Isomerism of CH3COX Molecules

As a final example, we consider torsional isomerism in l,2 diamino l,2-dicy-anoethylene (DADCE) which is known to be a precursor in the evolutionary synthesis of nucleic acids Some torsional isomers of DADCE are shown below  

Arguing as before, we expect pi nonbonded attraction to favor the cis geometry for a flxed conformation. Furthermore, sigma nonbonded attraction in the form of a hydrogen bond is expected to inake Cpv the most stable conformation in the cU series. A complete study of all possible torsional isomers of DADCE will be reported in the future. 

Experimentally, the cis isomer of DADCE seems to be more stable than the trams isomer Furthermore, in the crystalline state, the cis isomer is known to exist in the hydrogen bonded conformation 

We shall now examine molecules of the type CH3COX, where X can be a halide, a hydroxyl group, an alkoxy group, or an amido group. We shall first consider the conformational preference, i. e. staggered vs. eclipsed, imposed on such molecules by the presence of lone pairs or pi systems on the group X, c. g. X=F. 

Our analyas is illustrated by reference to the model system CH3COF and the dissection employed is shown below  

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