Canadian Journal of Chemistry p. 1196 - 1200 (1986)
Update date:2022-08-17
Topics:
McClelland, Robert A.
Soerensen, Poul E.
A temperature-jump relaxation study is reported for the equilibration: 3-hydroxyphtalide (SH) <*> o-formylbenzoate <*> (R-) <*> o-formylbenzoic acid (RH).A kinetic analysis is carried out in which SH and R- interconvert with catalysis in the ring opening direction by water and by added general bases.Excellent Broensted plots based upon a series of oxyacid buffer catalysts are obtained.These have slopes β for the base-catalyzed ring opening of 0.81 and α for the reverse acid-catalyzed ring closing of 0.19.A mechanism where S-, the conjugate base of SH, is a discrete intermediate can be ruled out on the basis of the Broensted values and the magnitudes of the rate constants.The lifetime of S- is estimated to lie in the range 1E-11-1E-15 s.Two mechanisms can be proposed.A fully concerted mechanism "enforced" by lifetimes less than 1E-13 s involves direct interconversion of SH and R- with no intermediate.A preassociated mechanism "enforced" by lifetimes in the 1E-11-1E-12 s range requires, in the ring closing direction, that an acid catalyst be hydrogen bonded to the carbonyl in R-.
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