Keto-enol/enolate equilibria in the 2-acetylcyclopentanone system. An unusual reaction mechanism in enol nitrosation

Article abstract of DOI:10.1039/b202981m

The keto-enol equilibrium of 2-acetylcyclopentanone (ACPE) was studied in water by analysing the effect that aqueous micellar solutions produce in the UV absorption spectrum of this compound. Aqueous solutions of anionic, cationic, or nonionic surfactants forming micelles have been used. The quantitative treatment of absorbance changes measured at the maximum absorption wavelength as a function of surfactant concentration gave the keto-enol equilibrium constant, K_E. In the same sense, the analysis of spectral changes measured as a function of pH in aqueous basic media allowed us to determine the acidity equilibrium constant, K_a. The combination of both equilibrium constants gives the acidity constant of the enol ionizing as an oxygen acid, pK_a^E = 7.72 and the acidity constant of the ketone ionizing as a carbon acid, pK_a^K = 8.12. The kinetic study of the nitrosation reaction of ACPE has been realized in aqueous strong acid media under several experimental conditions. As expected, the reaction is first-order with respect to ACPE concentration, but in sharp contrast to other β-dicarbonyl compounds, the dependence of both [H⁺] or [X^-] (X^- = Cl^-, Br^-, or SCN^-) is not simple first-order; instead a fractional order which varied from 1 to 0 was observed. The kinetic interpretation of these experimental facts has been done on the basis of a reaction mechanism that considers the formation of an intermediate in steady-state, which has been postulated as a chelate-nitrosyl complex. The quantitative treatment of the experimental data gave the values of every rate constant appearing in the proposed reaction mechanism.

Full text of DOI:10.1039/b202981m

View Article Online / Journal Homepage / Table of Contents for this issue
Keto–enol/enolate equilibria in the 2-acetylcyclopentanone system.
An unusual reaction mechanism in enol nitrosation
Emilia Iglesias
Dpto. de Qu ı´ mica F ı´ sica e E. Q. I. Facultad de Ciencias, Universidad de La Coru n˜ a,
1
5071-La Coru n˜ a, Spain. E-mail: qfemilia@udc.es
Received (in Montpellier, France) 20th March 2002, Accepted 16th May 2002
First published as an Advance Article on the web 6th September 2002
The keto–enol equilibrium of 2-acetylcyclopentanone (ACPE) was studied in water by analysing the effect that
aqueous micellar solutions produce in the UV absorption spectrum of this compound. Aqueous solutions of
anionic, cationic, or nonionic surfactants forming micelles have been used. The quantitative treatment of
absorbance changes measured at the maximum absorption wavelength as a function of surfactant
concentration gave the keto–enol equilibrium constant, K . In the same sense, the analysis of spectral changes
E
measured as a function of pH in aqueous basic media allowed us to determine the acidity equilibrium constant,
a
K . The combination of both equilibrium constants gives the acidity constant of the enol ionizing as an oxygen
acid, pK Ea ¼ 7.72 and the acidity constant of the ketone ionizing as a carbon acid, pKa ¼ 8.12. The kinetic
study of the nitrosation reaction of ACPE has been realized in aqueous strong acid media under several
experimental conditions. As expected, the reaction is first-order with respect to ACPE concentration, but in
K
+
ꢀ
ꢀ
ꢀ
ꢀ
sharp contrast to other b-dicarbonyl compounds, the dependence of both [H ] or [X ] (X ¼ Cl , Br , or
ꢀ
SCN ) is not simple first-order; instead a fractional order which varied from 1 to 0 was observed. The kinetic
interpretation of these experimental facts has been done on the basis of a reaction mechanism that considers the
formation of an intermediate in steady-state, which has been postulated as a chelate-nitrosyl complex. The
quantitative treatment of the experimental data gave the values of every rate constant appearing in the
proposed reaction mechanism.
form was slower than the ester hydrolysis; which is a kineti-
Introduction
cally indistinguishable situation. 2-Acetylcyclohexanone,
ACHE, is quite stable in either water or organic solvents.
Nevertheless, whereas e.g. ACHE exists entirely in the enol
form in dioxane solvent, a mixture of both the keto and the
enol tautomers is present in aqueous medium. Nevertheless,
a very surprising observation is the fact that the keto–enol
interconversion is slow in either direction. In alkaline medium,
the enolate is rapidly generated and quite stable, but the sub-
sequent acidification of the solution gives the enol in quantita-
tive proportions, which subsequently transforms into the keto
Despite the interest in determining the rate of the water enol-
reactions, the keto–enol interconversion has played a major
role in the development of correlations between rates and the
1
mechanisms of enol-reactions in solution. Information on
rates of enolization is often obtained from the study of bromi-
2
nation reaction of the enol, or of flash-photolysis in the case
,3
4
of monoketones.
Frequently, the keto and enol forms of many 1,3-dicarbonyl
compounds are both present in aqueous solutions in measur-
able proportions. Nevertheless, the enol tautomer is the predo-
minant form in apolar or non-hydrogen bond donor or
acceptor solvents. In the last few years, we have used aqueous
micellar solutions of various surfactants to determine the enol
content in water of several 1,3-dicarbonyl compounds, includ-
1
1
tautomer until the equilibrium proportions are reached.
Finally, 2-acetylcyclopentanone, ACPE, shows a similar beha-
viour to that observed with acyclic-1,3-diketones: in aprotic
solvents such as dioxane the enol tautomer is the majority
form; in aqueous acid medium, both tautomers exist in equili-
brium and their interconversion is fast.
5
–7
ing acyclic b-diketones or b-keto-esters.
We now have extended these investigations to the cyclic-1,3-
dicarbonyl compounds shown in Scheme 1. Ethyl cyclohexa-
none-2-carboxylate, ECHC, is stable in dry dioxane, but the
hydrolysis of the ester occurs at measurable rates in aqueous
In this work, we report the results obtained in the study of
the keto–enol/enolate equilibrium of ACPE under several
experimental conditions. Moreover, as nitrosation is an appro-
priate reaction to detect enols and enolates, kinetic studies of
the nitrosation of ACPE in aqueous acid medium have also
been performed. The kinetic results were interpreted on the
basis of an ‘unexpected’ reaction mechanism in enol-
nitrosation.
8
–10
solution.
In aqueous alkaline medium the enolate is rapidly
generated, but the ester also hydrolyses in this medium. It
could be possible that the conversion of the enol to the keto
Experimental
Materials
2-Acetylcyclopentanone (ACPE) an Aldrich product of the
maximum purity was used as supplied. Surfactants, sodium
Scheme 1 The structures of some cyclic-1,3-dicarbonyl compounds.
1
352
New J. Chem., 2002, 26, 1352–1359
DOI: 10.1039/b202981m
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

View Article Online
dodecyl sulfate (SDS), tetradecyltrimethylammonium chloride
TTACl), tetradecyltrimethylammonium bromide (TTABr)
and polyoxyethylene-9-dodecyl ether (C12 ), all from Sigma
(
E
9
or Aldrich of the highest purity, were used without further pur-
ification. All other reagents were supplied by Merck and used
as received. Solutions were prepared with doubly distilled
water obtained from a permanganate solution.
Methods
UV-vis absorption spectra and kinetic measurements were
recorded with a Kontron-Uvikon (model 942) double-beam
spectrophotometer provided with a thermostated cell holder.
The pH was measured with a Crison pH-meter equipped with
a GK2401B combined glass electrode and calibrated using
commercial buffers at pH 4.01, 7.01 and 9.26. All measure-
ꢁ
ments were performed at 25 C. Stock solutions of ACPE
(
grade). Aqueous solutions of ACPE (ꢂ4 ꢃ 10 M) were
obtained by diluting the appropriate volume of the dioxane
solution into the required volume of water.
Equilibrium constants were determined from spectroscopic
measurements. Scans of the spectra were monitored by work-
ing with a matched pair of quartz cells (sample + reference).
The latter contained all the reagents except for ACPE. Each
scan was registered after 15 min of thermostating.
ꢂ0.017 M) were prepared in dioxane (spectrophotometric
ꢀ
3
Fig. 1 Electronic absorption spectrum of ACPE in aqueous solutions
ꢁ
at 25 C under several experimental conditions: [1] alkaline medium
(
ꢀ
ꢀ5
[OH ] ¼ 0.16 M; [ACPE] ¼ 4.0 ꢃ 10 M); [2] acid medium of micel-
+
ꢀ4
lar solutions ([H ] ¼ 0.050 M; [SDS] ¼ 0.26 M; [ACPE] ¼ 2.8 ꢃ 10
+
ꢀ4
M); [3] acid medium ([H ] ¼ 0.050 M; [ACPE] ¼ 2.8 ꢃ 10 M), and
[4] basic medium of pH 6.78 (phosphate buffer 0.15 M;
ꢀ5
[ACPE] ¼ 4.0 ꢃ 10 M).
In aqueous basic media only a very strong absorption band
centered at 307 nm is observed (scans [1] and [4] in Fig. 1).
These experimental facts indicate that ACPE exists in aqueous
acid medium as a mixture of both the keto (lmax < 200 nm)
and enol (lmax ¼ 285 nm) tautomers, whereas the enolate
ion (lmax ¼ 307 nm) is the predominat species in basic med-
ium. The keto–enol/enolate transformations are rapid, in the
time-scale of these measurements. Scheme 2 reports the
involved equilibria.
Kinetic measurements were carried out under pseudo-first
order conditions, with a nitrite concentration at least 15-times
greater than that of ACPE. The nitrosation reaction was
observed by recording the decrease in absorbance at 285 nm
due to ACPE-enol consumption. The reaction was initiated
by injecting 30 mL of a stock solution of ACPE in dioxane
(
0.017 M) into a 1.0 cm quartz cuvette containing 3.0 mL of
acid solution of appropriate concentration, nitrite, and ionic
strength, which had been previously equilibrated in the instru-
ment cell holder for 15 min. In every case the integrated
method was followed, and the rate constants are derived from
the fits of the experimental data (absorbance–time, A–t) to the
Keto–enol equilibrium. The enol tautomer of a b-dicarbonyl
compound is stabilized by intramolecular hydrogen bonds;
therefore the enol concentration increases in aprotic solvents,
i.e. when intermolecular H-bonding with the solvent does not
compete. This means that the keto–enol equilibrium is very
sensitive to solvent nature. Aqueous micellar solutions are
microheterogeneous media, in which micellar aggregates pro-
vide a more aprotic solvent than does the bulk water phase.
An increase of the enol concentration is observed with the
increase of surfactant concentration, i.e. with the micellar
pseudophase, which is due to the stabilization of the enol form
in the micelles and to the fact that more hydrophobic sub-
strates, such as the enol, are well accepted by hydrophobic sol-
vents. The only tautomeric form uptaken by the micelle is the
enol form, since a clear isosbestic point (ꢂ232 nm) is observed
on varying the [surfactant]. We also assume that the enol is
standard exponential model given by eqn. (1), where A, A
and A are mean absorbance readings at times t, infinite, and
, respectively, and k represents the pseudo-first order rate
1
,
0
0
0
constant.
A ¼ A1 þ ðA0 ꢀ A1Þeꢀ
k0t
ð1Þ
Satisfactory correlation coefficients (r > 0.999) and residuals
were obtained in every case.
Results and discussion
1
.
Keto–enol/enolate equilibria
2
chemical characteristics similar to those observed for benzoy-
-Acetylcyclopentanone is a b-diketone that shows physico-
5
lacetone. ACPE is relatively soluble and very stable in
water. The UV absorption spectrum of ACPE in aqueous acid
solution shows strong absorption centered at 285 nm and a less
intense band at l < 200 nm (scan [3] in Fig. 1). In aqueous
acid micellar solutions of SDS (0.26 M) the intensity of the
absorption band at 285 nm increases strongly with respect to
that recorded in water, whereas the intensity of the band
observed at l < 200 nm decreases a little; and a 1–2 nm displa-
cement of the maximum absorption to higher wavelengths can
also be noted (scan [2] in Fig. 1). In dioxane solution, the
absorption maximum appears at 284 nm, showing higher
absorbance values than even in aqueous micellar solutions.
ꢀ
4
For example, at [ACPE] ¼ 2.8 ꢃ 10 M the absorbance read-
ings at l ¼ 285 nm were 0.472, 1.150, and 1.727, respectively,
for the aqueous, micellar (0.26 M of SDS), and dioxane
solution.
Scheme 2 Keto–enol/enolate equilibria of 2-acetylcyclopentanone in
water.
New J. Chem., 2002, 26, 1352–1359
1353

View Article Online
Scheme 3 The involved keto–enol equilibria of ACPE in aqueous
micellar medium.
located at the micellar interface since no detectable shift ( < 2
nm) in lmax is caused by the presence of micelles.
From the previous considerations, Scheme 3 might be pro-
posed, in which KH and EH represent, respectively, the keto
and enol tautomeric forms of ACPE, and Dn refers to the
micellized surfactant, i.e. [D
n
] ¼ [surfactant] ꢀ c.m.c. The
t
increase in absorbance at 285 nm of the aqueous micellar solu-
tions of ACPE with the increase in [surfactant] can be
described by eqn. (2), where l ( ¼ 1 cm) is the optical path-
length and eEH is the molar absorption coefficient of the enol.
A285 ¼ eEHlð½EHꢄ Þ þ ½EHꢄ Þ
ð2Þ
w
m
+
Taking into account that [ACPE] ¼ [KH] + [EH] + [EH] ,
Fig. 2 Absorbance readings of aqueous acid ([H ] ¼ 0.017 M, HCl)
t
w
w
m
ꢀ4
micellar solutions of ACPE (2.8 ꢃ 10 M) as a function of surfactant
and the expressions of both K
E
s
and K resulting from Scheme
concentration: (A) anionic surfactant of sodium dodecyl sulfate, SDS;
3
, eqn. (3) can be easily obtained to relate the absorbance read-
ings as a function of micellized surfactant concentration, [D ].
The solid lines in Fig. 2 correspond to the theoretical fit of eqn.
3) to the experimental points of A285 vs. [D ]. The optimized
parameters K and K /(1 + K ) were determined from this
(
B) cationic surfactant of tetradecyltrimethylammonium chloride,
TTACl, and nonionic surfactant of polyoxyethylene-9-dodecylether,
. The curves fit eqn. (3); for parameters, see Table 1.
n
12 9
C E
(
n
s
E
E
0
method by using A 285 (the absorbance of the solution in the
absence of surfactant) and c.m.c. as known parameters.
aqueous micellar solutions of SDS 0.26 M, and dioxane solu-
tions, was measured at 285 nm using a bandpass of 2 nm.The
measurements were taken in the presence of 0.050 M hydro-
chloric acid, except in the case of dioxane. The experimental
results are shown in Fig. 3. In every case, a perfectly linear cor-
relation between A285 and [ACPE] is observed, thereby indicat-
ing negligible association of the substrate molecules, i.e. the
absorption band at 285 nm obeys the Beer–Lambert law in
water, aqueous micellar solutions, or in dioxane solvent. It is
also noticeable that there is a higher enol content in dioxane
or in the presence of micelles than in water.
0
A
ð1 þ Ks½DnꢄÞ
2
85
A285 ¼
ꢀ
ꢁ
ð3Þ
KEKs
1
þ
½Dnꢄ
1
þ KE
The results obtained, presented in Table 1, indicate that ACPE
is near 30% enolized in water and the association constant of
the enol to micelles achieves low values; in other words, it is
a moderately hydrophobic species. Even though the hydropho-
bicity of TTAX (X ¼ Cl or Br) micelles (14 C-atoms in the
By remembering that A285 ¼ eEHl[EH]w (experiments in
hydrocarbon chain) is higher than that of SDS or C12E
9
water) or
A
285 ¼ eEHl([EH]
w
+ [EH]
m
)
(experiments in
micelles, the enol of ACPE binds stronger to anionic micelles
of SDS than cationic or nonionic ones. A possible contribution
of the sulfate head groups (H-bond acceptors only) in H-bond-
ing interactions with the enol could explain the results.
micelles), and that [ACPE] ¼ [KH] + [EH], eqns. (4) and (5)
t
may be deduced to express the absorbance variation as a func-
tion of ketone concentration in both aqueous or aqueous
micellar solutions, respectively.
eEH‘KE
1 þ KE
Beer–Lambert law behaviour. At ACPE concentrations in
the range (0.5 to 5) ꢃ 10 M, the absorbance of the aqueous,
A285
¼
½ACPEꢄt
ð4Þ
ꢀ
4
Table 1 Experimental conditions and optimized parameters obtained in the study of the influence of surfactant concentration on the UV-spectrum
of ACPE dissolved in aqueous HCl 0.050 M micellar solutions. Eqn. (3) fits the experimental data; Fig. 2 displays some of the results
0
A
ꢀ1
3
Surfactant
[ACPE] /M
t
285
c.m.c./M
K
s
/mol dm
K
E
/(1 + K
E
)
r
K
E
ꢀ
4
4
4
4
ꢀ3
SDS
2.8 ꢃ 10
3.1 ꢃ 10
2.8 ꢃ 10
2.8 ꢃ 10
0.548
0.600
0.463
0.473
4.0 ꢃ 10
3.7 ꢃ 10
3.0 ꢃ 10
3.5 ꢃ 10
23.7 ꢅ 0.7
12.8 ꢅ 0.2
15.6 ꢅ 0.5
13.0 ꢅ 0.9
0.299
0.252
0.283
0.260
0.999
0.999
0.999
0.999
5
7
4
5
0.43
0.34
0.39
0.35
ꢀ
ꢀ
ꢀ
ꢀ3
ꢀ3
ꢀ3
12 9
C E
TTACl
TTABr
Average K
E
¼ 0.38
1
354
New J. Chem., 2002, 26, 1352–1359

View Article Online
Fig. 3 Concentration dependence of the electronic absorption spectrum of ACPE (A) in aqueous acid solutions and (B) in aqueous micellar solu-
tions of SDS 0.26M; (C) Beer–Lambert plots at l ¼ 285 nm of ACPE dissolved in (˘) dioxane; (G) aqueous micellar solutions of SDS 0.26 M, and
in water (S) l ¼ 285 nm, (X) l ¼ 208 nm. See Table 2 for parameters.
eEH‘KE 1 þ Ks½SDSꢄ
a
Determination of the pK of ACPE. The absorption spectrum
m
A285¼
½ACPEꢄ
ð5Þ
t
of ACPE was analyzed as a function of pH. Phosphoric acid
(1)–hydrogen phosphate or hydrogen carbonate–carbonate
buffers of total buffer concentration 0.15 M were used to
obtain pH values in the range 6 to 11. The concentration of
1
þ KE
KsKE
1
þ 1
½SDSꢄm
þ KE
Least-squares fitting of the A285 vs. [ACPE] yielded the
t
ꢀ5
results of Table 2. The gradient of the straight line drawn with
the date measured in water has a value of S ¼ 1641 ꢅ 17
ACPE was kept constant and equal to 4.0 ꢃ 10 M. Typical
results, depicted in Fig. 4, show that the intensity of the
absorption band due to enolate, centered at 307 nm, increases
with pH. The dependence of absorbance measurements on pH
describes a sigmoid curve, typical of an acid–base titration (see
Scheme 2).
1
ꢀ
1
mol dm ; if the value of K
3
E
( ¼ 0.38) determined previously
is introduced in the expression of the gradient of A
[
vs.
285
ꢀ
1
ACPE]
t
given by eqn (4), one determines eEH ¼ 5956 mol
3
ꢀ1
dm cm . On the other hand, if we assume that ACPE exists
in dioxane entirely in the enol form, then the slope of the cor-
responding straight line equals the molar absorption coefficient
Enolate anions are ambident species, i.e. there are two sites
of protonation and the equilibrium enolization constant, K ,
E
linking the two tautomeric protonated forms, is a function of
the acidity constants of these two species. Thus, titration of
an equilibrium mixture of keto and enol tautomers yields a
mixed acid ionization constant, K , whose value corresponds
a
to the pH of the inflection point. This acid ionization constant
measured in water can be defined as in eqn. (6).
ꢀ
1
3
ꢀ1
of the enol, i.e. eEH ¼ 6015 ꢅ 106 mol dm cm . By intro-
ducing this value in the expression of S1 one gets
K
E
¼ 0.375, which agrees with the same value determined in
this study from the effect of surfactant concentration on
keto–enol equilibrium and listed in Table 1. These values
ꢀ1
3
5956 or 6015 mol dm cm ) determined for the molar
ꢀ1
(
absorption coefficient of the enolic form are lower than that
ꢀ
þ
K
E
K
E
1
2
3
ꢀ1
3
ꢀ1
½
E ꢄ ½H ꢄ
K K
K
a
Ka
K KE
a
often assumed (16 ꢀ 12) ꢃ 10 mol dm cm , but agrees
with previous work by us that shows that the extinction coeffi-
cient of the enol varies from one dicarbonyl compound to
t
a
a
Ka¼
¼ K
¼
¼
ð6Þ
E
½KHꢄ þ ½EHꢄ
þ Ka 1 þ KE 1 þ KE
6
The absorbance at 307 nm is given by: A307 ¼ eEHl[EH] +
another.
ꢀ
e
[
E
l[E ]. Hence, the mass balance equation: [ACPE]
EH] + [E ], along with the expressions of eqn. (6) allow us to
t
¼ [KH] +
In the same way, least-squares fitting of the A285ꢀ[ACPE]
t
ꢀ
measured in aqueous SDS micellar solutions of [SDS] ¼
ꢀ
3
obtain eqn. (7) which relates the absorbance measurements as
a function of pH. In this equation A and A refer to the absor-
bance readings at very low and very high pH, respectively.
0
.26 M (c.m.c. ¼ 4 ꢃ 10 M) to eqn. (5) gives a slope value
ꢀ
2 2 1
1
3
a
b
of S ¼ 3976 ꢅ 50 mol dm . From the ratio S /S one gets
ꢀ
1
3
K
s
¼ 20.3 mol dm if the K
E
value is assumed to be that
determined previously. This result compares quite well with
that determined at fixed [ACPE] and variable [SDS], a fact that
rules out the possibility of keto-tautomer association and dis-
approves the old assumption of a unique value for the molar
absorption coefficient of the enol of any dicarbonyl compound
at the maximum wavelength absorption.
pKa
pH
Aa10
þ Ab10
A307¼
ð7Þ
pKa
pH
1
0
þ 10
The solid sigmoid curve in Fig. 4(A) was constructed from
eqn. (7) with the experimental values of A
a
¼ 0.035 and
ꢀ1
3
ꢀ1
A
¼ 1.275 (which gives e
¼ 31900 mol dm cm ) and
b
E
the adjusted value of pK
a
¼ 8.25 ꢅ 0.02 (r ¼ 0.99
7
). >From
the latter value and by making used of KE determined pre-
Table 2 The slope values (S) of the plots of the absorbance readings
at 285 nm against [ACPE] obtained in different media
viously, the relationships of eqn. (6) give pK Ka ¼ 8.12 and
E
pKa ¼ 7.72, that is, the stronger acid (enol) of a pair of com-
ꢀ3
ꢀ1
3
Medium
[HCl]/mol dm
S/mol dm
r
K
E
pounds in equilibrium with a common anion (enolate) is the
minor component of the mixture.
Dioxane
Micellar
No acid
0.050
6015 ꢅ 106
3980 ꢅ 50
0.999
0.999
6
—
0.38
+
On the other hand, the experimental data of A307–[H ] may
7
1
1
be related through eqn. (8), where A and A correspond to
EH
E
(
Water
SDS 0.26 M)
the absorbance readings when all the [ACPE] is in the enol or
enolate forms, respectively. The solid line in Fig. 4(B) fits
0.050
1641 ꢅ 17
0.999
8
0.375
New J. Chem., 2002, 26, 1352–1359
1355

View Article Online
Table 3 Variation of the pseudo-first order rate constant, k
0
, as a
function of [nitrite] obtained in the kinetic study of the nitrosation
solutions 0.020 M at
of 2-acetylcyclopentanone in aqueous HClO
ionic strength 0.20 M, controlled with NaClO
4
ꢁ
4
, and 25 C
ꢀ3
ꢀ1
ꢀ3
k
0
/10
s
[nitrite]/10
M
1
2
3
4
4
6
8
1
1
1
.13
.22
.34
.50
.95
.15
.05
0.5
3.0
4.3
0.86
1.73
2.59
3.45
4.32
5.18
6.91
8.60
10.4
12.1
ꢀ
3
+
1
are listed in Table 4. It can be seen that k increases smoothly
0
M and [H ] ¼ 0.033 M (by using HClO
4
). The results
0
with the ionic strength, i.e. an effect which cannot be inter-
preted as a primary kinetic salt effect.
+
Influence of acidity. The influence of [H ] (controlled either
with HClO or HCl) was investigated at constant nitrite con-
4
centration and I ¼ 0.20 M. Fig. 5 shows the variation of the
+
overall rate constant as a function of [H ]. These ‘unusual’
+
versus [H ] profiles in ketone nitrosation reactions were
k
0
adapted to eqn. (10) with the values of b and d depicted in
Table 5.
þ
b½H ꢄ
k0¼
ð10Þ
þ
Fig. 4 (A) Spectrophotometric titration curve for the acid ionization
1 þ d½H ꢄ
ꢀ
5
of ACPE (4.0 ꢃ 10 M) in aqueous solutions of the buffers acetic
ꢀ2
ꢀ
One may note the higher d-value obtained from the experi-
ments performed in the presence of Cl than that determined
acid–acetate,
buffer]
¼ 0.15 M. Solid line fit eqn. (7). (B) Plot of absorbance read-
ing against [H ] according to eqn. (8).
HPO
4
–H
2
PO
4
,
and
carbonate–bicarbonate;
ꢀ
[
t
+
ꢀ
in the presence of ClO
4
.
E
ꢀ8
ꢀ
Influence of halide ion concentration. The influence of Cl ,
eqn. (8) to the experimental points when K ¼ (1.8 ꢅ 0.1)10
a
ꢀ
3
mol dm ; (1 + K
¼ 3.4 ꢅ 0.2; A1 ¼ 0.202 ꢅ 0.05, and
ꢀ
ꢀ
ꢀ
E
)/K
E
EH
Br , and SCN (in general X ) concentration on the nitrosa-
tion reaction of ACPE was investigated under several experi-
mental conditions of [nitrite], [H ], and ionic strength.
1
A
those previously reported.
¼ 1.30 ꢅ 0.03. All these results are in good agreement with
E
+
Representative results are plotted in Fig. 6. As can be seen,
the variation of the observed rate constant as a function of
1
þ
1
E
A
½H ꢄ þ A K
EH
E
a
A307¼
ð8Þ
1
þ KE
KE
ꢀ
[X ] can be summarized by eqn. (11) in every case, where k
0
w
E
K þ
a
þ
½H ꢄ
represents the observed rate constant measured in the absence
of X .
ꢀ
w
ꢀ
2
Nitrosation of the enol
k þ g½X ꢄ
0
k0¼
ð11Þ
ꢀ
1
þ Z½X ꢄ
The kinetic features of the nitrosation reaction of ACPE in
aqueous acid medium were investigated by monitoring the
decreasing absorbance at l ¼ 285 nm due to enol consump-
tion. The method of flooding was applied in obtaining the
experimental data of absorbance–time (A–t), which fit eqn.
The corresponding values of the unknown parameters g and
Z are collected in Table 6 along with the experimental condi-
(
1) perfectly. Thus, first-order reactions with respect to ACPE
Table 4 Variation of the pseudo-first order rate constant, k
function of ionic strength (I), controlled with NaClO , obtained in
the kinetic study of the nitrosation of 2-acetylcyclopentanone in aqu-
0
, as a
concentration, the limiting reagent, were observed under all
experimental conditions used.
4
ꢀ3
ꢁ
eous HClO
4
solutions 0.033 M at [nitrite] ¼ 1.6 ꢃ 10 M and 25 C
ꢀ
3
ꢀ1
Influence of [nitrite]. The influence of nitrite concentration
was analysed in aqueous HClO solutions 0.020 M, and
I/M
k
0
/10
s
4
ꢀ4
[
trolled with NaClO . The pseudo-first order rate constant,
ACPE] ¼ 1.7 ꢃ 10 M at ionic strength, I ¼ 0.20 M, con-
0.033
0.066
0.099
2.62
2.65
2.84
2.87
2.94
3.13
3.19
3.53
3.70
3.90
4
k
0
, increases proportional to [nitrite]. The results listed in
0
0
0
0
0
0
0
.17
.23
.30
.37
.47
.57
.67
Table 3 are consistent with eqn. (9) with a ¼ (1.20 ꢅ 0.02)
ꢀ1
3
ꢀ1
mol dm s (r ¼ 0.999
8
).
k0 ¼ a½nitriteꢄ
ð9Þ
Influence of ionic strength. The influence of ionic strength
controlled with NaClO
) was analyzed at [nitrite] ¼ 1.6 ꢃ
(
4
1
356
New J. Chem., 2002, 26, 1352–1359

View Article Online
ꢀ
0
Fig. 6 Influence of [X ] on the pseudo-first order rate constant, k ,
+
0
Fig. 5 Influence of [H ] on the pseudo-first order rate constant, k ,
for the nitrosation of ACPE performed in aqueous perchloric acid at
ꢁ
ꢁ
for the nitrosation of ACPE at ionic strength 0.20 M, 25 C and (A)
ionic strength 0.20 M (controlled with NaClO ), 25 C and (A)
4
ꢀ3
+
+
ꢀ3
[
with NaClO
nitrite] ¼ 1.6 ꢃ 10 M, [H ] controlled with HClO
4
and I controlled
X ¼ Br; [H ] ¼ 0.10 M, at (S) [nitrite] ¼ 1.6 ꢃ 10
M, (;)
ꢀ3
+
ꢀ3
ꢀ
ꢀ
4
; (B) [nitrite] ¼ 2.5 ꢃ 10 M, [H ] controlled with HCl
[
nitrite] ¼ 2.5 ꢃ 10
M; and (B)
X
¼ SCN
at [nitrite] ¼
ꢀ3
+
+
and I controlled with NaCl; the curves fit eqn. (10); for parameters,
see Table 5.
1.6 ꢃ 10 M and (S) [H ] ¼ 0.010 M, (G) [H ] ¼ 0.015 M. The
curves fit eqn. (11), for parameters, see Table 6.
tions. By observing the results obtained e.g. in the presence of
ꢀ
ꢀ
ꢀ2
6
ꢀ
ꢀ 14
X
Nitrosyl compounds act as nitrosating species and, despite
¼ Br , or 30 mol dm when X represents SCN .
ꢀ
+
Br , one may note that the Z-parameter depends on [H ], but
is not dependent on [nitrite], whereas the other two parameters
+
their lower reactivity with regard to that of NO , catalysis
by X is generally observed, due to the greater concentration
of the nitrosyl compounds resulting from the high value of
XNO in comparison to KNO .
For ketones with high enol content, like ACPE, the rate
+
vary with both [nitrite] and [H ]. On the other hand, it is also
ꢀ
ꢀ
remarkable that the Z-values vary with the nature of X :
ꢀ
ꢀ
ꢀ
values increase on going from Cl , Br , to SCN , i.e. Z-values
increase with the nucleophilic character of X .
K
ꢀ
determining step is, in general, the reaction between the enol
6
,10,15–17
and the nitrosating agent.
is first-order in [ketone], [H ], and [nitrite], as we have found
In such cases, the reaction
Reaction mechanism of nitrosation. In aqueous strong
mineral acid solutions, the effective nitrosating agents are those
+
6
in the nitrosation of e.g. benzoylacetone or ethyl cyclohexa-
none-2-carboxylate. Thereupon, eqn. (12) can be written to
account for the experimental facts, where k and k refer to
the nitrosation reaction paths by NO and XNO, respectively.
+
derived from protonation of nitrous acid: HNO
¼ 3 ꢃ 10
2
+ H =
dm being the
1
0
+
NO + H O, with K
ꢀ7
ꢀ1
3
mol
2
NO
1
3
corresponding equilibrium constant. In the presence of
non-basic nucleophiles (X ), such as Cl , Br , or SCN , equi-
librium formation of nitrosyl compounds, namely XNO, also
+ H =XNO + H
O, with KXNO being
the corresponding equilibrium constant, which at 25 C takes
1
2
+
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
þ
rate ¼ ðk1 þ k2½X ꢄÞ½ketoneꢄ½H ꢄ½nitriteꢄ
ð12Þ
ꢀ
+
occurs: X + HNO
2
2
ꢁ
ꢀ2
The results exposed in the current work for the case of
2-acetylcyclopentanone are not in agreement with eqn (12).
ꢀ3
ꢀ
ꢀ
on values of 1.14 ꢃ 10 when X ¼ Cl ; 5.1 ꢃ 10 when
0
Table 5 Experimental conditions and parameters obtained in the variation of the observed rate constant, k , for nitrosation of ACPE as a func-
+
tion of [nitrite], eqn. (9), or [H ], eqn. (10)
+
[H ]/M
ꢀ1
3
ꢀ1
ꢀ1
3
ꢀ1
ꢀ1
3
[
nitrite]/M
I/M
a/mol dm s
b/mol dm s
d/mol dm
Variable
0.020 (HClO
4
)
0.20 (NaClO
0.20 (NaClO
0.20 (NaCl)
4
)
)
1.20 ꢅ 0.02
—
0.101 ꢅ 0.003
0.79 ꢅ 0.03
—
3.1 ꢅ 0.3
16 ꢅ 1
ꢀ
3
3
1
2
.6 ꢃ 10
.5 ꢃ 10
variable (HClO
variable (HCl)
4
)
4
—
—
ꢀ
New J. Chem., 2002, 26, 1352–1359
1357

View Article Online
ꢀ
Table 6 Conditions and parameters obtained in the study of the influence of [X ] in the nitrosation of ACPE. Eqn. (11) fits the experimental data
of k versus [X ]; typical results are shown in Fig. 6
ꢀ
0
b
ꢀ
+
[H ]/M
a
w
ꢀ1
ꢀ1
3 ꢀ1
ꢀ1
3
X
[nitrite]/M
I/M
k /s
g/mol dm s
Z/mol dm
r
4 5
k /k
0
ꢀ
ꢀ
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
Cl
1.6 ꢃ 10
0.015
0.030
0.010
0.015
0.010
0.017
0.010
0.015
0.45
0.53
0.20
0.20
0.20
0.20
0.20
0.20
1.63 ꢃ 10
3.91 ꢃ 10
0.95 ꢃ 10
1.65 ꢃ 10
1.42 ꢃ 10
1.85 ꢃ 10
0.037 ꢅ 0.002
0.062 ꢅ 0.002
0.244 ꢅ 0.003
0.454 ꢅ 0.015
0.568 ꢅ 0.006
0.53 ꢅ 0.01
1.3 ꢅ 0.2
1.75 ꢅ 0.10
7.1 ꢅ 0.3
16 ꢅ 1
0.999
0.999
0.999
0.999
0.999
0.999
87.1
58.3
ꢀ3
Cl
Br
1.6 ꢃ 10
7
8
2
7
6
ꢀ
ꢀ
ꢀ3
1.6 ꢃ 10
855
ꢀ3
Br
Br
1.6 ꢃ 10
1053
705
ꢀ
ꢀ
ꢀ3
2.5 ꢃ 10
7.75 ꢅ 0.6
14.8 ꢅ 0.9
316 ꢅ 5
ꢀ3
Br
1.6 ꢃ 10
987
31600
30000
ꢀ
ꢀ
ꢀ3
ꢀ3
ꢀ3
SCN
SCN
1.6 ꢃ 10
1.1 ꢃ 10
1.5 ꢃ 10
9.98 ꢅ 0.10
0.9999
0.9999
5
ꢀ3
1.6 ꢃ 10
14.8 ꢅ 0.1
450 ꢅ 5
a
b
ꢀ1
3
In mol dm .
Controlled with NaClO
4
.
+
ꢀ
Recently, we have studied the nitrosation of 1,1,1-trifluoro-
3
K
the intermediate, eqn (13) can be easily derived.
XNO[nitrite][H ][X ], and by assuming a steady-state for
1
8
-(2-thenoyl)acetone in aqueous acid medium. Contrary to
the expected behaviour for a ‘usual’ nitrosation process invol-
ving enols, the rate equation determined for this trifluoroke-
tone was independent of [H ] and showed a dependence on
2
3
ꢀ
+
6kNOKNO þ kXNOKXNO½X ꢄ7 KE
þ
6
7
k0¼
½H ꢄ½nitriteꢄ ð13Þ
ꢀ
4
5
k3
k5
k4
1 þ KE
[
Br ] similar to that summarized by eqn (11) with the sole
w
þ
þ
ꢀ
1
þ
½H ꢄ þ ½H ꢄ½X ꢄ
k5
exception that k ꢂ 0. These experimental facts have been
0
interpreted by means of a reaction mechanism in which the
enolate is the species that undergoes nitrosation to give a che-
late-nitrosyl complex as an intermediate in steady-state, that is
formed in a reversible step from the attack of BrNO through
the enolate anion, and subsequently rearranges to give the
stable C-nitroso compound in the rate limiting step.
The comparison between eqn. (10) and (13) indicates that, in
the absence of X , the d-parameter equals the ratio of the rate
constants k
ꢀ
ꢀ
1
3
3
/k
5
, i.e. d ¼ 3.1 ꢅ 0.3 mol
dm ¼ k
3 5
/k (see
ꢀ
Table 5). Nevertheless, in the presence of [Cl ] ¼ 0.20 M,
ꢀ
ꢀ
d ¼ k /k + [Cl ]k /k ; then, from the observed value of
3
5
4
5
1
3
dm along with the value of
d ¼ 16ꢅ1 mol
3
k /
A rate equation of type (10) would also be derived if the
nitrosation occurs via the enolate anion. However, as the over-
all acidity constant of the keto–enol/enolate equilibrium is
ꢀ
1
3
k ( ¼ 3.1 ꢅ 0.3 mol dm ) determined in the study of the influ-
5
+
ence of [H ] in the absence of X , one gets k
ꢀ
ꢀ2
4
/k
5
¼ 64.5 mol
6
dm .
ꢀ
9
K
a
¼ 5.6 ꢃ 10 M (Scheme 2), the resulting expression of k
0
By assuming K
work, the b-values allow us to determine the reactivity of
ACPE enol toward the nitrosating agents NO or ClNO. In
E
¼ 0.38 determined in the first section of this
should be independent of acidity under the experimental con-
+
ditions of [H ] for data in Fig. 5.
+
An alternative explanation emerges if we consider that nitro-
sation occurs initially and reversibly at the oxygen atom of the
ꢀ
the absence of Cl , b ¼ kNOK K [nitrite]/(1+K ), then the
NO
E
E
experimental conditions and b-values reported in Table 5 give
–
OH enol (concurrently with the proton-loss) to form a che-
ꢀ
2
6
ꢀ1
kNOKNO ¼ 230 mol dm s . In the same way, the influence
late-nitrosyl complex intermediate, which then rearranges to
the stable C-nitroso compound. The corresponding reaction
mechanism is shown in Scheme 4.
+
of [nitrite] at constant [H ] ( ¼ 0.020 M) gave a ¼ 1.20 ꢅ 0.02
ꢀ1
3
ꢀ1
mol dm
s
. The comparison between eqn. (9) and (13)
K
ꢀ
+
[H ]/
shows that in the absence of X , a ¼ kNO
NO
K
E
A quite similar reaction scheme has been assumed to explain
the kinetic features observed in the nitrosation of aliphatic
nitro-compounds in aqueous acid medium, and several other
ꢀ
2
6
ꢀ1
(
good agreement with the value obtained in studying the influ-
1 + K ), which results in k
K
NO NO
¼ 218 mol dm s , in
E
1
9
2
0,21
ence of acidity. By assuming a mean value for k
6
K
¼ 224
ꢀ1
cases of NO rearrangements have been reported.
From Scheme 4, in which the enol form of ACPE is the
NO NO
ꢀ2
ꢀ1
ꢀ1
3
mol dm s , the value of b ( ¼ 0.79 ꢅ 0.03 mol dm s
)
reported in Table 5 along with the experimental conditions,
+
nitrosatable species either by NO or by XNO (in the presence
3
ꢀ3
9
ꢀ1
ꢀ
gives kClNO
K
¼ 4.6 ꢃ 10
mol
dm
s
(or
ClNO
6
of X ), by taking into account the mass balance equation,
¼ [KH] + [EH] which implies [EH] ¼ [ACPE]
ꢀ1
3
ꢀ1
kClNO ¼ 4.05 ꢃ 10 mol dm s for the reactivity of nitrosyl
[
(
ACPE]
1 + K ),
t
t E
K /
[XNO] ¼
+
+
chloride with the enol of ACPE).
The influence of [X ] was studied at low [H ], see Table 6;
E
that
[NO ] ¼ KNO[nitrite][H ];
ꢀ
+
+
thus the term (k
in other words, the denominator of eqn (13) simplifies to
3 5
/k )[H ] is negligible with respect to 1 (unity),
+
1 + [H ][X ]k
ꢀ
(
4
/k
5
), which by comparison with eqn (10) gives
4 5
/k . At this point one may understand the varia-
+
Z ¼ [H ]k
tion of Z with acidity reported in Table 6. For example, from
the two sets of experiments performed in the presence of
ꢀ
ꢀ2
6
Cl , it is possible to obtain k
4
/k
in good agreement with the same value of k
5
¼ 86.7 or 58.3 mol dm ,
4
/k
5
¼ 64.5
ꢀ2
6
mol dm determined in the study of the influence of acidity
ꢀ
at constant [Cl ] ¼ 0.20 M. The average value of the three pre-
ceding data is reported in Table 7. The same procedure has
4 5
been applied to determine the average values of k /k for the
ꢀ
ꢀ
case of Br and SCN which are also listed in Table 7. These
values increase with the nucleophilic character of X , in agree-
ꢀ
ment with the nature of the k -process, in the relative propor-
4
tion of 1:13:440. In this sense, the values of log(k
satisfactorily correlated with the Swain and Scott nucleophili-
4 5
/k ) are
Scheme 4 Postulated reaction mechanism of the nitrosation of
ACPE in aqueous acid media.
2
city parameter, n: log(k
2
4
/k
5
) ¼ ꢀ(2.9 ꢅ 0.5) + (1.5 ꢅ 0.1)n,
1
358
New J. Chem., 2002, 26, 1352–1359

View Article Online
ꢀ
ꢀ
ꢀ
Table 7 Kinetic rate constants obtained in the nitrosation of ACPE in
ꢁ
Cl < Br < SCN , i.e. in accordance with the Swain-Scott
nucleophilicity parameter.
aqueous acid medium at 25 C. See Scheme 4 for their meaning.
K
mol dm
XNO
/
k
XNO
mol dm s
/
4 5
k /k
ꢀ2
6
ꢀ1
3
ꢀ1
ꢀ2
6
X-NO
mol dm
Acknowledgements
+
ꢀ7 a
8
b
NO (X == H
O)
3.0 ꢃ 10
1.14 ꢃ 10
0.051
7.3 ꢃ 10
3.1
70
2
Financial support from the Direcci o´ n General de Investigaci o´ n
(
Ministerio de Ciencia y Tecnolog ´ı a) of Spain (Project
BQU2000-0239-C02-01) is gratefully acknowledged.
ꢀ3
6
ClNO
BrNO
SCNNO
4.2 ꢃ 10
6
1.2 ꢃ 10
900
31 000
4
30
6.5 ꢃ 10
a
ꢀ1
3 b
2 3 5
In mol dm . For the case of H O as nucleophile, i.e. k /k in
ꢀ
1
3
mol dm .
References
1
(a) J. Toullec, in The Chemistry of Enols, ed. Z. Rappoport, John
Wiley & Sons, Chichester, England, 1990, 323–398; (b) J. Toullec,
Adv. Phys. Org. Chem., 1982, 18, 1; (c) S. Fors e´ n and M. Nilson,
in The Chemistry of the Carbonyl Group, ed. J. Zabicky,
Interscience Publishers, London, 1970, 157–240.
(
r ¼ 0.99
7
). The positive and high gradient of the correlation
ꢀ
ꢀ
ꢀ
indicates that Cl , Br , or SCN are better nucleophiles than
water, and the nucleophilic character increases in the expected
order: Cl < Br < SCN . This finding supports the pro-
posed reaction mechanism, even though unexpected in enol
nitrosation reactions.
ꢀ
ꢀ
ꢀ
2
(a) A. Gero, J. Org. Chem., 1954, 19, 469; (b) A. Gero, J. Org.
Chem., 1961, 26, 3156; (c) J. P. Guthrie, J. Cossar and A. Klym,
J. Am. Chem. Soc., 1982, 104, 895; (d ) J. P. Guthrie, J. Cossar
and A. Klym, J. Am. Chem. Soc., 1984, 106, 1351.
On the other hand, the values of g reported in Table 6 allow
us to determine the reactivities of the nitrosating agents nitro-
syl chloride, nitrosyl bromide and nitrosyl thiocyanate. By
3
4
(a) R. Hochstrasser, A. J. Kresge, N. P. Schepp and J. Wirz,
J. Am. Chem. Soc., 1988, 110, 7875; (b) J.-E. Dubois, M. El-
Alaoui and J. Toullec, J. Am. Chem. Soc., 1981, 103, 5393.
(a) J. Andraos, Y. Chiang, A. J. Kresge, I. G. Pojarlieff, N. P.
Schepp and J. Wirz, J. Am. Chem. Soc., 1994, 116, 73; (b) Y.
Chiang, A. J. Kresge, Q. Meng, R. A. More O’Ferrall and Y.
Zhu, J. Am. Chem. Soc., 2001, 123, 11 562; (c) Y. Chiang,A, J.
Kresge, N. P. Schepp and R.-Q. Xie, J. Org. Chem., 2000, 65,
1175
comparing eqn. (11) and (13) one gets that g ¼
+
k
XNO
K
XNO[H ][nitrite]K
E
/(1 + K
E
). From this expression,
+
taking into account the [H ] and [nitrite] used in each case
Table 6), it is possible to calculate the second order rate con-
(
stants (kXNO) for the attack by ClNO, BrNO and SCNNO
reported in Table 7. The reactivity order found as
5
6
7
8
9
E. Iglesias, J. Phys. Chem., 1996, 100, 12 592.
+
NO > NOCl > NOBr > NOSCN is that expected, but the
E. Iglesias, J. Chem. Soc., Perkin Trans. 2, 1997, 431.
E. Iglesias, Langmuir, 2000, 16, 8438.
E. Iglesias, J. Org. Chem., 2000, 65, 6583.
values of kXNO are below those for the corresponding reactions
of aromatic amines (such as 1-naphthylamine) or the carba-
2
3
E. Iglesias, J. Phys. Chem. B, 2001, 105, 10 287.
2
4
nion of malononitrile, where both NOCl and NOBr react at
the diffusion limit. However, the enol of 2-acetylcyclopenta-
none proves to be the most reactive 1,3-dicarbonyl compound
10 E. Iglesias, J. Phys. Chem. B, 2001, 105, 10 295.
11 E. Iglesias, submitted for publication.
1
2
A. S. N. Murthy, A. Balasubramanian, C. N. R. Rao and T. R.
Kasturi, Can. J. Chem., 1962, 2267.
J. H. Ridd, Adv. Phys. Org. Chem., 1978, 16, 1.
(a) H. Schmid and E. Hallaba, Monatsh. Chem., 1956, 87, 560; (b)
H. Schmid and E. Fouad, Monatsh. Chem., 1957, 88, 631; (c) G.
Stedman and P. A. E. Whincup, J. Chem. Soc., 1963, 5796.
D. L. H. Williams, Nitrosation, Cambridge University Press,
Cambridge, 1988, ch. 2.
6
,18
in nitrosation among those studied.
1
1
3
4
Conclusions
15
16
17
In sharp contrast with the homologous substrate: 2-acetyl-
cyclohexanone, the keto–enol interconversion in ACPE system
is a rapid reaction in both directions. The keto–enol equili-
brium constant determined in water as KE ¼ 0.38, indicates
that the amount of the enol tautomer present at equilibrium
is higher than 25%. Nevertheless, the presence of micelles
enhances the total enol content by taking up this tautomer.
In aqueous basic media, the enolate is generated and the mea-
J. R. Leis, M. E. Pe n˜ a and D. L. H. Williams, J. Chem. Soc.,
Chem. Commun., 1987, 45.
(a) J. R. Leis, M. E. Pe n˜ a, D. L. H. Williams and S. D. Mawson,
J. Chem. Soc., Perkin Trans. 2, 1988, 157; (b) P. Roy and D. L. H.
Williams, J. Chem. Res. Synop., 1988, 122; (c) M. A. C. Reed and
D. L. H. Williams, J. Chem. Res. Synop., 1993, 342.
E. Iglesias, Langmuir, 2001, 17, 6871.
1
1
8
9
E. Iglesias and D. L. H. Williams, J. Chem. Soc., Perkin Trans. 2,
1
988, 1035.
sured acid ionization constant was pK
a
¼ 8.25.
2
0
A. Castro, E. Iglesias, J. R. Leis, M. E. Pe n˜ a, J. V a´ zquez-Tato and
D. L. H. Williams, J. Chem. Soc., Perkin Trans. 2, 1986, 1165.
The nitrosation of ACPE in aqueous strong acid media goes
through the enol tautomer to give an intermediate, that is pos-
tulated as the chelate-nitrosyl complex, which is in steady-state
21 A. Castro, E. Iglesias, J. R. Leis, J. V a´ zquez-Tato, F. Meijide and
M. E. Pe n˜ a, J. Chem. Soc., Perkin Trans. 2, 1987, 651.
+
2
2
3
T. H. Lowry and K. S. Richardson, Mechanism and Theory in
Organic Chemistry, 3rd edn., Harper & Row, Publishers, Inc.,
New York, 1987, p. 367.
whatever the nature of the nitrosating agent: NO , NOCl,
NOBr, or NOSCN. The reactivities in water of these reactants
with the enol of ACPE increase according to the expected
trend: NO > NOCl > NOBr > NOSCN. In the same sense,
2
J. Casado, A. Castro, E. Iglesias, M. E. Pe n˜ a and J. V a´ zquez-
Tato, Can. J. Chem., 1986, 64, 133.
24 E. Iglesias and D. L. H. Williams, J. Chem. Soc., Perkin Trans. 2,
+
the observed trend of the nucleophilic character, evaluated
from the ratio of the rate constants k /k , increases as
1989, 343.
4
5
New J. Chem., 2002, 26, 1352–1359
1359