Nucleophilicity basicity compared

Nucleophilicity roughly parallels basicity when comparing nucleophiles that have the same reacting atom. For example, OH- is both more basic and more nucleophilic than acetate ion, CH3CO2-, which in turn is more basic and more nucleophilic than H20. Since "nucleophilicity" is usually taken as the affinity of a Lewis base for a carbon atom in the Sfj2 reaction and "basicity" is the affinity of a base for a proton, it s easy to see why there might be a correlation between the two kinds of behavior. 

O-as- S p i ro p h os p h or a n e s 69, which display anti-apicophilicty, exhibit enhanced reactivity toward nucleophiles compared to 0-/ra r-spirophosphoranes 70 for example 69 produces hexacoordinate 157 on treatment with MeLi at 0°C while 70 is unreactive under the same conditions (Scheme 16). Compound 69 (R = Bn) is more reactive toward bases than 70 (R = Bn) with KHMDS deprotonating 69 (R = Bn) in THF at 0 °C but not 70 (R= Bn). -BuLi was sufficiently basic to deprotonate both 69 and 70 <2002JA13154>. Further differences are observed in the stability and stereochemistry of the brominated products formed between the reaction of the anion with BrCF2CF2Br. The effect of the bromide substituent in the O-m-derivative is such that it accelerates pseudorotation to the O-trans-isomer (Scheme 17). 

In comparing nucleophiles that have the same attacking atom, nucleophilicity parallels basicity. In other words, a more basic nucleophile is a more effective nucleophile. 

Nucleophilicity roug hly paratleU basicity when comparing... 

This long discussion of nucleophilicity has brought together many new concepts, such as steric hindrance and solvent effects, both of which we will meet again in our study of organic chemistry. Keep in mind, however, the central relationship between nucleophilicity and basicity in comparing two nucleophiles. 

The closeness of fit may be gauged from the experimental and theoretical rate vs. concentration curves for hydrolysis of p-nitrophenyl carboxylates catalysed by quaternary ammonium surfactant micelles (Figure 3). The shape of the curve is satisfactorily explained for unimolecular, bimolecular, and termolecular reactions. An alternative speculative model is effectively superseded by this work. Romsted s approach has been extended in a set of model calculations relating to salt and buffer effects on ion-binding, acid-dissociation equilibria, reactions of weakly basic nucleophiles, first-order reactions of ionic substrates in micelles, and second-order reactions of ionic nucleophiles with neutral substrates. In like manner the reaction between hydroxide ion and p-nitrophenyl acetate has been quantitatively analysed for unbuffered cetyltrimethylammonium bromide solutions. This permits the derivation of a mieellar rate constant km = 6-5 m s compared to the bulk rate constant of kaq =10.9m s . The equilibrium constant for ion-exchange at the surface of the micelle Xm(Br was estimated as 40 10. The... 

Methyllithium is a strong basic and a powerful nucleophile. Compared to another synthetically important organolithium compound, -butyllithium, methyllithium reacts slowly with tetrahydrofuran at room temperature. An etheral solution of methyllithium is stable for a long time. Most of the reactions involving methyllithium are conducted at low temperatures. Methyllithium is used for deprotonations and as a source of methyl anion. Ketones can be converted to tertiary alcohols using methyllithium as... 

IV-methyl pyrolidinone is used in most cases. Figure 5.31 summarizes the main reaction which can take place during the process and the corresponding rate constant. The formation of diamide has also been evidenced.140 The reactivity is governed by the electron affinity of the anhydride and the ionization potential or basicity of the diamine (see Section 5.2.2.1). When a diacid with a low electron affinity reacts with a weak nucleophilic diamine, a low-molecular-weight is obtained, because the reverse reaction is not negligible compared with the forward reaction. 

In comparing nucleophiles whose attacking atom is in the same row of the periodic table, nucleophilicity is approximately in order of basicity, though basicity is thermodynamically controlled and nucleophilicity is kinetically controlled. So an approximate order of nucleophilicity is NH2 >RO > OH > R2NH > ArO > NH3 > pyridine > F > H2O > CIO4, and another is R3C > R2N > RO > F (see Table 8.1). This type of correlation works best when the structures of the nucleophiles being compared are similar, as with a set of substituted phenoxides. Within such a series, linear relationships can often be established between nucleophilic rates and pAT values. 

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