Reactions of the simple nitroalkanes with hydroxide ion in water. Evidence for a complex mechanism

Article abstract of DOI:10.1039/c1ob05270e

Conventional kinetic analysis of the reactions of nitromethane (NM), nitroethane (NE) and 2-nitropropane (2-NP) with hydroxide ion in water revealed that the reactions are complex and involve kinetically significant intermediates. Kinetic experiments at the isosbestic points where changes in reactant and product absorbance cancel indicate the evolution and decay of absorbance characteristic of the formation of reactive intermediates. The deviations from 1st-order kinetics were observed to increase with increasing extent of reaction and in the reactant order: NM < NE < 2-NP. The apparent deuterium kinetic isotope effects for proton/deuteron transfer approach unity near zero time and increased with time toward plateau values as the reaction kinetics reach steady state. It is proposed that the initially formed preassociation complexes are transformed to more intimate reactant complexes which can give products by two possible pathways.

Full text of DOI:10.1039/c1ob05270e

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 4563
www.rsc.org/obc
PAPER
Reactions of the simple nitroalkanes with hydroxide ion in water. Evidence for
a complex mechanism†
Zhao Li,a,b Jin-Pei Cheng* and Vernon D. Parker*
b
a
Received 18th February 2011, Accepted 28th March 2011
DOI: 10.1039/c1ob05270e
Conventional kinetic analysis of the reactions of nitromethane (NM), nitroethane (NE) and
-nitropropane (2-NP) with hydroxide ion in water revealed that the reactions are complex and involve
2
kinetically significant intermediates. Kinetic experiments at the isosbestic points where changes in
reactant and product absorbance cancel indicate the evolution and decay of absorbance characteristic
of the formation of reactive intermediates. The deviations from 1st-order kinetics were observed to
increase with increasing extent of reaction and in the reactant order: NM < NE < 2-NP. The apparent
deuterium kinetic isotope effects for proton/deuteron transfer approach unity near zero time and
increased with time toward plateau values as the reaction kinetics reach steady state. It is proposed that
the initially formed preassociation complexes are transformed to more intimate reactant complexes
which can give products by two possible pathways.
Table 1 Equilibrium and rate data often cited in support of the
nitroalkane anomaly”
Introduction
“
1
The observation of the unusual inverse order of kinetic and ther-
modynamic acidities of the simple nitroalkanes (nitromethane =
NM, nitroethane = NE and 2-nitropropane = 2-NP) during their
reactions with hydroxide ion in water more than a half-century
Substrate
NM
NE
2-NP
a
a
b
pK
a
in water
Relative kapp
10.22
1.00
8.60
7.74
c
0.188
0.0114
2
ago was later referred to as the nitroalkane anomaly. Data for this
a
b
c
Ref. 1e. Ref. 1b,c. Ref. 3a corrected for symmetry factor (NM = 3, NE =
, 2-NP = 1).
novel relationship are reproduced in Table 1. This phenomenon
has perplexed physical organic chemists ever since its discovery
and a number of papers were published on the topic for reactions
2
3
4
5
in protic solvents, aprotic solvents. and mixed solvents.
carbon attached to the nitro group by the hydrogen bond. It was
The reactions of arylnitroalkanes in the presence of benzoic
acid buffers in DMSO were observed to be free of complications
from ion pairing and a 1-step proton transfer equilibrium was
8
later pointed out that hydrogen bonding to oxygen of the nitronate
product by the conjugate acid of the base is more likely than the
1
4
formation of an intermediate. Reference to a C kinetic isotope
4
6
proposed. A recent study led to the conclusion: “Calculations
also show that the ‘nitroalkane anomaly’, well established in
solution, does not exist in the gas phase.” It is important to
note at the outset that explanations of the nitroalkane anomaly
have generally involved a single transition state on the reaction
coordinate, i.e. the proton transfers have been assumed to take
place in a single step, sometimes followed by rapid hydrogen
9
effect study was cited as evidence against the proposed reaction
intermediate. We now report convincing experimental evidence
that the assumption that rate determining proton transfer is the
first step in the mechanism is not correct.
The present work is a part of our general effort to determine the
mechanisms of fundamental organic reactions. We have recently
10
shown that the reaction of 2,4,6-trinitroanisole (TNA) with
methoxide ion in methanol takes place by a reversible consecutive
mechanism (Scheme 1). The latter is contrary to the long-
accepted belief that formation of the 2,4,6-trinitro-1,3-dimethoxy-
cyclohexadienylide (I) and the 1,1-dimethoxy complex (II) take
place by a competitive rather than a sequential mechanism.
The tentatively proposed intermediate (III) was not observed
7
bonding or ion pairing. An exception to the latter was the
proposal of slow reaction of a hydrogen bonded substrate molecule
to an anionic intermediate in which the charge is localized on the
a
Department of Chemistry and Biochemistry, Utah State University, Logan,
UT, 84322, USA. E-mail: vernon.parker@usu.edu
b
Department of Chemistry, The State Key Laboratory of Elemento-Organic
1
11
but the H NMR spectrum had previously been recorded.
A
Chemistry, Nankai University, Tianjin, 300071, China
reversible consecutive mechanism was previously proposed for the
†
Electronic supplementary information (ESI) available: Details of
the kinetic data and profiles under various conditions. See DOI:
0.1039/c1ob05270e
proton transfer reaction between 1-nitro-1-(4¢-nitrophenyl)ethane
12
1
(NNPE) with hydroxide ion in aqueous acetonitrile.
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4563–4569 | 4563

View Article Online
according to Scheme 2 where the anionic products are represented
as two resonance forms showing the distribution of the negative
charge.
Scheme 2 Reactions of nitroalkanes with hydroxide ion in water.
The conventional first-order analyses of the reactions are
revealing in terms of whether or not the reactions conform to
3
first-order kinetics as is expected from the previous studies. The
latter is illustrated by the ln(1 - E.R.)–time profiles, where E.R. is
the extent of reaction, along with the corresponding least squares
correlation lines. The degrees of deviation of the experimental data
from the correlation lines were observed to be inversely related to
the kinetic reactivity of the nitroalkane, i.e. deviations are more
pronounced in the order, 2-NP > NE > NM. The latter was also
observed to increase with increasing E.R. and the data for 2-NP
began to deviate from that expected for first-order kinetics during
the first half-life of the reaction while deviations for the other two
substrates became significant at greater E.R. The relationships are
illustrated by the plots in the right column of Fig. 1 and in more
detail in the ESI.†
Scheme 1 Proposed mechanism for the reaction of TNA with methoxide
ion in methanol.
Results and discussion
Absorbance–time profiles for the evolution of products at 240 nm
over several half-lives for the reactions of NM, NE and 2-NP
with hydroxide ion in water at 298 K are illustrated by the plots
in the left column of Fig. 1. (Spectra of anions of nitroalkanes
13
in water have been reported. Reactant spectra and extinction
coefficients of NM, NE and 2-NP are shown in Fig. S4, electronic
supplementary information (ESI)).† It is evident that the reactions
go to completion and the absorbance values at the plateaus
indicate that the reaction products are stable during the time
In order to be able to describe the deviations of the nitroalkane
proton transfer reactions with hydroxide ion in more detail, the
least square correlations of ln(1 - E.R.)–time profiles were carried
14
1
out incrementally over the 2000 point profiles by two different
procedures. In the first, the analyses were carried out on 101 point
increments, beginning with data points 1 to 101, followed by data
points 101 to 201 and subsequent increments with the final one
being from data points 1701 to 1801. The data, summarized in
Table 2, clearly show that the apparent first-order rate constants
are E.R. dependent, decreasing continuously as the reactions
proceed toward completion.
periods of the experiments. H NMR spectra of reaction mixtures
after several half-lives show no significant signals indicative of side-
products. The acid–base reactions produce the conjugate bases
The data in Table 3 were obtained using a modification of the
pseudo first-order analysis procedure. The experimental procedure
Table 2 Variations of the apparent pseudo first-order rate constants for
the reactions of nitroalkanes with hydroxide ion in water over several half-
lives (5 half-lives for NM, 4 half-lives for NE, 3 half-lives for 2-NP)
a
k/s-1 (NM)b
k/s-1 (NE)b
k/s-1 (2-NP)c
Point range
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
–101
01–201
01–301
01–401
01–501
01–601
01–701
01–801
0.493
0.494
0.492
0.486
0.477
0.465
0.452
0.440
0.430
0.423
0.416
0.408
0.394
0.366
0.322
0.266
0.220
0.221
0.240
0.242
0.240
0.234
0.224
0.211
0.195
0.179
0.164
0.152
0.141
0.129
0.112
0.0885
0.0585
0.0289
0.0117
0.0215
0.01506
0.01447
0.01377
0.01299
0.01215
0.01131
0.01049
0.00970
0.00896
0.00824
0.00752
0.00677
0.00597
0.00512
0.00425
0.00344
0.00283
0.00261
01–901
01–1001
001–1101
101–1201
201–1301
301–1401
401–1501
501–1601
601–1701
701–1801
Fig. 1 Absorbance–time profiles (a, b, c) and ln(1 - E.R.)–time profiles
(
a¢,b¢,c¢) for the reactions of nitroalkanes (a,a¢ NM, b,b¢ NE, c,c¢ 2-NP)
a
b
c
Described in the text. [NaOH] = 20 mM and 50 mM in H
2
O at 298 K.
with hydroxide ion in water at 298 K.
4
564 | Org. Biomol. Chem., 2011, 9, 4563–4569
This journal is © The Royal Society of Chemistry 2011

View Article Online
Table 3 Apparent rate constants for the reactions of nitroalkanes with
value at low conversion and to increase with increasing E.R. This is
illustrated in Fig. 2 for the reaction of NE (0.1 mM) with hydroxide
ion (50 mM) and for NM and 2-NP in Fig. S1 and S3 (ESI†),
respectively. The KIEapp–time profiles show a similar trend (Figs.
S9 (NM), S10 (NE), and S11 (2-NP)†) but in this case the values
are near unity at zero time and approach a constant value at
longer times where the reactions reach a steady state.
hydroxide ion in water over the first half-life at 298 K
a
b
c
NM
NE
2-NP
1
app/s-1
k
app/s-1
k
app/s-
±
k
±
±
Segment
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.478
.560
.548
.545
.544
.538
.531
.527
.525
.523
.522
.521
.519
.519
.518
.517
.516
.515
.514
.514
.513
.512
.511
.511
0.056
0.010
0.002
0.002
0.001
0.003
0.004
0.004
0.004
0.004
0.004
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.006
0.006
0.006
0.006
0.280
0.276
0.273
0.271
0.269
0.265
0.261
0.259
0.258
0.257
0.256
0.256
0.255
0.255
0.254
0.254
0.253
0.253
0.252
0.252
0.251
0.251
0.251
0.250
0.010
0.005
0.004
0.002
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.003
0.003
0.003
0.0248
0.0223
0.0214
0.0210
0.0206
0.0200
0.0193
0.0187
0.0183
0.0179
0.0176
0.0173
0.0171
0.0170
0.0168
0.0167
0.0166
0.0165
0.0164
0.0163
0.0162
0.0162
0.0161
0.0161
0.0013
0.0007
0.0005
0.0008
0.0008
0.0008
0.0007
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
0.0006
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Fig. 2 Apparent KIE for the reactions of NE and NE-d as a function of
2
degree of conversion under the conditions shown.
The data discussed in the previous paragraphs clearly rule
out a simple 1-step mechanism for the proton transfer reactions
between the nitroalkanes and hydroxide ion. Some form of the
reversible consecutive mechanism (Mechanism 1 illustrated in
Scheme 3) provides a starting point to search for a mechanism
that is consistent with the experimental data. The integrated rate
a
[
NM] = 0.10 mM, [NaOH] = 20.0 mM, average of 3 sets of 12–20 stopped-
b
flow repetitions. [NE] = 0.10 mM, [NaOH] = 50.0 mM, average of 4 sets of
c
1
0–20 stopped-flow repetitions. [2-NP] = 0.10 mM, [NaOH] = 50.0 mM,
average of 3 sets of 15–20 stopped-flow repetitions.
15
law is available for this mechanism.
involved recording 2000 point absorbance–time (Abs–t) profiles
over about the first half-lives of the reactions and carrying out
a sequential first-order analysis on specific point segments of the
Abs–t profiles. The procedure also employed least squares linear
correlation but in this case 24 different analyses were carried out
over different point segments in the profile. The point segments
include points 1 to 11, 1 to 21, 1 to 31, 1 to 41, 1 to 51, 1 to 101, 1
to 201, 1 to 301, 1 to 401, 1 to 501, 1 to 601, 1 to 701, 1 to 801, 1 to
Scheme 3 The 2-step reversible consecutive mechanism applied to the
proton transfer reactions between the nitroalkanes and hydroxide ion in
water (Mechanism 1).
9
1
01, 1 to 1001, 1 to 1101, 1 to 1201, 1 to 1301, 1 to 1401, 1 to 1501,
to 1601, 1 to 1701, 1 to 1801 and 1 to 1901. If a reaction obeys
In the simplest form of Mechanism 1 (Scheme 3) the intermedi-
ate is a preassociation complex in which there are no new covalent
bonds formed. Under those circumstances, a steady state would be
expected to be reached early in the reaction. However, contrary to
the latter, the reactions of the simple nitroalkanes with hydroxide
ion in water reported here do not reach a steady state early in
the reactions as shown by the data in Table 2. For this reason
we believe that Mechanism 1 is unlikely for the proton transfer
reactions between the nitroalkanes and hydroxide ion reported
here.
A feature of the data in Tables 2 and 3 of great mechanistic
significance is that the time dependent kapp values for all three
substrates decrease from the values at short times toward steady-
state values at longer times. The only circumstance that gives
rise to this general behavior is when the absorbances monitored,
primarily due to the anionic products, also have contributions due
to intermediates. For purely product absorbance (Fig. 3a) the time
dependent rate constants (kinst) increase from zero to the steady
state value for mechanism 1 (Fig. 3b). The most significant point
with regard to the presence of an intermediate is that when the
apparent absorbance includes both that due to product and a
first-order kinetics the values of the rate constants for all of the
point segments are expected to be time independent and have the
same value. This procedure was described earlier but is repeated
here for clarity. A factor that should be taken into account of the
data in Table 3 in comparison to that in Fig. 1 is that the former
corresponds to 2000 points over the first half-life and the latter to
10
2
000 points over more than 4 half-lives. This difference means that
the data segments for Table 3 begin at shorter times than those in
Fig. 1.
What is immediately apparent in Table 3 is that in all 3 series
of reactions, involving NM, NE and 2-NP as substrates, is that
the kapp–time profiles begin at relatively high values of kapp and
decay with time toward steady state values. Although it might
be argued that the standard deviations of the short time data in
Tables S1 to S3 (ESI†) are relatively high, the mean values of three
determinations (Table 3) have a small degree of variance and are
certainly significant.
The apparent deuterium kinetic isotope effects (KIEapp) for all
three nitroalkanes were observed to be E.R. dependent, with a low
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4563–4569 | 4565

View Article Online
Fig. 4 Diode-array UV absorption spectra of the reaction between NE-d
20.37 mM) and hydroxide ion (100.0 mM) in water at 298 K: start time,
s; cycle time, 10 s; total run time, 300 s.
2
(
0
the isosbestic points. There is no true isosbestic point for the
absorbance–time curves illustrated in Fig. 4 due to the interference
of intermediate absorbance. How the latter can be determined in
the presence of absorbance from intermediates is shown in Fig. 6 in
the experimental section. The rationale for these experiments was
based upon the fact that reactant and product absorbance changes
with time are cancelled at the isosbestic point. The absorbance–
time data at the isosbestic points in Fig. 5 were obtained during the
reaction of (a) NE and (b) NE-d
2
with hydroxide ion in water at
2
98 K. Absorbance–time curves at isosbestic points are shown
in Figs. S5 to S8 (ESI†) for NE, NE-d
respectively.
2
, 2-NP and 2-NP-d ,
1
Fig. 3 Simulated concentration–time (a, c) and kinst–time (b, d) profiles
for the reversible consecutive mechanism with the rate constants shown.
k
f
f 0
¢ = k [Excess Reactant] .
significant contribution from an intermediate (Fig. 3c) the time
dependent rate constants decay from an initial value to a steady
state value (Fig. 3d). The very different form of the kinst–time
profiles for analysis of pure product absorbance (Fig. 3b) and
that in the presence of a significant concentration of intermediate
(
Fig. 3d) provides a very definitive mechanism probe.
Due to the fact that we do not have reliable methods to estimate
either the absorbance maximum wavelengths or the extinction
coefficients of the intermediates, it is pointless to attempt to fit
experimental data to simulated data such as shown in Fig. 3. The
simulation (Fig. 3) assumed that the intermediate and product
have equal extinction coefficients in the simulated experiment.
The presence of intermediates during the reactions was con-
firmed by carrying out the diode-array experiments (Fig. 4) near
Fig. 5 Abs–time profile at the isosbestic points for the reactions of (a)
NE, (b) NE-d with hydroxide ion in water at 298 K.
2
4
566 | Org. Biomol. Chem., 2011, 9, 4563–4569
This journal is © The Royal Society of Chemistry 2011

View Article Online
Simulations show that if the intermediate does not absorb
at the isosbestic point, a negative Abs–time profile is observed
due to decreases in absorbance of reactant and product ac-
companying the intermediate formation. This is also the case if
the intermediate absorbs with an extinction coefficient less than
that of the reactants and product. The fact that positive Abs–
time profiles were observed (Fig. 5, S5 to S8†) suggests that the
extinction coefficients of the intermediates are greater than that
of products and reactants at the isosbestic wavelengths in this
study.
It should be pointed out that the Abs–time profiles of the
intermediates in Fig. 5 (and Figs. S5 to S8†) were obtained at
substrate concentrations much greater than were used in the
kinetic experiments. The fact that absorbance due to reactants
and products cancel at the isosbestic points made the latter
determinations possible. The absorbance due to the intermediates
have a lesser effect on the Abs–time profiles under kinetic
conditions. The most important effect of intermediate absorbance
under kinetic conditions is that illustrated in Fig. 3a as compared
to that in Fig. 3c. For the case where only product absorbs (Fig. 3a),
there is an evident lag in the onset of product absorbance near
zero time while were both product and intermediate to absorb, the
absorbance rises sharply from zero time. These facts give rise to a
more dramatic effect in the corresponding kinst–time profiles. When
only product absorbs, kinst rises from 0 at zero time to a plateau
value when steady-state is achieved. When the intermediate also
absorbs at the wavelength where product absorbance is monitored,
The requirement of rate constants of similar magnitude is
demonstrated by the simulated kapp–time profile illustrated in
Fig. 3. With the three pertinent rate constants equal at a value of
-
1
0.02 s , nearly three half-lives are required to reach steady-state
(Fig. 3d). The inverse relationship between the thermodynamic
and kinetic acidity can be explained by either option shown in
◦
17
Mechanism 2. First, the E values become more negative in
the order NM > NE > 2-NP and the values of the equilibrium
constants for the formation of the reactant complexes are likely
to be in the same order. Taking these factors into account leads
to the prediction that the kinetic order for proton transfer to
hydroxide ion is NM > NE > 2-NP which is the reverse of the
thermodynamic acidity order of the substrates. The second option,
direct proton transfer within the reactant complex, is consistent
with the observed result simply from the fact that the relative
rates of reactions of substrates in a 3-step reaction cannot be
predicted from the thermodynamic stabilities of reactants and
products.
Some comment is necessary in order to explain how we view
the association and reactant complexes to differ. The interactions
-
between OH and the nitroalkanes in the former are expected to
be relatively weak and the two entities are expected to retain their
solvation shells to a large degree. For this reason, the HACET is
not expected to be favourable at this stage. We view the formation
of the reactant complex to involve extrusion of solvent from the
two entities and a change in geometry which allows the concurrent
transfer of an electron from hydroxide ion to the nitroalkane and a
hydrogen atom from the nitroalkane to hydroxide ion. The changes
taking place in going to the reactant complexes would also enhance
the favorability of the direct proton transfer.
k
inst decreases from a relatively large value near zero time to the
steady-state plateau value. The latter assumes that the extinction
coefficient of the intermediate is sufficiently large to cause an
increase in absorbance at the wavelength where product evolution
is monitored.
A question which arises from our work in relation to the
previous work on these reactions is: “Why was the complexity
in mechanism not observed in the earlier studies?” The work
Mechanisms that account for both our kinetic data and for the
inverse relationship between the kinetic and thermodynamic acidi-
ties of NM, NE and 2-NP are necessary. Mechanism 2 (Scheme 4)
involving either an intra-complex electron transfer coupled with
3a
of Bell and Goodall was presented in greater detail than other
kinetic work on this reaction series. The most important details
3a
presented which explain why the complexities that we have
observed went unnoticed are: (a) the reactions were carried
out for only about one half-life and (b) the rates of reaction
were followed by a pH-stat, a procedure less reliable than the
direct spectrophotometric monitoring of product formation used
here.
16
hydrogen atom transfer (HACET) or a polar reaction (kprod) as
shown in Scheme 4, accounts for the observations. In Mechanism
2
it is assumed that the association complex is in equilibrium
(
Kassoc) with reactants. Simulations of this mechanism showed that
to observe the “late” achievement of the steady-state, it requires
k
f
¢ (equal to kRC
Kassoc), k-RC and kHACET or kprod to be close in
magnitude. Our data do not allow us to differentiate between
the two possibilities for product forming reactions illustrated in
Scheme 4.
Experimental
Materials
3a
2
-NP-d
1
was prepared by using the literature procedure. The
, NE-d ) were obtained
other nitroalkanes (NM, NE, 2-NP, NM-d
3
2
commercially from Sigma-Aldrich and were subjected to an
1
additional purity check by H NMR spectrometry before use.
Distilled water was further purified by passing through a Barnsted
Nanopure Water System. Concentrated sodium hydroxide solu-
tion was commercially available from Fisher Scientific, which had
been determined by titration with standard oxalic acid solutions.
Kinetic experiments
Scheme 4 The 3-step reversible consecutive mechanism applied to the
reactions between the nitroalkanes and hydroxide ion in water (Mechanism
Kinetic experiments were carried out using a Hi-Tech SF-61
stopped-flow spectrometer installed in a glove box and kept under
2
).
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4563–4569 | 4567

View Article Online
◦
a nitrogen atmosphere. The temperature was controlled at 25 C
using a constant temperature flow system connected directly to the
reaction cell in the return pathway to a bath situated outside of
the glove box. All stopped flow experiments included recording
1
0–20 absorbance–time profiles at 240 nm. Each experiment
was repeated at least three times. The 2000 point absorbance–
time curve data were collected either over 1+ or 5+ half-lives
(
HL).
Absorbance–time (Abs–t) profiles for product evolution were
analyzed individually by two different procedures. The first step
in both procedures was to convert the Abs–t profiles to (1 -
E.R.)–time profiles. E.R. denotes extent of reaction, carried out
by dividing each absorbance value by the maximum absorbance
value obtained in the long-time (>10 HL) kinetic runs, when the
reaction goes to completion, such as the profiles shown in Fig. 1.
This procedure gave (1 - E.R.)–time profiles that decayed from
(
1 - E.R.) = 1 for either 1 or 5 HL depending on which analysis
procedure was used. For pseudo first-order kinetic analysis the (1 -
E.R.)–time profiles were converted to ln(1 - E.R.)–time profiles.
For further processing, the individual 5 HL (1 - E.R.)–time profiles
were first averaged to give the average profiles.
The first kinetic procedure used was simply least squares linear
correlation of the ln(1 - E.R.)–time profiles to give the apparent
pseudo first-order rate constants over either 1 or 5 HL. These
values are recorded in the Results section. The second kinetic
procedure, the modified pseudo first-order analysis, has been
described in detail in the former section. The second procedure
involved recording the Abs–time profiles over slightly more than
the 1st HL followed by the sequential 24 linear correlations
described in the Results section.
Fig. 6 DAbs–time profiles for the reaction of NE-d
2
(20.37 mM) with
hydroxide ion (50.0 mM) in water at 298 K near the isosbestic point,
67.5 nm (a: from 266.0 nm to 269.0 nm with an increment of 1.0 nm; b:
from 267.3 nm to 267.7 nm with an increment of 0.1 nm). DAbs = Abs
Abs
2
t
-
0
.
Conclusions
The proton transfer reactions of NM, NE and 2-NP with
hydroxide ion in water take place by a complex mechanism. This
conclusion is supported by conventional 1st-order analysis as well
as by a sequential 1st-order analysis designed to show changes
in the apparent pseudo-first-order rate constant if they exist. For
all three reactions the kapp were observed to be relatively large
at short times and decrease toward steady-state values later in
the reaction. Absorbance–time profiles at isosbestic points show
the presence of intermediates which absorb at these wavelengths.
The absorbances due to intermediates are most persistent for
Determination of isosbestic points
The diode-array kinetic experiments were carried out near the
apparent isosbestic points using an Agilent 8453 UV-vis. spec-
trophotometer. The start time and cycle time were set equal to
0
s and 1 s, respectively. The total run time was dependent upon
how long the reaction takes to reach completion. The resulting
Abs–t profiles, recorded at an interval of 1 s, were first converted
to DAbs–time profiles by subtracting the initial value from each
absorbance value.
It was pointed out earlier that true isosbestic points were
not observed due to the contributions of absorbance of the
NE-d
2
(compare the time scales in Figs. S5 and S6†) and 2-NP-
d
1
(compare the time scales in Fig. S7 and S8†) in which the
intermediates. This is very obvious for the reactions of NE-d
Fig. 4) and 2-NP-d (Fig. S8, ESI†) in which cases the lifetime
2
acidic protons are replaced by deuterons. The latter indicates that
the deuterium kinetic isotope effects are not associated with the
formation of the intermediates. This is also illustrated by the fact
that the KIE are E.R. dependent approaching unity near zero time
and increasing toward a plateau value as steady-state is achieved.
A reversible consecutive mechanism is proposed with two possible
product forming steps involving either a polar proton transfer
or an intra-complex H Atom Coupled Electron Transfer from a
(
1
of the intermediates appears to be the longest observed. The
latter is based upon the fact that the absorbance observed due
to the intermediate is substantially greater than all other cases
studied (see Fig. S5, S7, ESI†). The wavelength judged to be
most close to the true isosbestic point was selected based upon
the comparison of the DAbs–time profiles as shown in Fig. 6.
Although the absolute absorbance values are not known, due
to inherent instrumental error, the DAbs values are expected to
be reliable and have been verified by multiple determinations.
The DAbs–time profile judged to be most significant is that with
the long-time value approaching zero since the concentration of
intermediate is expected to approach zero as the reaction goes to
completion.
¯
“Reactant Complex”.
Acknowledgements
This work was supported by a grant (ARRA: CHE-0923654)
from the National Science Foundation. This support is gratefully
acknowledged.
4
568 | Org. Biomol. Chem., 2011, 9, 4563–4569
This journal is © The Royal Society of Chemistry 2011

View Article Online
References
Mishima, J. Am. Chem. Soc., 1999, 121, 10223; (c) C. F. Bernasconi and
P. J. Wenzel, J. Am. Chem. Soc., 2001, 123, 2430; (d) C. F. Bernasconi
and P. J. Wenzel, J. Org. Chem., 2003, 68, 6870; (e) J. R. Keeffe, S.
Gronert, M. E Colvin and N. L. Tran, J. Am. Chem. Soc., 2003, 125,
11730; (f) H. Yamataka and S. C. Ammal, ARKIVOC, 2003, 59; (g) M.
Sato, Y. Kitamura, N. Yoshimura and H. Yamataka, J. Org. Chem.,
2009, 74, 1268.
7 F. G. Bordwell and W. J. Boyle, Jr., J. Am. Chem. Soc., 1975, 97, 3447.
8 C. F. Bernasconi, D. A. V. Kliner, A. S. Mullin and J. X. Ni, J. Org.
Chem., 1988, 53, 3342.
1
(a) S. H. Maron and V. K. La Mer, J. Am. Chem. Soc., 1938, 60, 2588;
(
(
(
b) D. Turnbull and S. H. Maron, J. Am. Chem. Soc., 1943, 65, 212;
c) G. W. Wheland and J. Farr, J. Am. Chem. Soc., 1943, 65, 1433;
d) R. G. Pearson and R. L. Dillon, J. Am. Chem. Soc., 1953, 75, 2439.
2
3
A. J. Kresge, Can. J. Chem., 1975, 52, 1897.
(a) R. P. Bell and D. M. Goodall, Proc. R. Soc. London, Ser. A, 1966,
2
94, 273; (b) F. G. Bordwell, W. Boyle, Jr., J. A. Hautala and K. C. Yee,
J. Am. Chem. Soc., 1969, 91, 4002; (c) P. W. Fukyama, K. Flanagan,
F. T. Williams, Jr., L. Frainier, S. A. Miller and H. Shechter, J. Am.
Chem. Soc., 1970, 92, 4689; (d) A. J. Kresge, J. Am. Chem. Soc., 1970,
9 J. C. Wilson, I. Kallsson and W. H. Saunders Jr., J. Am. Chem. Soc.,
1980, 102, 4780.
9
9
2, 3210; (e) F. G. Bordwell and W. J. Boyle Jr., J. Am. Chem. Soc., 1971,
3, 512; (f) A. J. Kresge, H.-L. Chen, Y. Chiang, E. Murrill, M. A. Payne
10 V. D. Parker, Z. Li, K. L. Handoo, W. Hao and J.-P. Cheng, J. Org.
Chem., 2011, 76, 1250.
11 K. L. Servis, J. Am. Chem. Soc., 1965, 87, 5495; K. L. Servis, J. Am.
Chem. Soc., 1967, 89, 1508.
12 (a) Y. Zhao, Y. Lu and V. D. Parker, J. Am. Chem. Soc., 2001, 123, 1579;
(b) V. D. Parker, Pure Appl. Chem., 2005, 77, 1823.
13 F. T. Williams, Jr., P. W. K. Flanagan, W. J. Taylor and H. Shechter,
J. Org. Chem., 1965, 30, 2674.
14 V. D. Parker, J. Phys. Org. Chem., 2006, 19, 714. Reviewers sometimes
object to referring to ln(1 - ER) vs. time as the conventional
1st-order relationship. This is the general relationship which arises
from the integration of the 1-step mechanism where ER refers
and D. S. Sagatys, J. Am. Chem. Soc., 1971, 93, 413; (g) F. G. Bordwell
and W. J. Boyle Jr., J. Am. Chem. Soc., 1972, 94, 3907; (h) R. A. Marcus,
J. Am. Chem. Soc., 1969, 91, 7224; (i) N. Agmon, J. Am. Chem. Soc.,
1
980, 102, 2164; (j) W. J. Albery, C. F. Bernasconi and A. J. Kresge,
J. Phys. Org. Chem., 1988, 1, 29.
J. R. Keefe, J. Morey, C. A. Palmer and J. C. Lee, J. Am. Chem. Soc.,
4
5
1
979, 101, 1295.
(a) J. R. Gandler and C. F. Bernasconi, J. Am. Chem. Soc., 1992, 114,
31; (b) C. F. Bernasconi, D. Wiersema and M. W. Stronach, J. Org.
Chem., 1993, 58, 217; (c) C. F. Bernasconi and J.-X. Ni, J. Org. Chem.,
994, 59, 4910; (d) C. F. Bernasconi, M. Panda and M. W. Stronach,
6
1
to [product]/[reactant]
o
when [product] is monitored and to (1 -
J. Am. Chem. Soc., 1995, 117, 9206; (e) C. F. Bernasconi and R. L.
Monta n˜ ez, J. Org. Chem., 1997, 62, 8162; (f) C. F. Bernasconi and
K. W. Kittredge, J. Org. Chem., 1998, 63, 1994; (g) C. F. Bernasconi,
M. Ali and J. C. Gunter, J. Am. Chem. Soc., 2003, 125, 151; (h) C. F.
Bernasconi, M. P e´ rez-Lorenzo and S. D. Brown, J. Org. Chem., 2007,
[reactant]/[reactant] ) when [reactant] is monitored.
o
15 H. S. Johnston, Gas Phase Reaction Rate Theory, Ronald Press,
Berkeley, Appendix, p. 439. A pair of brackets was omitted in third
equation in (A-3) on p. 439.
16 It seems reasonable to suggest that electron tunneling will allow
thermodynamically unfavorable inter-complex electron transfer to take
7
2, 4416.
◦
6
(a) C. F. Bernasconi, P. J. Wenzel, J. R. Keeffe and S. Gronert, J. Am.
Chem. Soc., 1997, 119, 4008; (b) H. Yamataka, Mustanir and M.
place at rates greatly exceeding estimations based upon E differences.
17 D. H. Evans and A. G. Gilieinski, J. Phys. Chem., 1992, 96, 2528.
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4563–4569 | 4569