Abstract:
The proton-transfer reactions of the substituted phenol tetrabromophenolphthalein ethyl ester
with various aliphatic and aromatic amines in chlorobenzene have been studied over ranges of
temperature. Equilibrium constants and thermodynamic parameters have been determined spectro-
photometrically; rate constants and activation parameters for the forward and back reactions have
been determined with the aid of a microwave temperature-jump apparatus. As in the case of the
reactions with 2,4-dinitrophenol (preceding paper), the forward rate constants approach the value
for diffusion control, and are not correlated with the equilibrium constants but appear to reflect
steric factors. With some of the amines, spectrophotometric investigation revealed the presence
also of a hydrogen-bonded species, whose formation may precede the proton-transfer step; equi
librium constants were determined for several of these complexes. The kinetics have been analysed
on the basis of a two-stage mechanism, hydrogen-bond formation followed by proton-transfer.
Various considerations, including the absence of a Bronsted-type relation between rate and equilibrium
constants, indicate that the formation of a hydrogen-bonded complex or an encounter-complex is
rate-limiting and the proton-transfer relatively fast.
