Nucleophiles Electron-rich reactants that electrophiles

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the most common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to the electrophile. Radical reactions involve species that have an odd number of electrons. A bond is formed when each reactant donates one electron. 

Both electron richness and electron accessibility lead to the prediction that a carbon-carbon double bond should be nucleophilic. That is, the chemistry of alkenes should involve reactions of the electron-rich double bond with electron-poor reactants. This is exactly what we find The most important reaction of alkenes is their reaction with electrophiles. 

All compounds with a particular functional group react similarly. Due to the cloud of electrons above and below its TT bond, an alkene is an electron-rich molecule, or nucleophile. Nucleophiles are attracted to electron-deficient atoms or molecules, called electrophiles. Alkenes undergo electrophilic addition reactions. The description of the step-by-step process by which reactants are changed into products is called the mechanism of the reaction. Curved arrows show which bonds are formed and which are broken and the direction of the electron flow that accompany these changes. 

While die above reactions represent only a small fraction of die reactions known for palladium, they form the basis of a powerful methodology for building carbon structures. Several variations have been developed which utilize certain types of reactants and give particular types of products. All diese variations, however, contain a common theme. In each case an electron-deficient reagent (e.g., a vinyl halide or aromatic triflate) reacts with an election-rich reagent (e.g., an alkene, an organoborane, or an organotin) witii the formation of a new carbon-carbon bond. In that sense diese reactions are related to die reactions between carbon nucleophiles and carbon electrophiles discussed previously in diis chapter. They are quite different, however, because diey proceed only in the presence of Pd(0). In fact they proceed only in die coordination sphere of Pd(0). The ability of Pd(0) to catalyze diese reactions is nearly unique We will now examine some of die more common processes. 

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