Common ion rate depression

Added KI (.1 M) was observed to cause a 40% rate depression in the solvolysis of 2,2-diphenyl-1-anisyliodoethylene, 146, Y = OCH3, X = H. Such common-ion rate depressions are indicative of relatively stable and selective intermediate cations (133). 

Common-ion rate depressions and the involvement of vinyl cation ion pairs similar to those observed by Miller and Kaufman (131) were observed by Rappoport and Gal (136) in the solvolysis of 150 in acetic acid. Further interesting evidence for ion-pair involvement in the solvolytic generation of some vinyl cations comes from the investigation of cis- and trans-1,2-dianisyl-2-phenylvinyl halides, 151 (1937). In 80% ethanol, solvolysis... 

The effect of crown ethers on the alkylation of sodium diethyl n-butylmalonate by 1-bromobutane has been studied by Zaug et al. (1972). The absence of a common-ion rate depression in dimethylformamide (DMF) pointed to an ion pair being the kinetically active species. The addition of dicyclohexyl-18-crown-6 (a mixture of [20] and [21]) accelerates the alkylation in both benzene and tetrahydrofuran (THF) (Table 24). The rates reach a plateau, indicating that at a crown-ether concentration of 0.5 M the ion pair is fully converted to the crown ether-separated ion pair which is slightly less reactive than the uncomplexed ion pair in DMF. The rate constant in pure dimethoxyethane (DME) is equal to that observed in THF or benzene... 

Aqueous ethanolyses of adamantylideneadamantyl halides show Grunwald-Winstein sensitivity parameters (m) of 0.74 ( 0.06), 0.90 ( 0.01), and 0.88 ( 0.03) for the chloride, bromide, and iodide compounds, respectively. All reaction products are formed with retention of both the ring structure and the stereochemistry of the reaction centre. Observed common-ion rate depressions are consistent with a reaction pathway via a free solvated homoallylic carbenium ion. 

For several a-alkoxy carbenium ions rate constants for reaction with water were determined by Jencks and Amyes from the partial reversibility and associated common ion rate depression of hydrolysis of the corresponding azidoacetals, as is illustrated in Scheme 14.130... 

Values of for chloride ions have been determined by combining a rate constant for solvolysis ksoiv (for reactions for which the ionization step is ratedetermining) with a rate constant for the reverse reaction corresponding to recombination of cation and nucleophile. The latter constant may be found (a) by generating the cation by photolysis and measuring directly rate constants for reactions with nucleophiles or (b) from common ion rate depression of the solvolysis reaction coupled with diffusion-controlled trapping by a competing nucleophile used as a clock. 

Indeed, data on a system which is a good model for loss of chloride ion from [112] rather than [113] is available in the work of Johnson and Cornell (1980), who studied the relative rates of loss of chloride ion from [115] and [116]. The reactions can be followed readily only at fairly high temperatures (80-150°C), but have the classical characteristics of a unimolecular ionisation to give [117], including a common-ion rate depression for both... 

The phenomenon of common ion rate depression (Ingold, 1969) in the solvolysis of substrates yielding stable carbocations may also be utilized as a measure of selectivity. Thus for the simplified solvolysis scheme represented in eqn (35), the observed rate constant, 0bs, is given by eqn (36), where a = k-1/k2. Since a measures... 

Evidence supporting this scheme will be summarized briefly. The common ion rate depression (mass law effect) is diagnostic for the regeneration of neutral substrate, RX, from dissociated ions (53). If the recombination rate is increased by a salt, MX, it is apparent that the X— anions formed by ionization of RX and those corresponding to the added MX are chemically indistinguishable. Thus ionization of RX in such cases must afford dissociated ions with little or no interaction between R+ and X-. 

In addition to the kinetic effect, an exchange process could also be observed if the reaction medium contains isotopically labeled MX. Whenever a common ion rate depression has been observed, it has also been possible to demonstrate exchange. With some substrates, however, exchange has been found in the absence of a mass law effect. In such cases it is likely that dissociated ions are not involved. Solvolysis and exchange are thought to take place via attack on the solvent-separated ion pair (52). 

Four different probes gave short reactivity orders toward vinyl cations (1) common ion rate depression in solvolysis (2) competitive capture of solvolytically generated ions (3) direct reaction of a vinyl cation with nucleophiles and (4) competition between intra- and intermolecular nucleophilic capture. A short reactivity order is obtained in each case, but because of the different solvents and conditions the orders cannot be combined to a single series. However, a selectivity rule that governs the relative reactivities toward different vinyl cations in terms of a constant selectivity or a reactivity-selectivity relationship can be determined. 

According to the Winstein-Ingold solvolysis scheme (68), the observation of a rate decrease in the solvolysis of RX by either the formed or an added X- ( common ion rate depression", CIRD) serves as evidence for product formation from an intermediate free vinyl cation R+. The simplified solvolysis scheme, when ion pairs are neglected and both the solvent SOH and its conjugate base SO- may be present, is shown in Scheme II. 

Common ion rate depression for solvolysis of RX with the same R but with different leaving groups X was followed only for a single system. From the a values obtained in the solvolysis of ( )- and (Z)-l,2-dianisyl-2-phen-ylvinyl-X, 16, in AcOH-AcO- (equation 9), relative reactivities toward the derived ion 17 were measured (74) (nucleophile, relative reactivity) OMs-, 0.16 OAc, 1.0 Cl , 15.2 Br 45.5 and AcOH, 0.0024. From other data, I will probably be at the top of a similar order (75) and 2,4,6-trinitrobenzene-sulfonate at the bottom (76). 

Allred and Winstein found that acetolyses of (11) and (12) are accompanied by extensive isomerization, and after 70% reaction the mixture of p-bromobenzenesulfonates is composed of 69% of (11) and 31% of (12) regardless of the starting material. This is the result of return from free ions as shown by the observation of a small common-ion rate depression and of the conversion of the p-bromobenzenesulfonates into toluene-p-sulfonates in the presence of lithium toluene-p-sulfonate. The reaction also shows a special salt effect in the presence of lithium perchlorate, yet even at high salt concentrations some isomerization is still evident. Since return from solvent-separated ion pairs such as (23) is excluded in the presence of lithium perchlorate, and since the geometry of the initially formed ionic species does not allow direct rearrangement to occur, a common, symmetrical intimate ion pair, such as (21), is implicated in the isomerization. The solvolysis... 

Winstein, S., and Robinson, G. C. "Salt Effects and Ion Pairs in Solvolysis and Related Reactions. IX. The f/7rt o-3- j-anisyl-2-butyl system." /. Am Chcm. Soc., 80,169 (1958). Winstein, S., Klinedinst, P. R., Jr., and Robinson, G. C. "Salt Effects and Ion Pairs in Solvolysis and Related Reactions. XVII. Induced Common Ion Rate Depression and the Mechanism of the Special Salt Effect." /. Am. Chcm. Soc., 83,885 (1961). 

The acetolysis of chloromethylthiiran or 2-chloroethyl methyl sulphide does show a common-ion rate depression, and hence thiiranium ions were proposed as intermediates. Thiiranium ions may be intermediates in the annelation of the steroid precursor (18), and in the reaction of azetidinone (19) with iodine and... 

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