipsing-methyl barrier, there has been considerable speculation described recently by Allinger and coworkers", who conclude it to be 4.89 kcal mol... 

There are no uniformly accepted names for the various conformers. Other names used in the literature include tnins-staggered trans, antiparallel, antiperiplanar (lUPAC), anti gflufhe-staggered gauche, skew-staggered, skew, synclinal flUPAC) cis-eclipsed di, ecliptic, synperiplanar (lUPAC) gflut-Ae-eclipsed anticlinal (lUPAC), The lUPAC terms, antiperiplanar (ap). synperiplanar (sp), anticlinal (ac), and synclinal (sc), have not become widely accepted in macromolecular chemistry. 

H2O2 adopts a nonplanar structure of C2 symmetry. Although chiral, the molecule undergoes rapid racemization. The flat shape of the anti conformer would minimize steric repulsions, the 90° torsion angle of the syn conformer would optimize mixing between the filled p-type orbital of the oxygen (one of the lone pairs) and the LUMO of the vicinal 0-H bond. The observed anticlinal "skewed" shape is a compromise between the two conformers. 

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