79Br NMR spectroscopy as a practical tool for kinetic analysis

Article abstract of DOI:10.1002/mrc.2401

⁷⁹Br NMR spectroscopy has been used to monitor a series of reactions in which the bromide ion is produced, including the Menschutkin reaction of pyridine with a range of substituted benzyl bromides and a Heck coupling process. In cases where the process could also be monitored using ¹H NMR spectroscopy, the kinetic analyses using heteronuclear magnetic resonance spectroscopy were shown to be completely consistent. Both the utility of the process in following reactions which may be difficult to analyse using other techniques and the practical limitations associated with solvent choice are discussed. Copyright

Full text of DOI:10.1002/mrc.2401

Research Article
Received: 19 October 2008
Revised: 25 November 2008
Accepted: 13 December 2008
Published online in Wiley Interscience: 2 February 2009
(www.interscience.com) DOI 10.1002/mrc.2401
79Br NMR spectroscopy as a practical tool for
kinetic analysis
Si Jia Chan,a Andrew G. Howe,a,† James M. Hookb and Jason B. Harpera∗
79Br NMR spectroscopy has been used to monitor a series of reactions in which the bromide ion is produced, including the
Menschutkin reaction of pyridine with a range of substituted benzyl bromides and a Heck coupling process. In cases where
the process could also be monitored using 1H NMR spectroscopy, the kinetic analyses using heteronuclear magnetic resonance
spectroscopy were shown to be completely consistent. Both the utility of the process in following reactions which may be
difficult to analyse using other techniques and the practical limitations associated with solvent choice are discussed. Copyright
c
ꢀ 2009 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: NMR; 79Br; 1H; kinetic analysis; Menschutkin reaction
Introduction
methanol and an ionic liquid.[10,11] Herein we report an extension
of this latter approach which uses 79Br NMR spectroscopy.
The linewidth of signals in a 79Br NMR spectrum, as for all
quadrupolar nuclei, is markedly dependent on the environment
of the nucleus; a symmetrical electronic distribution around the
bromine centre results in, effectively, zero field gradient and hence
relatively sharp signals while quadrupolar relaxation (and hence
line broadening) is more significant in cases where the nucleus is
in an asymmetric environment.[8] Given our previous experience
with 35Cl,[10,11] it was anticipated that organobromine compounds
would not give rise to an observable signal, with bromide ion
producing the only signal observed in the 79Br NMR spectrum. The
methods used below are based on the fact that the NMR signal
is proportional to the number of nuclei present and hence the
concentration of the sample. Thus the amount of the bromide ion
evolved during the process can be readily determined.
The79Brnucleusmightbeconsideredtheslightlylessfavourable
of the bromine nuclei for NMR studies, as while it is the slightly
dominant isotope, it has the lower sensitivity and the signals
have larger linewidth.[12] However, in determining if bromine
NMR spectroscopy is useful for following the progress of a
reaction neither of these relatively small differences is particularly
significant, since only one signal is likely to be observed. Further,
the reactions examined involve generating a bromide ion in the
presence of one of the range of cations and the effect of a cation
on the 79Br NMR signal of bromide has been well-described.[13]
Finally, the resonance frequency of 79Br is similar to that of 13C
and thus it can be observed on a 13C channel without the need for
NMR spectroscopy offers a particularly powerful tool for following
the progress of chemical reactions, not least because it allows
continuous monitoring of the process without the need to sample
a reaction mixture but also because the precision and accuracy
of quantitative NMR spectroscopy have been shown to rival
chromatographic methods such as HPLC and GC.[1] However,
the commonly examined nuclei, such as1H, may not always
be appropriate to follow a given reaction, often as a result of
components of the reaction mixture contributing signals that
obscurethesignalofinterest. Thisisparticularlyproblematicwhen
solvents which are NMR silent (such as deuterated solvents for 1H
NMR spectroscopy) are either not available or are prohibitively
expensive.[2] While methods have been developed to overcome
the problem of overlapping resonances, such as single-scan 2D
methods,[3] the presence of heteroatoms in the systems offers
another alternative.
The principal advantage of heteronuclear magnetic resonance
1
spectroscopy over H NMR spectroscopy is the reduction in the
number of signals in a spectrum. This has been exploited for
following the kinetics of various processes using nuclei with spin
number, I, having a value of 1/2 including 15N,[4] 19F,[5] 29Si,[6] and
31P.[7] However not all reaction mixtures to be examined contain
these nuclei, hence other nuclei need to be considered.
Quadrupolar nuclei, those with I > 1/2, are generally harder to
study than those with I = 1/2 due to their dominant quadrupolar
relaxation characteristics and the fact that they often have poor
sensitivityduetolownaturalabundanceandmagnetogyricratio.[8]
However, both these factors do not eliminate such nuclei from
consideration as line broadening is less of an issue in spectra
with few signals, or even one in some cases, and the rapid
relaxation times allow a large number of scans to be accumulated
in a given time, overcoming the inherently low sensitivity. We
have previously reported the use of 17O NMR spectroscopy to
quantify oxygenated additives in fuels[9] and the utility of 35Cl
NMR spectroscopy to monitor a solvolysis process in mixtures of
∗
Correspondence to: Jason B. Harper, School of Chemistry, University of New
South Wales, Sydney NSW 2052, Australia. E-mail: j.harper@unsw.edu.au
†
Visiting from the Department of Chemistry and Chemical Biology, Harvard
University, 12 Oxford Street, Cambridge, MA 02138, USA.
a
SchoolofChemistry,UniversityofNewSouthWales,SydneyNSW2052,Australia
Analytical Centre, University of New South Wales, Sydney NSW 2052, Australia
b
c
Magn. Reson. Chem. 2009, 47, 342–347
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.

79Br NMR spectroscopy as a practical tool for kinetic analysis
a broadband probe.[12] The arguments presented here, however,
might equally well be seen as demonstrating the practical utility
and limitations of the use of 81Br NMR spectroscopy in following
chemical kinetics; to date neither of these nuclei has been used
to follow the progress of the reaction. (The use of 81Br NMR
spectroscopy in mechanistic elucidation to identify bromide ion
as a product has been reported.[14] Conceptually, such a method
might be extended to follow reaction kinetics as outlined here.)
It is worth noting that throughout the work presented here,
conditions such as reagent concentration and temperature reflect
those that are typical for a given process. While increasing the
concentration of species under consideration might increase the
signal to noise ratio of the spectra obtained and changing the
temperature might reduce the linewidth of a signal, it is following
the reaction progress under typical conditions which is required
for this technique to be practically useful.
modification of literature procedure[22] through the addition of
brominetoethyltrans-cinnamateandthechalcone8, respectively.
The alkyl bromide 12 was prepared through modification of
literature procedure,[20] from the corresponding alcohol 12 and
phosphorus tribromide and identified using the key resonance
in the 13C NMR spectrum at δC 74.6. Ethanol was distilled from
sodium ethoxide and acetone was purified using silver nitrate.[23]
All other reagents were used as received.
1H NMR and 79Br NMR spectra were either recorded on a Bruker
DMX 500 Avance spectrometer (500 MHz, 1H; 125.3 MHz, 79Br) or
on a Bruker DPX spectrometer (300.17 MHz, 1H; 75.19 MHz, 79Br)
using ca 0.8 ml of either sample or reaction mixture in a 5-mm
NMR tube. Spectra were referenced externally against TMS (1H; δH
0) and 0.05-mol l−1 sodium bromide in deuterium oxide (79Br; δBr
0). Typical instrument parameters used for 79Br NMR spectroscopy
were p90 = 13 µs (−4.0 dB), AQ = 20 ms, d1 = 1 ms and 5000
scans. Line broadening was set at 0.2 Hz and integration was done
manually due to a slight change in the chemical shift of signal due
to the bromide ion as the reactions proceeded and the bromide
ion concentration increased.
Kinetic analyses of the Menschutkin reactions were carried out
in acetonitrile containing the appropriate benzyl bromide 1 (ca
0.05 mol l−1) and pyridine 2 (0.50 mol l−1) at 298 K. In each case the
reaction was followed using both 1H and 79Br NMR spectroscopies
until more than 90% of the starting material was consumed.
Spectra were taken at regular intervals during the reaction and the
extent of reaction calculated through integration of the signals
corresponding to benzyl protons in the starting materials 1a–e
(ca δH 4.5) and the products 3a–e (ca δH 5.8), and the bromide ion
(ca δBr 75). (At least 20 of each type of spectrum were obtained
for each kinetic run.) From this information, the pseudo-first-order
rate constant for the reaction of each of the species 1a–e under
these conditions was calculated.
The reactions chosen for examination in this study are
summarised in Schemes 1–5 below and represent a range of
organicprocessesinwhichbromideisgenerated.TheMenschutkin
reaction of each of the substituted benzyl bromides 1a–e with
pyridine 2 to give the corresponding benzylpyridinium bromides
3a–e (Scheme 1) is representative of the substitution process
in which bromide is produced.[15] It also offers the advantages
of being readily followed using 1H NMR spectroscopy and that
variation of the substituent on the bromides 1a–e allows tuning
of the rate of the reaction in order to test the scope of this
spectroscopic method. The elimination of hydrogen bromide from
the dibromide 4 to give the ester bromides 5 and 6 (Scheme 2)[16]
and the conversion of the dibromoketone 7 to the corresponding
chalcone 8 (Scheme 3)[17] are examples of elimination processes.
The palladium catalysed coupling of the bromotoluene 9 and
the acetal 10 (Scheme 4)[18] is an example of the Heck reaction,
a process widely used in synthetic organic chemistry and can
1
be difficult to follow using H NMR spectroscopy.[19] Finally, the
The elimination of hydrogen bromide from the dibromide
4 at 343 K was followed by monitoring the reaction mixture
[dibromide 4 (ca 20 mg), sodium acetate (ca 30 mg) in d6-ethanol
solvolysis of the bromide 12 (Scheme 5) was considered as it
is a further example of the substitution process and it cannot
1
(1.0 ml)] using both H and 79Br NMR spectroscopies until more
1
be followed using H NMR spectroscopy due to overlapping of
than 90% of the starting material was consumed. Spectra were
taken at intervals during the reaction and the extent of reaction
calculated through integration of the signals corresponding to the
bromoalkyl protons in the starting material 4 (ca δH 4.9, 5.4) and
the bromoethylene proton in the product 6 (ca δH 8.2). (At least 20
of each type of spectrum were obtained for each kinetic run.) From
this information, pseudo-first-order rate constants for the reaction
under these conditions were calculated. No signal corresponding
to the bromide ion was observed in the 79Br NMR spectra.
signals corresponding to the starting material 12 and the product
13.[10,20]
Experimental
The benzyl bromides 1a–e were either commercially available
or prepared from commercially available starting materials using
literature methods.[21] The dibromides 4 and 7 were prepared by
a X = OMe
b X = Me
c X = H
d X = Cl
N
Br
CH3CN
N
Br
X
X
3a-e
1a-e
2
e X = NO2
Scheme 1. The Menschutkin reaction between the benzyl bromides 1a–e and pyridine 2.[15]
Br
CO2Et
Br
CO2Et
NaOAc
AcOH + Br
Br
CO2Et
Br
EtOH
4
5
6
Scheme 2. The elimination of hydrogen bromide from erythro-ethyl cinnamate dibromide 4.[16]
c
Magn. Reson. Chem. 2009, 47, 342–347
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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S. J. Chan et al.
O
Br
O
KOH
HOBr + Br
Br
Acetone
7
8
Scheme 3. Treatment of the dibromide 7 yields the chalcone 8.[17]
OEt
OEt
Br
Pd(OAc)2
NBu4OAc
OEt
t
AcOH + Br
K2CO3
KCl, DMF
OE
10
9
11
Scheme 4. Heck reaction of 4-bromotoluene 9 with acrolein diethyl acetal 10.[18]
Br
OH
Table 1. Pseudo-first-orderrateconstantsforthereactionsdescribed
in Scheme 1, as monitored using either 1H or 79Br NMR spectroscopy.
Rates are the average of at least three kinetic runs and reported
uncertainties represent two standard deviations from the mean
NaHCO3
H2O + CO2 + Br
CH3CN/H2O
1
Substrate
k
obs/s−1 − H
k
obs/s−1
−
79Br
1a
1b
1c
1d
1e
(2.2 0.1) × 10−3
(6.9 0.4) × 10−4
(3.7 0.2) × 10−4
(3.4 0.2) × 10−4
(2.2 0.1) × 10−4
(2.3 0.3) × 10−3
(6.2 0.6) × 10−4
(3.7 0.1) × 10−4
(3.3 0.3) × 10−4
(2.2 0.1) × 10−4
12
13
Scheme 5. Solvolysis of the tertiary alkyl bromide 12.[20]
The formation of the chalcone 8 from the dibromide 7 [ca
25 mg, dissolved in a d6-acetone solution of potassium hydroxide
(0.36 mol l−1, 1.0 ml)] at 298 K was monitored using both 1H and
79Br NMR spectroscopies until more than 90% of the starting
material was consumed. Spectra were taken at intervals during the
reaction and the extent of reaction calculated through integration
of the signals corresponding to the bromoalkyl protons in the
starting material 7 (ca δH 5.6, 5.8) and the bromide ion (ca δBr 53).
Reaction mixtures for the Heck coupling process were prepared
by dissolving 4-bromotoluene 9 (ca 45 mg), acrolein diethyl acetal
10 (ca 100 mg), palladium acetate (ca 40 µg, added through
addition of ca 2 mg ml−1 solution), potassium carbonate (ca
50 mg), potassium chloride (ca 20 mg) and tetrabutylammonium
acetate (ca 150 mg). The coupling reaction was monitored at 343 K
using both 1H and 79Br NMR spectroscopies until more than 90%
of the starting material was consumed. Spectra were taken at
intervals during the reaction and the extent of reaction calculated
through integration of the signals corresponding to the acetal
proton of the product 11 (ca δH 5.3) and the bromide ion (ca δBr
97). At least 20 of each type of spectrum were obtained for each
kinetic run.
due to the bromide ion was observed to change on modification of
the cation. There was a general, though not completely consistent,
trend of increasing linewidth with increasing electron-donating
natureofthesubstituentonthephenylring. Itisunlikelythatthisis
due to changes in ion-pairing in solution both because increasing
electron donation to the cationic centre is anticipated to result in
a decrease in ion-pairing and hence a decrease in line broadening
but also because previous studies[13] have demonstrated that ion-
pairing is of limited importance in these systems. It is more likely
that changing the electronic nature of the substituent alters the
solvent–anion interactions; this will be touched on below.
The reaction of each of the bromides 1a–e with pyridine
was monitored using both H and 79Br NMR spectroscopies (an
1
example of the latter is shown in Fig. 1) by interleaving data
acquisition and thus the method was directly evaluated. The rate
constants for these reactions were calculated using each set of
spectroscopic data and the values are summarised in Table 1. The
first key feature is the reproducibility of the kinetic data, with the
uncertainties in the rate constants being ca 10% at most. However,
more importantly, the rate constants determined using either 1H
or 79Br NMR spectroscopy for each of the substrates 1a–e were
the same within uncertainty. This indicates that this 79Br NMR
spectroscopic method can be used to accurately measure rates of
reaction in these systems. Further, these results demonstrate that
sufficiently accurate 79Br NMR spectra to determine rate constants
could be obtained for reactions with half-lives as short as 7 min (as
was the case for the bromide 1a).
For the solvolysis of the bromide 12, reaction mixtures were
prepared by adding 3-bromo-3,7-dimethyloctane 12 (165.9 mg,
0.75 mmol) to a mixture of saturated NaHCO3 solution, acetonitrile
and acetonitrile-d3 (1.2 ml, ratio 0.65: 0.29: 0.06). The production of
bromide ion was monitored at 298 K using 79Br NMR spectroscopy
until no further production of bromide ion was evidenced (ca δBr
97).
Results and Discussion
The elimination of hydrogen bromide from the dibromide 4
couldbereadilyfollowedusing1HNMRspectroscopyandpseudo-
first-orderrateconstantsfortheprocessobtained. However, under
the conditions of the experiment, no signals were observed in the
Initialstudiesofthesalts3a–efromtheMenschutkinreactionwere
conducted to evaluate whether these salts would give reasonable
79Br NMR spectra. Interestingly, the line broadening of the signal
c
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Magn. Reson. Chem. 2009, 47, 342–347

79Br NMR spectroscopy as a practical tool for kinetic analysis
Figure 1. Stacked plots of 79Br NMR spectra for a Menschutkin reaction of the bromide 1b with pyridine at 298 K. The first spectrum was that acquired
after 145 s and each subsequent spectrum occurs in increments of 217 s. The signal corresponds to bromide anion.
1
79BrNMRspectrum.Previously,linebroadeninginsuchspectrahas
been observed on going from aqueous to alcoholic solvents,[24]
though in this case it demonstrates a limitation of the process
in terms of solvent choice when attempting to obtain spectra of
dilute solutions in short time frames.
components of the reaction mixture obscured signals in the H
NMR spectra due to the starting material and the product. As such,
79Br NMR spectroscopy represents a practical method of following
this process though it is worth noting that under the conditions
used, the half-life of the process (ca 8 min) is at the limit of that
which can be practically followed using 79Br NMR spectroscopy.
Next considered was the Heck coupling of reagents 9 and 10.
This type of reaction might be considered typical of those for
which 79Br NMR spectroscopy might be useful as the reaction
mixture contains many components, resulting in complex 1H NMR
spectra that are not readily integrated. This is demonstrated in
Figs 3 and 4, which illustrate the relative precision of the two
techniques in these cases. While in this case, the signal due to the
acetal proton on the coupled product 11 can be identified, it is
not readily integrated due to the presence of large nearby signals.
The result is Fig. 3, which clearly shows sigmoidal character but
a significant degree of scatter in the observed data. The same
problems are not present in the 79Br NMR spectra and, as such, the
same sigmoidal shape is observed but with substantially greater
precision.
A brief survey of some common solvents demonstrated that
under these conditions (ca 0.05 mol l−1 bromide, 5000 scans)
signals were observed in the 79Br NMR spectra for acetonitrile,
acetone, N,N-dimethylformamide (DMF) and water while no
signals were observed for methanol, ethanol and isopropanol.
It is clear by considering viscosity values for these solvents[25]
that viscosity alone cannot account for the lack of an observable
signal. Studies on the linewidth of the signal due to the bromide
ion in methanol/water systems, which we have reproduced below
under our conditions (Fig. 2), have attributed the change to a
combination of the viscosity and the dielectric constant of the
medium.[24] It is perhaps useful to consider this at the molecular
level,andtheeffectofthesolventonthesymmetryoftheelectronic
distribution (and hence field gradient) about the bromine atom. In
systems where the symmetry is preserved, such as acetonitrile,[14]
a signal is observed. In systems where hydrogen bonding to
the bromide ion occurs, contributions from the ion–solvent
and solvent–solvent interactions are significant. If these two
interactions cancel one another out, as is the case in water,[14,26]
the result is spherical symmetry and a signal is observed. If they
do not, as is the case for methanol where the solvent–solvent
interactions are notably different,[14,27] an asymmetric bromide
environment results in significant line broadening. Irrespective,
these results demonstrate that as a practical tool for chemical
kinetics, 79Br NMR spectroscopy is not appropriate for reactions in
whichalcoholsareusedasthesolvent, unlessinasmallproportion.
The reaction of the cinnamate dibromide 4 was also attempted
using d6-acetone as a solvent and replacing the sodium acetate
with an acetone soluble base, tetrabutylammonium acetate. The
signal due to the bromide ion in 79Br NMR spectra could be readily
observed, however the reaction had gone to completion prior to
the acquisition of the first spectrum. As such, investigations were
discontinued and the elimination of bromine from the chalcone
dibromide 7 was considered. At 298 K in d6-acetone, this process
was readily followed using 79Br NMR spectroscopy, however other
The final reaction considered was the solvolysis of the bromide
12 to the corresponding alcohol 13. The starting material 12 and
the product 13 have significant overlapping resonances in the 1H
NMR spectrum which, along with the presence of protonated
solvent, makes following the reaction impossible using this
method (Figure S1, Supporting Information). (We have previously
examined using 35Cl NMR spectroscopy for following reactions
of the corresponding chloride for similar reasons.)[10,11] However,
when the reaction was followed using 79Br NMR spectroscopy,
the production of the bromide ion could be readily observed
without interference of other signals (Figure S2, Supporting
Information) demonstrating that this method can be used for
systems in which 1H NMR spectroscopy is inappropriate. It is worth
noting that the nature of the base is significant here as, when
triethylamine was used, no signal corresponding to the bromide
was observed despite being detected using other methods (such
as addition of silver(I)). It is thought that this is due to interaction
of the bromide with the protonated triethylamine, introducing
asymmetry about the bromide atom and hence significant signal
broadening.
c
Magn. Reson. Chem. 2009, 47, 342–347
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S. J. Chan et al.
Figure 2. Stacked plots of 79Br NMR spectra for solutions containing ca 0.05 mol l−1 sodium bromide at 298 K. The first spectrum is taken in 10% v/v
methanol in water and each subsequent spectrum has the amount of methanol increased by 10%.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
80
160
240
320
0
80
160
240
320
time/min
time/min
Figure 4. Graph of the integral corresponding to the appearance of the
79Br NMR signal at δBr 97 of the reaction of 4-bromotoluene 9 and acrolein
diethyl acetal 10 with 5 mmol l−1 palladium (II) acetate at 343 K.
Figure 3. Graph of the integral corresponding to the appearance of the
1H NMR signal at δH 5.25 of the reaction of 4-bromotoluene 9 and acrolein
diethyl acetal 10 with 5 mmol l−1 palladium (II) acetate at 343 K.
Acknowledgements
Conclusions
AGH would like to acknowledge the support of a Herchel
Smith Harvard Summer Undergraduate Research Fellowship. JBH
acknowledges financial support from the University of New South
Wales Faculty Research Grants Scheme. The contributions of M.
Pratt, R. Young, D. Goldstein, D. Berger and H. Lau in undertaking
preliminary experiments is also gratefully acknowledged as is the
support from the staff of the NMR Facility of the UNSW Analytical
Centre. All authors would like to thank E. Davies, C. E. Rosella and
H. M. Yau for constructive comments during the preparation of
this manuscript.
The results described demonstrate that 79Br NMR spectroscopy
can be readily used to follow processes in which a bromide ion
is generated. These can range from simple first order processes
to more complicated systems and in the case of the former rate
constantscalculatedusingthisandotherspectroscopictechniques
are in good agreement. Some limitations in terms of the choice
of solvent have been shown, based on line broadening of the
bromide signal, however the overall utility has been demonstrated
by considering reactions that cannot be readily analysed using
conventional NMR techniques. It is anticipated that a range of
processes in which bromide ions are generated might now be
easily monitored.
Supporting information
Supporting information may be found in the online version of this
article.
c
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79Br NMR spectroscopy as a practical tool for kinetic analysis
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