Ritchie electrophiles

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

The N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

Ritchie (1975) has found that the reactivity of nucleophiles toward a number of different types of electrophilic centers can be correlated remarkably well by a very simple equation (192), where kNu is the rate constant for reaction of a... 

The nucleophilicity of the amine is another factor affecting reactivity, and changes in it have been sometimes responsible for the observed scattering in the Brpnsted plots. The Ritchie equation80 (equation 11) has been applied to a variety of reactions in which nucleophilic addition to, or combination with, an electrophilic center is rate-limiting. 

RITCHIE EQUATION ELECTROPHILE NUGLEOPHILIC GATALYSIS NUCLEOPHILIC COMPETITION NUCLEOPHILICITY ALPHA EFFECT ELECTROPHILICITY... 

As pointed out by Mayr,28 Ritchie,15 and Hine33,34 KR also measures the relative affinities of R+ and H30+ for the hydroxide ion. It can be regarded as providing a general affinity scale applicable to electrophiles other than carbocations.33,35 It can also be factored into independent affinities of R+ and H30+ as shown in Equations (2) and (3). Such equilibrium constants have been denoted If by Hine.33 AR corresponds to the ratio of constants for reactions (2) and (3) and, in so far as Kc for H30+ is the inverse of Kw the autoprotolysis constant for water, KR = KCKW... 

Rappoport and TaShma s work removed a major difficulty for Ritchie s analysis and helped pave the way for Mayr to exploit fully the wide applicability and simplicity of Equation (29) for predicting rates of reactions of electrophiles with nucleophiles. Mayr pointed out that Equation (29) could be rewritten as Equation (30), in which log ka corresponds to the rate constant for reaction of the electrophile under study with a reference nucleophile266 (chosen as water by Ritchie) which, in so far as it is characteristic of the... 

Mayr initially defined a set of electrophilic parameters for the benzhydryl cations using a reference nucleophile, which was chosen as 2-methyl-1-pentene.268,269 Values of E were then defined as log k/k0, where k0 refers to a reference electrophile (E= 0), which was taken as the 4,4 -dimethoxybenzhydryl cation. Plots of log k against E for other alkenes are thus analogous to the plots of logk against p fR in Fig. 7 except that the correlation is referenced to kinetic rather than equilibrium measurements. However, they differ from plots based on the Swain-Scott or Ritchie relationships in which log k is normally plotted against a nucleophilic parameter, that is, n or N+, rather than E. 

In this connection, it is helpful to look first at the reactivity of the anions. There is no generally acceptable measure of nucleophilic reactivity since both the scale and order of relative reactivities depend on the electrophilic centre being attacked (Ritchie, 1972). However, in the present reaction, the similarity in the reactivity of the different anions is remarkable. Thus, the Swain and Scott n-values (cf. Hine, 1962) indicate that the iodide ion should be 100 times more reactive than the chloride ion in nucleophilic attack on methyl bromide in aqueous acetone. In the present reaction, the ratio of the rate coefficients for iodide ions and chloride ions is 1.4. This similarity led to the suggestion that these reactions are near the diffusion-controlled limit (Ridd, 1961). If, from the results in Table 5, we take this limit to correspond to a rate coefficient (eqn 19) of 2500 mol-2 s 1 dm6 then, from the value of ken for aqueous solutions at 0° (3.4 x 109 mol-1 s 1 dm3 Table 1), it follows that the equilibrium constant for the formation of the electrophile must be ca. 7.3 x 10 7 mol-1 dm3. This is very similar to the equilibrium constant reported for the formation of the nitrosonium ion (p. 19). The agreement is improved if allowance is made for the electrostatic enhancement of the diffusion-controlled reaction by a factor of ca. 3 (p. 8) the equilibrium constant for the electrophile then comes to be ca. 2.4 x 10-7. 

An analysis of these results in terms of solvent effects leads to the observation of similarities with Ritchie s work on the N+ relation. Thus the constant selectivities obtained in the solvolysis reactions of certain methyl derivatives (Table 9) may indicate the existence of a basic similarity between the rate-determining process in these reaction and in the electrophile-nucleophile combination reactions correlated by the IV+ relation. The failure of the methyl halides to conform to this pattern might suggest that their substitution reactions are fundamentally different, and that the free energy of activation is dependent on factors other than desolvation. 

Figure 10 shows that the same relative reactivities of terminal tt-systems, which have been determined with respect to AnPhCH+ as the reference electrophile, can also be observed with respect to AnaCH +, which is 3 orders of magnitude less electrophilic, or with Tol2CH +, which is 2 orders of magnitude more electrophilic than AnPhCH +, i.e., Ritchie s constant selectivity relationship [160] is also observed for this type of reactions. 

See also electrophilicity Ritchie equation SWAIN-SCOTT EQUATION. 

Equation (31) has an electrophilic ( ) and a nucleophilic (N component and the value, s, is a nucleophile specific parameter. This equation has a close family relationship with the Ritchie and Swain-Scott equations (Chapter 2) and the Edwards equation (29). The equation successfully correlates rate constants for a wide range of disparate structures and... 

One of the most intriguing reactions of nucleophiles is the simple (on paper) combination of cations and nucleophiles. In Chapter 11, Ritchie continues his study of rates and equilibria for these electrophile-nucleophile combination (ENC) reactions. Ritchie s determination of rates and equilibria for a wide range of nucleophilic reactions (particularly ENC reactions) has provided a fundamental set of data for evaluating key concepts of physical organic chemistry in general and of nucleophilicity in particular. The present work examines a correlation, observed previously (see also Chapters 3, 9, and 16), between one-electron oxidation potential and nucleophilicity. Here the... 

A major contribution of Ritchie s has been his observation that a large number of nucleophiles show a constant selectivity toward a variety of electrophiles. In LFER terms, the reactivity of the nucleophile can be given by a single parameter with no selectivity coefficient (such as Brpnsted (5 or Swain-Scott s) (26b) ... 

Equation 7 is also related to the Ritchie equation 9, applied to nucleophile-electrophile bond-forming reactions. The formal similarity and the apparently unusual constant selectivity common to both suggest the possibility of a closer relation. However, our Nx values are in principle related to identity rates, which as pointed out by Ritchie et al. (23) do not exist for these one-bond-forming reactions and cannot be a part of his N+ values. 

The rates of addition of nucleophiles to carbonyl groups and the rates of elimination from the tetrahedral intermediates constitute another class, probably similar to the activated aromatic nucleophilic substitution. The carbonyl group is an electrophile, and no obvious source of any barrier exists, outside of desolvation. Therefore, a resemblance to Ritchies systems is found. No obvious relation between our kinetic nucleophilic characters (Nx) and the additions occurs, but a possible parallel to the equilibrium methylating powers, KYX (in Tables I and II), of the conjugate methylating agent of the... 

In water, N3 is much less reactive in aromatic nucleophilic substitution than expected from its reactivity toward carbocations, that is, its N+value. Ritchie (43) initially developed his N+ scale from nucleophilicities toward preformed carbocations and the scale fits the data for nucleophilicities toward many electrophiles, regardless of their charge. However, in water, and similar hydroxy lie solvents, the nucleophilicity of azide ion, relative to that of other anions, seems to be related to the carbocation-like character of the electrophile. An acyl derivative with its sp2 carbonyl group is somewhat akin to a carbocation stabilized by an alkoxide group, >C=0 <-— >C+-0 , just as a triarylmethyl carbocation is stabilized by electron delocalization into the aryl groups and azide ion is a good nucleophile toward these electrophiles. As compared with anions such as OH- or CN , azide ion, in water, is very reactive toward carbocations and in deacylation but is relatively unreactive toward dinitrohaloarenes (44). 

The thermal bleaching reaction was recently shown, through linear free enthalpy relationships, to be useful in designing organic reactions of the electrophile-nucleophile type and predicting the rates of these reactions [94]. Study of the reactions of di- and triarylmethyl cations with various nucleophiles afforded a scale of nucleophilicity in weakly polar non-nucleophilic solvents [94] and provided support for a relationship with the Ritchie [82] N scale of nucleophilicity [87] in water. 

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