A-Se2 mechanism

The A-Se2 mechanism is exemplified by some aromatic electrophilic substitutions ... 

The quantity on the left side of the equation is referred to as the logarithm of the rate constant corrected for protonation often the correction term is negligible. If the activity coefficient term on the right side is negligible, Eq. (8-104) predicts a linear relationship between the corrected log kobs and —//o, the slope being unity. A similar treatment of the A-Se2 mechanism also predicts a linear plot of log fcobs against —Ho-... 

For the A-SE2 mechanism we have reaction (la), with EZ = HA, followed by reaction (2a) with E = H. Clearly, the kinetics follow... 

The criterion which has been extensively employed to distinguish A-l mechanisms from A-2 or A-SE2 mechanism has been the linearity of plots of reaction rate coefficient versus the acidity function h0. The acidity function (see Volume 2, p. 358) is a measure of the proton-donating ability of a medium (as measured by its tendency to protonate a base B) which is given by equation (11), viz. 

Thus it can be seen that evidence for the A-l mechanism, even if one accepted that this followed from a linear rate coefficient-acidity function correlation, was scant. On the other hand, there have been a very large number of carefully documented studies in which general acid catalysis has been observed leading to the A-Se2 mechanism for the reaction, or it has been shown that the conclusions from an acidity function dependence are not rigorous. One such study has already been described above, and Satchell478 also found that in the detritiation of [4,6-3H2]-l,2,3-trimethoxybenzene by potassium bisulphate, dichloro- and tri-fluoroacetic acids, plots of log kl versus —H0 were linear with a slope of ca. 1.0... 

The kinetics of this reaction can be very conveniently followed by measuring the volume of carbon dioxide liberated in a given time, and the earlier studies employed this technique, though more recently spectrophotometric (uv) methods have been favoured. Most of the kinetic studies have been directed towards determining whether the SE1 or A-SE2 mechanisms, equilibria (257) and (258) or (259) and (260), respectively,... 

Further evidence regarding the mechanism was provided by LynnandBoums643 , who found a pH-dependent carbon-13 isotope effect in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers. The dependence was interpreted in favour of the A-SE2 mechanism, for an increase in acetate concentration would increase kL t and hence partitioning of the intermediate so that k 2 becomes more rate-determining. 

Support for the A-SE2 mechanism comes also from the measurement643b of... 

Among the facts supporting this mechanism (which is an A-Se2 mechanism because... 

This is an A-Se2 mechanism (p. 466). There is a great deal of evidence for it, including ... 

A1 mechanism 27 A-Se2 mechanism 28 A2 mechanism 29 The Bunnett-Olsen method 31... 

A1 mechanism 31 A-Se2 mechanism 34 A2 mechanism 36 Other mechanisms 38 Amide hydrolysis 51... 

By analogy with equation (12), the assumption made regarding the linearity of activity coefficient ratios is equation (45) (slope parameter j), and the resulting Bunnett-Olsen equations that apply to kinetic measurements are equations (46) and (47) for unprotonated and protonated substrates, respectively.156 These apply to the A1 and A-SE2 mechanisms for the A1 and A2 mechanisms they may require correction for partial substrate protonation as in equations (25) and (26) above. For A2 reactions an additional term such as the log water activity has to be added as in equation (33). These equations have been widely tested and work quite well.155-160 The difference between the Bunnett-Olsen and the excess acidity kinetic methods (discussed below) is that the Bunnett-Olsen method features an additive combination of the slope parameters e and , whereas the excess acidity method features a multiplicative one. There seems to be no theoretical justification for the former. Also the Bunnett-Olsen method still uses H0, whereas acidity functions are not needed for the excess acidity approach see above. 

Thus for these reactions m is necessarily less than unity, a result that has now been widely observed in practice,117,118,120,161,180,181 and thus the m1 value offers a clear distinction between the A1 and A-SE2 mechanisms, which is not the case with the H0 correlations discussed above. A number of different excess acidity plots according to equation (56), covering a wide reactivity range, are shown in Fig. 9. These are for the hydration of oc-methylstyrene,120 equation (58), and the mechanistically similar hydration of phenylacetylene 118 for the isomerization of m-stilbene 120 and for the detritiation of tritiated benzene, equation (28) above."7 As can be seen, all four plots are good straight lines the references cited may be consulted for the details. The slopes look steep, but m values for carbon protonation approximate 1.8,36 and the nfi values are all calculated to be... 

Excess acidity correlations have been used to show that some aromatic sulfonic acid desulfonations have an A-SE2 mechanism.188,189 This mechanism (alternative terminologies are Ad-E2 and A(E) +A(N))190 has also been found to apply to the hydration of acetylene itself,191 to ynamines192 and to many other alkynes,193-195 as well as to many different alkenes196-199 and vinyl ethers.200-203 The excess acidity method has been used to evaluate aA values for several alkene hydrations.204 205... 

Among the facts supporting this mechanism (which is an A-Se2 mechanism because the substrate is protonated in the rate-determining step) are (1) lsO labeling shows that in ROCH=CH2 it is the vinyl-oxygen bond and not the RO bond that cleaves 497 (2) the reaction is subject to general acid catalysis 498 (3) there is a solvent isotope effect when D2O is used.498 Enamines are also hydrolyzed by acids (see 6-2) the mechanism is similar. Ketene dithioacetals R2C=C(SR )2 also hydrolyze by a similar mechanism, except that the initial protonation step is partially reversible.499 Furans represent a special case of enol ethers that are cleaved by acid to give 1,4 diones. Thus... 

The possibility of detecting an isotope effect on proton transfer from an undissociated acid only arises for the A-SE2 mechanism. The... 

Sometimes the observation of general acid catalysis may be sufficient to identify the mechanism with considerable confidence. For example, in aromatic proton exchange catalyzed by ammonium salts, the only structurally attractive A-2 mechanism, shown in equations (4) and (5), violates the principles of microscopic reversibility. Thus the elimination of the A-l mechanism leaves only the A-SE2 mechanism. In a more... 

Taken all together it does not appear likely that the thermodynamic quantities of activation will be as useful as isotope effects in identifying A-SE2 mechanisms. 

Scheme 2.1. The A-SE2 mechanism for acid-catalyzed hydrogen exchange.

---