Neutral nucleophiles, second-order rate concentration

For neutral nucleophiles, we have utilized a series of ring-substituted N,N-dimethylanilines. The second-order rate coefficients should now be independent of nucleophile concentration, and this was confirmed by showing that log (k/k0 obtained from the product ratios, was independent of the amine concentration for 0.008 to 0.08 M N,N-dimethyl-p-toluidine. The log (k/k0) values could also be conveniently determined for m-CH3-, H-, p-Br-, and m-Cl-substituted derivatives (equation 13). For the m-N02 derivative, even at 0.32 M, the dominant reaction is solvolysis and only an approximate value for log (k/k0) could be obtained. A Hammett plot against the tabulated a values (43) (omitting the approximate m-N02 data) led to a linear plot and a slope (p value) of —2.77 0.15 (r = —0.996). This value is similar to values for reaction with other ethyl derivatives, derived from kinetically determined k values —3.60 for reaction with ethyl iodide in nitrobenzene at... 

One of the most comprehensive studies has been carried out by Bruice et al. [19] who studied the rate of solvolysis of neutral, positively and negatively charged esters when incorporated into non-functional and functional micelles of neutral, positive and negative charges. The second-order rate constants for alkaline hydrolysis, /cqh [0H ] were found to decrease with increasing concentration of surfactant for all cases studied. The association of the esters with non-nucleophilic micelles must either decrease the availability of the esters to OH attack or provide a less favourable medium for the hydrolysis reaction to occur. This is another circumvention of the simple electrostatic rules as the kinetic effect seems to have nothing to do with the concentration or restriction of access of the hydroxyl ions in the Stern layer of the micelles. Presumably the labile ester bond is not positioned near the surface of these micelles, but the molecules are oriented as shown in Fig. 11.2. 

The usual kinetic law for S/v Ar reactions is the second-order kinetic law, as required for a bimolecular process. This is generally the case where anionic or neutral nucleophiles react in usual polar solvents (methanol, DMSO, formamide and so on). When nucleophilic aromatic substitutions between nitrohalogenobenzenes (mainly 2,4-dinitrohalogenobenzenes) and neutral nucleophiles (amines) are carried out in poorly polar solvents (benzene, hexane, carbon tetrachloride etc.) anomalous kinetic behaviour may be observed263. Under pseudo-monomolecular experimental conditions (in the presence of large excess of nucleophile with respect to the substrate) each run follows a first-order kinetic law, but the rate constants (kQbs in s 1 ruol 1 dm3) were not independent of the initial concentration value of the used amine. In apolar solvents the most usual kinetic feature is the increase of the kabs value on increasing the [amine]o values [amine]o indicates the initial concentration value of the amine. 

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... 

The order of decreasing / —/ certainly follows the order expected, for water is a better nucleophile than formic acid, and better in neutral ethanol than in formic acid-dioxan, while iodide ion is better still. The interesting thing is that both the first two reactions are kinetically of type, the rates being unaltered by addition of sodium formate, while the third reaction is of intermediate type, the rate varying with addition of alkali but not being first order in alkali concentration. Yet if our analysis is correct, the second and third reactions must both involve strong nucleophilic participation. 

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