Experimental
Materials
N,N-Diethylhydroxylamine and N,N-dibenzylhydroxylamine
were purchased from Aldrich in the purest grades available and
were used without further purification. Solvents were also
of the purest grades commercially available and, with the
exception of CCl4, DMSO and HMPA, were deoxygenated by
bubbling with nitrogen and used without purification. The
tetrachloromethane used in the IR experiments was distilled
from phosphorus pentaoxide and great care was then taken to
avoid any contact with moisture in the atmosphere. DMSO and
HMPA were dried over CaH2 and their purity was checked by
IR spectroscopy.
Fig. 2
Both hydroxylamines are very good hydrogen-atom donors
ؒ
to the DPPH radical. Thus, in the absence of hydrogen-
bonding, the kЊ values are 76 and 143 dm3 molϪ1 sϪ1 for diethyl-
and dibenzyl-hydroxylamine, respectively. For comparison, the
kЊ value for hydrogen-atom abstraction by DPPH from phenol
ؒ
is only 0.2 dm3 molϪ1 sϪ1 1
.
ؒ
ꢀH2 Values for hydroxylamines
Kinetic measurements with DPPH
ؒ
Our IR-derived αH2 values for dibenzylhydroxylamine (Table 2)
may be compared with values which can be calculated from the
reported4 equilibrium constants for 1 : 1 complex formation in
CCl4 at 25 ЊC of this compound with three calibrated3 HBAs.
Thus, for diethyl ether (βH2 = 0.45),2 DMSO (βH2 = 0.78)2 and
triethylamine (βH2 = 0.67)2 the reported4 values of Ki/molϪ1
dm3 (with the calculated values of αH2 in parentheses) are 2.3
(0.44), 11 (0.37) and 14 (0.46). Surprisingly, the Ki (and hence
the αH2 ) values do not correlate with the relative HBA abilities
of the three bases (as given by βH2 ). For the strongest base,
DMSO, there is fair agreement with our own measurements
(see Table 2). What of the other two bases? Abraham et al.3
found that for a number of rather weak acids, certain acid–
base combinations had to be excluded from their general
scheme. Dialkylhydroxylamines are certainly weak acids and
diethyl ether is one of the “excluded” bases. However, pyrid-
ine (βH2 = 0.62) is also an excluded base and yet the measured
The maximum of the DPPH absorption band in the UV
(ca. 330 nm) was determined for each solvent using a Hewlett
Packard 8425A diode array spectrophotometer. Deoxygenated
stock solutions of the hydroxylamine, (2.5–50) × 10Ϫ3 mol
dmϪ3, were very rapidly mixed (∼1.3 × 10
s) with equal
Ϫ3
Ϫ4
ؒ
volumes of a deoxygenated stock solution of DPPH (1 × 10
mol dmϪ3) in the same solvent using a Biosequential SX-18 MV
stopped-flow reaction analyzer (Applied Photophysics). The
ؒ
pseudo-first-order decay of the DPPH was monitored at its
band maximum.
IR Spectroscopic measurements
Spectra were recorded on a Midac M FTIR spectrometer using
a CaF2 cell with a pathlength of ∼2.5 mm. Stock solutions con-
taining 5 × 10Ϫ3 mol dmϪ3 of the hydroxylamine were prepared
in tetrachloromethane which had been freshly distilled from
P2O5. Aliquots of 4.5 cm3 of these solutions were mixed with
0.5 cm3 aliquots containing (50–200) × 10Ϫ3 mol dmϪ3 of
DMSO or HMPA in the freshly distilled CCl4. The intensities
of the non-hydrogen-bonded OH stretching band at ca. 3600
cmϪ1 were then recorded on solutions containing 4.5 × 10Ϫ3 mol
dmϪ3 hydroxylamine and five different concentrations of the
HBA in the range (5–20) × 10Ϫ3 mol dmϪ3. The equilibrium
constant, Ki, for 1 : 1 complex formation between the hydroxy-
lamine and HBA was calculated as described above. The αH2
values were then calculated from Ki.
ؒ
rate constant for the (PhCH2)2NOH ϩ DPPH reaction in
pyridine falls on the line correlating log kS with βH2 (see
Fig. 1). This line yields an αH2 value compatible with those
derived from our IR measurements (Table 2), despite the fact
that it is based on rate constants measured in solvents having
smaller βH2 values than pyridine. We are therefore reluctant to
believe that some dialkylhydroxylamine–base combinations
exhibit special features which would exclude them from the
general scheme of Abraham et al.3 We consider it more likely
that Ki values for the H-bonding of dibenzylhydroxylamine
to diethyl ether and to triethylamine were overestimated,
something which is easier to do with weak acid–weak base
combinations than with the weak acid–strong base combin-
ations we employed.
Acknowledgements
This paper is dedicated to the memory of Professor Lennart
Eberson. We thank the Ministero dell’Universitá e della
Ricerca Scientifica e Tecnologica (M.U.R.S.T.) for financial
support for one of us (P.A.) and we thank Professor M. H.
Abraham for providing us with ref. 4.
The KSE-derived αH2 values for both hydroxylamines are in
reasonable agreement with the values determined by IR spec-
troscopy (using DMSO and HMPA as the HBAs, see Table 2).
There is no significant difference in the mean αH2 values for
diethyl- and dibenzyl-hydroxylamine and therefore the overall
mean αH2 = 0.29 should be appropriate for the majority of
sterically non-hindered N,N-dialkylhydroxylamines. However,
αH2 = 0.29 will not be a “universal” value for all hydroxylamines.
Larger, perhaps much larger, αH2 values will apply when electron-
withdrawing groups are attached to the nitrogen atom, both in
ring systems such as N-hydroxypyrroles and N-hydroxyindoles
and in acyclic systems such as bis(trifluoromethyl)hydroxyl-
amine. The range of αH2 values for hydroxylamines may even
rival the range found for 4-substituted phenols, viz. 0.550
for 4-methoxyphenol1 to 0.824 for 4-nitrophenol.3 Experiments
are planned to explore this important facet of hydroxylamine
chemistry.
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