Electrophilic/nucleophilic reactants

What about the second reactant, HBr As a strong acid, HBr is a powerful proton (H+) donor and electrophile. Thus, the reaction between HBr and ethylene is a typical electrophile-nucleophile combination, characteristic of all polar reactions. 

It might seem that the Vs,min and Vs,max on a suitable molecular surface should indicate sites susceptible to electrophilic and nucleophilic reactants, respectively. Such reasoning has had some success in the past [3,14,16,17], but it is not reliable. For example, shown in Figure 17.2 is the electrostatic potential on a surface of anisole (methoxybenzene), 1, which is well known to undergo electrophilic attack at the ortho and para positions. 

Most C,H-acidic compounds can be condensed with aldehydes or ketones to yield alkenes. Some of these reactions have also been realized on insoluble supports, with either the C,H-acidic (nucleophilic) reactant or the electrophilic reactant linked to the support. Some illustrative examples are listed in Table 5.6. Polystyrene-bound malonic esters or amides, cyanoacetamides, nitroacetic ester [95], and 3-oxo esters undergo Knoevenagel condensation with aromatic or aliphatic aldehydes. Catalytic amounts of piperidine and heating are generally required, although reactive substrates can react at room temperature. 

Where the catalyst is less nucleophilic, e.g. potassium fluoride or solid caesium fluoride, only one fluoride ion is likely to coordinate at all firmly to the silane. The nucleophilic reactant will then also be able to coordinate to the electrophilic silicon atom, itself receiving further activation in the process, and reaction ensues by intramolecular transfer about the hexacoordinate silicon atom as demonstrated in the GTP process. Less nucleophilic substrates such as alkyl halides are unreactive in these circumstances. 

To predict which of the two alkyne carbons, C or C, HNC will preferentially attack, one now invokes the local HSAB principle [119], which says that interaction is favored between electrophile/nucleophile (or radical/radical) of most nearly equal softness. The HNC carbon softness of 1.215 is closer to the softness ofC (1.102) than that of (0.453) of the alkyne, so this method predicts that in the reaction scheme above the HNC attacks C in preference to C, i.e. that reaction should occur mainly by the zwitterion A. This prediction agreed with that from the more fundamental approach of calculating the activation energies as the difference of ttansition state and reactant energies. This kind of analysis worked for -CH3 and -NH2 substituents on the alkyne, but not for -F. 

Besides the typical (normal) PTC reactions involving nucleophilic reactant anions and cationic catalyst, it is reasonable to believe that the PTC technique can be applied to reactions involving electrophilic reactant cations such as aryldiazonium and carbonium cations and anionic catalysts. In such reversed phase transfer catalysis (RPTC), a cationic reactant in the aqueous phase is continuously transferred into the organic phase in the form of a lipophilic ion pair with a lipophilic, non-nucleophilic anionic catalyst, and reacts with the second reactant in the organic phase. 

Label electrophilic/nucleophilic and acidic/basic sites of all reactants, and number identical atoms in tbe starting material and product. 

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