Kinetic study of the chlorine transfer from n-chlorosuccinimide to amino compounds

Article abstract of DOI:10.1002/(SICI)1099-1395(199708)10:8<631::AID-POC924>3.0.CO;2-W

A kinetic study of the reactions of N-chlorosuccinimide (NCS) with glycine (GIy), sarcosine (Sar), 2-methylalanine (2MA), proline (Pro) and pyrrolidine (Pyr) was carried out. The reactions were found to be first order with respect to both NCS and the amine or amino acid and order - 1 in proton concentration. In order to calculate the experimental activation parameters, the effect of temperature on the reaction rates was studied. The ionic strength and buffer concentration were found to have no effect on the rate constant. A reaction mechanism involving Cl⁺ transfer from NCS to the amine or amino acid to form an N-chloro compound is proposed.

Full text of DOI:10.1002/(SICI)1099-1395(199708)10:8<631::AID-POC924>3.0.CO;2-W

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 631–636 (1997)
KINETIC STUDY OF THE CHLORINE TRANSFER FROM
N-CHLOROSUCCINIMIDE TO AMINO COMPOUNDS
J. M. ANTELO,* F. ARCE, J. CRUGEIRAS AND M. PARAJÓ
Departamento de Química Física, Facultad de Química, Universidad de Santiago, 15706 Santiago de Compostela, Spain
A kinetic study of the reactions of N-chlorosuccinimide (NCS) with glycine (Gly), sarcosine (Sar), 2-methylalanine
(2MA), proline (Pro) and pyrrolidine (Pyr) was carried out. The reactions were found to be first order with respect to
both NCS and the amine or amino acid and order ؊1 in proton concentration. In order to calculate the experimental
activation parameters, the effect of temperature on the reaction rates was studied. The ionic strength and buffer
+
concentration were found to have no effect on the rate constant. A reaction mechanism involving Cl transfer from NCS
to the amine or amino acid to form an N-chloro compound is proposed. © 1997 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 10, 631–636 (1997) No. of Figures: 6 No. of Tables: 7 No. of References: 23
Keywords: chlorine transfer; N-chlorosuccinimide; amino compounds; kinetics
Received 15 July 1996; revised 14 February 1997; accepted 28 February 1997
INTRODUCTION
EXPERIMENTAL
Nitrogen-containing organic compounds can be oxidized to
N-chloro compounds by a wide variety of chlorinating
agents, including N-chlorosuccinimide (NCS). Because the
N-chloro compounds are themselves chlorinating agents,
Amino acid and amine solutions were prepared by direct
weighing of commercial products: glycine (Carlo Erba),
sarcosine and 2-methylalanine (Merck), proline (Sigma)
and pyrrolidine (Aldrich). Acetic acid–sodium acetate,
sodium dihydrogenphosphate–disodium hydrogenphos-
phate and boric acid–sodium borate buffers were made by
direct weighing from acetic acid, sodium dihydrogenphos-
phate and boric acid (all Merck p.a. grade chemicals),
respectively, and addition of an appropriate volume of
sodium hydroxide solution to obtain the desired pH. The
ionic strength was kept constant at I=0·5 by adding
1
Higuchi et al. classified them according to their capacity to
+
transfer Cl ions to another substrate.
One of the most salient features of NCS is its instability
in aqueous solution, possibly due to its photochemical
2
decomposition via radical intermediates. NCS has been
used as a halogenating reagent in various studies in aqueous
3
solution. Thus, Higuchi and Hasegawa studied the forma-
tion of dimethylchloramine by using the initial-rate method.
They postulated two potential reaction pathways, even
though the experimental results suggested that the process
NaClO
4
.
Succinimide (SI) and NCS were purified by recrystalliza-
tion prior to use. NCS was freshly prepared and stored in a
UV-opaque flask in order to avoid decomposition.
8
+
consisted of a Cl ion transfer.
Recent kinetic studies on the oxidation reactions of amino
Because the reactions were very fast, they were moni-
tored by using an Applied Photophysics stopped-flow
spectrophotometer. The temperature of the solutions was
kept constant at 25±0·1 °C by means of water circulating
via a thermostated bath. The equipment was furnished with
two syringes that were used to inject the amine or amino
acid and the NCS solution at the required pH, respectively.
On mixing the amine or amino acid solution with the
NCS solution, an absorption band with a maximum at about
4, 5
acids with NCS and NBS (N-bromosuccinimide) have led
to the proposal of a reaction mechanism in which an acyl
hypohalite forms initially, and then decomposes to an
aldehyde. This behaviour departs markedly from that of
similar reactions where sodium hypochlorite was used as
6, 7
the halogenating reagent.
In order to clarify the mechanism of this type of reaction,
in this work we carried out a kinetic study of the formation
of N-chloro compounds by use of NCS as the halogenating
reagent.
9, 10
260 nm, typical of N-chloro compounds,
was observed.
Figure 1 shows the reaction spectrum for N-chloroglycine
formation.
The reactions were studied in the dynamic spectrophoto-
metric mode; absorbance values were recorded at the
*
Correspondence to: J. M. Antelo.
1997 by John Wiley & Sons, Ltd.
©
CCC 0894–3230/97/080631–06 $17.50

6
32
J. M. ANTELO ET AL.
Table 2. Influence of glycine concentration on the formation rate
Ϫ4
constant for N-chloroglycine, with [NCS]=5ϫ10
M, pH=6·80,
I=0·5 (NaClO ), T=25 °C
4
Ϫ1
[Gly] (
M)
kobs (s
)
Ϫ3
Ϫ2
Ϫ2
Ϫ2
Ϫ2
5·0ϫ10
1·5ϫ10
2·5ϫ10
3·5ϫ10
4·5ϫ10
2·03±0·04
6·04±0·08
10·09±0·20
13·86±0·20
17·43±0·40
reagent.
The influence of the initial concentration of amine or
amino acid was studied by varying it while keeping constant
those of the other reactants. Table 2 gives the results
obtained for N-chloroglycine. The plot of log kobs against log
Figure 1. Spectra for N-chloroglycine formation. T=25 °C,
Ϫ4
Ϫ3
[NCS]=6ϫ10
M, [Gly]=6ϫ10
M, pH=6·8, ⌬t=12 ms
[Gly] was a straight line of slope 0·98±0·01, so the reaction
wavelength of the absorption maximum for the N-chloro
compound formed in each case.
order with respect to glycine was unity.
The influence of the proton concentration on the forma-
tion rate constant for N-chloroglycine at constant ionic
strength I=0·5 was studied by using buffer solutions of
acetic acid–sodium acetate, sodium dihydrogenphosphate–
disodium hydrogenphosphate and boric acid–sodium
borate. The results are shown in Table 3. A plot of log kobs vs
pH was a straight line of slope 0·94±0·01, so the rate
constant was inversely proportional to the proton concentra-
tion for the N-chloro compounds studied.
The effect of the buffer concentration on the reaction rate
was studied using acetic acid–acetate buffer solutions of
different concentrations. The rate constant did not vary with
the buffer concentration, and was also found to be
independent of the nature of the buffer.
The reactions were studied using the isolation method,
the amino acid being present in at least a tenfold excess over
the NCS. Absorbance–time data were fitted by first-order
integrated rate expressions and the slope was used to
calculate the pseudo-first-order rate constant.
After the reaction order had been determined, the
influence of the reagent concentrations and the different
experimental variables affecting the reaction medium (tem-
perature, buffer concentration, ionic strength and pH) were
investigated.
pH measurements were made with a Radiometer PHM82
pH meter that was calibrated with commercially available
buffer solutions of pH 4·01, 7·00 and 10·02.
We also studied the influence of the ionic strength on the
observed rate constant by varying the concentration of
RESULTS
NaClO with constancy of all other experimental parame-
4
A preliminary batch of experiments was conducted at
different NCS concentrations and constant concentrations of
all other reactants. The observed pseudo-first-order rate
constants given represent the means of 5–10 kinetic runs.
Table 1 shows the rate constants obtained for the reaction
with N-chloroglycine over the NCS concentration range
studied. Based on these results, the initial NCS concentra-
tion can be assumed not to influence the rate constant,
which confirms that the reaction is first order in this
ters. The reaction rate was found to be independent of ionic
strength in the range studied (0·03–0·5 ).
M
In order to calculate the experimental activation parame-
ters for the formation of the N-chloro compounds, the
influence of temperature on the reaction was investigated.
Figure 2 shows the results obtained for N-chloroglycine and
N-chloropyrrolidine, which conformed to the Arrhenius
equation and the theory of absolute rates, and were used to
‡
calculate the activation energy (E
entropy (⌬S ) for the N-chloro compounds studied.
), enthalpy (⌬H ) and
a
‡
Table 1. Influence of initial concentration on the formation rate
Ϫ2
constant for N-chloroglycine, with [Gly]=3·0ϫ10
M, pH=6·80,
MECHANISM AND DISCUSSION
I=0·5 (NaClO ), T=25 °C
4
Two mechanisms have been postulated for the reaction of
NCS, with (a) nucleophilic attack on the chlorine atom of
the N-chlorosuccinimide by the unprotonated amino nitro-
Ϫ1
[
NCS] (
M
)
kobs (s
)
Ϫ4
Ϫ4
Ϫ3
Ϫ3
3
7
1
2
·0ϫ10
·5ϫ10
·5ϫ10
·0ϫ10
9·33±0·20
9·45±0·22
9·30±0·25
9·23±0·20
9·47±0·22
4, 5
gen and (b) the other, previously reported, involving
nucleophilic attack of the carboxyl group of the amino acid
on the chlorine atom to yield an acyl hypochlorite (Scheme
1).
Ϫ3
2·75ϫ10
Both are in keeping with the experimental observation
©
1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 631–636 (1997)

CHLORINE TRANSFER FROM N-CHLOROSUCCINIMIDE TO AMINO COMPOUNDS
Table 3. Influence of proton concentration on the formation rate constant for N-
633
Ϫ4
Ϫ3
chloroglycine, with [NCS]=5ϫ10
M, [Gly]=5ϫ10
M
, I=0·5 (NaClO ), T=25 °C
4
Ϫ1
Ϫ1
pH
kobs (s
)
pH
kobs (s
)
Boric acid–borate:
Acetic acid–acetate:
8·90
8·60
8·11
182±2
90±1
28·6±0·3
5·41
4·98
4·42
0·0823±0·005
0·030±0·001
Ϫ3
Ϫ4
9·1ϫ10 ±5ϫ10
Ϫ3
Ϫ4
3
·86
3·8ϫ10 ±0·2ϫ10
Dihydrogenphosphate–
hydrogenphosphate:
7
7
6
·47
·09
·36
8·5±0·2
3·62±0·04
0·66±0·01
that the rate of reaction was independent of ionic strength,
since this suggests that at least one of the reactants in the
rate-controlling step is electrically neutral.
reaction rate will be given by
v=k [D][NCS]+k [C][NCS]
2
3
In order to account for the experimental behaviour
observed, and taking into account the potential species and
equilibria involved, the mechanism depicted in Scheme 2 is
proposed, according to which the amino acid, with an
unprotonated amino group, reacts with NCS to give an N-
chloroamino acid.
The concentrations of C and D can thus be expressed as
functions of the acidity and the total amino acid (Aa)
concentration:
+
2
+
[
C]=K K [Aa]/([H ] +K [H ]+K K )
I II I I II
+
+
2
+
[
D]=K [Aa][H ]/([H ] +K [H ]+K K )
e
I
I
I
I
Because the reaction takes place via the amino acid with
an unprotonated amino group, the only reactive species
present in the reaction medium will be C and D. Hence the
The first step of the process involves the amino acid
protonation equilibria, where K and K are the macroscopic
I
II
equilibrium constants for the loss of the first and second
proton, respectively. The values of the microscopic con-
stants can be estimated from K and K by assuming the
I
II
microscopic constant KЈ to be equal to the acidity constant
I
11
for the corresponding amino acid ester (K ). Also, since
K ӷ[H ]ӷK , under the conditions used in this work,
e
+
I
II
these expressions may be simplified to
+
[
C]=K [Aa]/[H ]
II
[
D]=K [Aa]/KI
e
so that the rate law may be written as
v=(k K [Aa]/K +k K [Aa]/[H ])[NCS]
+
2
e
I
3
II
and the first-order pseudo-constant is given by
+
kobs =k K [Aa]/K +k K [Aa]/[H ]
2
e
I
3
II
Figure 2. Influence of temperature on the formation rate constant
for (᭹) N-chloroglycine (T=21·4, 26·1, 32·7, 39·6, 45·1 °C) and
Therefore, the reaction is first order with respect to the
total amino acid concentration, consistent with experimental
evidence, and the rate constant is linearly dependent on the
reciprocal of the proton concentration, with slope k K [Aa]
(
᭺) N-chloropyrrolidine (T=18·3, 24·7, 31·0, 37·0 °C).
Ϫ
4
Ϫ
3
[NCS]=5ϫ10
M, [Gly]=[Pyr]=5ϫ10
M, pH=6·87, I=0·5
(
NaClO4 )
3
II
and intercept on the ordinate k K [Aa]/K . Our experimental
2
e
I
results lead to a straight line without a significant intercept,
indicating that the term containing k is much smaller than
2
that containing k and that the reaction takes place through
3
species C preferentially. Therefore, the expression for
observed rate constant will be given by
+
kobs =k K [Aa]/[H ]
3
II
Scheme 1
1997 by John Wiley & Sons, Ltd.
This equation can be used to calculate the second-order
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 631–636 (1997)
©

6
34
J. M. ANTELO ET AL.
Scheme 2
+
rate constant, k . A plot of k [H ] against the amino acid
of the correlation by pyrrolidine, which has no carboxyl
group, rules out the formation of an acyl hyopchlorite
3
obs
concentration should be a straight line with zero intercept,
the slope of which can be used to calculate k provided pK
is known at the ionic strength used (I=0·5). Table 4 shows
Ϫ
intermediate. Moreover, if COO were the active form, a
3
II
1
2
Ϫ
pH-dependent region (at pH close to pK of the COO
a
the k values calculated for the different N-chloro com-
pounds studied.
group of the amino acid) should be expected, contrary to the
experimental behaviour.
3
The alternative mechanism (chlorination of the carboxy-
late group) also leads to the prediction that k
proportional to [Aa]/[H ]. However, the correlation
is
obs
+
between k3 and the pKa of the amino compound
(slope=0·9±0·1; Figure 3), and in particular the satisfaction
Table 4. Values of pK and formation rate constants (k ) for the N-
II
3
chloro compounds studied
Ϫ1 Ϫ1
12
Amino acid
Gly
k3 (l mol
s
)
pK (25 °C, I=0·5)
II
5
(6·3±0·4)ϫ10
9·778
10·10
10·20
10·64
11·31
6
2MA
(1·29±0·04)ϫ106
Sar
Pro
Pyr
(2·09±0·06)ϫ10
6
(5·7±0·1)ϫ10
(1·55±0·04)ϫ10
7
Figure 3. Correlation between the N-chlorination rate constant, k3 ,
and pK values for the nitrogen-containing compounds studied
a
©
1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 631–636 (1997)

CHLORINE TRANSFER FROM N-CHLOROSUCCINIMIDE TO AMINO COMPOUNDS
635
Table 5. k (Br)/k (Cl) ratios obtained for the reactions of NBS and
with a given nitrogenous compound is expected to be
related directly to the electrophilicity of the halogenating
agent.
3
3
NCS with the different compounds studied
1
5
Amino acid
Gly
k3 (Br)
k (Br)/k (Cl)
3
3
N-Bromoamines form more rapidly than do the N-
chloroamines. The rate constants for the reaction of NXS
with amines and amino acids are 1·8–9·1 times greater for
X=Br than for X=Cl. Such an order of reactivity of NBS
and NCS is in the direction expected from the electro-
philicities of NXS, which are in the order of
NIS>NBS>NCS because of the increasing electronegativi-
ties and decreasing polarizabilities in the same order. In
addition, the Br atom can better delocalize part of the total
electron density of the negatively charge activated complex
in its d orbitals than can the Cl atoms, thus providing the
transition state with additional stability.
6
2·7ϫ106
4·3±0·3
9·1±0·4
1·8±0·1
2·7±0·1
3·0±0·1
2MA
11·8ϫ10
6
Sar
Pro
Pyr
3·56ϫ106
15·5ϫ106
47·4ϫ10
Table 6. Influence of succinimide concentration on the formation
Ϫ4
rate
constant
for
N-chloroglycine.
[NCS]=5ϫ10
M,
Ϫ2
[Gly]=2·5ϫ10
M, pH=6·70, I=0·5 (NaClO4 )
Ϫ1
[SI] (
M)
kobs (s
)
From the k (Br)/k (Cl) ratio of 1·8 to 9·1, we can say that
3
3
the halogen dependence is not really high as compared with
N-chloro- and N-bromoamine hydrolysis. From the Brøns-
0
0
0
0
0
·025
·050
·100
·150
·200
7·50±0·10
7·60±0·12
7·57±0·11
7·45±0·10
7·60±0·10
22
ted plot (see Figure 3), ␤=0·9±0·1 for NCS and ␤=0·8±0·1
13
for NBS, which suggests that there is nearly full bond
formation between the amine and the halogen in the
transition state. According to a later S 2-like transition state,
N
a small halogen influence is observed.
The influence of succinimide concentration on the
reaction rate constant was also investigated; as can be seen
from Table 6, this variable had no appreciable effect. The
study was carried out in phosphate buffer at pH 6·7 and a
constant ionic strength I=0·5. The lack of influence
observed excludes a potential reversible character for the
reaction.
Similar results have been observed previously for the
+
transfer of Br from NBS to these amino compounds, the
13
slope of the corresponding regression line being 0·8±0·1.
Table 5 compares the values of k (Br) and k (CI) obtained
3
3
for each amino compound under the same reaction condi-
tions, and shows that bromonium ion transfer is faster than
chloronium ion transfer. Excellent correlations between rate
constants and nucleophile basicity have previously been
reported for the N-halogenation of amines and amino acids
The calculated k values can be used to obtain the overall
3
activation enthalpy and entropy (Table 7) for the process by
using the following equation:
6, 7
14
by sodium hypochlorite or sodium hypobromite and for
several other chemically controlled reactions involving
chloronium ion transfer, such as the reactions of chlor-
-
-
k3K
II
k
B
⌬S
reac
⌬H
reac
1
5, 16
amines with amines, amino acids and peptides
and the
ln
=ln
+
Ϫ
ͩ
ͪ ͩ ͪ
17
18
19
20
T
h
R
RT
oxidation of iodide, bromide, cyanide, nitrite or
2
1
sulphite by hypochlorous acid or N-chloramines, which
involve nucleophilic attack on the chlorine, followed by
cleavage of the Cl–O bond and the loss of an OH group. In
every case where the reaction is not diffusion controlled, the
rate constant increases with increasing basicity of the amino
group of the substrate.
where
-
-
⌬S
=⌬S°+⌬S
reac
II
3
-
-
3
⌬
H
=⌬H°+⌬H
reac
II
where ⌬S° and ⌬H°, the overall entropies and enthalpies for
II
II
When the reaction is chemically controlled rather than
diffusion controlled, the reactivity of a halogenating agent
the equilibria between the neutral and anionic forms of the
amino acid, are known. Hence the activaction parameters
12
Table 7. Activation parameters for the formation of the N-chloro compounds
‡
Ϫ1
Ϫ1
‡
Ϫ1
studied, with ⌬S values in J mol
K
and E and ⌬H values in kJ mol
a
Amino
acid
-
-
12
12
-
3
-
3
⌬H
⌬S
⌬H°II
⌬S°II
⌬H
⌬S
Ea
reac
reac
Gly
64·7±0·4
74±1
59±1
Ϫ112±1
Ϫ86±4
Ϫ124±3
Ϫ101±2
44·4
47·7
41·0
43·3
Ϫ38·5
Ϫ35·1
Ϫ57·7
Ϫ58·6
20
26
18
21
Ϫ74
Ϫ51
Ϫ66
Ϫ42
67·4±0·6
76·1±1·1
61·5±1·1
67·0±0·6
66·6±1·7
2
MA
Sar
Pro
Pyr
64·4±0·6
©
1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 631–636 (1997)

6
36
J. M. ANTELO ET AL.
S. Vivekanamdam, Bull. Chem. Soc. Jpn. 63, 2397–2403
1990).
. J. M. Antelo, F. Arce and M. Parajó, Int. J. Chem. Kinet. 27,
37–647 (1995).
. D. W. Margerum, E. T. Gray and R. P. Huffman, in
Organometals and Organometalloids, Occurrence and Fate in
the Environmental, edited by F. E. Brickman and J. M.
Bellama, ACS Symposium Series, No. 82, pp. 278–291.
American Chemical Society, Washington, DC (1978).
(
6
7
6
Scheme 3
-
-
⌬
(
S and ⌬H for step 3 in Scheme 2 can also be calculated
8. D. D. Perrin and W. L. F. Armarego, Purification of Laboratory
Chemicals, 3rd ed. Pergamon Press, Oxford (1988).
3
3
Table 7).
9
. J. Kleinberg, M. Tecotzky and L. F. Audrieth, Anal. Chem. 26,
388–1389 (1954).
In conclusion, the N-chlorination of amines and amino
acids by reaction with NCS takes place by a mechanism
involving transfer of Cl from NCS to the unprotonated
nitrogen atom of the amino compound. This reaction falls
into the general class of nucleophilic substitution reactions
1
+
10. H. H. Sisler, F. J. Neth, R. S. Drago and D. Yaney, J. Chem.
Educ. 76, 3906–3909 (1954).
1. J. T. Edsall, ACS Monogr. No. 90, 75–115 (1943).
2. A. E. Martell and R. M. Smith, Critical Stability Constants.
Plenum Press, New York, Vol. 5 (1982); Vol. 6 (1989).
1
1
+
23
+ 7
+ 14
involving transfer of a cation such as NO , Cl or Br ;
for the reaction of NCS with amino compounds, the rate-
controlling step can be considered as a concerted process in
which the amino compound nucleophilically displaces the
leaving group (the succinimide anion) of the halogenating
agent. Since both NCS and the unprotonated amino group of
the amino compound are electrically neutral, charge separa-
tion must take place in the transition state (Scheme 3), a
hypothesis supported by the large negative entropies
calculated for step 3, which suggest that the transition state
is highly solvated.
13. J. M. Antelo, F. Arce and J. Crugeiras, J. Chem. Soc., Perkin
Trans. 2 2275–2279 (1995).
1
1
1
1
4. J. E. Wajon and J. C. Morris, Inorg. Chem. 21, 4258–4263
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5. M. P. Snyder and D. W. Margerum, Inorg. Chem. 21,
545–2550 (1982).
6. M. Ferriol, J. Gazet and M. T. Saugier-Cohen Adad, Int. J.
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(
2
7. K. Kumar, R. A. Day and D. W. Margerum, Inorg. Chem. 25,
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(
1
9. L. M. Shurter, P. P. Bachelor and D. W. Margerum, Enriron.
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©
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 631–636 (1997)