Neutral nucleophiles, second-order rate

Second-order rate constants for the reactions of phenacyl bromide with a number of anionic or neutral nucleophiles in 3 2 (v/v) acetone-water have been measured at several temperatures.141 Correlation analysis with the Bronsted equation or Swain-Scott equation is not satisfactory. Better results were obtained with the two-parameter Edwards equation. 

The second-order rate constant for the reaction between methoxycarbonyl-acetylene and piperidine increases with increasing solvent polarity. This can be attributed to the increased solvation of the strongly dipolar activated complex, which is formed from neutral molecules [88], Analogous solvent effects have been observed for the nucleophilic addition of aziridine to 3-dimethylaminopropynal [89] and the addition of diethylamine to / -alkoxyvinyl methyl ketones [793],... 

Kevill and Lin s (27) earlier study of the ethanolysis of the triethyloxonium ion, at 0.0 °C, has been extended to a consideration of the competition between the solvent and added nucleophile, either anionic or neutral, for reaction with the substrate (equation 11). This study is related to earlier studies, at 25.0 °C, of competition between water and added nucleophile for reaction with methyl bromide (36), methyl iodide (37), or the cyclic pen-tamethyleneiodonium ion (38) and of competition between methanol and added nucleophile for reaction with methyl iodide (39) or trans-Pt(py)2Cl2 (39). The nucleophilicities are usually expressed, relative to the solvent, in terms of the Swain-Scott equation (36) (equation 12). In equation 12, k and k0 are second-order rate coefficients for reaction of a substrate with the added nucleophile and with the solvent, n is a measure of the nucleophilicity of the added nucleophile, and s is a measure of the sensitivity of the substrate toward changes in nucleophilicity. The value of s is taken as unity for the standard substrate. 

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... 

On the basis of these results the mechanism shown in Scheme 13 was proposed in which the ki step is rate limiting. In support of rate-limiting nucleophilic attack, Fischer and Dotz cite the fact that the second-order rate constants (Table 22) increase strongly as the Z substituent becomes more electron withdrawing (the reasons why electron withdrawing substituents enhance nucleophilic additions even with neutral nucleophiles have been discussed under Phosphine and phosphite nucleophiles ), and that they correlate well with the equilibrium constants of (n-... 

Nucleophilicities relative to a standard solvent can be quantified by the Swain-Scott equation (12)66, in which k and k0 are the second-order rate constants for reactions of the nucleophile and solvent respectively, and s is a measure of the sensitivity of the substrate to nucleophilicity n. By this definition, the nucleophilicity of the solvent is zero. For all reactions examined, there will be competition between attack by solvent (present in large excess) and reaction with added anionic nucleophiles. Hence, only n values well above zero can be obtained with satisfactory reliability. In the original work66, the solvent was water and all but one of the substrates were neutral s was defined as 1.0 for methyl bromide and was calculated to be 0.66 for ethyl tosylate the lowest reliable n value reported was 1.9 for picrate anion, but a value of < 1 for p-tosylate anion was reported66 in a footnote. 

The kinetics of the reactions of [PtCl3L] (L = dmso, Et2S, PMc3, PEt3, PPh3, or AsEt3) with many neutral or anionic nucleophiles, L, to produce trans-[PtCl2LL ] have been followed in 95% methanol and the second-order rate constants 2 (the reactions follow the usual two-term rate law) used to make comparisons... 

For acetaldehyde, the half-life of the exchange reaction is on the order of 1 min under neutral conditions, but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 sec The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. Hydration is catalyzed by both base and acid. Basic catalysts function by assisting deprotonation of water, giving the more nucleophilic hydroxide ion ... 

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 second-order rate constants for the reaction of the anions formed from imidazoles, pyrimidines, and purines with quinone methides and benzhydrylium ions in DMSO and in water have been measured and their N and % values in the log k = Sf N+E) equation determined. As expected, the nucleophiles are more reactive in DMSO than in water and the anions are 10" -10 times more reactive than their neutral counterparts. However, the solvent effects vary markedly with each nucleophile. The % values range from 0.50 to 0.77, whereas the N values for the anionic nucleophiles vary from approximately 15 to 21 in DMSO and from 10 to 12 in water. The Brpnsted basicity is correlated with the nucleophilicity in DMSO but not in water. 

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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