Nucleophilic constant determination

Figure 2.5. Nucleophile selectivities determined from product analysis for the reactions of ring-suhstituted 1-phenylethyl derivatives (X-l-Y) with azide ion, acetate ion and methanol in 50 50 (v/v) water/trifluoroethanol. The selectivities are plotted against the appropriate Hammett substituent constant or a. Leaving group Y ( ) ring-suhstituted benzoates ( ) chloride (T) dimethyl sulfide (A) tosylate.

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

When the carbocations are generated by Laser flash photolysis, the ion pair collapse with the nucleophilic counterion Cl- is so fast [136] that the decay cannot be followed with the instrumentation used for these experiments, i.e., only those carbocations which manage to escape from the [Aryl2CH + Cl ] ion pair can be observed. Consequently, all rate constants determined for the Laser photolytically produced carbocations refer to the reactions of the nonpaired entities. 

A. Williams, Bonding in Phosphoryl (-PO3 ") and Sulphuryl (-SO3 ) Group Transfer between Nitrogen Nucleophiles as Determined from Rate Constants for Identity Reactions, J. Am. Chem. Soc., 1985, 107, 6335. 

In the case of nitrite nucleophile, however, the favourable situation was met that the role of this anion in aromatic nucleophilic substitutions had been thoroughly investigated and host-guest complexation happened under the experimental conditions of catalytic rate constant determination. 

The acyl-enzyme is an ester which was shown by both chemical and physical methods to result from the attachment of the acyl moiety of the substrate to Ser-195 of the enzyme. 2. A base of pX, = 7 is required for the reaction. 3. This base is the imidazole ring of His-57. 4. The deacylation is a nucleophilic reaction and a series of substituents in the acyl group yields a Hammett p-constant of +1.6 [18]. 5. The reaction is first order with respect to the nucleophile, as determined from the kinetics of the methanolysis reaction [19]. 6. The nucleophile reacts in the protonated form with the acyl-enzyme, as evidenced by the pH-dependence of the reaction of acetyl-, iso-... 

Theoretical studies of the gas-phase hydrolysis or methanolysis of methylsul-fonyl chloride indicated a concerted Sn2 process involving a four-membered cyclic transition state. The tertiary amine-catalysed hydrolysis of benzenesul-fonyl chloride was shown to be inhibited by chloride ion and a nucleophilic mechanism of catalysis was favoured. Kinetic studies" of the solvolysis of p-substituted benzenesulfonyl chlorides in aqueous binary mixtures with acetone, methanol, ethanol, acetonitrile and dioxime showed that the reactions were third order processes, with first order rate constants determined mainly by the molar concentrations of the protic solvent, so that the reaction rates appear to be dominated by solvent stoichiometry. The solvolyses in methanol and ethanol yield both an alcoholysis (ap) and a hydrolysis product (hp). Solvolyses of electron-rich arylsulfonyl chlorides, under neutral or acidic conditions, exhibited surprising maxima in solvent-dependent S values as defined by Equation 15. 

We can see that the nucleophilic constant can be determined from the electrode potentials and basicities - see Table 2.7.3) and also from the nucleophile polarizabilities and basicities (E, ). Both E values are in good agreement. 

Equilibrium constants for reactions of various nucleophiles with a,a,a-trifluo-roacetophenone to give tetrahedral adducts were determined [5J] (equation 39). In the equilibria, all nucleophiles were found to be less reactive with trifluoro-acetophenone than with aldehydes [55] (equation 39). 

The rate constant /ct, determined by means of Eq. (6-47) or (6-48), may describe either general base or nucleophilic catalysis. To distinguish between these possibilities requires additional information. For example, in Section 3.3, we described a kinetic model for the N-methylimidazole-catalyzed acetylation of alcohols and experimental designs for the measurement of catalytic rate constants. These are summarized in Scheme XVIIl of Section 3.3, which we present here in slightly different form. 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

Zahler and elaborated in a series of papers by Miller and co-workers and in Bunnett s publications, many of which are cited in Section I, D. It should be pointed out that the effects of substituents on nucleophilic substitutions show important differences from their effects on other reactions or on equilibria which involve competition for a lone-pair of electrons on another group or stabilization of negative charge on some atom of the reacting moiety. The cr-constants for nucleophilic substitutions differ from those determined in the latter work in that they show the response of the substituent to a strong demand for stabilization of negative charge on the substituent itself, especially by resonance. 

Fig. 19 Variation of the intermolecular stretching force constant ka with nucleophilicity Nb for several series of halogen-bonded complexes B- XY, where B is one of a series of Lewis bases and XY is Cl2, Br2, BrCl, C1F or IC1. N% were assigned by use of Eq. 4 with the choice of ICi = 10.0, hence the perfect straight line for the B- -IC1 series. The lines for the other series are those obtained by least-squares fits to the ka values using the Mb determined from the B- IC1 series. Points for E P- -IC1, E N- -C1F and EEN- BrCl are anomalous and were excluded from the fits (see text for discussion)... 

In order to determine the efficiency of the polymers as reagents in nucleophilic catalysis, it was decided to study the rate of quaternization with benzyl chloride. Table I shows the second-order-rate constants for the benzylation reaction in ethanol. Comparison with DMAP indicates that poly(butadiene-co-pyrrolidinopyridine) is the most reactive of all the polymers examined and is even more reactive than the monomeric model. This enhanced reactivity is probably due to the enhanced hydrophobicity of the polymer chain in the vicinity of the reactive sites. 

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