Kinetics and mechanism of the aminolysis of 4-nitrophenyl and 2,4-dinitrophenyl 4-methylphenyl carbonates in aqueous ethanol

Article abstract of DOI:10.1002/kin.10046

The reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol-water, at 25.0°C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo-first-order rate coefficients (k_obs) are obtained. Plots of k_obs against [amine] are linear, with k_N as slopes. A biphasic Broensted-type plot for k_N is obtained for the aminolysis of MPNPC, with slopes β₁ = 0.2 (high pK_a) and 3₂ = 0.9 (low pK_a). This is in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T^±), and a change in the rate-determining step, from formation to breakdown of T^± the amine pK_a decreases. For the aminolysis of MPDNPC, a slightly curved Broensted-type plot for k_N is obtained, with β₁ = 0.1 (low pK_a) β₂ = 0.55 (high pK_a). This is consistent with a concerted mechanism.

Full text of DOI:10.1002/kin.10046

Kinetics and Mechanism
of the Aminolysis of
4-Nitrophenyl and
2,4-Dinitrophenyl
4-Methylphenyl Carbonates
in Aqueous Ethanol
´
ENRIQUE A. CASTRO, MONICA ANDUJAR, PAOLA CAMPODONICO, JOSE G. SANTOS
´
´
Facultad de Quımica (502), Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22, Chile
Received 6 July 2001; accepted 29 November 2001
DOI 10.1002/kin.10046
ABSTRACT: The reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-me-
thylphenyl 2,4-dinitrophenyl carbonate (MPDNPC) with a series of secondary alicyclic amines
are subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0 C, ionic strength 0.2 M
(KCl). Under amine excess over the substrate, pseudo-first-order rate coefficients (kobs) are ob-
¨
tained. Plots of kobs against [amine] are linear, with kN as slopes. A biphasic Bronsted-type plot
for kN is obtained for the aminolysis of MPNPC, with slopes
(low pKa). This is in accordance with a stepwise mechanism, through a zwitterionic tetrahedral
= 0.2 (high pKa) and
= 0.9
1
2
intermediate (T ), and a change in the rate-determining step, from formation to breakdown of
¨
T
as the amine pKa decreases. For the aminolysis of MPDNPC, a slightly curved Bronsted-type
plot for kN is obtained, with
with a concerted mechanism.
2002
= 0.1 (low pKa) and
= 0.55 (high pKa). This is consistent
1
2
C
2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 309–315,
reactions have been found to proceed through a zwitte-
rionic tetrahedral addition intermediate (T , stepwise
INTRODUCTION
mechanism) [1,2,4] and other reactions take place with
no intermediate (concerted mechanism) [3].
Much attention has been drawn to the kinetics and
mechanism of the aminolysis of carboxylic acid deriva-
tives such as esters [1] and alkyl aryl carbonates [2,3].
Nonetheless, little is known on the kinetics of the
aminolysis of diaryl carbonates [4]. Some of the above
Gresser and Jencks [4] found a linear Bro¨nsted-type
plot with slope 1.0 for the reactions of quinuclidines
with phenyl 4-nitrophenyl carbonate (PNPC) in water.
This was attributed to a stepwise mechanism, with rate-
determining decomposition of T to products [4]. It
was also found in this work that the same aminolysis
of phenyl 2,4-dinitrophenyl carbonate (PDNPC) in the
same solvent exhibits a curved Bro¨nsted plot, which
Correspondence to: Enrique A. Castro; e-mail: ecastro@puc.cl.
Contract grant sponsor: FONDECYT.
Contract grant number: 2110081.
2002 Wiley Periodicals, Inc.
c

310
CASTRO ET AL.
was explained by a stepwise mechanism with a change
in the rate-limiting step as the amine basicity varies [4].
Recently, we have studied kinetically the reactions
of secondary alicyclic amines with 4-methylphenyl 4-
nitrophenyl thionocarbonate (MPNPTOC) in aqueous
MgSO4, filtered under vacuum, and the solvent evapo-
rated off.
The crystallized (diethyl ether) MPNPC melted at
136.2–137.1 C, andwasidentifiedasfollows:1HNMR
(200 MHz, CDCl3) ∂ 2.38 (s, 3H), 7.19 (m, 4H), 7.48
(d, 2H, J = 9.1 Hz), 8.30 (d, 2H, J = 9.1 Hz); 13C
NMR (50 MHz, CDCl3) ∂ 20.67 (CH3), 120.41 (C-
20/60), 121.76 (C-2/6), 125.37 (C-3/5), 130.22 (C-30/50),
136.53 (C-40), 145.58 (C-4), 148.60 (C-10), 151.23 (C-
1), 155.36 (C O). Anal. Calcd. for C14H11NO5: C,
61.54; H, 4.06; N, 5.13. Found: C, 61.72; H, 3.88; N,
5.11.
ethanol, where a linear Bro¨nsted plot of slope
0.25 was found, consistent with a stepwise mechanism
where the formation of T is rate-limiting [5].
=
In order to shed more light on the mechanism of
the aminolysis of diaryl carbonates and with the aim
to assess the effects of different groups on the kinet-
ics and mechanism, we investigate in this work the
reactions of 4-methylphenyl 4-nitrophenyl carbonate
(MPNPC) and 4-methylphenyl 2,4-dinitrophenyl car-
bonate (MPDNPC) with secondary alicyclic amines in
aqueous ethanol. We compare the kinetic results be-
tween the aminolyses of MPNPC and MPDNPC to
evaluate the effect of the leaving group. We also com-
pare the title reactions with the quinuclidinolysis of
PNPC and PDNPC in water to study the effects of
the amine nature and solvent. Lastly, we compare the
aminolysis of MPNPC with the same aminolysis of
MPNPTOC in the same solvent [5] to assess the effect
of the electrophilic group (CO vs. CS).
The crystallized (diethyl ether) MPDNPC melted
1
at 136.7–137.4 C, and was identified as follows: H
NMR (200 MHz, CDCl3) ∂ 2.38 (s, 3H), 7.19 (m,
4H), 7.66 (d, 1H, J = 8.9 Hz), 8.58 (dd, 1H, J1 = 8.9
Hz, J2 = 2.7 Hz), 9.04 (1H, J = 2.7 Hz); 13C NMR
(50 MHz, CDCl3) ∂ 20.91 (CH3), 120.36 (C-20/60),
122.13 (C-6), 126.28 (C-3), 129.52 (C-5), 130.31
(C-30/50), 136.92 (C-40), 141.20 (C-2), 145.47 (C-4),
148.36 (C-1), 148.63 (C-10), 150.33 (C O). Anal.
Calcd. for C14H10N2O7: C, 52.83; H, 3.17; N, 8.80.
Found: C, 52.74; H, 2.93; N, 8.68.
Kinetic Measurements
EXPERIMENTAL
Materials
These were carried out by means of a Hewlett
Packard 8453 diode array spectrophotometer in 44 wt%
ethanol–water, at 25.0 0.1 C, ionic strength 0.2 M
(maintained with KCl). The reactions of MPNPC were
followed at 400 nm (appearance of 4-nitrophenoxide
anion) except that with piperazinium ion, which was
followed at 330 nm (appearance of 4-nitrophenol).
On the other hand, the aminolysis of MPDNPC was
studied at 360 nm (following the appearance of 2,4-
dinitrophenoxide anion). All reactions were investi-
gated under excess of the amine over the substrate
(20-fold at least). The initial substrate concentration
The secondary alicyclic amines were purified as de-
scribed [6]. The products of the reactions, the 4-
methylphenyl carbamates of piperidine and morpho-
line, were synthesized as reported [7].
The substrates, MPNPC and MPDNPC, have not
been synthesized previously, according to our knowl-
edge. They were prepared as follows: To a solution of
4-nitrophenol (2.01 g, 14.5 mmol) or 2,4-dinitrophenol
(2.67 g, 14.5 mmol) in THF (10 ml) in a Schlenk
round-bottomed flask, a solution (9.1 ml, 14.5 mmol)
of 1.6 M butyllithium (Aldrich) was added slowly
under nitrogen atmosphere. The product, lithium
4-nitrophenoxide or lithium 2,4-dinitrophenoxide, was
rapidly transferred to a compensation funnel, under
nitrogen. In another Schlenk round-bottomed flask,
tolyl chloroformate (Aldrich, 2.47 g, 14.5 mmol) was
dissolved in anhydrous THF (10 ml) under nitrogen
and the flask placed in an ethanol–liquid nitrogen bath
(ca. 40 C). The compensation funnel was attached to
the flask and lithium 4-nitrophenoxide or lithium 2,4-
dinitrophenoxide solution added dropwise with stirring
for 2 h. The mixture was left overnight with stirring
under nitrogen at ambient temperature. Chloroform
(50 ml) was added to this mixture and the solution
washed with water. The organic layer was dried with
5
was 2.5 10 M. Pseudo-first-order rate coefficients
(kobs) were found for all reactions; these were de-
termined by means of the infinity method [plots of
ln(A
A) vs. time, where A and A are the ab-
∞
∞
sorbances at infinity and t times].
Three pH values were employed in the reactions
with each amine. These pH values were maintained by
the amine as its own buffer (pH near the pKa of its
conjugate acid), except in the reaction of MPNPC with
1-(2-hydroxyethyl)piperazinium cation (HPA), where
the pH was maintained by partial ionization of 1-(2-
hydroxyethyl)piperazinium dication (HPAH). The pKa
value of the conjugate acid of HPA is 5.6 under the
experimental conditions of the reactions [8].
The experimental conditions of the reactions and the
kobs values are shown in Tables I and II.

AMINOLYSIS OF 4-NITROPHENYL AND 2,4-DINITROPHENYL 4-METHYLPHENYL CARBONATES
311
Table I Experimental Conditions and kobs Values for the Aminolysis of MPNPCa
103 [N]tot (M)c
103 kobs (s
)
No. of Runs
b
1
Amine
pH
FN
Piperidine
10.52
10.82
11.12
9.41
9.71
10.01
8.79
9.09
9.39
8.18
8.48
8.78
7.33
7.63
7.93
5.07
5.37
5.67
0.333
0.50
0.667
0.333
0.50
0.667
0.333
0.50
0.667
0.333
0.50
0.667
0.333
0.50
0.667
0.333
0.50
0.3–2.7
0.3–3.0
0.3–3.0
1–10
1–10
1–10
0.6–6.0
0.6–6.0
0.6–6.0
10–100
10–100
10–100
10–100
10–100
10–100
10–90
3.7–44.0
7.1–71.0
15.0–100
9.9–56.0
8.1–84.0
11.0–120
5.2–54.0
10.0–80.0
11.0–110
8.5–65.0
10.0–95.0
13.0–120
0.74–6.8
9
9
9
Piperazine
10
10
10
10
10
10
9
10
10
10
10
10
9
1-(2-Hydroxyethyl)piperazine
Morpholine
1-Formilpiperazine
Piperazinium ion
0.97–9.1
1.40–13.0
0.022–0.13
0.023–0.25
0.033–0.37
10–100
10–100
10
10
0.667
aIn 44 wt% ethanol–water, at 25 C, ionic strength 0.2 M (KCl).
bFree amine fraction.
cConcentration of total amine (free base plus protonated forms).
Table II Experimental Conditions and kobs Values for the Aminolysis of MPDNPCa
103 [N]tot (M)c
103 kobs (s
)
No. of Runs
b
1
Amine
pH
FN
Piperidine
10.32
10.82
11.12
9.41
9.71
10.01
8.79
9.09
9.39
8.18
8.48
8.78
7.33
7.63
7.93
5.07
5.37
5.67
4.3d
0.24
0.50
0.667
0.33
0.50
0.667
0.333
0.50
0.667
0.333
0.50
0.667
0.333
0.50
0.667
0.333
0.50
0.85–3.41
0.85–3.41
0.427–2.99
0.824–2.88
0.843–2.95
0.411–2.47
0.416–2.91
0.416–2.91
0.416–2.50
0.20–2.40
0.20–2.40
0.20–2.00
0.20–2.40
0.80–2.40
0.40–2.40
0.428–3.0
0.402–2.81
0.429–3.16
8.76–30.7
8.92–31.2
9.01–31.5
14.3–67.0
42.3–149
31.4–152
25.0–96.6
32.6–131
21.5–169
4.59–35.1
9.69–54.7
13.0–59.8
3.49–33.4
3.21–44.8
6.26–46.0
1.48–8.99
5.95–12.7
3.65–16.2
0.272–1.65
0.417–2.25
0.304–2.58
0.390–1.19
0.462–1.41
0.575–1.62
7
7
7
6
6
7
7
7
6
7
5
6
7
5
6
7
6
7
6
6
6
Piperazine
1-(2-Hydroxyethyl)piperazine
Morpholine
1-Formilpiperazine
Piperazinium ion
0.667
0.022
0.029
0.034
1-(2-Hydroxyethyl)piperazinium ion
4.6d
4.9d
aIn 44 wt% ethanol–water, at 25 C, ionic strength 0.2 M (KCl).
bFree amine fraction.
cConcentration of total amine (free base plus protonated forms).
dReactive species is HPA (see text, pKa of conjugate acid 5.6 [8]). Buffer is due to partial ionization of the dication HPAH (see text).

312
CASTRO ET AL.
Table III Values of pKa for the Conjugate Acids of
Secondary Alicyclic Amines and kN Values for the
Reactions of These Amines with MPNPC and MPDNPCa
Product Studies
The 4-methylphenyl carbamates of piperidine and mor-
pholine were identified as one of the final products of
the reactions of both substrates with these two amines.
This was carried out by comparison of the UV–vis
spectra after completion of these reactions with those
of authentic samples of the above carbamates, under
the same experimental conditions. 4-Nitrophenoxide
anion or 2,4-dinitrophenoxide anion were identified
as the other product of the aminolysis of MPNPC or
MPDNPC, respectively. This was achieved by compar-
ison of the UV–vis spectra after completion of these
reactions with those of authentic samples of sodium 4-
nitrophenoxide or sodium 2,4-dinitrophenoxide under
the kinetic conditions.
1
kN (s 1 M
)
Amine
pKa
MPNPC
50
17 0.3
2.7 0.1
MPDNPC
Piperidine
Piperazine
1-(2-Hydroxyethyl)- 9.09
piperazine
10.82
9.71
1
84
103
36
3
4
1
Morpholine
8.48
1.8 0.1
34
1
1-Formylpiperazine 7.63
1-(2-Hydroxyethyl)- 5.60
piperazinium ion
0.19 0.01
9.8 0.4
1.41 0.04
Piperazinium ion
5.37 0.0057 0.0002 1.26 0.07
aBoth the pKa and kN values were determined in 44 wt% aqueous
ethanol, at 25.0 C, ionic strength 0.2 (KCl).
RESULTS AND DISCUSSION
Bro¨nsted break results from a change in the rate-
determining step, from breakdown of the zwitterionic
tetrahedral intermediate (T ) to products (k2 step),
to T formation (k1 step) as the amine basicity in-
creases [1b,2,4]. Also, a linear Bro¨nsted-type plot for
the aminolysis of MPNPC could be drawn, and this
would be also consistent with the mechanism depicted
by Scheme 1, where the k2 step would be rate-limiting.
The curved lines of the Bro¨nsted plots in Fig. 1
were calculated by means of a semiempirical equation
[Eq. (3)] based on the existence of the intermediate T
in Scheme 1 [1b,2b,2c,6,9,11]. Similar equations have
been reported, which satisfactorily account for step-
wise mechanisms [4,12].
The kinetic law obtained under the reaction conditions
is that described in Eq. (1), where P is 4-nitrophenoxide
anion (4-nitrophenol in the reaction of MPNPC with
piperazinium ion) or 2,4-dinitrophenoxide anion, S is
the substrate (MPNPC or MPDNPC), and kobs is the
pseudo-first-order rate coefficient (excess of amine was
used throughout).
d[P]
= kobs[S]
(1)
dt
Plots of kobs against [NH] at constant pH are linear
in accordance with Eq. (2), where NH represents a sec-
ondary alicyclic amine free base, and k0 and kN are the
rate coefficients for hydrolysis and aminolysis of the
substrates, respectively.
In Eq. (3), 1 and 2 are the Bro¨nsted slopes at high
o
o
and low pKa, respectively, and kN and pKa are the
corresponding values at the center of the curvature.
kobs = k0 + kN[NH]
(2)
o
o
log kN kN
=
pKa pKa
2
For the reactions of MPDNPC with PI and PA, the k0
valueswerenegligiblecomparedtotheaminolysisterm
in Eq. (2).
log[(1 + a)/2]
1) pKa pKa
(3)
o
log a = (
2
The second-order rate coefficients for aminolysis
(kN) were obtained as the slopes of plots of Eq. (2)
and were pH-independent. These values, together with
those of the pKa of the conjugate acids of the amines,
are shown in Table III. Figure 1 shows the Bro¨nsted-
type plots obtained for kN. The plots are statistically
correctedwith p = 2fortheaminiumions, except p = 4
for piperazinium dication, and q = 1 for the amines,
except piperazine with q = 2 [5–10]. As seen in Fig. 1,
the Bro¨nsted plots for the aminolysis of both substrates
are nonlinear.
Figure 1 Bro¨nsted-type plots (statistically corrected) for kN
obtained in the reactions of MPNPC ( ) and MPDNPC ( )
with secondary alicyclic amines in 44 wt% ethanol–water,
25.0 C, ionic strength 0.2 (KCl).
The nonlinear Bro¨nsted plot for MPNPC can be
explained by the mechanism described in Scheme 1,
where NH represents a secondary alicyclic amine. The

AMINOLYSIS OF 4-NITROPHENYL AND 2,4-DINITROPHENYL 4-METHYLPHENYL CARBONATES
313
quinuclidine compared to an isobasic secondary amine.
This fact shifts the pKa position of the Bro¨nsted break
o
toward the right, i.e., toward larger pKa values [15].
The rate constant for nucleofuge expulsion from T
o
(k2 in Scheme 1) also influences the pKa value [15].
The magnitude of k2 does not change significantly with
the amine nature since the amino moiety in T cannot
exert its push to expel the nucleofuge because it lacks
an electron pair [4].
On the other hand, the change of solvent from water
to ethanol–water should also mean a larger k value,
1
in view that the transition state for amine expulsion is
less polar than the intermediate T [4]. The value of k2
should be little affected by the solvent nature because
of the fact that both the transition state for leaving group
expulsion and the intermediate T are highly polar [4].
Therefore, the effect of the larger nucleofugalities of
quinuclidines than isobasic secondary amines and the
effect of the change of solvent from water to ethanol–
water should point in opposite directions.
Scheme 1
Addition of a methyl group to the “nonleaving” moi-
The Bro¨nsted curves in Fig. 1 were calculated with
ety of the substrate should not change the k nor the
1
o
o
the following parameters: log kN = 1.38, pKa = 10.5,
k2 value significantly since the inductive effects of phe-
noxy and 4-methylphenoxy are similar. It is known that
the inductive effects from groups attached to the cen-
tral carbon of a T intermediate are more important
than the resonance effects [16]. The Hammett induc-
tive substituent constant for 4-methylphenoxy is un-
known to us, but those for 4-methylphenyl and phenyl
= 0.2, and
= 0.9 (n = 7, R2 = 0.991) for MP-
1
2
o
o
NPC and log kN = 1.46, pKa = 8.9,
= 0.1, and
1
= 0.55 (n = 8, R2 = 0.991) for MPDNPC. The er-
2
o
rors of the slopes are 0.1, and those of pKa and log
o
kN are 0.2 and 0.1, respectively.
The values
= 0.2 and
= 0.9 found for the
1
2
aminolysis of MPNPC are in accord with the val-
ues reported for other reactions governed by stepwise
are = 0.12 for both [17]. Consequently, it is reason-
I
able to assume that the I values for 4-methylphenoxy
and phenoxy are also similar. Therefore, the push ex-
erted by these two groups in a tetrahedral intermediate
T to expel either the amine or the nucleofuge should
be similar [4].
mechanisms:
9,11,12].
= 0.1–0.3 and
= 0.8–1.1 [1b,2,4–
1
2
Nevertheless, the value
= 0.55 obtained for the
2
aminolysis of MPDNPC is not in agreement with
the value of the Bro¨nsted slope when breakdown of
the tetrahedral intermediate to products is the rate-
determining step. The slight curvature of the Bro¨nsted
plot for the reactions of MPDNPC is consistent with
a concerted mechanism [13], although a stepwise pro-
cess cannot rigorously be excluded.
o
In conclusion, the larger pKa value found for the
reactions of quinuclidines with PNPC in water [4]
compared to that for the reactions of secondary al-
icyclic amines with MPNPC in ethanol–water (this
work) should be due to the greater nucleofugality of
quinuclidines than isobasic secondary amines from T ,
which is only partially compensated by the change of
solvent.
If the mechanism for the reactions of MPDNPC with
secondary alicyclic amines were stepwise, a compari-
son of the kN values for these reactions with those of
MPNPC would show that the former is much more re-
active than the latter toward secondary amines when
breakdown of T to products is rate-determining (e.g.,
kN for MPDNPC is ca. 300-fold greater than MPNPC in
the low pKa region). This is reasonable in terms of the
stronger electron withdrawal of 2,4-dinitrophenoxy,
relative to 4-nitrophenoxy, from both the substrate and
T , resultingingreatervaluesofboth K1 (=k1/k 1)and
A linear Bro¨nsted-type plot of slope = 1.0 was
found by Gresser and Jencks in the reactions of PNPC
with quinuclidines in water [4]. These tertiary alicyclic
amines cover a pKa range very similar to that of the
secondary alicyclic amines used in this work. The rea-
son why the pKa value at the center of the Bro¨nsted
o
curvature (pKa ) is lower for the reactions of MPNPC
o
(pKa = 10.5, this work) than that for the reactions of
o
PNPC (pKa > 11.5 [4]) is the following.
It was found that quinuclidines are better leaving
groups from a zwitterionic tetrahedral intermediate
(T ) than isobasic secondary alicyclic amines [14].
This means a larger k (see Scheme 1) value for a
1

314
CASTRO ET AL.
The value of k2 for the intermediate 1 has been esti-
k2, respectively. Nevertheless, if formation of T were
rate limiting for the aminolysis of MPDNPC at the high
pKa region of the Bro¨nsted plot, their k1 values would
be much larger than those for the aminolysis of MP-
NPC. As seen in Fig. 1, the k1 values are only slightly
larger for MPDNPC, despite the different electrophilic
ability of the carbonyl groups of the two compounds.
This analysis suggests that the aminolysis of these two
substrates occur by different mechanisms, and gives
support to a concerted mechanism for the aminolysis
of the dinitro derivative.
mated as (0.9–1.5) 107 s 1 [5]. Substitution of S by
O
in 1 (to yield 2) should result in a larger k2 value,
in view of the stronger driving force of O , compared
to that of S , in a tetrahedral intermediate to form a
double bond and expel a nucleofuge [21].
The concerted aminolysis of MPDNPC in aqueous
ethanol (this work) seems to be in contrast to the step-
wise mechanism found for the reactions of PDNPC
with quinuclidines in water [4]. Assuming the effect of
Me addition to the nonleaving group on the stability of
the intermediate T is negligible, the above change in
mechanism should be due to the solvent change. In fact,
secondary alicyclic amines should stabilize T in view
of their lower nucleofugalities compared to isobasic
quinuclidines (see above). On the other hand, the less
polarsolvent, aqueousethanol, shoulddestabilizeT in
comparison to water, because of the ionic nature of this
intermediate [4,18,19]. Therefore, the above change in
mechanism could be due to a great destabilization of
T in aqueous ethanol, despite its stabilization by sec-
ondary alicyclic amines.
It has been reported that the reactions of secondary
alicyclic amines with methyl 2,4-dinitrophenyl carbon-
ate in water are driven by a stepwise mechanism [20].
Substitution of methoxy by 4-methylphenoxy as the
nonleaving group and the change of solvent, from water
to aqueous ethanol, should both destabilize the inter-
mediate T and therefore, it is reasonable a change to
a concerted mechanism for the reactions of MPDNPC
in aqueous ethanol.
In the reactions of secondary alicyclic amines with
4-methylphenyl 4-nitrophenyl thionocarbonate (MP-
NPTOC) in 44 wt% ethanol–water, the plots kobs vs
[NH] are nonlinear upwards, indicating a variable or-
der in amine (between 1 and 2) [5]. These plots were
explained through a reaction mechanism similar to that
described in Scheme 1, but with an additional path:
a proton transfer (k3 step, partially rate-determining)
from T to an amine (NH) to yield and an anionic
intermediate (T ), which decomposes rapidly to prod-
ucts [5].
In order to compare the k3 values for the aminoly-
sis of MPNPTOC and MPNPC, those for the pKa of
the tetrahedral intermediates 1 and 2 should be previ-
ously known. The pKa value of 1 has been estimated
as 5.4 pKa units lower than that of the corresponding
protonated amine [5]. This was achieved by employ-
ing Hammett inductive
values for the substituents
I
attached to the central carbon of 1, using Jencks’ pro-
cedure [16]. Employing = 0.03 and –0.26 for S
I
and O , respectively [17], and
= –9.2 [22], the
I
pKa difference between 1 and 2 can be determined.
This gives pKa = –9.2 (0.03 + 0.26) = –2.67. There-
fore, the pKa of 2 should be 5.4 2.7 = 2.7 pKa units
less than that of the corresponding aminium ion. This
means that the proton transfer from any of these zwitte-
rionic intermediates (1 or 2) to the free amine to yield
the corresponding anionic intermediate is thermody-
namically favorable and diffusion controlled [16,23].
Therefore, the k3 value for 1 and 2 should be ca.
1
1
1010
M
s
1 M in water [13a,23,24] and 2 109 s
1for piperazinium ion and 4 109 s
M
for the
1
1
other secondary alicyclic amines in 44 wt% ethanol–
water [5,25].
Therefore, the simple reaction mechanism found in
the aminolysis of MPNPC [Eq. (2) and Scheme 1], rela-
tive to the less simple one for the corresponding thiono-
carbonate, should be due to the fact that for the latter
reactions k2 k3[amine], whereas k2 > k3[amine] for
the reactions of the carbonate (being the range of amine
concentration about the same in the reactions of a given
amine with both substrates).
We thank FONDECYT (projects 1990561 and 2010081) for
financial assistance to this work.
Forthesereactionsitwasestimatedthatk3[NH] k2
[5]. The reason why the order in amine is unity for the
same aminolysis of MPNPC seems to be that (1) the
value of k2 is greater for the aminolysis of MPNPC
compared to the corresponding thionocarbonate, and
(2) the value of k3 is similar for the reactions of both
substrates, as explained below.
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