Ea. Castro et al., KINETICS AND MECHANISM OF THE PYRIDINOLYSIS OF ALKYL ARYL THIONOCARBONATES, Journal of organic chemistry, 62(8), 1997, pp. 2512-2517
The reactions of methyl 4-nitrophenyl, ethyl 4-nitrophenyl, and ethyl
2,4-dinitrophenyl thionocarbonates (MNPTOC, ENPTOC, and EDNPTOC, respe
ctively) with a series of 3- and 4-substituted pyridines are subjected
to a kinetic investigation in water, 25.0 degrees C, ionic strength 0
.2 M (maintained with KCl). Under amine excess, pseudo-first-order rat
e coefficients (k(obsd)) are obtained, which are linearly proportional
to the free-pyridine concentration. The second-order rate coefficient
s (k(N)) are obtained as the slopes of these plots. The Bronsted-type
plots found for the two mononitro derivatives coincide in one straight
line (same slope and intercept) of slope beta = 1.0. The EDNPTOC pyri
dinolysis shows a curved Bronsted-type plot with slopes beta(1) = 0.1
(high pK(a)), beta(2) = 1.0 (low pK(a)), and pK(a)(0) = 6.8 (pK(a) val
ue at the center of curvature). These plots are consistent with the ex
istence of a zwitterionic tetrahedral intermediate (T+/-) on the react
ion pathway whereby expulsion of aryloxide anion from T+/- is rate det
ermining (k(2) step) at low pK(a) for EDNPTOC (and in the whole pK(a)
range for MNPTOC and ENPTOC), and there is a change to rate-limiting f
ormation of T+/- (k(1) step) at high pK(a) for EDNPTOC. Comparison of
these Bronsted plots among them and with similar ones permits the foll
owing conclusions: (i) There is no variation of k(N) by substitution o
f methoxy by ethoxy as the nonleaving group of the substrate, (ii) The
pK(a)(0) value is smaller far the less basic aryloxide nucleofuge due
to a larger k(2) value. (iii) The change of C=S by C=O as the electro
philic center of the substrate results in larger values for both k(-1)
(amine expulsion rate) and k(2), and also a larger k(-1)/k(2) ratio f
or the carbonyl derivative. There is also an increase of k(1) by the s
ame change. The K-1/k(2) (=k(1)k(2)/k(-1)) values are larger for the p
yridinolysis of methyl 2,4-dinitrophenyl and methyl 4-nitrophenyl carb
onates compared to the corresponding thionocarbonates (EDNPTOC and MNP
TOC, respectively). (iv) Pyridines are more reactive than isobasic sec
ondary alicyclic amines toward ENPTOC when either the k(1) step of the
k(2) step is rate limiting. This is explained by the softer nature of
pyridines than alicyclic amines (k(1) step) and the greater nucleofug
ality (k(-1)) of the latter, amines than isobasic pyridines, leading t
o a larger k(2)/k(-1) ratio for pyridines (k(2) is little affected by
the amine nature), and therefore a larger K(1)k(2) value when the k(2)
step is rate determining.