Kinetics and mechanism of the reactions of polyallylamine with aryl acetates and aryl methyl carbonates

Article abstract of DOI:10.1002/poc.1007

The reactions of polyallylamine (PAA) with 4-nitrophenyl acetate (NPA), 2,4-dinitrophenyl acetate (DNPA), 2,4,6-trinitrophenyl acetate (TNPA), 4-nitrophenyl methyl carbonate (NPC), 2,4-dinitrophenyl methyl carbonate (DNPC) and 2,4,6-trinitrophenyl methyl carbonate (TNPC) at pH 7.0-11.5 were subjected to a kinetic investigation in aqueous solution at 25.0°C and an ionic strength of 0.1 M (KC1). Potentiometric titration curves were obtained at different polymer concentrations under the same conditions as for the kinetic measurements. The degree of dissociation (a) and pK_app values for PAA at each pH were found from the titration curves. The shape of these curves shows a conformational change of the polymer at α > 0.7. Similar behavior was observed through the dependence of logk_N on either pH or a, where k_N is the second-order rate constant for the title reactions. The K_N value is influenced by the electrostatic interactions in the polymer chain and the conformational changes that PAA undergoes in solution. The Bronsted-type plots (logk_N vs pk_app) are linear with slopes (β values) of 0.5, 0.4, 0.5, 0.7, 0.6 and 0.7 for the reactions of PAA with NPA, DNPA, TNPA, NPC, DNPC and TNPC, respectively. These data are consistent with concerted mechanisms. The k _N values increase in the sequence TNPA > DNPA > NPA and TNPC > DNPC > NPC. These results are in accordance with those found for the reactions with monomeric amines, which are due to the increasing nucleofugality of the leaving groups, and also the increasing electrophilic character of the carbonyl carbon, as more nitro groups are added to the substrate. Acetates are more reactive than the corresponding methyl carbonates, which can be explained by the larger electron-releasing effect exerted by MeO relative to Me. PAA destabilizes the putative tetrahedral intermediate relative to the monomeric amines and the stability of tetrahedral intermediates would decrease in the sequence pyridines > anilines > secondary alicyclic amines > quinuclidines > PAA. Copyright

Full text of DOI:10.1002/poc.1007

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 129–135
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1007
Kinetics and mechanism of the reactions of
polyallylamine with aryl acetates and aryl
methyl carbonates
1
2
1
1
1
Enrique A. Castro, Gerardo R. Echevarr ı´ a, Alejandra Opazo, Paz S. Robert and Jos e´ G. Santos *
1
Facultad de Qu ı´ mica, Pontificia Universidad Cat o´ lica de Chile. Casilla 360, Santiago 094411, Chile
2
Departamento de Qu ı´ mica F ı´ sica, Universidad de Alcal a´ , 28871 Alcal a´ de Henares, Spain
Received 16 August 2005; revised 28 September 2005; accepted 2 October 2005
ABSTRACT: The reactions of polyallylamine (PAA) with 4-nitrophenyl acetate (NPA), 2,4-dinitrophenyl acetate
(DNPA), 2,4,6-trinitrophenyl acetate (TNPA), 4-nitrophenyl methyl carbonate (NPC), 2,4-dinitrophenyl methyl
carbonate (DNPC) and 2,4,6-trinitrophenyl methyl carbonate (TNPC) at pH 7.0–11.5 were subjected to a kinetic
ꢀ
investigation in aqueous solution at 25.0 C and an ionic strength of 0.1 M (KCl). Potentiometric titration curves were
obtained at different polymer concentrations under the same conditions as for the kinetic measurements. The degree
of dissociation (ꢀ) and pKapp values for PAA at each pH were found from the titration curves. The shape of these
curves shows a conformational change of the polymer at ꢀ > 0.7. Similar behavior was observed through the
dependence of logk on either pH or ꢀ, where k is the second-order rate constant for the title reactions. The k value
N
N
N
is influenced by the electrostatic interactions in the polymer chain and the conformational changes that PAA
undergoes in solution. The Brønsted-type plots (logk vs pKapp) are linear with slopes (ꢁ values) of 0.5, 0.4, 0.5, 0.7,
N
0
.6 and 0.7 for the reactions of PAA with NPA, DNPA, TNPA, NPC, DNPC and TNPC, respectively. These data are
consistent with concerted mechanisms. The kN values increase in the sequence TNPA > DNPA > NPA and
TNPC > DNPC > NPC. These results are in accordance with those found for the reactions with monomeric amines,
which are due to the increasing nucleofugality of the leaving groups, and also the increasing electrophilic character of
the carbonyl carbon, as more nitro groups are added to the substrate. Acetates are more reactive than the
corresponding methyl carbonates, which can be explained by the larger electron-releasing effect exerted by MeO
relative to Me. PAA destabilizes the putative tetrahedral intermediate relative to the monomeric amines and the
stability of tetrahedral intermediates would decrease in the sequence pyridines > anilines > secondary alicyclic
amines > quinuclidines > PAA. Copyright # 2006 John Wiley & Sons, Ltd.
KEYWORDS: polyallylamine; aryl acetates; aryl methyl carbonates; kinetics; mechanism
ꢁ
INTRODUCTION
ionic tetrahedral intermediate (T ) and others by a
concerted pathway (a single step). Structure–reactivity
correlations, such as the Brønsted-type relationship, have
The study of the reactions involving synthetic macromole-
cules can serve as a model for more complex enzymatic
processes. In general, a synthetic polymer can concentrate
and/or repel a reactive molecule in its vicinity or, when the
synthetic polymer is functionalized with some catalytically
active functions, some effects on the kinetics and mechan-
ism of the reactions are observed.
The mechanisms of the aminolysis of aryl acetates,
alkyl aryl carbonates and diaryl carbonates
2
–7
helped to clarify the mechanisms.
The apparent acid strength of a polyamine-conjugated
acid depends not only on the concentration and type of
added salt but also on the degree of dissociation. The
variation of these allows a range of basicities of the
polyamine without a change in its structure. This has
been used advantageously in mechanistic studies where
1
2
3–5
4b,6,7
8
with
Brønsted plots are represented. The dissociation beha-
vior of polyamines and polyamino acids has been studied
monomeric amines have been well established. Some of
these reactions are known to proceed through a zwitter-
8,9
using potentiometric titration curves.
A systematic study of the kinetics and mechanism of the
aminolysis of aryl acetates by poly(ethylenimine) (PEI)
*
Correspondence to: J. G. Santos, Facultad de Qu ´ı mica, Pontificia
Universidad Cato
´
lica de Chile. Casilla 360, Santiago 094411, Chile.
E-mail: jgsantos@puc.cl
Contract/grant sponsor: MECESUP; Contract/grant numbers: PUC-
8
was carried out by Arcelli and Concilio. In the absence of
a simple salt, a mechanism has been reported that involves
substrate–polyelectrolyte interactions similar to the action
of an enzyme. On the other hand, in the presence of salt a
stepwise mechanism, through a zwitterionic tetrahedral
0
004; RED QUIRNICA vch-01.
Contract/grant sponsor: FONDECYT; Contract/grant number:
020538.
Contract/grant sponsor: DGI; Contract/grant number: BQU2003-
7281.
8a
1
ꢁ
8b
0
intermediate (T ), was obtained.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 129–135

1
30
E. A. CASTRO ET AL.
O
Table 1. Mean values of degree of dissociation (ꢀ ) and
m
pKapp values of the protonated PAA at different pH and
concentration ranges
R
C O Ar
a
R = Me, MeO
Ar = 4-nitrophenyl, 2,4-dinitrophenyl, 2,4,6-trinitrophenyl
2
10 [N] (M)
b
c
pH
.00
7.50
8.00
ꢀm
pKapp
tot
7
4.96–31.4
5.15–31.4
0.063
0.112
0.233
0.328
0.444
0.586
0.882
0.939
1.0
8.17
8.40
8.52
8.81
9.10
9.35
9.63
9.33
NH2
n
0.113–0.975
0.106–0.887
0.100–0.827
0.104–0.946
0.034–0.882
0.034–0.257
0.034–0.256
0.034–0.342
8
9
9
.50
.00
.50
Polyallylamine (PAA)
Scheme 1
10.50
1
1
1
0.50
1.00
1.50
Some kinetic studies on the hydrolysis of esters with
1.0
10
11
polyelectrolytes containing amine, imidazole, pyri-
a
ꢀ
12
In aqueous solution at 25 C, ionic strength 0.1 M (KCl).
Concentration of total amino groups (free base plus protonated forms).
ꢀm is the mean ionization degree of PAA at each pH at the concentration
dine, and other groups have described increased activity
of these polyamines with respect to their monomeric
counterparts. On the other hand, modified polyelectrolytes,
b
c
range stated.
13
10
such as PEI and polyallylamine (PAA) with various
hydrophobic groups (alkyl or benzyl), were investigated in
order to determine the effect of the environment on the
mechanism. In general, these modified polyamines accel-
erate the hydrolysis reactions relative to unmodified
polymers; changes in the mechanism have also been
Determination of pKapp
The potentiometric titration of PAA was carried out at
ꢀ
25.0 ꢁ 0.1 C, under nitrogen, by means of a Radiometer
autotitrator equipped with a PHM-62 pH-meter, an ABU-
11 autoburette, a TTT-60 titrator, an REA-160 recorder, a
TTA-60 thermostatic support, a G-2040 glass electrode
and a K-4040 calomel electrode. In each experiment, 10
ml of the polymer solution at different concentrations
10a
observed.
The formation of the Schiff bases of pyridoxal-5-phos-
1
4a
14b
phate and pyridoxal with PAA as bearers of —NH —
2
groups is controlled by the same mechanism, although the
rate-determining step is different.
In this paper, we report a kinetic study of the reactions
of PAAwith 4-nitrophenyl acetate (NPA), 2,4-dinitrophe-
nyl acetate (DNPA), 2,4,6-trinitrophenyl acetate (TNPA),
ꢃ3
ꢃ2
(1 ꢂ 10 to 5.1 ꢂ 10 M) and an ionic strength of 0.1 M
(KCl) were titrated with NaOH (0.01–0.1 M). The poly-
mer concentration range in the potentiometric titration
was similar to that in the kinetic measurements. The
apparent dissociation constant (pKapp) was calculated
4
-nitrophenyl methyl carbonate (NPC), 2,4-dinitrophenyl
9d
methyl carbonate (DNPC) and 2,4,6-trinitrophenyl
methyl carbonate (TNPC) (Scheme 1). This, together
with the investigation of the dissociation behavior of
PAA, will allow the determination of the mechanism
and the assessment of the effect of the amine group
bearer and of the leaving and non-leaving groups of the
substrate on the mechanism. We also compared the
kinetic results of this study with those reported for the
reactions of monomeric amines with the same substrates.
according to the equation
pKapp ¼ pH þ logð1 ꢃ ꢀÞ=ꢀ
ð1Þ
where ꢀ is the degree of dissociation, which is the ratio of
the free amino groups to the total polyallylamine con-
centration (expressed in monomeric units); the free
amino group concentration was determined from the
volume of NaOH added at each pH.
Table 1 shows the pKapp and ꢀm values of PAA at
9a,9d,17
different pH and at various polymer concentration ranges
(ꢀ is the mean degree of ionization of PAA at each pH at
the concentration range stated).
EXPERIMENTAL
Materials
m
Polyallylamine hydrochloride (PAA), from Polyscience,
Warrington, PA, USA, MW ¼ 60 000 Da, was used
without further purification. 4-Nitrophenyl acetate
Kinetic measurements
PAA solutions were prepared freshly in the correspond-
ing external buffer of 0.01 M at ionic strength 0.1 M (KCl),
at the desired pH. The reactions were initiated by addition
of 30 ml of a stock solution of the corresponding acetate
to 2.5 ml of polymer solutions thermostated at
(
trophenyl acetate (TNPA) were synthesized as described
NPA), 2,4-dinitrophenyl acetate (DNPA) and 2,4,6-trini-
15
previously. 4-Nitrophenyl methyl carbonate (NPC),
,4-dinitrophenyl methyl carbonate (DNPC) and 2,4,6-
trinitrophenyl methyl carbonate (TNPC) were synthe-
2
ꢀ
25 ꢁ 0.1 C. The initial concentration of the substrates
16
ꢃ5
sized by a standard procedure. All other reagents
were of analytical grade.
was 5 ꢂ 10 M. The kinetic measurements were carried
out by following spectrophotometrically the production
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 129–135

REACTIONS OF POLYALLYLAMINE WITH ARYL ACETATES AND METHYL CARBONATES
131
a
Table 2. Experimental conditions and kinetic results for the reactions of PAA with TNPA, DNPA and NPA
2
10 [N] (M)
b
2
10 k (s
ꢃ1
ꢃ1 ꢃ1
ꢃ1 ꢃ1
kN (M s )
pH
)
No. of runs
kNobs (M
s
)
tot
obs
TNPA
c
7
7
8
8
9
9
1
1
.00
.50
.00
.50
.00
.50
5.41–31.4
5.41–31.4
4.20–53.0
9.75–131
2.09–9.15
2.38–16.0
4.30–33.0
6.85–59.0
11.8–110
21.7–75.0
6
6
6
5
10
6
1.84 ꢁ 0.08
4.39 ꢁ 0.4
8.9 ꢁ 0.5
18 ꢁ 3
29 ꢁ 1
39 ꢁ 4
38 ꢁ 2
53 ꢁ 9
85 ꢁ 4
c
c
d
d
d
0.113–0.938
0.106–0.887
0.100–0.831
0.104–0.863
0.034–0.882
0.034–0.256
38 ꢁ 2
68 ꢁ 7
116 ꢁ 12
129 ꢁ 6
e
e
0.05
10.0
11
5
114 ꢁ 5
257 ꢁ 16
257 ꢁ 16
DNPA
c
7
7
8
8
9
9
1
1
1
.00
.50
.00
.50
.00
.50
4.96–12.8
5.15–16.9
1.04–4.16
1.54–13.3
0.240– 2.20
0.400–3.60
0.620–6.00
1.36–12.9
2.71–15.5
5.91–14.9
9.59–16.3
5
6
6
0.42 ꢁ 0.03
0.94 ꢁ 0.05
2.4 ꢁ 0.1
3.7 ꢁ 0.8
6.3 ꢁ 0.5
13.4 ꢁ 0.6
14.9 ꢁ 0.4
29 ꢁ 2
6.7 ꢁ 0.5
8.4 ꢁ 0.4
10.2 ꢁ 0.4
11 ꢁ 2
c
c
d
d
d
0.113–0.938
0.106–0.887
0.100–0.927
0.104–0.946
0.034–0.882
0.0341–0.341
0.0342–0.257
6
11
11
11
6
14 ꢁ 1
23 ꢁ 1
e
e
e
0.50
1.00
1.50
16.9 ꢁ 0.5
29 ꢁ 2
6
33 ꢁ 3
33 ꢁ 3
NPA
c
c
c
d
d
d
7
7
8
8
9
9
1
1
.00
.50
.00
.50
.00
.50
5.06–16.6
5.15–16.9
0.056–0.277
0.073–0.588
0.013-0.110
0.025–0.193
0.027–0.257
0.081–0.549
0.371–0.612
0.935–1.58
6
6
6
6
6
6
6
7
0.019 ꢁ 0.002
0.041 ꢁ 0.003
0.100 ꢁ 0.002
0.21 ꢁ 0.03
0.28 ꢁ 0.03
0.62 ꢁ 0.05
1.04 ꢁ 0.08
2.20 ꢁ 0.2
0.300 ꢁ 0.003
0.370 ꢁ 0.002
0.43 ꢁ 0.01
0.64 ꢁ 0.09
0.63 ꢁ 0.07
1.06 ꢁ 0.09
0.117–0.975
0.106–0.887
0.111–0.927
0.104–0.863
0.034–0.257
0.034–0.341
e
e
b
0.50
1.00
1.11 ꢁ 0.09
2.2 ꢁ 0.2
a
ꢀ
In aqueous solution at 25 C, ionic strength 0.1 M (KCl).
b
c
d
e
Concentration of total amino groups (free base plus protonated forms).
.01 M phosphate buffer.
.01 M borate buffer.
.01 M carbonate buffer.
0
0
0
of 4-nitrophenoxide (400 nm), 2,4-dinitrophenoxide
355 nm) and 2,4,6-trinitrophenoxide (355 nm) anions
Product studies
(
using a Hewlett-Packard Model 8453 diode-array spec-
trophotometer. Under amine excess, pseudo-first-order
rate constants (kobs) were found for all reactions. The
plots of kobs vs total amine concentration are linear, with
kNobs as slope. Phosphate, borate and carbonate buffers
The presence of 4-nitrophenoxide, 2,4-dinitrophenoxide
and 2,4,6-trinitrophenoxide anions as products of the
reactions was determined spectrophotometrically by
comparison of the UV–visible spectra at the end of the
reactions with those of authentic samples under the same
conditions.
(
0.01 M) were used at appropriate pH ranges.
The experimental conditions and the values of kobs and
To determine whether these reactions are nucleophilic or
amine-catalyzed hydrolysis, an HPLC study was carried
out to check the presence of acetic acid as a product of the
reaction of DNPA. Acid acetic analysis by HPLC con-
sisted of an IC pack ion-exclusion column (7.8 mm
i.d. ꢂ 300 mm), 2.5 mM H SO –acetonitrile–methanol
kNobs for the reactions of PAA with TNPA, DNPA and
NPA are summarized in Table 2 and those for the
reactions with TNPC, DNPC and NPC in Table 3.
2
4
ꢃ1
Transmission electron microscopy
(95:3:2) as mobile phase at a flow-rate of 1.0 ml min
and a photodiode-array detector coupled to a computer.
The detector was set at 210 nm. Acetic acid was identified
by comparing the peak retention time of the reactions
with that of a standard. Only traces of acetic acid were
found in the reaction of DNPA at pH 7.0, showing that
amine-catalyzed hydrolysis and/or spontaneous hydrolysis
of DNPA are negligible compared with direct amine
attack.
PAA solutions (6.6 ꢂ 10ꢃ3 M), at the same conditions as
those of the kinetic measurements, at pH 8.0 and 11.0,
were prepared and submitted to a transmission electron
microscopy (TEM) study. The photographs show
that the macromolecule adopts a rod-like conform-
ation at pH 8.0 and a random coil conformation at
pH 11.0.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 129–135

1
32
E. A. CASTRO ET AL.
a
Table 3. Experimental conditions and kinetic results for the reactions of PAA with TNPC, DNPC and NPC
2
10 [N] (M)
b
2
10 k (s
ꢃ1
ꢃ1 ꢃ1
ꢃ1 ꢃ1
kN (M s )
pH
)
No. of runs
kNobs (M
s
)
tot
obs
TNPC
c
7
7
8
8
9
9
1
1
1
.00
.50
.00
.50
.00
.50
6.07–15.5
4.99–16.4
2.15–6.19
2.39–13.3
0.604–1.47
1.49–5.47
1.52–11.1
2.99–15.3
3.28–18.8
11.0–58.7
30.7–117
5
6
5
6
6
6
6
6
6
0.44 ꢁ 0.01
0.99 ꢁ 0.04
1.69 ꢁ 0.03
6.4 ꢁ 0.5
19 ꢁ 1
6.9 ꢁ 0.2
8.7 ꢁ 0.3
7.9 ꢁ 0.1
21 ꢁ 2
c
d
d
d
d
0.256–0.768
0.195–0.812
0.0723–0.542
0.0686– 0.515
0.0319–0.239
0.0327–0.245
0.036–0.270
42 ꢁ 2
26 ꢁ 2
44 ꢁ 3
e
e
e
0.50
1.00
1.50
67 ꢁ 6
71 ꢁ 6
206 ꢁ 17
382 ꢁ 36
209 ꢁ 17
380 ꢁ 36
DNPC
c
7
7
8
8
9
9
1
1
1
.00
.50
.00
.50
.00
.50
4.97–16.5
5.11–16.8
0.330–1.79
0.556–3.46
0.0628–0.425
0.0718–1.10
0.170–1.32
0.503–4.22
0.927–4.65
2.20–7.39
12
6
6
5
6
5
12
7
0.12 ꢁ 0.01
0.25 ꢁ 0.01
0.43 ꢁ 0.03
1.35 ꢁ 0.07
2.4 ꢁ 0.1
1.70 ꢁ 0.14
2.09 ꢁ 0.08
2.1 ꢁ 0.1
4.1 ꢁ 0.2
5.4 ꢁ 0.2
7.8 ꢁ 0.3
12.5 ꢁ 0.5
7.9 ꢁ 0.4
13 ꢁ 1
c
d
d
d
d
0.107–0.895
0.106–0.879
0.0722–0.542
0.114–0.946
0.071–0.882
0.0697–0.697
0.0354–0.265
4.5 ꢁ 0.2
e
e
e
0.50
1.00
1.50
4.6 ꢁ 0.2
7.9 ꢁ 0.4
4.58–7.46
6
13 ꢁ 1
NPC
c
7
7
8
8
9
9
1
1
1
.00
.50
.00
.50
.00
.50
6.07–16.9
4.99–16.4
0.256–1.07
0.208–0.866
0.0648–0.486
0.0686–0.515
0.0319–0.239
0.0654–0.245
0.0335–0.201
0.0192–0.0648
0.059–0.301
0.0156–0.0389
0.023–0.067
0.017–0.064
0.0628–0.251
0.106–0.286
0.331–0.593
1.27–1.76
5
6
6
6
6
6
6
5
5
0.0041 ꢁ 0.0002
0.022 ꢁ 0.0007
0.0291 ꢁ 0.002
0.066 ꢁ 0.001
0.110 ꢁ 0.007
0.43 ꢁ 0.02
0.060 ꢁ 0.003
0.182 ꢁ 0.006
0.15 ꢁ 0.01
0.219 ꢁ 0.003
0.25 ꢁ 0.02
0.74 ꢁ 0.03
0.9 ꢁ 0.1
c
d
d
d
d
e
e
e
0.50
1.00
1.50
0.81 ꢁ 0.09
1.45 ꢁ 0.07
1.45 ꢁ 0.07
3.0 ꢁ 0.3
3.0 ꢁ 0.3
a
b
c
d
e
ꢀ
In aqueous solution at 25 C, ionic strength 0.1 M (KCl).
Concentration of total amino groups (free base plus protonated forms).
.01 M phosphate buffer.
.01 M borate buffer.
.01 M carbonate buffer.
0
0
0
RESULTS AND DISCUSSION
The difference in acidic strength between a monomeric
amine and the amino groups of a polyamine is well
9d
known. Moreover, basicity changes with the degree of
dissociation (ꢀ) of these polyamines and, therefore, they
9a,9d
10
9
show different pK values at different pH.
a
Figure 1
shows the titration curves of pKapp vs ꢀ at several PAA
concentrations. As can be seen, pKapp depends on both ꢀ
and the concentration of the polymer, owing to a change
in the charge density of the polyion.
The curves in Figure 1 show that proton release
from PAA decreases with increase in the degree of
dissociation for the highest polymer concentration
8
ꢃ2
used (5.1 ꢂ 10 M) and that PAA does not exhibit a
0
.0
0.2
0.4
0.6
0.8
1.0
cooperative transition (hydrogen bond or hydrophobic
9a
interactions) in accordance with the behavior described.
Figure 1 shows that for the more dilute solutions the
α
Figure 1. Potentiometric titration of PAA, expressed as
pKapp versus degree of dissociation (ꢀ), in aqueous solution
pKapp values increase up to ꢀꢄ0.7 and then decrease for
ꢀ
> 0.7. These results can be explained by a transition or
ꢀ
at 25.0 C and an ionic strength of 0.1 M (KCl). PAA con-
ꢃ3
ꢃ3
conformational change of the PAA from rod-like con-
formation to a random coil form, as has been reported for
centrations (M): 1.1 ꢂ 10
(^), 3.66 ꢂ 10
(*),
4
)
ꢃ3
ꢃ3
ꢃ2
6.59 ꢂ 10 (4), 9.16 ꢂ 10 (&) and 5.1 ꢂ 10
(
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 129–135

REACTIONS OF POLYALLYLAMINE WITH ARYL ACETATES AND METHYL CARBONATES
d,17
133
9
3
2
1
0
1
other polyelectrolytes.
with pH was confirmed by TEM.
The conformation change
A linear dependence of the pseudo-first-order rate
constant (kobs) on PAA concentration at each pH was
observed, according to the equation
kobs ¼ k þ kNobs½Nꢅtot
ð2Þ
0
where k and k
0
are the rate coefficients for spontaneous
Nobs
-
hydrolysis and aminolysis of the substrates, respectively.
These results suggest that there is no substrate-polyelec-
trolyte association because the presence of this association
shows limiting plots, similar to those described for PEI in
7
8
9
10
11
12
pH
8a
the absence of KCl.
Figure 3. Variation of logkN as a function of pH for the
reactions of PAA with NPC (*), DNPC (4) and TNPC (&) in
No differences in kobs values were observed when
reactions were carried out with different fractions of the
polymer (water/acetone) and the commercial polymer,
thus disregarding the presence of oligomers in the com-
mercial polymer.
ꢀ
aqueous solution at 25.0 C and an ionic strength of 0.1 M
(KCl)
tively. As can be seen, below pH 9.5 k increases with
N
In most of the studied reactions, k is negligible in Eqn
0
increase in pH or free amine molar fraction. The presence
of protonated amino groups in the polymer leads to a rod-
like conformation of the polymer by repulsion of the
charges, where each molecule behaves independently.
Above pH 10.5, a new increase in kN with increasing
pH is observed. When the amino groups are almost
completely deprotonated, interactions between the side
chains are permitted and PAA adopts a random coil
conformation, which must be more reactive in order to
(
terms are important.
The nucleophilic rate constant (k ) is obtained as the
N
2), except those at the higher pH media, where the two
ratio between of kNobs to the corresponding ꢀm. The
values of k for the reactions of PAA with NPA, DNPA,
N
TNPA are given in Table 2 and those of NPC, DNPC and
TNPC in Table 3.
The nucleophilic rate constants in the reaction of PAA
with carbonates are smaller than those for the corre-
sponding acetates at the same pH values. This result can
be explained through the larger resonance effect exerted
by MeO in the carbonates relative to Me in the acetates,
rendering the former carbonyl carbon atom less positively
charged and, therefore, inhibiting amine attack. This is in
accord with what has been found for the reactions with
explain the k increase when ꢀ becomes 1. The change in
N
conformation of PAA is also supported by the TEM study.
This behavior is independent of the substrate because the
breaks of the above plots take place at approximately the
same pH value (ꢆ10.0) for the three substrates. A referee
suggested that at pH > 10.5 it is possible that a small but
important substrate-polyelectrolyte binding might be
present, this complex being responsible for the rate
increase when the polyamine is completely deprotonated.
Figures 2 and 3 also show that for the same pH values,
the kN values increase in the sequences TNPA >
DNPA > NPA and TNPC > DNPC > NPC. These se-
quences are the same as those for the reactions with
2
f,2g,3a
monomeric amines.
Figures 2 and 3 show the dependence of logk on pH
for the aminolysis of the acetates and carbonates, respec-
N
2
1
0
2f
monomeric amines, due to the increasing nucleofugality
of the leaving groups, and also the increasing electro-
philic character of the carbonyl carbon, as more nitro
groups are added to the substrate.
Figures 4 and 5 show Brønsted-type plots for the
reactions of PAA with acetates and methyl carbonates,
respectively, studied at the pH range 7.0–9.5, where the
polymer adopts a linear conformation. The plots are
linear with the following slopes (ꢁ): TNPA 0.5, DNPA
7
8
9
10
11
12
0
.4, NPA 0.5, TNPC 0.7, DNPC 0.6 and NPC 0.7. These
ꢁ values are similar to those found for the aminolysis
monomeric amines) of carbonyl compounds when the
mechanism is concerted, as in the reactions of secondary
pH
(
Figure 2. Variation of logkN as a function of pH for the
reactions of PAA with NPA (*), DNPA (4) and TNPA (&) in
2f
2f
4b
ꢀ
alicyclic (SA) amines with TNPA, TNPC, DNPC
aqueous solution at 25.0 C and an ionic strength of 0.1 M
4b
(
KCl)
and phenyl 2,4-dinitrophenyl carbonate (ꢁ ¼ 0.41, 0.36,
J. Phys. Org. Chem. 2006; 19: 129–135
Copyright # 2006 John Wiley & Sons, Ltd.

1
34
E. A. CASTRO ET AL.
However, when the amine group is in the backbone of
2
1
8
the polymer this difference is about 3–3.5 pK units. In
b
the case of the reaction of PAA with NPA, the curvature
of the Brønsted plot should be centered at pK > 10
app
22
(
pK of 4-nitrophenol ꢄ 7), which lies outside the pK
a
app
range used. Nevertheless, if the mechanism were step-
wise, the rate-determining step would be the expulsion of
2
the nucleofuge and the ꢁ value should be 0.8–1.1. The ꢁ
0
8
value obtained (0.5) is relatively low, suggesting that the
concerted mechanism governs the PAA aminolysis of
NPA. In the case of the reactions of PAA with DNPA and
.0
8.5
pKapp
9.0
TNPA the curvature should be located at pK ꢄ 7.0–7.5
app
22
(
2
pK of 2,4-dinitrophenol ꢄ 4) and 3.3–3.8 (pK of
a
a
22
,4,6-trinitrophenol ¼ 0.3), respectively. Both values
Figure 4. Brønsted-type plots for the reactions of PAA with
are smaller than the lowest pK of the experimental pK
a a
range, indicating that, at the pK range employed, the
NPA (*), DNPA (4) and TNPA (&) in aqueous solution at
a
ꢀ
2
5.0 C and an ionic strength of 0.1 M (KCl)
Brønsted slope would correspond to that where the first
2
step is rate-determining (ꢁ ). However the experimental
1
slope values found (ꢁ ¼ 0.4 and 0.5 for the reactions of
0
.48 and 0.39, respectively). The slopes are also in
DNPA and TNPA, respectively) are larger than those
2
reported for ꢁ (0.1–0.3), suggesting a concerted me-
accordance with those obtained for the reactions of SA
amines with S-(2,4-dinitrophenyl) and S-(2,4,6-trinitro-
phenyl) O-ethyl thiocarbonates (ꢁ ¼ 0.56 and 0.48, re-
1
chanism for the reactions of PAA with those substrates.
In the case of the reactions of the carbonate series, the
ꢁ values are in upper limit of those found for the
18
spectively), the reactions of quinuclidines with these
19
two substrates (ꢁ ¼ 0.54 and 0.47, respectively) and
aminolysis (monomeric amines) of carbonyl compounds
2d,4b,7c
20
those of anilines with the latter compound (ꢁ ¼ 0.54).
when the mechanism is concerted.
The same
These ꢁ value are also in agreement with those found in
the concerted reactions of SA amines and anilines with 4-
methylphenyl 2,4-dinitrophenyl carbonate and 4-chloro-
phenyl 2,4-dinitrophenyl carbonate in 44 wt% ethanol–
argumentation about the absence of curvature in the
reactions of PAA with acetates can be made concerning
the reactions of PAAwith aryl methyl carbonates in order
to show that the latter reactions are also concerted.
Furthermore, it is well known that for a stepwise mechan-
ism, the replacement of Me by MeO destabilizes the
7c
water (ꢁ ¼ 0.44–0.68). It is known that the linearity of
the Brønsted plot and the slope value are not sufficient to
21
18
prove that a mechanism is concerted. Also, it must be
ensured that there is no break in the Brønsted-type plot
for a hypothetical stepwise mechanism.
Stepwise aminolyses of aryl acetates (with monomeric
tetrahedral intermediate. This is due to an inductive
electron-withdrawing effect of the MeO group in the
putative tetrahedral intermediate, which enhances the
21
ꢃ
push exerted by O in the intermediate to expel either
ꢀ
18
the amine or the nucleofuge. Therefore, if the mechan-
amines) show values of pKa (pK at the curvature center
a
of the Brønsted-type plot) about 4–5 pK units greater than
2
the pK of the conjugate acid of the leaving group.
a
ism for the reactions of PAA with the acetates is con-
certed, then it should be also concerted for the reactions
with carbonates.
The dependence of the nucleophilic rate constant
kN), obtained for the reactions of PAA with the acetate
(
series, on the pK of the amine (pK ) and the pK of the
a
app
a
2
leaving group (pK ) is given by Eqn (3) (R ¼ 0.94,
lg
1
0
2
n ¼ 18) and that for carbonates by Eqn (4) (R ¼ 0.939,
n ¼ 18):
logk ¼ ð0:45 ꢁ 0:13ÞpK
ꢃ
N
app
ð3Þ
ð4Þ
ð0:29 ꢁ 0:02ÞpK ꢃ ð2:0 ꢁ 1:0Þ
lg
-
1
8
.0
8.5
9.0
9.5
logk ¼ ð0:71 ꢁ 0:13ÞpK
ꢃ
N
app
pKapp
ð0:28 ꢁ 0:02ÞpK ꢃ ð4:8 ꢁ 1:1Þ
lg
Figure 5. Brønsted-type plots for the reactions of PAA with
The values for the sensitivity to the nucleophile
(ꢁnuc ¼ 0.45 and 0.71 for acetates and carbonates,
J. Phys. Org. Chem. 2006; 19: 129–135
NPC (*), DNPC (4) and TNPC (&) in aqueous solution at
ꢀ
2
5.0 C and an ionic strength of 0.1 M (KCl)
Copyright # 2006 John Wiley & Sons, Ltd.

REACTIONS OF POLYALLYLAMINE WITH ARYL ACETATES AND METHYL CARBONATES
135
O
δ -
O
REFERENCES
δ – = – 0.29
δ+ = + 0.45
Me
C
Ar
δ+
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6
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are near the expected value for a concerted mechanism.
Scheme 2 shows the transition state for the acetate esters
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A, Concilio C. J. Org. Chem. 1996; 61: 1682–1688.
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of these substrates with PAA are concerted (this work).
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attributed to greater instability of the tetrahedral inter-
mediate formed with the polyamine or else to a transition
state for the concerted path that is more stable (relative to
reactants) than that for the stepwise reaction. In the same
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, 147–169.
4a
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2e
SA amines and NPC with SA amines and quinuclidi-
4b
nes are stepwise, in contrast to the reactions of these
substrates with PAA, which are concerted. These results
show the destabilization of the putative tetrahedral inter-
mediate formed with PAA.
1
1
1
It is known that SA amines and quinuclidines destabi-
lize the putative tetrahedral intermediate relative to
anilines and pyridines owing to a better leaving ability
of the two former amines from the intermediate. The
sequence of leaving abilities of monomeric amines
1
9
39; (b) Garc ´ı a del Vado MA, Echevarr ´ı a G, Basagoit ´ı a A, Santos
JG, Garc ´ı a-Blanco F. Int. J. Chem. Kinet. 1998; 30: 1–6.
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1
1
19
is quinuclidines > SA amines > anilines > pyridines.
Therefore, considering that the mechanism of the title
reactions is concerted, it is possible that the leaving
ability of PAA from the putative tetrahedral intermediate
is the greatest compared with monomeric amines. There-
fore, the stability of tetrahedral intermediates would
decrease in the sequence pyridines > anilines > SA
amines > quinuclidines > PAA.
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1
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6
Acknowledgements
1. Williams A. Free Energy Relationships in Organic and Bio-
Organic Chemistry. Royal Society of Chemistry: Cambridge,
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We thank MECESUP (Chile) (Projects PUC-0004 and
RED QUIMICA UCH-01), FONDECYT (Chile) (Project
1020538) and DGI (Spain) (Project BQU2003-07281) for
financial assistance.
2
2
3. Castro EA, Pavez P, Santos JG. J. Org. Chem. 2002; 67: 4494–
4497.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 129–135