Non-steady-state kinetic study of the S 2 reaction between
N
p-nitrophenoxide ion and methyl iodide in aprotic solvents
containing water. Evidence for a 2-step mechanism
Yun Lu, Kishan L. Handoo and Vernon D. Parker*
Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, USA
Received 5th September 2002, Accepted 7th October 2002
First published as an Advance Article on the web 8th November 2002
Non-steady-state kinetic studies reveal that the SN2
reaction between p-nitrophenoxide ion and methyl iodide
in acetonitrile containing water follows a 2-step mechan-
ism involving the formation of a kinetically significant
intermediate.
effectively serve to differentiate it from the irreversible second-
order mechanism [eqn. (2)] for reactions that are not accom-
(2)
panied by significant primary kinetic isotope effects. These are
Introduction
(
a) extent of reaction–time profiles that differ significantly from
We report the results of non-steady-state kinetic studies of the
SN2 reactions of methyl iodide with p-nitrophenoxide ion in
acetonitrile containing varying amounts of water. The extent of
reaction–time profiles for these reactions deviate significantly
from those expected for the classical single-step displacement of
iodide ion by the nucleophile. The kinetic data are consistent with
the reversible consecutive second-order mechanism [eqn. (1)].
that expected for mechanism (2), (b) initial rate constants (kinit),
which are significantly larger (if reactant decay is monitored) or
significantly smaller (if formation of product is monitored)
than the apparent pseudo first-order rate constants (kpfo) meas-
ured at longer times and (c) the ratio of times necessary to reach
an extent of reaction equal to 0.50 and that required to reach
extent of reaction equal to 0.05 (t0.50/t0.05) which are either sig-
nificantly greater than 13.5 (reactant monitored) or less than
13.5 (product monitored). For reactions following mechanism
(2), kinit/kpfo values are predicted to be equal to 1.00 and t0.50/t0.05
values are expected to be equal to 13.5. The latter are also the
limiting values for the mechanism probes for mechanism (1)
under conditions where the two mechanisms are kinetically
indistinguishable.
In recent years the emphasis of studies of the S 2 reactions
N
of alkyl halides with nucleophiles has shifted from reactions in
1–3
4–6
solution to gas-phase reactions. In solution, the generally
accepted mechanism for the classical S 2 reactions of methyl
N
iodide with nucleophiles involves a single transition state
7
between reactants and products, and the most recent studies
have been directed toward adding detailed knowledge of the
factors affecting the structure of the transition state. To the best
of our knowledge, experimental data have not been presented
which questions the single step mechanism for the reactions of
alkyl halides with nucleophiles in solution.
The experimental data summarized in Table 1 were derived
from stopped-flow absorbance–time curves in which absorb-
ance due to p-nitrophenoxide at 420 nm was monitored. The
data are inconsistent with the irreversible second-order mech-
The reactions of various methyl derivatives with phenoxide
ion have been compared to those of p-nitrophenoxide ion in
sulfolane solution. The selectivity, defined as the ratio of rate
anism for the S 2 reaction between p-nitrophenoxide ion and
N
methyl iodide. In all cases, kinit/kpfo are significantly greater than
unity and t0.50/t0.05 are significantly greater than 13.5. On the
other hand, the values of the mechanism probes suggest that
the data are consistent with the reversible consecutive second-
order mechanism (2).
8
constants for the reactions of phenoxide to that of p-nitro-
phenoxide, was observed to increase from 2.8 for reaction with
ϩ
(
CH ) O to 28 for reaction with CH I. No deviations from the
3
3
3
classical S 2 mechanism were reported.
The conclusions expressed in the previous paragraph are
strongly reinforced by extent of reaction–time profiles shown
in Fig. 1 (0.04 v/v % water), Fig. 2 (0.6 v/v % water) and Fig. 3
(2 v/v % water). In all three figures the experimental data are
indicated with solid circles, the theoretical best-fit data (mechan-
ism 1) are shown as solid lines and the response expected for
N
Our recent work employing non-steady-state kinetic meas-
urements has shown that previously undetected kinetically
significant intermediates are involved in a number of organic
reactions including proton transfer reactions of arylmethyl rad-
9
–11
ical cations,
the proton transfer reaction between a nitro-
12,13
alkane and hydroxide ion,
NADH model compound and the Diels–Alder reaction
a hydride transfer reaction of an
4
the concerted S 2 (mechanism 2) are represented by the lower
N
1
lines. The uniformly excellent fit between experimental data and
theoretical data for mechanism (1) is the outstanding feature
in all three figures. This is contrasted by the large deviations
shown between the experimental data and the theoretical lines
for mechanism (2).
15
between anthracene and tetracyanoethylene. The purpose of
the work reported here was to attempt to obtain data which
implicate a kinetically significant intermediate in the classical
SN2 reaction between methyl iodide and a nucleophile. The
latter reaction has long been considered the prototype for
single-step concerted reaction mechanisms.
13
The procedure for fitting experimental and theoretical data
involves systematically increasing k from the apparent rate
f
constant evaluated in the conventional manner (kapp) and at
each k varying k until the best fit is found at the particular k .
The input files for the fitting program consisted of extent of
reaction–time profiles for all three concentrations of methyl
iodide and the iterations of rate constants were applied concur-
f
p
f
Results and discussion
There are three experimental observations characteristic of the
reversible consecutive second-order mechanism [eqn. (1)] which
(
1)
3
6
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 6 – 3 8
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3