A-Se2 reactions

The reason for these discrepancies may arise from the very important discovery that a single acidity function cannot describe the protonation of indicator bases of even the same charge type12-19 and Katritzky et al.19 have emphasised that this renders the attempt by Bunnett20 to obtain a more satisfactory diagnosis of the A-l, A-2, and A-SE2 reactions by the use of w and w values less reliable than it at one time appeared to be. 

The decarboxylation of 2,4-dimethoxybenzoic acid by 2.5-8.0 M sulphuric acid at 60.5 °C has also been studied and division of the first-order rate coefficient by the molar concentration of sulphuric acid gave constant values (2.94+0.25 x 10 6) throughout the acid range so that the A-SE2 reaction was again indicated643. 

Both the Se2 (front) and Se2 (back) mechanisms are designated DeAe in the lUPAC system. With substrates in which we can distinguish the possibility, the former mechanism should result in retention of configuration and the latter in inversion. The reaction of allylsilanes with adamantyl chloride and TiCU, for example, gives primarily the antiproduct via a Se2 reaction. When the electrophile attacks from the front, there is a third possibility. A portion of the electrophile may assist in the removal of the leaving group, forming a bond with it at the same time that the new C—Y bond is formed... 

Fig. 9 Excess acidity plot against X for some A-SE2 reactions at 25°C a-methylstyrene hydration (open circles) 120,182 phenylacetylene hydration (open and closed squares) 118,160,183,184 cw-stilbene isomerization (closed circles) 120,185 and tritiated benzene T-exchange (open and closed triangles).117,147,186,187...

Aromatic substitution, a quantitative treatment of directive effects in, 1, 35 Aromatic substitution reactions, hydrogen isotope effects in, 2, 163 Aromatic systems, planar and non-planar, 1, 203 Aryl halides and related compounds, photochemistry of, 20, 191 Arynes, mechanisms of formation and reactions at high temperatures, 6, I A-Se2 reactions, developments in the study of, 6,63... 

In the second mechanism, the first and second steps are concerted. In the case of hydrolysis of 2-(p-nitrophenoxy)tetrahydropyran, general acid catalysis was shown470 demonstrating that the substrate is protonated in the rate-determining step (p. 259). Reactions in which a substrate is protonated in the rate-determining step are called A-Se2 reactions.471 However, if protonation of the substrate were all that happens in the slow step, then the proton in the transition state would be expected to lie closer to the weaker base (p. 259). Because the substrate is a much weaker base than water, the proton should be largely transferred. Since the Brpnsted coefficient was found to be 0.5,the proton was actually transferred only... 

Equation (113) allows us to arrive at an estimate of the importance of transfer effects for A-SE2 reactions in which the transferred hydrogen acquired by the product does not readily undergo isotopic exchange out of the product. Under such conditions the ratio of the first two terms in the first factor of equation (113) measures the ratio of protium to deuterium in the product, i.e. 

A-Se2 reactions (mainly alkylmereuric halide cleavages) which have been investigated by Kreevoy s group from a similar point of view (Williams and Kreevoy, 1968). The case of 2-dichloro-methylene-l,3-dioxolan (3, DMD, for short) (Gold and Waterman, 1968b) illustrates most of the interesting features of all these examples. 

Arynes, mechanisms of formation and reactions at high temperatures, 6, 1 A-Se2 reactions, developments in the study of, 6, 63... 

There have now been a number of tests of equation (9) for A-SE2 reactions (see Table 1), and some of the results are shown in Figs. 2a-c. It is accurately obeyed by catalytic coefficients for carboxylic acids, but other classes of acids give rise to systematic deviations. Phosphoric acid and bisulfate ion seem always to be above the line generated by carboxylic acids, and H+, if its conjugate base, H20, is considered to be present in 55m concentration, is always below this line. Other homogeneous classes of acids, such as anilinium ions, may give rise to separate Bronsted lines (Thomas and Long, 1964). These deviations would seem to imply that the three-center model is at least oversimplified. 

It may ultimately develop that A-SE2 reactions can be distinguished from A-2 reactions by this pattern of deviations. At the moment, however, there do not appear to be enough examples, studied in sufficient detail, to reach a definitive conclusion. 

Table 8 lists some A-SE2 reactions which have been shown to be facilitated by complexing. 

The anomalously large isotope effects observed in the proton abstraction from nitroalkanes by certain pyridine bases (Funderburk and Lewis, 1964 Bell and Goodall, 1966) have been attributed to tunneling. There seems to be no other reasonable attribution. No unambiguous effect of this type has yet been identified in an A-SE2 reaction. Since the expected value in the absence of tunneling is not certain (Section IIA3) the anomaly would have to be quite large to be interpreted confidently. 

For A-Se2 reactions measurement of AS° associated with kh/kd seems to be the tool most likely to identify tunneling. High precision can be obtained (Kreevoy et al., 1966b), and the method is easily adaptable to long temperature ranges, and (if alcoholic solvents are used) to low temperatures. 

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