Chemistry Reference
In-Depth Information
was refluxed for five hours. About 25 ml of water was then added and the mixture was cooled in ice.
Colorless crystals formed and were filtered off, sucked partially dry, and sublimed at about one Torr. Yield
of di-
tert
-butyl ketoxime: 3.3 g (46 %).
19
Some 24 years later, a simpler and more elegant synthesis of this ketoxime (and other symmetric di-
tert
-
alkyl ketoximes) was developed by Mendenhall and coworkers (Equation 6.7).
20
In their new procedure the
alkylating agent was generated
in situ
from sodium and the tertiary chloride, thus conveniently avoiding
expensive, pyrophoric reagents.
=
=
2Na
+
t
-RCl
+
t
-RCN
→
NaCl
+
t
-R
2
C
NNa
→→
t
-R
2
C
NOH
(6.7)
6.2.2 Synthesis of di-
tert
-butyliminoxyl
19
The ketoxime (0.3 g) in 30 ml benzene was shaken 90 min with 0.7 g silver oxide. The resulting blue
solution was filtered and the filtrate was concentrated at 25
◦
C to a blue oil (0.22 g, 74 %) which was
distilled twice
in vacuo
by the bulb-to-bulb procedure with the receiver immersed in liquid nitrogen.
Di-
tert
-Butyliminoxyl is a blue liquid, freezing point
21
◦
C,
n
294K
D 1.4452,
d
295K
−
0.824, with a weak
absorption extending from about 530 nm to beyond 800 nm,
ε(
800 nm
)
=
5
.
1 in benzene, 5.0 in 95 %
ethanol, and 4.7 in cyclohexane.
19
The IR spectrum shows an intense absorption at 1610 cm
−
1
from C
=
N and/or N-O stretching vibrations.
A peak at 1650 cm
−
1
in the parent ketoxime (C
=
N vibration) does not appear in the spectrum of the radical.
However, peaks at 2960, 1480, 1390, and 1366 cm
−
1
are also present in the IR spectrum of di-
tert
-butyl
ketone and can, therefore, be assigned to C-H vibrations.
The EPR spectrum of di-
tert
-butyliminoxyl in dilute solutions is split into a triplet due to interac-
tion of the unpaired electron with the
14
N nucleus. The magnitude of the
14
N splitting shows a very
14
N splitting of di-
tert
-butyl nitrox-
much smaller dependence on the solvent than is the case for the
ide. For example, at 23
◦
C(Me
3
C)
2
C
=
NO
•
has
a
N
=
31
.
32 G in benzene and 31.55 G in acetic acid,
whereas (Me
3
C)
2
NO
•
has
a
N
44 G in benzene and 16.46 in acetic acid.
19
The lack of a sig-
nificant solvent effects on both the EPR and visible spectra can be rationalized by noting that the
nitrogen atom in the iminoxyl is approximately
sp
2
hybridized and is, therefore, less polarizable and
less basic than the
sp
3
hybridized nitrogen atom in the nitroxide. The orbital energies and electron dis-
tribution in the iminoxyl are, therefore, less affected by a change in solvent polarity than is the case for
the nitroxide.
The three main lines in the EPR spectrum of (Me
3
C)
2
C
=
.
15
NO
•
show additional hyperfine splittings by
the hydrogen atoms. At 23
◦
C, the relative intensities of these lines are not consistent with 18 equivalent
protons. However, the multiplet structure at
=
50
◦
C was simulated using two groups of nine protons with
−
a
H
481 and 0.772 G; these correspond to the protons on the anti and syn
tert
-butyl groups,
respectively.
21
(
9H
)
=
0
.
50
◦
C, the multiplet structure changes because
inversion of the C
=
NO
•
moiety becomes faster and the distinction between the anti and syn
tert
-butyl
protons becomes blurred. At 72
◦
C, inversion is rapid on the EPR time scale (
k
inv
=
As the temperature was increased above
−
10
6
s
−
1
1
.
3
×
)
and
all eighteen protons become equivalent. Under these conditions,
a
H
627 G, which is the mathemat-
ical average of the anti and syn proton hyperfine splittings given above. This proves that the spin on
both sets of protons has the same sign. This sign was determined by taking the
=
0
.
1
H NMR spectrum of
neat (Me
3
C)
2
C
=
NO
•
.
21
1600 Hz)
shifted to
low
field.
21
The average spin density on the
tert
-butyl protons is therefore
positive
in sign.
The paramagnetic shift relative to (Me
3
C)
2
C
=
NOH is
At room temperature this spectrum shows a
single
broad peak (
H
∼
7 ppm, which yields
a
H
61 G, in
excellent agreement with the EPR results. This positive spin on the
tert
-butyl protons in (Me
3
C)
2
C
=
NO
•
contrasts with the
−
45
.
(
av
)
=+
0
.
radical, (Me
3
C)
2
NO
•
, where the spin on the protons is negative,
a
H
11 G.
21
π
=
0
.
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