Mass-law effect

The notion of concurrent SnI and Sn2 reactions has been invoked to account for kinetic observations in the presence of an added nucleophile and for heat capacities of activation,but the hypothesis is not strongly supported. Interpretations of borderline reactions in terms of one mechanism rather than two have been more widely accepted. Winstein et al. have proposed a classification of mechanisms according to the covalent participation by the solvent in the transition state of the rate-determining step. If such covalent interaction occurs, the reaction is assigned to the nucleophilic (N) class if covalent interaction is absent, the reaction is in the limiting (Lim) class. At their extremes these categories become equivalent to Sn and Sn , respectively, but the dividing line between Sn and Sn does not coincide with that between N and Lim. For example, a mass-law effect, which is evidence of an intermediate and therefore of the SnI mechanism, can be observed for some isopropyl compounds, but these appear to be in the N class in aqueous media. 

If the X formed during the reaction can decrease the rate, at least in some cases, it should be possible to add X from the outside and further decrease the rate in that way. This retardation of rate by addition of X is called common-ion effect or the mass law effect. Once again, addition of halide ions decreases the rate for diphenylmethyl but not for tert-butyl halides. 

This would have both a solvent effect and a mass law effect on the rate of ester formation. The error is systematic, since it is most serious for the slower ester formation reactions, and consequently the p value calculated by Jaffe144 from the data of Hartman and Borders142 is not accurate. Later workers allowed for this side-reaction 46 or used aromatic sulphonic acids rather than HC1 as the catalyst145147. However, whatever the exact p values, it is quite clear that the polar effects of substituents on acid-catalyzed ester hydrolysis and formation are small. 

Satchell87 found that the acylation, in nitromethane, of 0-naphthol by acetyl chloride to be catalysed by hydrogen chloride, and that added chloride ions depress the rate by a mass law effect. Both unionised and ionised acetyl chloride take part in the acylation and although the ionisation is kinetically independent of the amount of hydrogen chloride, the observed increase in rate by added hydrogen chloride can be explained most easily on the basis that the ionisation is dependent on the presence of HC1, the mechanism being... 

Choride ion is considerably less reactive than the azide ion. Thus, although values of kc 1/ kn2o have been quite widely available from mass law effects of chloride ion on the solvolysis of aralkyl halides, normally the reaction of the chloride ion cannot be assumed to be diffusion controlled and the value of kn2o cannot be inferred, except for relatively unstable carbocations (p. 72). Mayr and coworkers251 have measured rate constants for reaction of chloride ion with benzhydryl cations in 80% aqueous acetonitrile and their values of logk are plotted together with a value for the trityl cation19 in Fig. 7. There is some scatter in the points, possibly because of some steric hindrance to reaction of the trityl cations. However, it can be seen that chloride ion is more... 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

Moreover, the rate constants for these substitutions appear to decrease as the reaction progresses. This is because the initial dissociation is significantly reversible. As the concentration of Cl resulting from the reaction builds up, recombination of Cl with the 5-covalent intermediate, Co(en)2Cl f, begins to compete with the reaction of the intermediate with NOj, Br, or SCN, and a smaller and smaller percentage of the acts of dissociation result in net reaction. The same effect may be observed by carrying out the reaction in the presence of an excess of ionic chloride here, the observed rate of substitution is greatly decreased. This, the so-called mass-law effect, constitutes one of the surest tests for the dissociation mechanism. 

Intermediates and causes them to abstract hydride Ions more rapidly from Isobutane or any other potential donor. Increased hydride transfer converts more of the carbonlum Ions at the add Interface to saturates faster, yielding product while minimizing polymerization and side reactions. It Is also likely that the surfactants physically block alkyl Ions from one another in the surface film and thus Impede Ion + olefin polymerization. In such a film the carbonlum Ion concentration must also be lower than In the absence of surfactant and mass law effects will therefore also lead to less polymerization and cracking. The fact that steady state hydride transfer rates In H2SO are subject to control through the use of acid modifiers which act In the bulk acid and at the acid-hydrocarbon Interface Is the key to the control of sulfuric acid alkylation. 

This effect is a variation of Le Chatelier s principle, and is called the common ion (or mass-law) effect. The first step of the SN1 reaction is to produce the leaving group, and if there is already a large concentration of this present, the forward reaction is disfavoured as it would produce more of that species. 

This phenomenon is a direct consequence of the mass-law effect on ionization equilibria in electrolytic solution. 

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). 

Additional support for the zwitterion-carbene mechanism comes from the observation of the mass-law effect of added common ion salts In the presence of 0.22 M sodium bromide, the initial rate of the second-order reaction of 3-bromo-3-methyl-l-butyne is depressed by about 40%. The same concentration of either sodium perchlorate or sodium nitrate depresses the rate by only about 18%. 

In early phases of the reaction, [L ] is close to zero, so the term fc i[L ] is also about zero. Making the assumption thatfc i[L ] + fc2[Y ] w k2[Y ] allows us to cancel the C2[Y ] terms in the numerator and denominator of equation 8.16, which leads to the approximate expression of equation 8.13. As the reaction proceeds, [L ] increases and the apparent rate of the reaction may decrease if 1 [L ] becomes significant relative to /c2[Y ]. Adding L to the solution would also decrease the rate of formation of R—Y, a phenomenon known as the common ion effect or mass law effect. In solvolysis reactions, in which the solvent is the nucleophile, the concentration of the nucleophile is effectively constantandcannotbe varied.Thereactionisthereforemoreproperly described as pseudo-first order, since only the concentration of the substrate can be varied. 

Esters can be hydrolyzed in either basic or acidic solution. In acidic solution, the reaction is reversible, and the position of equilibrium depends on the relative concentrations of water and the alcohol. In aqueous solution, hydrolysis occurs in alcoholic solution, the equilibrium is shifted in favor of the ester by the mass law effect. Hydrolysis in aqueous alkaline solution is essentially irreversible. The reverse of the final proton transfer is extremely unfavorable because the carboxylate anion is a much weaker base than the alkoxide ion, and the equilibrium is far to the right. The... 

The position of the equilibrium depends on the relative concentrations of water and the alcohol. In aqueous solution, hydrolysis occurs in alcoholic solution, the equilibrium is shifted in favor of the ester by the mass law effect. In aqueous alkaline solution ester hydrolysis is essentially irreversible ... 

---