The nitro anomaly and Bronsted βnuc values in SN2 reactions on chlorine

Article abstract of DOI:10.1002/poc.482

The kinetics of chlorine transfer reactions between N-chlorosuccinimide (NCS) and four carbon nucleophiles (the conjugated bases of phenyldinitromethane, Meldrum's acid, phenylmalononitrile and phenylnitro-methane) in water were determined. A plot of log k for the S_N2 reactions vs the pK_a of the first three conjugated acids of the nucleophiles gave a straight line with a slope (β_nuc) of 1.8. The data point for the mononitro derivative, phenylnitromethane, deviates negatively from the line by 6.7 log units. This deviation is typical of proton transfer reactions and was recently shown to occur also in S_N2 reactions on bromine. Copyright

Full text of DOI:10.1002/poc.482

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2002; 15: 540–543
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.482
The nitro anomaly and Brùnsted bnuc values in SN2 reactions
on chlorine†
Linoam Eliad and Shmaryahu Hoz*
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel 52900
Received 5 October 2001; revised 26 December 2001; accepted 4 January 2002
ABSTRACT: The kinetics of chlorine transfer reactions between N-chlorosuccinimide (NCS) and four carbon
nucleophiles (the conjugated bases of phenyldinitromethane, Meldrum’s acid, phenylmalononitrile and phenylnitro-
methane) in water were determined. A plot of log k for the SN2 reactions vs the pKa of the first three conjugated acids
of the nucleophiles gave a straight line with a slope (bnuc) of 1.8. The data point for the mononitro derivative,
phenylnitromethane, deviates negatively from the line by 6.7 log units. This deviation is typical of proton transfer
reactions and was recently shown to occur also in SN2 reactions on bromine. Copyright  2002 John Wiley & Sons,
Ltd.
KEYWORDS: halophilic reactions; SN2 on chlorine; nitro anomaly
INTRODUCTION
One of the most unique and fascinating features of proton
transfer reactions is the nitro anomaly. This phenomenon
has three different modes of appearance.1–4 The most
relevant to our study is the one discovered by Pearson and
Dillon in 1953.1 They reported a linear correlation
between the kinetics and the thermodynamics of
deprotonation reactions of carbon compounds. However,
the reactivity of nitroethane and nitromethane deviated
negatively from this linear correlation by several orders
of magnitude. The presence of another activating group
largely attenuates this anomaly. Thus, dinitromethane,
for example, fits the linear correlation well. We have
recently shown5 that an anomalous behavior similar to
the above is found in the bromine transfer reactions
between carbon donors and carbon nucleophiles.
The present study was aimed at exploring the
possibility of the existence of the nitro anomaly also in
chlorine transfer reactions, in which an N-donor (N-
chlorosuccinimide) is used instead of a carbon donor.
RESULTS AND DISCUSSION
Successive chlorination
The reactions shown in Eqns (1)–(4) were studied. All the
reactions were carried out in buffered aqueous solution.
Of the four nucleophiles used in this study, two (PNMA
and MAA) are capable of undergoing a second chlorina-
tion [Eqns (5) and (6)].
Repetitive scanning in the 200–400 nm range showed
that at pH 7.9 PNM is deprotonated to give the
*Correspondence to: S. Hoz, Department of Chemistry, Bar-Ilan
University, Ramat-Gan, Israel 52900.
E-mail: shoz@mail.biu.ac.il
†Presented in part at the 8th European Symposium on Organic
Reactivity (ESOR-8), Cavtat (Dubrovnik), Croatia, September 2001.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 540–543

THE NITRO ANOMALY IN CHLORINE TRANSFER REACTIONS
541
Table 1. Second-order rate constants for the Cl transfer
reactions in water at 20°C
Nucleophile
K (l molÀ1 sÀ1
)
PDNMA
MAA
PMNA
PNMA
66
2.7 Â 103
3.8 Â 105
9.7 Â 102
corresponding anion over 1.5 h (at this pH 96% of the
carbon acid is ionized). Addition of 7 equiv. of NCS to
the ionized solution resulted in an immediate disappear-
ance of the anion absorption. Addition of 1 equiv.
resulted in the disappearance of about half of the
absorption. This is interpreted as being due to a rapid
deprotonation and a successive chlorination of the
monochloro derivative obtained in the first step. The
kinetics were studied using up to a 9-fold excess of NCS
utilizing a second-order rate equation and taking into
account the fact that 2 equiv. of NCS are consumed for
each equivalent of PMNA.
and distort the kinetics of the reaction. Therefore, we
studied this reaction independently and found it to be first
order in each of the components, PMNA and PMNCl,
with a rate constant of (3.5 Æ 2) Â 104 l molÀ1 sÀ1. Thus,
this rate constant is much lower than the rate constant of
the chlorination of PMNA and therefore will not interfere
with the kinetic measurements.
The Brùnsted equation
The relative reaction rates in the second chlorination
process of the Meldrum’s acid are entirely different.
Whereas PNM undergoes a slow deprotonation and its
chloro derivative undergoes a rapid deprotonation (see
discussion below on the nitro anomaly), Meldrum’s acid
and its monochloro derivative are rapidly deprotonated.
On the other hand, whereas the chlorination step of the
monochloro derivative of PNM is fast, that of the
monochloro Meldrum’s acid anion is relatively slow.
Thus, when a fourfold excess of NCS was added to MAA,
the absorption at lmax = 260 nm had completely disap-
peared. However, when the excess of NCS amounted to
only 40%, the absorption of MAA vanished and a new
peak was observed at l = 275.5 nm. Repetitive scanning
experiments and single-wavelength monitoring at
l = 260 and 275.5 nm showed that the monochloro
derivative anion is immediately obtained and when a
threefold excess of NCS is used; the half-life of MAA is
ca 5 s and that of the monochloro derivative is ca 50 s.
Therefore, under the reaction conditions (small excess of
NCS), the second chlorination step hardly interferes with
the determination of the rate constant for the first
chlorination step.
The rate constants for the reactions studied [Eqns (1)–(4)]
are given in Table 1. The corresponding Brønsted-type
plot is shown in Fig. 1. Three of the points form a straight
line with a slope of 1.8 (r = 0.9933) but the point for
PNMA deviates negatively from the line.
There are essentially two types of linear free energy
relationships (LFER) correlating kinetics with thermo-
dynamics. In the first, the kinetics are correlated with the
thermodynamics of the same process (as in Brønsted
plots10 for proton transfer reactions). The second is when
the rate axis represents a certain reaction, e.g. SN2, and
the equilibrium axis represents not the thermodynamics
of the said reaction but rather that of a model reaction,
e.g. proton transfer reaction (the slope in this case is
marked bnuc whereas in the former it is b). The first type
seems to represent more truly the concept which under-
Dimerization
When PMNA reacts with PMNCl, a dimer is obtained
[Eqn. (7)].
This reaction was previously reported by a few
groups6–9 and also observed in our studies of bromine
transfer reactions.5 Since PMNCl is formed in the
reaction of PMNA with NCS, it may react with PMNA
Figure 1. Brùnsted-type plot for chlorine transfer reactions
from NCS to carbon nucleophiles
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 540–543

542
L. ELIAD AND S. HOZ
Table 2. Ab initio calculated energies (B3LYP/6±31 ‡ G*) for the protonated, chlorinated and anionic species in Eqns (8) and (9)
and the related reaction energies
E (a.u.)
E (kcal)
H trans.
Species
RH
RCI
R
Cl trans.
CH3CNÀ
À132.76169
À224.9874
À245.02479
À449.51259
À345.81191
À360.68517
À592.34378
À684.56368
À704.60902
À909.09182
À805.39867
À820.23034
À132.15665
À224.4512
À244.45022
À448.99953
À345.25361
À360.13127
32.1
À11.1
13.0
55.3
8.4
37.5
À4.3
28.9
À
CH3(CN)2
À
CH3NO2
À
CH2NO2)2
À25.6
À
CH3COCHCOCH3
Succinimide
2.8
lines the LFER since b reflects the relative sensitivity of
the transition states and ground states to the variation of
substituents in the same (and not in model) reaction.
Bordwell and Clemens reported bnuc values close to 1
for electron transfer reactions11 and values in the range
0.3–0.4 for the related nucleophilic (SN2) reactions by
carbon nucleophiles on carbon centers.12 Since the bnuc
value for our reactions is much larger (1.8), we suspected
that this might be due to an unexpectedly low sensitivity
to substituents in the ground states for proton transfer
compared with chlorine transfer. In this case the high
‘kinetic sensitivity’ (1.8) for a chlorine transfer reaction
is obtained because the kinetics are compared with the
thermodynamics of a model reaction (proton transfer). In
other words, if the thermodynamics of a chlorine transfer
reaction are also more sensitive to substituent variation
than a proton transfer reaction, a plot of the kinetics vs the
thermodynamics of the chlorine transfer reaction itself
will give a normal slope (0–1).
In order to explore this possibility, we conducted ab
initio calculations (Gaussian 98, B3LYP/6–31 ‡ G*)13
on the energetics of the equilibria for proton transfer and
chlorine transfer reactions shown in Eqns (8) and (9) for a
series of model carbon nucleophiles.
The energies obtained for all the species involved are
given in Table 2 and the data are presented graphically in
Fig. 2. The plot of the reaction energies for chlorine
transfer reaction vs those for the proton transfer reaction
gives a reasonably linear correlation with r = 0.9955 and
a slope of 1.06. The slope of nearly unity shows that there
are no unusual differential substituent effects in the two
series, proton and chlorine transfer reactions. Therefore,
assuming that any differential solvent effects, if they
exist, are cancelled out, plotting the kinetics of the
chlorine transfer reaction vs the pKa of the corresponding
conjugated acids of the nucleophiles is in fact identical
with plotting the kinetics of chlorine transfer reaction vs
the thermodynamics of the same reaction. Therefore, the
slope of the plot in Fig. 1 implies that the sensitivity to
substituents of the transition state for the SN2 reaction on
chlorine is nearly double that in the ground state. At the
moment we are unable to offer any explanation as to why
the geometric/electronic structure of the transition state is
so much more sensitive to substituents than the end
product.
We will now focus on the deviation from the line of the
point for PNMA (Fig. 1). As mentioned in the
Introduction, Pearson and Dillon1 showed that proton
transfer reactions involving nitromethane are slower than
expected on the basis of their thermodynamic acidity. It is
important to point out that this behavior is unique to the
case of a single nitro activating group. In cases where
there is an additional negative charge stabilizing group
the anomaly is largely attenuated. Thus, for example, in
the case of two nitro groups on the same carbon, the
anomaly is not doubled but rather disappears as the point
for dinitromethane is nearly on the line of the Brønsted
plot for the other carbon acids.
Figure 2. Ab initio calculated chlorine transfer vs proton
transfer energies
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 540–543

THE NITRO ANOMALY IN CHLORINE TRANSFER REACTIONS
543
In our case, we studied the reaction of nitromethane
with a phenyl group a to the nitro group. However, since
relative to the nitro group the acidifying effect of a phenyl
group is small, the anomaly is not removed and the point
for PMNA deviates negatively from the Brønsted line.
We have not discussed the mechanism of the reaction,
which could either be a simple SN2 reaction or involve an
electron transfer process. Clearly the coupling reaction
between PMNA and CPMN proceeds through an electron
transfer process. However, we have not found any
indication for such a coupling process in any of the other
reactions we have studied. While we are unable to rule
out this possibility, we are more inclined to believe that
the Cl transfer in the reactions that we have studied are
SN2 type and display the nitro anomaly rather than to
suggest that we have discovered that the nitro anomaly is
manifested not only in atom transfer reactions but also in
electron transfer reactions.
The reactions of PMNA with NCS were performed at
pH 7.51, monitoring the disappearance of the absorption
of PMNA (lmax = 280 nm). Although very low concen-
trations of the reactants were used, because of the fast rate
of the reaction, we were able to monitor only the second
half of the reaction. As a result, the experimental error in
the second-order rate constant is Æ10%. The reaction of
PNMA with NCS was performed at pH 8, monitoring the
disappearance of the absorption of PNMA (lmax = 293
nm). The experimental error based on repetitive measure-
ments was 6.5%. The disappearance of the absorption of
PDNMA in its reaction with NCS was monitored at
l
max = 373 nm at pH 8.0. The experimental error in this
case was Æ5%. The reactions with MAA were performed
at pH 8 using an excess of MA in order to minimize the
effect of the consecutive chlorination. The reactions were
monitored by following the disappearance of the MAA
absorption (lmax = 260 nm) with an estimated experi-
mental error of 5%.
EXPERIMENTAL
Instrumentation
REFERENCES
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using a Radimeter PH52 digital pH meter.
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Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 540–543