TY - JOUR
T1 - Azide-Water Competition in Solvolysis Reactions. A Revisit of the Reactivity-Selectivity Relationship 2
AU - Ta-Shma, Rachel
AU - Rappoport, Zvi
PY - 1983/9
Y1 - 1983/9
N2 - The suggested linearity between the logarithms of the solvolysis rate constant (ksolv) of RX and the selectivity (kN/kw) in capture of the intermediate cation R+ by water (W) and N3 (N) in 80% acetone was reinvestigated by using extended literature data. The assumptions involved in deriving the kn/kw values, such as first-order dependency on the nucleophile concentration in its reaction with R+ and the temperature, solvent, nucleofuge, and other extrapolations required for obtaining the parameters, were discussed. The extended log ksolv vs. log (kn/kw) plot, which is based on the revised set of values, is not linear and composed of five regions; (a) Ritchie's constant selectivity region; (b) a border region where the selectivity increases more than the reactivity and which may be an artifact; (c) an apparent reactivity-selectivity (RS) region with an appreciable scatter of the points, where log (kn/kw) decreases on decreasing ksolv (the reaction with N3- in this region is diffusion controlled and the plot reflects a log ksolv vs. log kw relationship); (d) a low ksolv and nearly constant low selectivity region where hindrance to nucleophilic approach to the intermediate may sterically enforce a preassociation ion pair mechanism on the reaction; (e) an inverted RS region, where the selectivity increases on decreasing ksolv and where the reaction with N3- is mostly SN2. The various mechanistic regions were discussed in relationship to the existence and the lifetimes of the cationoid intermediates.
AB - The suggested linearity between the logarithms of the solvolysis rate constant (ksolv) of RX and the selectivity (kN/kw) in capture of the intermediate cation R+ by water (W) and N3 (N) in 80% acetone was reinvestigated by using extended literature data. The assumptions involved in deriving the kn/kw values, such as first-order dependency on the nucleophile concentration in its reaction with R+ and the temperature, solvent, nucleofuge, and other extrapolations required for obtaining the parameters, were discussed. The extended log ksolv vs. log (kn/kw) plot, which is based on the revised set of values, is not linear and composed of five regions; (a) Ritchie's constant selectivity region; (b) a border region where the selectivity increases more than the reactivity and which may be an artifact; (c) an apparent reactivity-selectivity (RS) region with an appreciable scatter of the points, where log (kn/kw) decreases on decreasing ksolv (the reaction with N3- in this region is diffusion controlled and the plot reflects a log ksolv vs. log kw relationship); (d) a low ksolv and nearly constant low selectivity region where hindrance to nucleophilic approach to the intermediate may sterically enforce a preassociation ion pair mechanism on the reaction; (e) an inverted RS region, where the selectivity increases on decreasing ksolv and where the reaction with N3- is mostly SN2. The various mechanistic regions were discussed in relationship to the existence and the lifetimes of the cationoid intermediates.
UR - http://www.scopus.com/inward/record.url?scp=0020822178&partnerID=8YFLogxK
U2 - 10.1021/ja00357a020
DO - 10.1021/ja00357a020
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AN - SCOPUS:0020822178
SN - 0002-7863
VL - 105
SP - 6082
EP - 6095
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 19
ER -