Stereoelectronic effects and the ester function

This Chapter deals with the stereoelectronic effects which control the cleavage of tetrahedral intermediates during the formation or the hydrolysis of an ester. Since these effects are also operative in the ester function itself, a discussion of the functional group will first be presented. 

In the ester function, it was proposed that two types of electronic effects referred to as the primary and the secondary electronic effects (1-3), are present. The role and the relative importance of these two types of electronic effects can be understood by analyzing them in relation to the geometry and the relative stability of the two forms, Z and E, of the ester function. 

It is well known that esters are planar and that the Z form is much more stable (=3 kcal/mol) than the form (4). The E form is of course observed in small ring lactones but as soon as the size of the lactone ring allows the Z form to exist, the form is no longer observed (5). Even in the case of tert-butyl formate, where there must be a strong steric repulsion 

The primary electronic effect is due to the delocalization of electron pairs between the ether oxygen and the carbonyl group of the ester function as expressed by the resonance structure 2, and 3. Resonance structures 

2 and 2 show the delocalization of an electron pair between the carbon and oxygen of the carbonyl group (1 2 and resonance structures and 

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