Electron sources ranking

In this chapter we shall explore the classification and ranking of electron sources into the 12 common generic classes of electron sources. These generic classifications are based on chemical reactivity, and just like you do not expect a pig to fly, these... 

The rankings of allylic electron sources reflect the electron-releasing group trends... 

The pi bond is the electron source. For allylic sources, the electron pair donor stabilizes the resultant cation. The simple alkene reactivity trend reflects the stabilization of the resultant carbocation by alkyl substitution and delocalization with other double bonds. The ranking of simple double bonds as electron sources basically is the more substituted, the more reactive. 

The products of an addition-elimination path very often undergo a second attack by the nucleophile (Ad] j + Ep then a second Adj ) because the product of the first addition-elimination is also a reactive electron sink. If the addition-elimination product is more reactive than the original starting material, a second addition almost certainly will occur. A ranking of the reactivity of electron sinks and sources is very important for path selection. For each reaction, the useful limit must be found for each reactivity spectrum. How far down the reactivity trend for the electron source can we go and still have the reaction with a particular electron sink occur at a useful rate There are four ways that the reaction may stop after the initial AdN -t- Ep ... 

Organometallics follow the addition-elimination pathway (AdN + Ep) characteristic of this sink. However, the products of the addition-elimination path often undergo a second attack by the organometallic because the AdN + Ep product is often more reactive than the original substrate (see Section 9.2). The reactivity ranking of the electron sinks and sources is important in this decision. 

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