Ester bond, antibonding orbital

The additional stabilizing secondary electronic effect in the Z form might be larger than that (=1.4 kcal/mol) observed in acetals, because the carbonyl bond in esters is a more polarized bond than the C—OR bond in acetals and therefore, the antibonding orbital of the o C—0 bond should be of lower energy allowing increased overlap. It has been postulated on that basis (1-3) that the greater stability of the Z form (=3 kcal/mol) by comparison with the form in esters would be due mainly to this secondary electronic effect. 

Furthermore, the oxygen atom of the carbonyl group in the amide function has an electron pair oriented antiperiplanar to the polar C-N bond there is therefore an electronic delocalization caused by the overlap of that oxygen electron pair orbital with the antibonding orbital of the C-N sigma bond (o ) as shown in two dimensions by structure 5 and in three dimensions by structure . This additional n-o delocalization is referred to here as a secondary electronic delocalization. Thus, amides are similar to esters because they both have the primary electronic effect and one secondary electronic effect. This is in contrast with Z esters which have two secondary electronic effects besides the primary electronic effect. 

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