Polarized multiple bonds allylic sources

The polarized multiple bond makes adjacent C-H bonds acidic deprotonation generates the enolate, an allylic electron source. Often there is more than one possible regiochem or stereochem of the enolate, which can be selected by reaction conditions (see Section 9.3). 

Under equilibrium conditions (thermodynamic control), the allylic source adds to the polarized multiple bond (path AdN). However, the allylic source can also serve as a base and may deprotonate the sink, creating a mixture of sources and sinks and thus a messy statistical mixture of products. Clean products result if the source is just the deprotonated sink or if the sink has no acidic protons. With ketones, the equilibrium of the attack step favors the starting materials, and therefore the reaction goes to completion only if driven by a following elimination. In the next Adisj2 example, the source is the deprotonated sink. The product is an aldehyde-alcohol, or aldol, a name now used for the general process of an enol (acidic media) or enolate (basic) reacting with an aldehyde or ketone. 

The polarized multiple bond of the nitrile makes the adjacent CH bonds acidic (p/ a = 25) deprotonation by a strong base gives the nitrile enolate, an allylic eleetron source. 

There are other paths that fit the medium, sources and sinks protonation of an anion, p.t., and addition to a polarized multiple bond by the oxygen of the enolate by AdgS or AdN2. There are two sites that can serve as a base or as a nucleophile on this ambident allylic source. We have three choices to evaluate proton transfer to oxygen,... 

The ester enolate is an allylic source that can serve as a base or a nucleophile. The original ester has acidic hydrogens within range of the enolate. The ester carbonyl is an electrophilic sink (a polarized multiple bond with attached leaving group). 

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