A further study of acetylacetone nitrosation

Article abstract of DOI:10.1039/c2ob26073e

The nitrosation of acetylacetone (AcAc) has been revised in an aqueous acid medium of perchloric acid and buffers of mono-, di-, or tri-chloroacetic acid. The results show that in the presence of buffers, under conditions of [nit] ? [AcAc] (nit = sodium nitrite) the reaction cannot be studied by UV-Vis spectroscopy, contrary to the recently published paper by Garcia-Rio et al. (J. Org. Chem., 2008, 73, 8198). The present study also corroborates the previously published mechanism of AcAc nitrosation, where no base-catalysis was observed. Contrarily, the low effect of buffers was attributed to the formation of nitrosyl chloro-, dichloro- or trichloro-acetate salts that are new nitrosating agents.

Full text of DOI:10.1039/c2ob26073e

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A further study of acetylacetone nitrosation†
Cite this: Org. Biomol. Chem., 2013, 11,
059
Emilia Iglesias* and Isabel Brandariz
1
The nitrosation of acetylacetone (AcAc) has been revised in an aqueous acid medium of perchloric acid
and buffers of mono-, di-, or tri-chloroacetic acid. The results show that in the presence of buffers, under
conditions of [nit] ≪ [AcAc] (nit = sodium nitrite) the reaction cannot be studied by UV-Vis spectroscopy,
contrary to the recently published paper by García-Rio et al. (J. Org. Chem., 2008, 73, 8198). The present
study also corroborates the previously published mechanism of AcAc nitrosation, where no base-catalysis
was observed. Contrarily, the low effect of buffers was attributed to the formation of nitrosyl chloro-,
dichloro- or trichloro-acetate salts that are new nitrosating agents.
Received 4th June 2012,
Accepted 25th July 2012
DOI: 10.1039/c2ob26073e
www.rsc.org/obc
Introduction
The reaction of ketones with a variety of nitrosating agents
including nitrous acid, alkyl nitrites, nitrosyl halides, or dini-
(
trogen trioxide) to give nitroso ketones or keto oximes –
depending on the structure of the substituent adjacent to the
carbonyl group – has already been discussed in a comprehen-
Scheme 1
1
sive account by Touster that covers the literature on the topic
up to 1953.
By analogy with other reactions of ketones with electro-
philes, such as halogenation, the obvious expectation is that
nitrosation occurs by electrophilic addition to the enol form of
the ketone. Singer and Vamplew published in 1957 the first
kinetic studies of the nitrosation of acetone (Ac) – using
nitrous acid – and they proposed a reaction mechanism
whereby the electrophilic nitrosation occurs at the carbonyl
oxygen atom followed by a rapid internal rearrangement of the
2
NO-group to the adjacent C-atom (Scheme 1). The rate con-
8
−1 −1
stant of acetone nitrosation is faster (k ∼ 10 M
s
) than
Scheme 2
ni
the enolization rate constant (k
e
∼ 10 M−1
−
5
s
−1
).
Later in 1977, the synthetic and analytic study of Rogic
et al. demonstrated that the overall mechanism of the nitrosa-
tion of ketones is an electrophilic addition of the nitrosating
agent to the double bond of the enol present in the equili-
brium. Depending on the experimental conditions and/or on
3
In 1987, the Williams’ research group carried out a detailed
4
–7
kinetic study of the nitrosation of ketones.
The first study
on the topic that includes the nitrosation of acetone (Ac),
ethyl-methyl-ketone (EMK), acetylacetone (AcAc), and 1,3-
dichloroacetone was performed under conditions of [ketone]
e
the ketone, the enolization step, k , or the nitrosation step, kni,
can determine the reaction rate (Scheme 2).
≫
[HNO ], with [HNO ] > 2 mM in order to be able to follow
2 2
the reaction by UV-vis spectroscopy at λ = 385 nm where
nitrous acid, the limiting reagent, absorbs. The nitrosation of
simple ketones, such as Ac or EMK, in the presence of nucleo-
−
−
philes (X ), e.g. Br at 0.5 M, shows zero-order kinetics, and
Department of Química Física e E. Q. I. Facultad de Ciencias, Universidad de La
Coruña, 15071-La Coruña, Spain. E-mail: emilia.iglesias@udc.es
Electronic supplementary information (ESI) available: Fig. S1 of the reaction
the zero-order rate constant increases proportionally to both
†
+
+
ketone concentration and H -concentration, i.e., rate = k [H ]
e
spectrum in conditions of excess nitrite over AcAc and Tables S1–S3, listing the
[
ketone]. These kinetic observations point to a reaction mech-
anism where the nitrosation takes place via the enol form that
experimental conditions and the observed rate constants obtained in the nitrosa-
3 2
tion of AcAc in the presence of the buffers Cl Ac, Cl Ac and ClAc, are provided.
See DOI: 10.1039/c2ob26073e
is generated in the rate limiting step of the reaction; in other
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words, it is the enolization reaction that is being studied. In
−
the absence of Br , or at low halide ion concentration, the
reaction rate depends on [HNO
now the attack of the nitrosating agent on the enol form, kni
By contrast, the nitrosation of acetylacetone (AcAc), also under
experimental conditions of [AcAc] ≫ [HNO ] (with [HNO ] =
.1 mM and the ionic strength 0.5 M controlled with NaClO ),
2
], i.e., the rate limiting step is
.
2
2
2
4
+
is first-order in nitrous acid as well as first-order in [AcAc], [H ]
and [nucleophile catalyst], with nucleophile catalyst Cl , Br ,
SCN , or SC(NH
−
−
−
−
2 2
) , generally denoted by X , see eqn (1). In
this equation, k is the third-order rate constant measured in
1
−
+
+
+
the absence of X (nitrosation by NO , HNO
2 2
NOH O )), whereas k represents the nitrosation rate constant
by XNO, HNO + H + X ⇄ XNO. The lower reactivity of the
2
+ H ⇄ NO
Scheme 3
+
(
+
−
2
simple first-order catalysis, instead a fractional order which
decreases from 1 to 0 on increasing [X ] (or [H ]) was observed.
These kinetic observations were interpreted on the basis of an
enol of AcAc than that of Ac or EMK, together with the high enol
−
+
content of AcAc (the keto–enol equilibrium constant, K
E
, being
8,9
of the order of 0.20 or 0.40), explains the different behaviour.
“
unexpected” reaction mechanism in enol nitrosation, in
ꢀ
þ
Rate ¼ ðk1 þ k2½X ꢁÞ½H ꢁ½AcAcꢁ½HNO2ꢁ
ð1Þ which the enol is the species that undergoes nitrosation to
give a chelate–nitrosyl complex (I) in steady-state, which is gen-
erated in the reversible step from the attack of the nitrosating
agent, XNO, and subsequently rearranges to give the stable
The nitrosation of AcAc was also studied in acetonitrile
using tert-butylnitrite (tBN) as the nitrosating agent under
experimental conditions of [AcAc] ≫ [tBN], with [tBN] ∼ 1 mM,
by noting the decreasing absorbance at ca. 380 nm, due to
nitrous acid or tBN consumption, or by noting the increasing
absorption at 220 nm, due to oxime formation.
Some years later, our group applied the use of aqueous
micellar solutions to study the keto–enol equilibrium of 1,3-
dicarbonyl compounds, such as AcAc.
measured KE = 0.40 was used in the analysis of the kinetic
results obtained in the nitrosation of AcAc under experimental
conditions of [HNO ] ≫ [AcAc] (with [AcAc] = 0.17 mM and
12
C-nitroso compound in the rate limiting step, Scheme 3.
Quite a similar reaction scheme has been proposed in the
13
nitrosation of 2-acetyl-cyclohexanone, 3-chloro-2,4-pentane-
dione or 3-ethyl-2,4-pentanedione.
1
0
14
15,16
Recently, García-Rio et al.
revised the nitrosation of
AcAc in an aqueous medium. The authors proposed two reac-
tion paths that involve, first, nitrosation through the C-atom
adjacent to the carbonyl – a step subject to nucleophilic cataly-
sis – and, second, nitrosation through the O-atom of the enol –
a step subject to acid–base catalysis. The experimental con-
ditions used in this work were [NaNO ] (= [nit]) = (4–5) × 10
9
,11
The value of the
2
[
nit] = 3.1 mM (nit means sodium nitrite)), uncontrolled ionic
−5
2
strength and following the nitrosation reaction by noting the
decreasing absorbance at 274 nm due to the AcAc enol.
Taking into account the slight difference in ionic strength
conditions used in the work of Williams’ group and our
group, along with the different values used for K , the concor-
dance between both sets of results is good, see Table 1.
We continue to extend the nitrosation kinetic studies to
other 1,3-dicarbonyl compounds. Hence, the nitrosation of
-acetylcyclopentanone (ACPE) performed in an aqueous
strong acid medium was, as expected, first-order with respect
to both [ACPE] and [nit], but, in sharp contrast to other enol-
−3
M; [AcAc] = (0.5–5) × 10 M, and different acidity conditions.
According to the authors, the reaction rates were measured by
monitoring the formation of the oxime product at 275 nm!
The aim of this work is to demonstrate that, firstly, it is not
possible to carry out the study of the nitrosation of AcAc by
UV-vis spectroscopy at 275 nm under the experimental con-
ditions of ref. 15 and, secondly, the reaction mechanism valid
for the nitrosation of AcAc is that previously published in ref. 5
and 9, where no acid–base catalysis was observed.
5
9
E
2
+
−
nitrosation reactions, the dependence of [H ] or [X ] is not a
Results
Table 1 Experimental conditions and values of k
1
determined in the present
Under the same experimental conditions used in the work of
work and compared with other published values
15
García-Rio et al., we recorded the reaction spectra for the
−
5
−3
nitrosation of AcAc; [nit] = 5 × 10 M; [AcAc] = 1 × 10 M, and
This
work
+
Ref. 9
Ref. 5
Ref. 15
[H ] = 0.20 M (HClO ). As can be seen in Fig. 1 the band cen-
4
tered at 273 nm, due to the enol of AcAc, does not change sig-
nificantly with time, because AcAc is in great excess over
nitrite, and, on the other hand, the absorption band due to
HNO (observed between 350–400 nm) cannot be detected due
2
to the low nitrite concentration. However, in aqueous perchlo-
ric acid, an absorption band appears at approximately 230 nm,
λ/nm
I/M
230
0.20
274
Variable
0.17 × 10
3.0 × 10
0.20
4.2
385
0.50
0.0487
2.1 × 10
0.138
5.9
275 !!!
1.0
Variable 5.3 × 10
−
3
5
−3
−4
[
[
[
AcAc]/M 1 × 10
−
−3
−3
−5
nit]/M
5 × 10
0.20
5.4
(4–5) × 10
+
H ]/M
0.13
8.45
Variable
10.4
−
2
k
1
/mol
6
−1
dm s
1
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Fig. 1 (a) Reaction spectra (scans at 3 min intervals) for the nitrosation of AcAc at [nit] = 5 × 10−5 M, [AcAc] = 1 × 10−3 M, and [H ] = 0.20 M (HClO
increasing absorption due to the oxime formation at 230 nm; -o-o- spectrum of AcAc in the absence of nit; (…) spectrum at infinite time, and (b) absorbance values
at 230 nm fit to the first-order integrated rate equation: A = A − (A − A )exp(−kobst) with A = 0.8433 ± 0.0007; A − A = 0.6310 ± 0.0009; kobs = (1.088 ±
+
) showing the
4
t
∞
∞
o
∞
∞
o
−3 −1
0
.004) × 10
s .
1
0
which has been used by Williams et al. in the study of the
nitrosation of AcAc by tBN in acetonitrile. Therefore, under
these experimental conditions, we measured the rate constant
of the reaction by noting the increasing absorption at 230 nm.
Fig. 1b shows the corresponding fit to the first-order integrated
rate equation.
For comparison purposes, we list in Table 1 the third-order
+
rate constant, k
1
, corresponding to rate = k
1
[H ][nit][AcAc],
determined in the present study in aqueous perchloric acid,
along with the published values and the corresponding exper-
1
imental conditions. The k values, determined either by
Williams or by us, are in good agreement, but are nearly half
the values reported in ref. 15 (see Table 1). We wonder if the
substrate studied in the work of García-Rio et al. is really AcAc.
Because the reaction of AcAc cannot be monitored by UV-vis at
2
75 nm under the experimental conditions of the work and
the k values differ very significantly either from those pub-
lished or from that carefully measured in the present study.
Fig. 2 Repeat scans each 3 min showing the decreasing absorption band due
1
+
to enol in the reaction of [AcAc] = 1.0 mM and [nit] = 2 mM at [H ] = 0.20 M
4
(HClO ); (o) absorption spectrum of AcAc 1.0 mM in aqueous perchloric acid
Fig. 2 shows the reaction spectrum under conditions of 0.20 M in the absence of nitrite.
similar concentration values of AcAc and nit. There is a signifi-
cant decrease of the absorption band centered at 273 nm due
to enol consumption as the reaction proceeds, which draws noting the decreasing absorbance at 273 nm was k
1
=
two well-defined isosbestic points, and also decreasing absor- 5.06 mol− dm
2
6
s . This value matches that measured in
−1
+
bance between 350–400 nm due to nitrous acid. Following the aqueous perchloric acid at the same [H ] of the buffer; this
reaction under these experimental conditions of second order fact indicates that in 0.41 M of total Cl
with [AcAc] ∼ [nit] at [H ] = 0.20 M, the third-order rate con- the effect of the buffer is negligible.
3
Ac buffer concentration
+
2
6
−1
stant is k
the published data in ref. 5 and 9.
Fig. 3 shows the reaction spectrum under the same exper- (ClAc) acids, the nitrosation of AcAc was firstly studied in
imental conditions of [AcAc] and [nit] of Fig. 2, but now in an aqueous perchloric acid at I = 0.30 M (NaClO ). Fig. 4 shows
aqueous buffer of trichloroacetic–trichloroacetate (Cl Ac) 0.41 the variation of k (= kobs/[nit]) as a function of [H ] under con-
M. Decreasing absorption bands due to either AcAc–enol or ditions of constant ionic strength, 0.30 M (NaClO ), and vari-
HNO
can also be seen. The same spectrum recorded at [nit] = able ionic strength. The effect of ionic strength is a slight
× 10− M does not show any variation, because of the strong increase of k
absorption of the buffer below 260 nm, approximately, and of 0.20 M, approximately, the k
1
= 4.1 mol− dm s , again in good agreement with
In order to further investigate the effect of the buffers tri-
chloroacetic, dichloroacetic (Cl Ac) and monochloroacetic
2
4
+
3
o
4
2
5
+
5
o
, nearly a 20% increase; however, at [H ] <
-values correlate almost linearly
o
+
+
the low absorption of nit in the 340–400 nm region. The third- with [H ] (dotted line), but if one takes the whole [H ] interval,
order rate constant measured under these conditions by a smooth curve can be drawn. The same figure also shows the
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is necessary to take into account that: [AcAc] = [KH] + [EH] (KH
=
keto form, EH = enol form); [nit] = [HNO ] (the pK of HNO is
2 a 2
17
3.13), and the equilibrium of nitrosating agent formation,
+
+
+
−7
−1
3 18
HNO
2
+ H ⇄ NO (NO OH
2
), KNO (= 3.5 × 10 mol dm ).
þ
kniKNOðKE=1 þ KEÞ½nitꢁ ½H ꢁ
kobs ¼
ð2Þ
þ
1
þ ðka=kcÞ ½H ꢁ
The non-linear regression analysis of the experimental data
according to eqn (2) affords the following values of k /k = 2.8 ±
at I = 0.30 M,
at uncontrolled ionic
strength, see the solid lines in Fig. 4. These values give kni
a
c
−
1
−1 −1
0
.2 M and kni
K
NO
K
E
/(1 + K
E
) = 5.6 ± 0.1 M
s
−
1
−1 −1
and 1.9 ± 0.1 M and 4.23 ± 0.07 M
s
7
−1 −1
7
−1 −1
results (kni = 5.6 × 10 M
s
and 4.2 × 10 M
s , respect-
4
,9
ively) in good agreement with those already published (kni
=
7
−1 −1
7
−1 −1
6
.0 × 10 M
s
and 4.4 × 10 M
s
E
by using K = 0.4).
From the data reported in the caption of Fig. 2 of ref. 15 we
Fig. 3 Repeat scans each 3 min showing the decreasing absorption band due
to enol in the reaction of [AcAc] = 1.0 mM and [nit] = 2 mM in a buffer of tri-
simulate the expected results of k (= kobs/[AcAc]) in order to
o
compare them with those measured in the present work.
These results are shown in Fig. 5(a), along with the values
obtained in the present study. As can be seen the difference is
clearly evident.
3
chloroacetic–trichloroacetate [Cl Ac] = 0.41 M.
In the same sense, Fig. 5(b) shows the result of the influ-
ence of the monochloroacetic acid–monochloroacetate buffer
at pH 2.69. As can be seen, k correlates linearly with [buffer]
o
at constant pH, but the effect of [buffer] observed in this study
is much lower than that published in ref. 15, even though both
studies afford rate constants of the same order of magnitude.
Discussion
The observed experimental facts in the present revision study
can be explained on the basis of the already published reaction
mechanism by noting some details. The nitrosation of AcAc
occurs by the NO-transfer from the nitrosating agent XNO to
the O-atom of the carbonyl group to form, in the first step, the
nitrosyl complex intermediate in steady state, which rearranges
Fig. 4 Variation of k
acid medium of (●) HClO
trolled ionic strength; (▲) HClO
strength 0.30 M (NaClO ); (o) Cl
acetate ratios and [AcAc] = 0.21 mM, [nit] = 6.7 mM; ionic strength 0.50 M, and
△) Cl
Ac and (∇) ClAc – see all points close to the 0–0 interception axis – at
o
(= kobs/[nit]) for the nitrosation of AcAc in an aqueous
to the final C-nitroso compound or to the oxime – when poss-
4
at [AcAc] = 0.27 mM; [nit] = 8.3 mM, and uncon-
+
at [AcAc] = 0.21 mM; [nit] = 6.7 mM, and ionic ible – in the rate limiting step, Scheme 3. In general, high [H ]
4
−
4
3
Ac 0.53 M at variable trichloroacetic/trichloro-
and/or high [X ] are required to achieve the steady-state
approximation, but the exact conditions depend on the nature
of the diketone. The small effect of the acetic acid–acetate
buffer and its chloroderivatives is due to the formation of
nitrosyl acetate, which increases the reaction rate as do ClNO,
BrNO or SCNNO.
(
2
3
ionic strength 0.35 M and equal AcAc and nit concentrations as for Cl Ac. Solid
lines fit to eqn (2).
ko values measured in buffers of Cl Ac (open circles), Cl Ac
Several key points have not been considered in the work of
3
2
(
0
open triangles), and ClAc (inverse open triangles close to the García-Rio et al. The first is concerned with the formation of
–0 interception axis). The influence of Cl Ac has been studied nitrosyl acetate (AcONO) and its chloroderivatives. Acetate is a
at I = 0.55 M; values of k determined at [Cl Ac] = 0.50 M and stronger base than e.g. trichloroacetate; therefore, the base
3
o
3
+
variable [H ] agree perfectly with those obtained in aqueous catalytic effect should be better observed in acetic buffer than
perchloric acid solutions at the same acidity; at fixed [H ] a in trichloroacetic buffer. By contrast, in the sense of forming
+
small increase of 1.35-fold is observed when total [Cl Ac] nitrosyl compounds, trichloroacetate resembles more, for
3
−
−
−
increases from 0.3 to 0.52 M.
instance, Cl , Br or SCN , than acetate does. In this regard,
1
9
The results in Fig. 4 can be easily explained on the basis of Kozlov et al. found a 30% yield of AcONO against 44% of
the reaction in Scheme 3. The observed rate constant derived ClAcONO or 40% of Cl CCONO in the reaction of the corre-
3
from this scheme is that of eqn (2). To arrive at this equation, it sponding acid with NaNO2.
1
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(= kobs/[AcAc] (●) determined in this work noting the increasing absorbance at 230 nm at [AcAc] = 1 mM and [nit] = 6 × 10− M in aqueous
5
Fig. 5 (a) Plot of k
perchloric acid of ionic strength 0.60 M, and (▲) values of k
ClAc, calculated from data in ref. 15 (●) and obtained in the present study (▲).
o
o
o
calculated from data in ref. 15; (b) plot of k against the total buffer concentration of monochloroacetic,
In the present work, we found that, at pH values close to reaction of silver acetate and nitrosyl chloride. On the other
the pK of the corresponding buffer, an increase of e.g. 1.7-fold hand, the preparation of acyl nitrites from an aqueous mixture
a
the total buffer concentration results in an increase of the of the corresponding carboxylic acid and sodium nitrite
observed rate constant, kobs, of 1.05, 1.20, and 1.35, respectively, yielded 44% of ClCH COONO and 40% of Cl CCOONO and
2
3
for chloroacetate, dichloroacetate, and trichloroacetate buffers, the spectral characterization of the system reveals that at high
see Table S1.† That is, the catalytic effect is opposite to the HNO concentration, the equilibrium AcONO + HNO ⇄ N
strength of the base component of the buffer, which means that + AcOH is established.
the catalytic effect cannot be attributed to the base catalysis. In From the point of view of kinetic studies, the work of
2
2
2 3
O
1
9
2
4
this respect, it is worth recalling that the buffering capacity of Casado et al. shows that the nitrosation of the amino group
Cl Ac, Cl Ac or ClAc is small and one has to be very careful in amino acids proceeds via the initial formation of nitrosyl
3
2
when working with these buffers; otherwise, the effect of pH carboxylate followed by an intramolecular NO migration from
could be wrongly interpreted as a buffer concentration effect. oxygen to nitrogen. This research group also found that
As the second point, assuming that the experiments of ref. AcONO is the only nitrosating species in the nitrosation of
1
5 were performed under [HNO ] ≪ [AcAc], and thus [HNO ] N-methyl-phenylamine, even though its concentration is very
2
2
≪
[buffer], according to the published results on azide nitrosa- low; by contrast, in the nitrosation of morpholine (a less basic
2
0
tion in acetate buffer, the first postulated step of the reaction amine) the nitrosation occurs solely by N O , but the catalysis
AcO + HNO2 + H ⇄ AcONO is strongly shifted to nitrosyl observed by the acetic acid–acetate buffer under conditions
acetate formation, and the second step of AcONO + NO
2
3
−
+
−
2
⇄
where the rate controlling step is not the nitrosation is due to
−
25
N O + AcO is negligible, especially in Cl Ac or Cl Ac. This is the increase in N O , in accordance with the work of Kozlov
2
3
2
3
2 3
1
9
20
the reason why in the nitrosation performed in acetic acid et al. and Stedman.
buffer under conditions of [HNO
2
] ≫ [substrate], only nitrosa-
Further evidence on behalf of the reaction mechanism
involving the nitrosyl complex formation, stated in Scheme 3,
tion by N O is observed.
2
3
There are several bibliographic references on the character- is the ability of 1,3-diketones to form a chelate complex with
3
+
2+
+
+
ization of nitrosyl acetate (or acetyl nitrite) and its chloroderi- metal ions, such as Fe , Cu , Tl , etc. (NO is also a
vatives. Kennedy et al. have developed a model that predicts, cation).
2
1
26,27
The formation of nitrosyl or metal ion complexes
first, that AcONO is more reactive than N O and, second, the increases the acidity of the proton of the enol several thousand
2
3
−
equilibrium constant of AcONO formation, i.e. AcO + HNO
2
+
times. In fact, when metal ions react with 1,3-diketones, the
, which means final product is invariably the metal–enolate complex and the
that at the same pH value, [AcONO] ≈ 20[NO ]. In a very recent majority of metal ions only react with either the enolate ion or
+
−6
−2
H ⇄ AcONO, was determined as 4.4 × 10
M
+
2
2
study the formation enthalpy at 298 K of acetyl nitrite has the enol tautomer.
been determined as ΔH ° = −58.2 kcal mol , using density
−1
f
functional B3LYP and CRS-QB3 computations; this study also
shows an intramolecular transfer reaction (isomerization) with
a low energy barrier of 3.6 kcal mol in the acetyl nitrite,
Conclusions
−
1
where the NO moves to the CvO oxygen in the motion of a The experimental conditions of acetylacetone nitrosation in
stretching frequency. In the same sense, a communication of water have been revised very carefully and it can be concluded
23
1
982
describes the preparation of acetyl nitrite in the that the reaction cannot be followed at 275 nm under
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conditions of [nit] ≪ [AcAc]. The kinetic results obtained in
the present study are in complete agreement with those pre-
viously published by Williams or by us, but disagree with the
results of García-Rio: the effect of buffers derived from acetic
acid cannot be attributed to base catalysis. Several literature
references on the formation of nitrosyl acetate – and its chloro-
derivatives – in aqueous solutions of sodium nitrite, on the
characterization of these species, and on their influence on
nitrosation reactions must be considered. Therefore, the
kinetic features of acetylacetone nitrosation are consistent
with the reaction mechanism proposed in Scheme 3, which
can be simplified depending on the experimental conditions,
and the small effect of buffers is due to the increase of nitro-
sating agent concentration because of the formation of nitrosyl
salts derived from acetate or its chloroderivatives.
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