Kinetic and equilibrium studies of anilinoalkanesulfonate formation

Article abstract of DOI:10.1039/b803070g

¹H NMR studies of the reactions of some hydroxyalkanesulfonates, 1, with aniline derivatives, 2, show the formation at equilibrium of anilinoalkanesulfonates, 3. Kinetic studies in water are consistent with a mechanism involving dissociation of

Full text of DOI:10.1039/b803070g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Kinetic and equilibrium studies of anilinoalkanesulfonate formation†
Michael R. Crampton,* Peter M. Lowry and Ian J. Smith
Received 21st February 2008, Accepted 28th March 2008
First published as an Advance Article on the web 6th May 2008
DOI: 10.1039/b803070g
1H NMR studies of the reactions of some hydroxyalkanesulfonates, 1, with aniline derivatives, 2, show
the formation at equilibrium of anilinoalkanesulfonates, 3. Kinetic studies in water are consistent with
a mechanism involving dissociation of 1 to give the parent aldehyde, which reacts with 2 to give a
carbinolamine. Acid-catalysed dehydration of the carbinolamine yields an iminium ion which reacts
rapidly with sulfite to give the product 3. Study of the variation with pH of the rate constants of the
reaction indicates a change in the nature of the rate-determining step from carbinolamine formation to
carbinolamine dehydration as the pH is increased. Variations in values of rate and equilibrium
constants with the nature of 1 and 2 are discussed in terms of electronic and steric effects.
Introduction
The formation of aminoalkanesulfonates either by reaction of
amines with hydroxyalkanesulfonates or by reaction of the ap-
propriate imine with bisulfite is well known.1–3 One mechanistic
proposal involved an SN2 reaction between the amine and proto-
nated hydroxyalkanesulfonate.4,5 However there is strong evidence
against this route,6 and the accepted mechanism involves reaction
with sulfite of the iminium ion formed from the amine and the
free aldehyde, present at low concentration in equilibrium with
the hydroxyalkanesulfonate.7
One reason for the current interest in these compounds is their
use in the synthesis of azo-dyes for ink-jet printing.8–10 A major
problem with their use is that the reactions forming them may
fail to go to completion or, once formed, the required products
may partially revert to reactants. Hence to use these reactions
efficiently, quantitative information regarding the kinetics and
equilibria of their formation is essential. There is a brief report
of the reaction of ammonia with hydroxymethanesulfonate11
and a fuller study of the formation of anilinomethanesulfonate
from aniline and hydroxymethanesulfonate.7 The latter concluded
that the rate-limiting step in the overall reaction is likely to be
either carbinolamine formation from amine and aldehyde, or
carbinolamine dehydration. However, since this study covered a
narrow acidity range, pH 6–8, definite conclusions could not be
drawn.
Here we report kinetic and equilibrium results for the
reactions in water of hydroxypropanesulfonate, 1a, with a
series of ring-substituted anilines, 2, to form the corre-
sponding anilinopropanesulfonates, 3a. Some data for reac-
tions of 2-methylhydroxypropanesulfonate, 1b, and 2,2-dimethyl-
hydroxypropanesulfonate, 1c, with aniline are included for com-
Scheme 1
parison. The results are interpreted7 in terms of Scheme 1. Values
of the overall equilibrium constant, K, defined in eqn (1) were
measured either using 1H NMR spectroscopy or by combination
of values of the rate constants kf and kr for the forward and reverse
reactions (Scheme 1).
[3]
kf
K =
=
(1)
[1][2] kr
Measurements here were made over a much larger acidity range,
pH 5–10, than previously.7 The results clearly show that the
rate-determining step changes from carbinolamine formation to
carbinolamine dehydration as the pH is increased. This work also
Chemistry Department, Durham University, Durham, DH1 3LE, UK
† Electronic supplementary information (ESI) available: Supplementary
Tables S1–S11. See DOI: 10.1039/b803070g
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Table 2 Chemical shifts of HX (the a-hydrogen) in 1a and in adducts 3a
extends from one to four the number of hydroxyalkanesulfonates
studied and examines in detail the effects of ring substituents in
the anilines.
formed with ring-substituted anilines
d/ppm
Dda/ppm
1a
4.15
0
Results and discussion
Ring substituent in 3a
H
4-Me
4-Cl
4.31
4.19
4.24
4.28
4.30
4.34
4.46
+0.16
+0.04
+0.09
+0.13
+0.15
+0.19
+0.31
1H NMR measurements
The spectrum of 1a, formed from equimolar propanal and sodium
bisulfite in deuterium oxide, shows a doublet of doublets at d
4.15 attributed to Hx, the a-hydrogen. Due to the chirality of the
adjacent carbon atom the b-hydrogens, HA and HB give distinct
resonances. Shifts and coupling constants are in Table 1. In the
presence of aniline, new bands attributed to anilinopropanesul-
fonate, 3a, are gradually formed, the change taking several hours.
Shifts are given in Table 1, where the HA, HB and HX correspond
to that for 1a. Changes in shift of the aromatic hydrogens of
the aniline ring, the methyl hydrogens and of hydrogens labelled
HB were small. However, on reaction, significant changes in the
values of shifts for HX and HA were observed, and integration after
equilibrium had been reached allowed the calculation of a value
for the equilibrium constant, K, of 70 10 dm3 mol−1. In a related
fashion, new bands observed in the spectra of 1b and 1c in the
presence of aniline are attributed to the formation of 3b and 3c.
NMR data and equilibrium constants are in Table 1.
3-Cl
3-CN
3-NO2
4-NO2
a Dd = shift in 3a − shift in 1a.
2, no bands were observed from adducts formed by displacement
of both the anilino hydrogens.
Kinetic and equilibrium measurements
Aniline in water shows a strong UV absorbance band at 230 nm
(e = 8 × 103 dm3 mol−1 cm−1). In the presence of hydrox-
yalkanesulfonates, 1, the absorbance maximum slowly moves to
longer wavelength, 240 nm, with increased intensity. Related shifts
in absorbance maximum were observed with ring-substituted
anilines in the presence of 1. Spectrophotometric measurements at
an appropriate wavelength, usually 250 nm, were used for kinetic
studies. With a concentration of 1 in a large excess over that of
aniline (which was 1 × 10−4 mol dm−3) a single first-order process
was observed, designated kobs. Measurements were made at 25 ◦C
in solutions whose pH was controlled with dilute buffers (0.02–
0.04 mol dm−3). As will be shown, the analysis is simplified by
working with slight excesses of free sulfite added either in the
form of sodium sulfite or sodium bisulfite depending on the
pH. The sum of these concentrations will be designated as [sulfite]
as indicated in eqn (2).
Changes of shift in the spectrum of N-methylaniline in the
presence of 1a are relatively small. However, integration of bands
due to the N-Me hydrogens, at d 2.65 in the parent and d 2.82 in
the product, in solutions containing various known excesses of 1a
allowed the calculation of a value of 1.1 0.1 dm3 mol−1 for K.
Changes in the spectrum of 1a in the presence of six ring-
substituted anilines are consistent with the formation of the
corresponding anilinopropanesulfonates. Values of shifts for HX
(the a-hydrogen) in 1a and in the products 3a are listed in
Table 2. It should be noted that in all cases spectra recorded
with stoichiometric concentrations of 1 and 2 in the range 0.02–
0.10 mol dm−3 gave no evidence for the presence of species other
than the reactants or products 3. The thermodynamic stabilities of
the hydroxyalkanesulfonates, 1, are high12,13 so that there was little
dissociation to give aldehyde and bisulfite. For 1 in a large excess of
[sulfite] = [SO32−] + [HSO3
]
(2)
−
Specimen results for reactions of 4-methylaniline and 3-
chloroaniline at pH 8.0 are in Tables 3 and 4, and data for other
ring-substituted anilines are provided as ESI in Tables S1–5†.
Examination of Table 3 shows that at a constant concentration
of 1a values of kobs depend inversely on the sulfite concentration.
Hence values of the product kobs[sulfite] are independent of the free
sulfite concentration. Since the formation of 3 from 1 and 2 is an
equilibrium process, kobs may be expressed in terms of the overall
Table 1 1H NMR dataaat 23 ◦C for hydroxyalkanesulfonates, 1, and corresponding anilinoalkanesulfonates, 3, in D2O
d/ppm
J/Hz
HX
HA
HB
Me
JAX
JBX
JAB
JAMe = JBMe
Ka/dm3 mol−1
1a
1b
1c
3a
3b
3c
4.15
4.09
4.03
4.31
4.35
4.25
1.84
2.06
—
2.02
2.30
—
1.53
—
—
1.5
—
—
0.90
0.91, 0.94
0.97
0.90
0.94, 0.96
1.03
3.2
5.2
—
3.2
4.4
—
10
—
—
10
—
—
15
—
—
15
—
—
7.5
6.8
—
7.5
7
—
—
—
70 10
62 10
34 10
—
a Values of K, defined in eqn (1), determined from NMR integrals for systems at equilibrium. Specimen data in Table 8.
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Table 3 Kinetic and equilibrium data for reaction of 1a with 4-methylanilineain water at 25 ◦C and pH 8.0
[1a]/10−3 mol dm−3
[sulfite]/10−3 mol dm−3
kobsb/10−3 s−1
k
obs[sulfite]/10−6 mol dm−3 s−1
DAbsc
Kd/dm3 mol−1
8.0
17
17
34
53
53
71
90
90
3.0
4.0
5.0
7.0
8.0
13
10
11
20
3.26
3.07
2.22
3.19
3.31
1.85
3.37
3.44
1.91
9.8
12.3
12.0
22.3
26.5
24.1
33.7
37.8
38.3
0.342
0.509
0.504
0.683
0.764
0.780
0.823
0.866
0.860
63
59
57
60
56
61
59
62
60
a Concentration is 1 × 10−4 mol dm−3
with DAbs∞ = 1.02.
.
b Measured at 255 nm. c Change in absorbance accompanying reaction. d Calculated as DAbs/(DAbs∞ − DAbs)[1a]
forward, kf, and reverse, kr, rate constants by eqn (3). Correspond-
ingly, plots of kobs[sulfite] versus [1] were linear, allowing the values
(at a given pH) of kf[sulfite] and kr[sulfite] to be determined from
the slopes and intercepts respectively. The results in Table 3 give
values for the reaction of 1a with 4-methylaniline at pH 8.0 of
kf[sulfite] 3.7 × 10−4 s−1 and kr[sulfite] 6.4 × 10−6 mol dm−3 s−1.
Their combination gives a value for K (= kf[sulfite]/kr[sulfite]) of
58 dm3 mol−1. The value of K calculated independently using the
absorbance changes accompanying reaction is 60 dm3 mol−1. Data
for the corresponding reaction of 3-chloroaniline are in Table 4.
Values obtained are kf[sulfite] 8.0 × 10−5 s−1 and kr[sulfite] 8.4 ×
10−7 mol dm−3 s−1, whose combination gives a value for K of
95 dm3 mol−1, in good agreement with the value of 97 dm3 mol−1
from absorbance measurements.
and kr[sulfite] were determined at several pH values in the range
pH 5–10. Representative results for the reaction of 1a with aniline
are given in Table 5 and other results are given as ESI in Tables
S6–11†. The results show that when pH ≥ 7, the rate coefficients
show little dependence on acidity, while when pH < 7 values show
moderate increases with increasing acidity. Nevertheless, values of
K determined either from kinetic or absorbance measurements
are independent of pH over the range examined. The latter
observation is expected since there will be no change in the
ionisation state of the reactants 1 and 2 or the product 3 in this
pH range.
Reverse reaction
For the reaction of propanal with aniline and sulfite an alternative
k
obs[sulfite] = kf[sulfite][1] + kr[sulfite]
(3)
method was used to evaluate the term kr[sulfite] relating to the
1
reverse reaction. Thus H NMR studies in CD3CN showed that
For reactions of 1a with aniline and five ring-substituted
anilines, and for reaction of 1b with aniline, values of kf[sulfite]
reaction of propanal (0.2 mol dm−3) with equimolar aniline gave
the imine 4 in high yield.
Table 4 Kinetic and equilibrium data for reaction of 1a with 3-chloroanilineain water at 25 ◦C and pH 8.0
[1a]/10−3 mol dm−3
[sulfite]/10−3 mol dm−3
kobsb/10−4 s−1
k
obs[sulfite]/10−6 mol dm−3 s−1
DAbsc
Kd/dm3 mol−1
8.0
17
34
53
71
3.0
4.0
7.0
8.0
10
4.65
5.09
5.49
6.17
6.69
7.23
1.40
2.24
3.62
4.95
6.56
7.95
0.60
0.87
1.02
1.12
1.18
1.21
100
106
91
92
97
90
11
96
a Concentration is 1 × 10−4 mol dm−3
with DAbs∞ = 1.35.
.
b Measured at 250 nm. c Change in absorbance accompanying reaction. d Calculated as DAbs/(DAbs∞ − DAbs)[1a]
Table 5 Variation with pH of values of rate and equilibrium constants for reaction of 1a with aniline
pH
5.2
5.9
7.0
7.6
8.0
9.1
10.0
kf[sulfite]/10−4 s−1
kr[sulfite]/10−6 mol dm−3 s−1
Ka(kinetic)/dm3 mol−1
Kb(Abs)/dm3 mol−1
14
3.7
1.85
1.40
1.90
0.74
0.88
18
77
78
84
64
84
74
73
60
52
76
76
71
68
70
4.7
2.2
2.2
2.3
1.0
1.2
a K = kf[sulfite]/kr[sulfite]. b Calculated from changes in absorbance accompanying reaction, as in Tables 3 and 4.
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is not compatible with rate-limiting reaction of sulfite with
iminium ions, the k4 step. Hence carbinolamine formation, k2, or
dehydration, k3, will be the slow step in the overall reaction. Jencks
and co-workers have shown14–17 that in general in the reactions of
carbonyl compounds with amines, carbinolamine dehydration is
rate-limiting in alkaline solutions (pH > 7). In the present case
the assumption that k3, the acid-catalysed dehydration of carbino-
lamine, is rate-limiting and that steps leading to the carbinolamine
are at equilibrium, leads7 to eqn (4). Here K1 is the equilibrium
constant for dissociation of the dianion of 1 to give aldehyde
and sulfite, and K2 is the equilibrium constant for formation
of carbinolamine from aldehyde and aniline. Correspondingly,
eqn (5) will apply to the reverse reaction.
Bands were observed at d 7.90 (CH, t, J = 5 Hz), d 7.03–
7.36 (Ar), d 1.81 (CH2, m) and d 1.19 (CH3, t, J = 7.5 Hz).
The reaction was repeated in acetonitrile and a small volume
of the solution was mixed with a large excess of aqueous sulfite
solution, so that the final stoichiometric concentration of imine
was ca. 1 × 10−4 mol dm−3 with a ratio of [sulfite] : [imine] of
greater than ten. UV studies indicated two processes on mixing
the imine with sulfite; a fast reaction, too rapid for measurement,
giving increased absorption at 250 nm and compatible with
initial formation of the anilinopropanesulfonate 3a, and a slow
decomposition with decreasing absorbance at 250 nm. The final
spectrum was consistent with the regeneration of aniline. Kinetic
measurements showed a first-order process whose rate constant
was measured in buffered solution with ionic strength 0.1 mol dm−3
using potassium chloride. It should be noted that here the final
concentrations of aniline and of hydroxypropanesulfonate, 1a, are
ca. 1 × 10−4 mol dm−3, so that in contrast with the earlier data,
where concentrations of 1a are much higher, the reaction flux in
the forward direction is negligible compared to the flux in the
reverse direction; hence kobs = kr. At each pH studied, values of
kobs were measured at six sulfite concentrations in the range 1 ×
10−3 mol dm−3 to 2 × 10−2 mol dm−3, and values were found to
be inversely dependent on [sulfite], so that the term kobs[sulfite]
was constant. Values in the pH range 5.8 to 9.9 are shown in
Fig. 1. Measurements of kr[sulfite] showed that values were, within
experimental error, independent of ionic strength in the range I =
0.05 to 0.20 mol dm−3.
+
⎛
⎜
⎞
⎟
[H ] + KHSO
−
3
(4)
(5)
kf [sulfite] = k3K1K2Ka
⎜
⎝
⎟
⎠
K
−
HSO3
+
⎛
⎜
⎞
⎟
[H ] + KHSO
−
k−3
K4
3
kr[sulfite] =
⎜
⎝
⎟
⎠
KHSO
−
3
−
In these eqns KHSO is the acid dissociation constant of
3
hydrogen sulfite,7,18,19 and its value, 8 × 10−8 mol dm−3, indicates
that both the forward and reverse reactions should show a change
in dependence on acidity around pH = 7. When KHSO > [H+], at
−
3
higher pH, values should become independent of acidity, and when
[H+] > KHSO , at lower pH, then the logarithms of values should
−
3
increase linearly with increasing pH. The experimental results in
Table 5 and Tables S6–11† show that values of both kf[sulfite] and
kr[sulfite] do become approximately independent of acidity when
pH > 7. Values for the reverse reaction, determined both from
the equilibration process and from independent measurements
are shown in Fig. 1, where they are compared with the acidity
dependence predicted from eqn (5). This shows that while there
is acceptable agreement with eqn (5) in alkaline solutions, when
pH < 7 experimental values fall below those predicted. It should be
noted that the assumption that dehydration of the carbinolamine
occurs without acid catalysis predicts an inverse relation with
acidity when pH > 7, which is not observed.
The other possibility is that formation of the carbinolamine,
the k2 step, is rate-limiting in the forward direction. At pH
values above those where the parent anilines (2) are protonated,
the k2 step is not expected to show strong dependence on pH,
although weak general acid catalysis is possible.7,14,16 In the present
work we have worked at low buffer concentrations where such
effects should be small. The assumption that carbinolamine
formation/decomposition is rate-limiting leads to eqns (6) and
(7) for the forward and reverse reactions respectively.
Fig. 1 Variation with pH of values of log(kr[sulfite]) in the reaction of
propanal with aniline and sulfite. Experimental points determined from
the equilibration process are shown as ᭺, and from the reverse reaction
as ●. The broken line is calculated from eqn (5) with k−3/K4 = 1.6 ×
-
([H+ ] + KHSO
)
−
3
(6)
(7)
kf [sulfite] = k2K1Ka
[H+ ]KHSO
−
10−6 mol dm−3 s−1 and KHSO = 8 × 10−8 mol dm−3
.
3
3
([H+ ] + KHSO
)
−
k−2
3
kr[sulfite] =
K3K4 [H+ ]KHSO
−
Rate-determining step
3
The forward reaction has a first-order dependence on the aniline
concentration, showing that the k1 step in Scheme 1, formation
of free aldehyde from the hydroxyalkanesulfonate, 1, is not rate-
limiting. Similarly, the inverse dependence on sulfite concentration
These expressions predict that when pH > 7, values should
show an inverse dependence on acidity, which is not observed
experimentally. When pH < 7 values should become independent
of acidity.
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Table 6 Summary of rate and equilibrium constants for formation of anilinoalkanesulfonates at pH 8.0 and 25 ◦C in water
Aniline
R
Hydroxyalkane sulfonate
R1
R2
pKa
kf[sulfite]/10−4 s−1
kr[sulfite]/10−6 dm3 mol−1 s−1
Kb/dm3 mol−1
a
H
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
Me
Me
H
4.60
5.08
4.15
3.46
2.75
2.47
4.60
4.60
4.85
1.9
3.6
1.1
0.79
0.24
0.18
2.0
2.3
6.0
1.4
0.84
0.21
0.14
3.3
80
60
81
4-Me
4-Cl
3-Cl
3-CN
3-NO2
H
94
115
133
60
34
1.1
H
NMe
—
—
a Values for anilinium ions, from ref. 20. b From data in Tables 1, 3–6 and S1–10.
Overall, our results (Fig. 1) are consistent with the idea that
in alkaline solutions, where pH > 7, acid-catalysed carbinolamine
dehydration is rate-limiting. As the solutions are made increasingly
acidic, in the pH range 7 → 5, carbinolamine formation becomes
rate-limiting.
k−3
kr[sulfite] =
(9)
k4
q(observed) = q(k−3 step) − q(K4 process)
(10)
The values of q for each of those processes will be positive since
they both involve reduction of positive charge on the iminium ion.
However, that for the equilibrium process is expected to be larger
since it involves full neutralisation of charge. Hence overall q has
a negative value.
Substituent effects
Values of rate and equilibrium constants measured at pH 8.0 are
summarised in Table 6. Hammett plots, versus r−, show good
correlations, with q values of −1.47 for the forward reaction
and −1.81 for the reverse reaction. At this pH, acid-catalysed
carbinolamine dehydration will be rate-limiting and eqn (4)
reduces to eqn (8).
The results in Table 6 show that values of K, the overall
equilibrium constant for formation of anilinoalkanesulfonates, 3,
from hydroxyalkanesulfonates, 1, and anilines, 2, show only small
variation. Both electronic and steric factors may be important. In
the reactions of hydroxypropanesulfonates, 1a, with substituted
anilines, 2, the substituent R is remote from the reaction centre
so that only electronic factors are involved. Here the values of
K change by only a factor of ca. 2 as the substituent is changed
from 4-Me to 3-NO2, indicating that the electronic effect of the
CHEt(SO3−) group is rather similar to hydrogen. Interestingly, the
value of K for reaction of 1a with N-methylaniline is considerably
reduced, even allowing for the statistical factor of two relative
to aniline. This is probably a consequence of increased steric
repulsion with the ethyl and sulfite groups as the substituent on
nitrogen is changed from H to Me.
Increasing methyl substitution in the hydroxyalkanesulfonate
from 1a (R1 = R2 = H) to 1b (R1 = H, R2 = Me) and 1c (R1 = R2 =
Me) results in small decreases in the value of K. Here both steric
effects in the reactants, 1, and products, 3, must be considered,
and the small decreases observed are likely to indicate that the
anilinoalkanesulfonates, 3, are rather more sterically demanding
than the hydroxyalkanesulfonates, 1. Further evidence that this
is the case comes from the knowledge7 that for the reaction
of hydroxymethanesulfonate (formed from formaldehyde) with
kf[sulfite] = k3K1K2Ka
(8)
For the reactions involving hydroxypropanesulfonate, 1a, values
of K1 and Ka will be invariant. Hence it is values of k3 and K2
which may be affected by ring substitution. K2 is the equilibrium
constant for carbinolamine formation, and the data for the cor-
responding reaction involving formaldehyde7 show no variation,
within experimental error, with ring substitution. Hence it is likely
that in the present system the q value reflects changes in value of
k3. This step, as shown in Scheme 2, is likely to involve transfer
of positive charge in the transition state towards the aniline ring.
Hence a negative value of q is expected. The value obtained, −1.47,
can be compared with a value of q = −2.8 for protonation of the
parent anilines.
For the reverse reaction at this pH the rate-determining step will
be hydration of the iminium ion. Eqn (5) reduces to eqn (9) and
the observed q value will depend on both the k−3 step and the K4
equilibrium, as shown in eqn (10).
Scheme 2
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aniline the value of K is 780 dm3 mol−1, which is a factor of
ten larger than for the corresponding reaction of 1a.
2 give a single first-order process, and the results are consistent
with the mechanism of Scheme 1. Variation of rate constants with
pH indicate that in alkaline solution, pH > 7, the rate-determining
step is acid-catalysed dehydration of the carbinolamine followed
by rapid reaction with the sulfite of the iminium ion formed
in this process. In more acidic solutions the results show that
carbinolamine formation becomes rate-limiting. Values of the
overall equilibrium constant show relatively little variation with
the nature of the ring substituent in the aniline or with the degree of
methyl substitution at the b-carbon atom of the parent aldehydes.
The results also allow determination of the values of rate constants
for carbinolamine formation from the aldehydes and anilines. The
results presented will be of interest in industrial processes which
use the anilinoalkanesulfonates as intermediates.
Carbinolamine formation
Our results indicate that as the acidity of the solutions increases the
rate-limiting step changes from the dehydration of carbinolamine
to its formation. This change is likely to have occurred by pH 5,
so that the forward rate term may be expressed as eqn (11).
k2K1Ka
kf [sulfite] =
(11)
K
−
HSO3
Values obtained at this pH for reaction with a series of
substituted anilines are given in Table 7. Values of Ka, the acid
dissociation constant of 1a, and K1, the equilibrium constant
for dissociation of the dianion into propanol and sulfite, are
available,12,13 allowing values of k2 to be calculated. The values
in Table 8 show a dependence on substituent, q = −1.6, similar
to that shown in the reactions of anilines with formaldehyde,
where q = −1.8. However, values for the formaldehyde reaction
are ca. 250 times greater than those for reactions involving
propanal, consistent with the greater electrophilicity expected for
formaldehyde.7,21,22
Experimental
Aniline, its substituted derivatives, and aldehydes were the purest
available commercially. Solutions of sodium sulfite and sodium
hydrogen sulfite were prepared in high purity water and concen-
trations checked by iodine titration.
◦
1H NMR spectra were measured at 23 1 C in D2O using a
Bruker Avance 400 MHz spectrometer with a 30 degree pulse, 4
second acquisition and 1 second recycle delay, or with a Varian
Inova 500 MHz instrument with a 45 degree pulse and 4 second
acquisition time. Conditions were chosen to avoid saturation,
confirmed by the correspondence of spectra measured on different
instruments. Some early measurements were made with a Varian
Mercury 200 MHz instrument. Spectra of solutions containing
equimolar solutions of aldehydes and sodium hydrogen sulfite
confirmed the near-quantitative formation of hydroxyalkanesul-
fonates, 1. Changes in spectrum in the presence of aniline and its
derivatives showed the formation at equilibrium of anilinoalka-
nesulfonates, 3. Values for equilibrium constants, defined in eqn
(1), were determined from integrated peak intensities. A specimen
calculation for reaction of 1b with 2, R = H, is in Table 8.
Conclusion
1H NMR studies show that reactions of hydroxyalkanesulfonates
1 derived from propanal, isobutyraldehyde(2-methylpropanal)
and trimethylacetaldehyde (2,2-dimethylpropanal) with substi-
tuted anilines, 2, proceed smoothly to equilibrium to yield the
corresponding anilinoalkanesulfonates. There is no evidence for
the formation of species with anything other than 1 : 1 overall
stoichiometry. Kinetic studies in water with 1 in large excess over
Table 7 Values of k2 calculated from the forward reaction of 1a at pH 5.0
and 25 ◦
C
UV spectra and kinetic measurements were made in water at
25 0.2 ◦C using a Shimadzu UV-2102 PC spectrometer fitted
with a Peltier temperature control, or with Perkin Elmer k12
or k2 instruments, for which the temperature was controlled by
circulation of thermostatted water. The temperatures of actual
reaction solutions were checked regularly. The pH of solutions
was controlled using dilute buffers containing borax, phosphate,
acetate or bicarbonate, and pH values of reaction solutions were
checked using a Jenway 3020 pH meter. First-order rate constants
were evaluated using standard methods and are precise to 5%.
Aniline
R
kf[sulfite]/10−4 s−1
k2a/dm3 mol−1 s−1
4-Me
H
4-Cl
3-Cl
3-CN
3-NO2
20
14
5.0
3.0
1.3
1.0
760
530
190
110
50
38
a Calculated from eqn (11) with Ka 2.0
×
10−11 mol dm−3
, K1
0.0105 mol dm−3 and KHSO 8.0 × 10−8 mol dm−3
.
−
3
References
Table 8 Specimen calculation of K for reaction of 1b with 2, R = H
[1b]stoich/dm3 mol−1 [2]stoich/dm3 mol−1 [3b]eq/dm3 mol−1 Ka/dm3 mol−1
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0.063
0.054
0.071
0.070
0.090
0.053
0.041
0.041
0.036
64
64
60
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K =
a Calculated as
using integrals
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for peaks due to [1b] and [3b] at equilibrium.
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2410 | Org. Biomol. Chem., 2008, 6, 2405–2411
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The Royal Society of Chemistry 2008
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