point (~10) from the slope of the curve and the average of these
readings was taken as the pKa for the sulfamate. A pKa value
for +NH3SO2OC6H4NO2-4 was calculated using the Advanced
Chemistry Development (ACD) computer program.37
varying concentrations of Et2NH.9 In a partial product run a spent
rate (a solution that has been allowed to react for ≥10 half lives) in
water solution of 1a (original concentration 1 ¥ 10-4 M) which had
been reacted at 50 ◦C at pH 3 gave an absorbance for 4-nitrophenol
which was within 91% of a ‘mock infinity’ solution at pH 3 made
up with 1 ¥ 10-4 M 4-nitrophenol and 1 ¥ 10-4 M sulfamic acid.
In another type of ‘bulked-up’ product run the sulfamic acid
produced on complete reaction of 1a (initial concentration 4.58 ¥
10-4 M) was determined gravimetrically as barium sulfate and the
amount of BaSO4 obtained corresponded to 98% of the expected
sulfate. It is important to note that hydrolysis of sulfamic acid
produced in the rate runs in this current work will not occur under
Kinetics
The Cary 100 and the Kontron Uvikon 941 UV–vis instruments
were used mainly for the rate studies. The substrate concentration
was normally 1 ¥ 10-4 M. A suitable kanal. was chosen and the
increase in absorbance due to the production of phenol/phenoxide
product against time was plotted and from this the infinity
◦
absorbance was deduced. A plot of log(Ainf - At) vs. t, where Ainf
=
the prevailing conditions. At 50 C and even at pH 1 the t1/2 of
sulfamic acid is ~18 days.40
the absorbance at infinity and At = absorbance at time, t, was then
made and from the slope of the straight line the rate constant was
obtained using a linear least square method. Runs were repeated in
duplicate or triplicate and followed for at least 4 half lives. The rate
constants obtained were usually reproducible to within the limits
stated in the Table S5 footnotes. Where comparison is possible with
other independent work the agreement between the rate constants
determined in this current work and literature data is very good.
For example, Blans and Vigroux38 report a rate constant of 1.8 ¥
10-5 s-1 for hydrolysis of compound 1i under identical conditions
to those used in Table S6; the value from this Table is 2.06 ¥ 10-5 s-1.
Thus, the two rate constants are within 7% of each other. A rate
constant of 9.85 ¥ 10-6 s-1 has ◦been reported for the hydrolysis of
1a at pH 2, m = 1.0 M KCl at 25 C6 and in this work a rate constant
of 10.1 ¥ 10-6 s-1 was found under the same conditions and thus
the difference in rates is less than 3%. Using the same conditions
the rate of disappearance of 1a was followed at 265 nm to give a
rate constant of 8.64 ¥ 10-6 s-1 in the present studies which is in
reasonable agreement with the above values.
For compounds 4, reaction stoichiometry was checked by
comparing the UV–vis spectra of spent reaction mixtures with
those of “mock infinity” solutions prepared from calculated
amounts of 4-nitrophenol and the relevant N-benzylsulfamic acid
at some selected pHs. In all cases, calculated yields were close to
100%.
Acknowledgements
The Corrib and Millennium funds of NUI, Galway and the
Institute of Chemistry of Ireland are thanked for grants. N. P.
M. thanks Donegal Co. Co. for a grant. Professor T. W. Bentley,
University of Wales, Swansea, UK is thanked for helpful advice
and J.-B. Malaubier is thanked for help with the Schemes and
Figures.
References
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controlled at 25 ◦C and on mixing the two solutions voltage
is recorded at millisecond intervals and the OLIS-KINFIT pro-
gramme converts this to absorbance at 400 nm and produces a rate
constant.
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Product studies
Reaction of 1a in aqueous ACN at 37 ◦C with Et2NH present
gave quantitative recoveries by HPLC of sulfamate, sulfamide and
4-nitrophenoxide with varying mixtures of water and ACN and
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