Lone pairs nucleophilicity trend

The enthalpies of reaction for nucleophilic carbencs depend on the stereoelec-tronic properties of the ligands affecting the availability of the carbene lone pair. An example of electronic influence is the 3.5 kcal/mol enthalpy difference between the isosteric pair IMes and IMesCI that shows the electron-withdrawing nature of Cl compared to H. This trend again is in line with electron donor/withdrawing ability of arene substituents. The effect in this la.st case is a long range electronic... 

The reactivity sequence furan > selenophene > thiophene > benzene has also been observed in the nucleophilic substitutions of the halogenonitro derivatives of these rings.21,22 This shows that the observed trend does not depend on the effectiveness of lone-pair conjugation of the heteroatoms NH, O, Se, and S and the 77-electron density at the carbon atoms. It is interesting to note that a good correlation is observed between molecular ionization potentials (determined from electron impact measurements) and reactivity data in electrophilic substitution, in that higher reactivities correspond to lower ionization potentials182 pyrrole furan < selenophene < thiophene benzene (see Table VII). This is expected in view of a... 

Electron availability decreases as the orbital the electrons occupy is made more stable. As the hybridization of the nitrogen lone pair goes from sp to sp to sp, the lone pair gets less basic and less nucleophilic as indicated by the following trend. 

An exception to this hybridization trend is that an ester s carbonyl oxygen sp lone pairs are much more basic and more nucleophilic than its ether oxygen sp lone pairs. When an ester s carbonyl lone pair is protonated, a delocalized cation is formed. When an ester s ether oxygen lone pair is protonated, the cation is less stable because it is not delocalized and the electron withdrawal by the adjacent carbonyl destabilizes it. 

Let us consider a few principles that we can use for understanding trends in nucleophilicity. When applying these principles, our point of reference is the nucleophilic atom, which possesses the lone pair that is used to form a bond with the electrophile. As we work our way through these principles, we will see similarities to explanations given earlier (Section 27A on www.masteringchemistry. com), but it is important that we take careful note of the differences. 

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