Acidity and basicity of primary N-phenylnitramines: Catalytic effect of protons on the nitramine rearrangement

Article abstract of DOI:10.1002/poc.461

Para-substituted N-phenylnitramines were prepared either by oxidation of diazonium salts or by nitration under alkaline or acidic conditions. Isotopic [¹⁵N-NO₂] labelling indicated that the bands characteristic of the N-nitro group appear in the 1318-1323 and 1585-1607 cm^-1 regions. In the nitrogen NMR spectra, the nitramino group gives two resonances at -193 ± 3 (NH) and -32 ± 3 ppm (NO₂). The chemical shifts in proton and carbon NMR spectra are predictable, based on increments and the additivity rule. The spectral data indicate the lack of conjugation between the nitramino group and another substituent bound to the ring. It seems to contradict the well-known fact that substituents strongly (ρ = 4) influence the rate of nitramine rearrangement. The acidities of primary N-phenylnitramines (3.77 < pK_A < 5.62) are similar to those of benzoic acids and weakly dependent (ρ = 1) on the electronic character of a substituent. Based on the analogy with benzoic acids, it has been calculated that basicities of nitramines (pK_B ≈ 21) are extremely low. Consequently, addition of protons to an intact nitramine molecule, as the preliminary step of the rearrangement, seems to be improbable. Migration of the N-nitro group precedes protonation; the latter process facilitates transformation of intermediates into stable final products. Copyright

Full text of DOI:10.1002/poc.461

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2002; 15: 115–122
DOI:10.1002/poc.461
Acidity and basicity of primary N-phenylnitramines:
catalytic effect of protons on the nitramine rearrangement
Zdzisraw Daszkiewicz,* Grzegorz Spaleniak and Janusz B. Kyzior
Institute of Chemistry, University of Opole, 45-052 Opole, ul. Oleska 48, Poland
Received 5 June 2001; revised 8 October 2001; accepted 8 October 2001
ABSTRACT: Para-substituted N-phenylnitramines were prepared either by oxidation of diazonium salts or by
nitration under alkaline or acidic conditions. Isotopic [15N-NO2] labelling indicated that the bands characteristic of
the N-nitro group appear in the 1318–1323 and 1585–1607 cmÀ1 regions. In the nitrogen NMR spectra, the nitramino
group gives two resonances at À193 Æ 3 (NH) and À32 Æ 3 ppm (NO2). The chemical shifts in proton and carbon
NMR spectra are predictable, based on increments and the additivity rule. The spectral data indicate the lack of
conjugation between the nitramino group and another substituent bound to the ring. It seems to contradict the well-
known fact that substituents strongly (ꢀ = 4) influence the rate of nitramine rearrangement. The acidities of primary N-
phenylnitramines (3.77 < pKA < 5.62) are similar to those of benzoic acids and weakly dependent (ꢀ = 1) on the
electronic character of a substituent. Based on the analogy with benzoic acids, it has been calculated that basicities of
nitramines (pKB ꢀ 21) are extremely low. Consequently, addition of protons to an intact nitramine molecule, as the
preliminary step of the rearrangement, seems to be improbable. Migration of the N-nitro group precedes protonation;
the latter process facilitates transformation of intermediates into stable final products. Copyright  2001 John Wiley
& Sons, Ltd.
KEYWORDS: nitramines; amphiprotic compounds; NMR spectroscopy; rearrangement; reaction mechanism
*
INTRODUCTION
protonation on the amide nitrogen is a preliminary
step of the reaction.
Primary and secondary nitramines are susceptible to
rearrangement under the influence of an acid (Scheme 1).
The nitramine rearrangement involves migration of the
N-nitro group to the ortho or para position of an aromatic
ring. There are at least three theories explaining the
mechanism of the nitro group migration:
The first assumption seems to be experimentally
confirmed;4 the second one will be discussed in this
paper.
N-Methyl-N-phenyl nitramine and its ring-substituted
derivatives are the most frequently used as model
compounds in the investigations on the mechanism of
nitramine rearrangement. The acid–base equilibria can-
not be established because in acidic solutions the
rearrangement occurs immediately. Acidity requirements
are strongly dependent on the substituents bound to the
ring. N-Methyl-N-phenylnitramine can be rearranged in
0.01 M perchloric acid, its 4-nitro derivative rearranges in
0.5 M HClO4,5 whereas N-methyl-N,2,4-trinitroaniline
requires concentrated sulfuric acid.6 The most simple
explanation is that electron-withdrawing substituents on
*
the nitro group remains covalently bonded to the
aromatic ring during migration, three or five nodes
from the migration origin;1
the nitro group migrates as the NO2 radical; the
*
process occurs in the solvent cage, which is
responsible for its intramolecularity;2
heterolytic cleavage of the N—N bond provides
*
nitronium cations, forming a p-complex with an
aromatic moiety, which is subsequently trans-
formed into final products.3
The nature of the rearrangement remains controversial;
however, there is a common agreement in two aspects:
*
rearrangement is intramolecular;
*Correspondence to: Z. Daszkiewicz, Institute of Chemistry, Uni-
versity of Opole, 45-052 Opole, ul. Oleska 48, Poland.
E-mail: zdasz@uni.opole.pl
Scheme 1
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 115–122

116
Z. DASZKIEWICZ, G. SPALENIAK AND J. B. KYZIOs
Scheme 2
the ring decrease the proton affinity of the nitramine
group. In fact, the problem is more complex.
X-ray diffraction studies have indicated that the
nitramino group in N-(4-chlorophenyl)-N-methylnitra-
mine is planar with a short N—N bond.7 Its geometry is
nearly the same as in N,N-dimethylnitramine.8 The latter
compound was studied with the use of photoelectron
spectroscopy. The results, supported with quantum-
mechanical calculations, revealed that the outer valence
shell consists of three p-orbitals, spread all over the
NNO2 group.9 This means that there is no unshared
electron pair on the amide nitrogen; consequently,
protonation at this site must completely rearrange the
charge distribution within the nitramino group. More-
over, our recent studies on the nitro derivatives of N-
phenyl-N-methylnitramine demonstrated that the elec-
tron-acceptor substituents do not influence the geometry
of the N-methylnitramino group.10 It is inferred that we
cannot see a basic centre and its interaction with a ring
substituent; consequently, a catalytic effect of an acid on
the nitramine rearrangement cannot be explained in a
simple way.
Figure 1. Arnett's plot, correlation between the acidity
pKA&A) and basicity pKA&B) of some amphiprotic compounds
The aim of this work is to evaluate the proton-donating
properties of nitramines, i.e. the right-hand equilibrium
given in Scheme 2, and calculate a hypothetical
equilibrium of the basic dissociation of primary phenyl-
nitramines.
were excluded for obvious reasons. In these amphiprotic
ions or molecules, a protonation site and an acidic centre
are separated. In N-phenylnitramine, protons are added or
abstracted from the same amide nitrogen atom, hence
estimation of its acidity may provide some information
on its proton affinity.
Two presumptions were made:
*
the mechanism of rearrangement is the same in the
primary and secondary nitramine series;
the acidic and basic properties of amphiprotic
*
compounds are interrelated.
The latter requires some comment. The problem has
been investigated by Arnett with the final conclusion that
there is no correlation between acidity, expressed as
pKA(A), and basicity, given as pKA(B), of the conjugated
acid.11 There is, however, a linear correlation between
pKA(A) and pKA(B) provided that the same functional
group is the basic and acidic centre. Arnett’s plot,
completed with some other data, is presented on Fig. 1.
The correlation is surprisingly good, considering that
the pKA values are defined in three acidity–basicity
scales, i.e. Hammett’s Ho acidity scale, the Sørensen pH
EXPERIMENTAL
The proton and carbon NMR spectra were obtained using
a Tesla BS 567A (100 MHz) spectrometer. For the
registration of 15N NMR spectra a Bruker DPX instru-
ment (250 MHz) was used. The FTIR spectra were
recorded on a Philips PU 9804 apparatus. A Hewlett-
Packard GC system HP 6890 with mass selective detector
5973, equipped with a direct inlet and on-column inlet,
was used for registration of the mass spectra and
chromatographic control of the reactions. An HP-1
methyl siloxane capillary column (30 m  320 mm Â
0.25 mm) was employed. A Beckman DU 640B spectro-
meter was used for registration of the UV–VIS spectra.
scale and the H scale for strongly basic solutions. On the
À
other hand, several species do not obey the simple
relation given in Fig. 1. Monocations of aminopyridines,
monoanions of phthalic acids and several heterocyclic
compounds, such as imidazole, pyrazole and indazole,
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 115–122

ACIDITY AND BASICITY OF PRIMARY N-PHENYLNITRAMINES
117
The pKA values were measured using CI-316 pH-meter
(Elmetron). The aqueous solutions of nitramines
(1.377 Â 10À3 < c < 0.015 M) were triturated with
0.0542 M sodium hydroxide.
water (50 ml). The solution was cooled to ‡5°C and
diazotized with an excess of sodium nitrite (29.35 g,
0.28 mol) dissolved in a minimum amount of water
(30 ml).
Ammonium
tetrafluoroborate
(29.35 g,
0.28 mol), as a saturated aqueous solution, was added
and the precipitate of diazonium tetrafluoroborate was
collected by filtration. It was added to the cold (À5°C)
aqueous potassium hydroxide (60.0 g of 50% aq. KOH)
and the suspension was poured into potassium cyano-
ferrate(III) (200.0 g, 0.60 mol) dissolved in water
(1200 ml) containing potassium hydroxide (60.0 g). The
solution was stirred for 18 h at room temperature. A
yellow precipitate was collected by filtration, purified and
identified as 4,4'-azoxybenzene. The filtrate was cooled
in an ice bath and neutralized with 30% sulfuric acid.
Extraction with methylene chloride (3 Â 100 ml) gave a
multicomponent mixture, which was re-extracted with
1 M aq. KOH. The alkaline solution was neutralized as
before; the precipitate was collected by filtration, dried in
vacuum and crystallized from n-hexane. N-(4-Methoxy-
phenyl)-nitramine (13.45 g, 40%) was obtained as
colourless crystals melting at 77–79°C. MS, m/z (int.):
24'-Methanesulfonyl)-N-phenylnitramine 2proce-
dure A, 2b)
4-(Methanesulfonyl)-aniline (3.42 g, 0.02 mol) was dis-
solved in nitromethane (35 ml). Absolute nitric acid
(1.05 ml, 25 mmol) was added to cold acetic anhydride
(10 ml), the solution was maintained for 15 min at room
temperature and combined with the substrate solution.
The mixture was left for 0.5 h at room temperature and
cooled to À20°C. The crude product was collected by
filtration and crystallized twice from toluene, yielding
(4'-methanesulfonyl)-N-phenylnitramine (2.20 g, 51%)
as colourless prisms, m.p. 179–181°C. Calc. for
C7H8N2O4S (216.21): 38.88% C, 3.73% H. Found:
‡
Á
39.11% C, 3.70% H. MS, m/z (int.): 216 (M , 26), 201
(2), 170 (69), 156 (26), 108 (100), 91 (40), 81 (53), 63
(72). IR (KBr): 1613, 1597, 1581, 1502, 1467, 1385,
1311, 1287.
‡
Á
168 (M , 17), 153 (4), 122 (100), 107 (15), 95 (69), 80
(23), 79 (25), 65 (19), 52 (35). IR (KBr): 1592, 1581,
1512, 1466, 1436, 1396, 1327, 1306, 1252, 1220.
N-24-Fluorophenyl)-nitramine 2procedure B, 2d)
4-Fluoroaniline (3.33 g, 0.03 mol) was dissolved in dry
toluene (150 ml); methylmagnesium iodide in isoamyl
ether (3.5 ml of 1 M solution, 35 mmol) was added and
the mixture was stirred intensely at the boiling point for
3 h. It was cooled to room temperature and n-butyl nitrate
(4.00 g, 33 mmol) in toluene (4 ml) was added. The
mixture was stirred for 3 h at room temperature. Water
(30 ml) acidified with acetic acid (3 ml) was added
dropwise and the layers were separated. A toluene
solution was extracted (3 Â 30 ml) with 1 M aqueous
potassium hydroxide. The extract was neutralized with
4 M hydrochloric acid at 0°C; the crude product was
collected with methylene chloride. The solution was
dried over anhydrous sodium sulfate, filtered, diluted
with n-hexane and cooled. The crude product was
collected by filtration and crystallized from a mix-
ture of methylene chloride and n-hexane. N-(4-Fluoro-
phenyl)-nitramine (2.11 g, 45%) was obtained as white
leaflets, m.p. 100–101°C. Calc. for C6H5FN2O4 (156.12):
46.16% C, 3.23% H. Found: 46.25% C, 3.33% H. MS,
RESULTS AND DISCUSSION
We have prepared a series of para-substituted N-
phenylnitramines by nitration of corresponding anilines;
standard procedures were slightly modified to facilitate
isolation of the products in the pure state (scheme 3). The
derivatives containing electron-withdrawing substituents
were obtained in sulfolane or nitromethane solutions by
the action of acetyl nitrate (procedure A). The acetyl
group, as the electron acceptor, is not strong enough,
hence 4-aminoacetophenone cannot be N-nitrated in this
way. More basic anilines were nitrated with n-butyl
nitrate after pretreatment with a strong base (procedure
B); sodium hydride and Grignard reagents were alter-
natively employed. We could not find reaction conditions
(solvent, strong base) that were applicable in the
preparation of any primary nitramine. N-Phenylnitramine
was prepared from aniline by diazotation and oxidation
‡
Á
m/z (int.): 156 (M , 14), 110 (100), 90 (6), 83 (92), 63
(14), 57 (34). IR (KBr): 1621, 1598, 1513, 1433, 1381,
1335, 1294, 1250, 1207.
N-24-Methoxyphenyl)-nitramine 2procedure C, 2g)
4-Anisidine (24.63 g, 0.20 mol) was dissolved in a
mixture of concentrated hydrochloric acid (50 ml) and
Scheme 3
Copyright  2001 John Wiley & Sons, Ltd.
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118
Z. DASZKIEWICZ, G. SPALENIAK AND J. B. KYZIOs
Table 1. Para-substituted N-phenylnitramines
No.
Substituent X
Procedure
Yield (%)
M.p. (°C), (solvent)
Lit. m.p.
2a
2b
2c
2d
2e
2f
NO2
SO2Me
Br
A
A
B
B
C
B
C
54
112–113 (CH2Cl2)
111–11212
–
51
179–181 (PhMe)
25
92–100 (Et2O–n-hexane)
100–101 (CH2Cl2–n-hexane)
41.5–42 (Et2O–n-hexane)
51–52 (CH2Cl2–n-hexane)
77–79 (n-hexane)
101–10212
–
F
45a
26b
28c
40
H
Me
OMe
46–4713
5212
2g
7914
a
With the use of sodium hydride, the reaction does not occur.
Procedure B, using the MeMgI/BuONO2 system gives 6% yield.
Sodium hydride was used as a strong base; the labelled compound was prepared analogously.
b
c
(procedure C). The compound details are collected in
Table 1.
accompanied by a broad absorption with several sub-
maxima up to ca 2900 cmÀ1. This indicates the presence
of hydrogen bonds. Such specific interactions also
usually evolve some additional bands in the IR spectra
in the lower wavenumbers regions. To our surprise, the
nitramines 2a–2g give identical spectra in solutions and
in potassium bromide pellets; the bands characteristic of
the N-nitro group appear at the same wavenumbers. The
nature of the intermolecular interactions in crystal lattices
seems to be obscure.
The carbon NMR spectra of the primary nitramines do
not differ significantly from those of corresponding
N-methyl-N-phenylnitramines,15 although the chemical
shifts of ring carbon atoms are lower in the former,
usually by 1–5 ppm. The substituent-induced chemical
shift differences (SCSD, increments) of the nitramino
group derived from the spectrum of the parent compound
and the average values from all the spectra are nearly the
same. The result indicates that there is no electronic
interaction between the nitramino group and another
substituent in a conjugated position.16 Despite the
diversity of the substituents investigated (from NO2 to
OMe), the additivity rule is obeyed in all the spectra. The
ipso increment of the nitramino group (Sipso = ‡ 7.6)
differs markedly from that of the nitro (Sipso = ‡ 19.9)
and amino (Sipso = ‡ 20.7) groups, and resembles that of
the methyl group (Sipso = ‡ 9.3).17 Consequently, the
interaction between the nitramino group and the aromatic
Example procedures are given in the experimental
section. The diversity of the procedures reflects the
different susceptibilities of nitramines to rearrangement;
those containing electron-acceptor groups are more
resistant to diluted acids and elevated temperature,
whereas electron-releasing substituents facilitate isomer-
ization.
The compounds 2a–2g were characterized using
spectroscopic methods, and the results were compared
with previously published data on analogous N-methyl-
N-phenylnitramines.15
The spectra of primary and secondary phenylnitra-
mines, with the same substituents on the ring, differ in
some aspects. The FTIR spectra will be considered first;
in those of N-methyl-N-phenylnitramines the bands
characteristic of the N-nitro group (1285–1299 and
1517–1536 cmÀ1) were recognized using [15N-NO2]
labelling.15 However, their primary counterparts give
no peaks in this region. Among the strong bands at 1585,
1596 cmÀ1 and 1305, 1322 cmÀ1, in the spectrum of 2f,
two of them can be assigned to the N-nitro group. Isotopic
labelling [15N-NO2] shifts the asymmetric stretch to
1562 cmÀ1 and symmetric stretching vibrations to
1293 cmÀ1. The bands at 1585 and 1305 cmÀ1 remain
unchanged, so they must be skeletal vibrations. The result
was extrapolated for other nitramines, which give very
similar spectra; some relevant data are collected in Table
2.
Asymmetric stretching vibrations of the N-nitro group
and the N—H stretch give rise to strong bands that appear
in narrow regions; the influence of substituents is
negligible. On the other hand, the frequencies vary in a
regular way; hence, we cannot conclude whether the
nitramino group interacts with the second substituent or
not.
All the aforementioned spectra were registered in
diluted (ca 0.2 M) solutions, using deuterated methylene
chloride as the solvent, because it is transparent in the
regions of interest. In the solid-state spectra, two strong
bands at ca 3250 and 3160 cmÀ1 were observed,
Table 2. The IR spectra of para-substituted N-phenylnitra-
mines &in CD2Cl2)
ꢁ(NO2) ꢁ(NO2)
N—H
region
‡
No. Substituent
s
asym.
symm.
2a
2b
2c
2d
2e
2f
NO2
SO2Me
Br
0.720
0.778
1607
1607
1601
1591
1587
1596
1585
1318
1319
1320
1320
1323
1322
1322
3367
3370
3373
3374
3375
3376
3376
0.232
F
0.062
0.000
H
Me
OMe
À0.170
2g
À0.268
Copyright  2001 John Wiley & Sons, Ltd.
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ACIDITY AND BASICITY OF PRIMARY N-PHENYLNITRAMINES
119
Table 3. The carbon &13C) and nitrogen &15N) NMR spectra of para-substituted primary N-phenylnitramines in DMSO-d6;
chemical shifts ꢃ are given in ppm versus tetramethylsilane as the internal standard, and versus nitromethane as the external
standard
Substituent
C-1
C-2
C-3
C-4
NH
NO2
NO2
SO2Meb
Br
141.6
140.1
135.2
132.2
135.1
133.4
128.6
119.9
123.5
123.5
125.1
122.0
122.2
125.0
124.9
128.3
132.0
116.0
128.1
129.5
114.2
144.2
137.8
119.0
160.5
126.8
136.5
158.2
À190.3
À191.2
À32.2a
À34.4
À35.2
À30.6
À31.1
À30.4
À29.4
Not observed
À194.9
F
H
Not observed
À193.7
Mec
OMed
À195.5
a
À11.6 (C-NO2).
b
43.5 (SO2Me).
c
20.5 (ArMe).
55.4 (OMe).
d
ring must be of an inductive nature. Structural data
confirm this conclusion: the substituent (NNO2) is nearly
perpendicular to the plane of the aromatic ring.7,18 Such
twisted conformations exclude mesomeric interaction of
the aromatic sextet and the p-electron system of the
nitramino group. Theoretical calculations by Anulewicz
et al.,18 although based on false (mesomeric) premises,
gave a result (ꢂ = ‡ 0.36) very close to the experimental
characteristics of the secondary nitramino group (ꢂ = ‡
0.57).19 We believe that the nitramino group is an
electron-demanding substituent, but its mesomeric inter-
another substituent on the ring may be considered as a
fact. The data collected in Table 4 indicate that the
nitramine proton is strongly deshielded; hence, it must
display some acidic properties.
Another interesting feature of the spectra reported is
the strong dependence of chemical shifts on the solvent
used. It is not limited to the N—H proton; its interaction
with the solvent molecules influences significantly the
absorption of ring protons, especially those in the ortho
positions.
The first attempts to measure pKA values of primary
phenylnitramines, using the spectrophotometric method,
were unsuccessful. The spectra registered in Britton–
Robinson buffer solutions, at 2 < pH < 10, demonstrated
that the UV–VIS spectra of dissociated and undissociated
nitramines are too similar to enable the measurements of
concentrations of both forms in the equilibrium. The
exceptions were the nitramines containing a strong,
action with the ring is not possible. The resonances in 15
N
NMR spectra of the amide (193.1 Æ 3 ppm) and nitro
(32.4 Æ 3 ppm) nitrogen atoms appear within a narrow
range, indicating that the intramolecular interaction
across the ring can be neglected (see Table 3).
The proton NMR spectra confirm the observation—the
lack of conjugation between the nitramino group and
Table 4. Proton NMR spectra of primary para-substituted N-phenylnitramines in DMSO-d6; chemical shifts ꢃ are given in ppm
versus tetramethylsilane as the internal standard
Primary nitramines
Secondary nitramines15
Substituent
H-2,6a
H-3,5a
N—H
Others
H-2,6
7.47
H-3,5
8.25
NO2b
NO2
SO2Me
SO2Meb
Br
Br
F
H
7.70
7.80
7.70
7.78
7.66
6.39
7.29
8.25
8.34
8.06
8.05
7.40
7.59
7.44d
Not observed
13.06
–
–
Not observed
12.91
3.22
3.15
–
7.57
7.20
7.26
8.06
7.59
7.09
Not observed
8.81c
–
13.79
–
7.42–7.46e
Not observed
13.70
–
7.27–7.54
Me
OMe
7.34
7.33
7.24
7.00
2.32
3.77
7.26
7.23
7.16
6.95
13.45
a
Doublets with the coupling constants 8.9–9.4 Hz.
In acetone-d6.
b
c
The spectrum registered in deuterated benzene.
Two multiplets, due to the coupling with the fluorine atom.
Proton on C-4 gives a multiplet centred at 7.29 ppm.
d
e
Copyright  2001 John Wiley & Sons, Ltd.
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120
Z. DASZKIEWICZ, G. SPALENIAK AND J. B. KYZIOs
Table 5. Acidic pKA&A) and basic pKA&B) dissociation
constants of para-substituted N-phenylnitramines &at 24°C)
estimated in aqueous solutions by the potentiometric
method
Para-substituent
KA
pKA(A)
pKA(B)
NO2
SO2Me
Br
1.70 Â 10À4
1.62 Â 10À4
5.37 Â 10À5
3.16 Â 10À5
4.07 Â 10À5
2.34 Â 10À5
2.19 Â 10À5
2.40 Â 10À6
3.77a
3.79b
4.27
À7.69
À7.67
À7.05
À6.76
À6.90
À6.59
À6.55
À5.33
Cl
4.50c
4.39
F
H
Me
OMe
4.63d
4.66
5.62
a
pKA(A) = 3.91 by spectrophotometric method.
pKA(A) = 3.80 by spectrophotometric method.
At 15°C, from Ref. 23.
b
c
d
4.75 < pKA(A) < 4.91, depending on the temperature in the region of 1 to
18°C; measured by the conductometric method.24
Figure 2. The UV spectra of N,4-dinitroaniline in aqueous
solutions of various acidities &pH)
electron-demanding substituent; the spectra of N,4-
dinitroaniline are presented in Fig. 2.
Acidities of primary phenylnitramines were measured
by the standard potentiometric method.20 Saturated,
aqueous solutions of the nitramines were titrated with
diluted aqueous solution of sodium hydroxide at room
temperature. The results, collected in Table 5, are the
average numbers from six pH measurements.
As is to be expected, the electron-withdrawing
substituents facilitate ionization of nitramines due to
the stabilization of anionic species. Acidities vary within
nearly two units on the logarithmic scale and resemble
closely those of ring-substituted benzoic acids. Their
acidities, i.e. pKA(A) values, were estimated in diluted
aqueous solutions,21 and their basicities, expressed as
Figure 3. Arnett's plot, correlation between the acidic and
basic dissociation constants of the ring-substituted benzoic
acids, extrapolated to the phenylnitramine region
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 115–122

ACIDITY AND BASICITY OF PRIMARY N-PHENYLNITRAMINES
121
Scheme 5
the preliminary step of the rearrangement. Such a pre-
sumption is acceptable if we ignore two well-known facts:
Scheme 4
*
the N-nitro group can migrate to an aromatic ring
without intervention of protons;25,26
there is no basic centre within the nitramino group.7–9
*
pKA(B) values, were also measured in the water–sulfuric
acid binary mixtures.22 There is a simple relationship
between the proton-releasing and proton-demanding
properties of benzoic acids, as shown in Fig. 3.
Otherwise, we can rationalize these facts assuming
that reversible shift of the N-nitro group precedes
protonation. There are two possible routes of transforma-
tion of the substituent into its mobile nitrito form
(Scheme 4). Homolytic cleavage of the N—N bond
and recombination within the solvent cage provides N-
nitrite. Migration to the ortho position, via cyclic
transition-state, gives C-nitrite. Further migrations occur
rapidly as the [3, 3] sigmatropic shifts in accordance with
Woodward–Hoffmann rules.
It should be pointed out that the shift of the N-nitro
group to the ring forms a basic centre, which is absent in
the intact molecule, viz. the imino group. Its protonation
prevents retro-migration of the nitro group, facilitates
expulsion of ring protons and restitution of the aromatic
system. The influence of ring substituents on the basicity
of anilines is strong (ꢀ = 3) and corresponds well to the
Hammett equation applied to the kinetics of the nitramine
rearrangement. Transformation of a labile nitrito form
into the stable final product follows a probable dissocia-
tion–recombination pathway, as indicated by the chemi-
cally induced dynamic nuclear polarization (CIDNP)
effect observed during rearrangement of some primary
nitramines.27
The same correlation can be used for the prediction of
‘basicities’ of primary N-phenylnitramines; some rele-
vant data in the last column of Table 5 indicate that
protonation of these species can be expected in
moderately concentrated sulfuric acid but not in diluted
aqueous solutions. Half protonation requires concentra-
tions between 67 and 82% of sulfuric acid; in such
solutions the decomposition of a nitramine prevails over
its rearrangement. Consequently, protonation of the
nitramine, as the preliminary step of the rearrangement,
seems to be impossible.
The same conclusion can be reached in another way,
considering that the Sørensen (pH) and Hammett (Ho)
acidity scales are compatible. If so, the relationship
pKA(B) ‡ pKB(B) = 14 should be obeyed, and, conse-
quently, the basicities of N-phenylnitramines in aqueous
solutions will be expressed with dissociation constants
of 2.0 Â 10À22 < KB (B) < 4.7 Â 10À20 (Scheme 5). The
basicities of nitramines are too low to explain the
catalytic effect of an acid. Moreover, the reaction
constant r of the basic dissociation of nitramines exceeds
unity negligibly, whereas the kinetic r constant of the
rearrangement is close to the maximum value (ꢀ = 4).5 If
migration of the N-nitro group occurs within a protonated
molecule, the influence of ring substituents on the proton
affinity and the rate of rearrangement should be parallel.
The spectral properties of para-substituted N-phenyl-
nitramines, as well as their acidic properties (pKA),
indicate that the substituents do not influence signifi-
cantly the electronic structure and chemical properties of
the nitramino group. The proton affinity of the nitramino
group seems to be insufficient to explain the catalytic
effect of an acid. The conclusions are well confirmed
with the data reported above, but they seem to contradict
some theories of the nitramine rearrangement. Now we
wish to demonstrate that our results are in agreement with
commonly known facts and that the apparent discrepan-
cies emerge from misinterpretations.
[15N-NO2]-N-(2,6-Dibromophenyl)-nitramine and its
2,6-dichloro analogue were rearranged in a strongly
acidic solution.27,28 The 15N NMR spectra, registered
during the rearrangement, indicated the enhancement
of the absorption of the 4-nitro group in the prod-
ucts. The results with [15N-NO2]-N-methyl-N-phenyl-
nitramine were similar, except that the product composi-
tion was much more complex. According to Kaptein’s
rules, the nuclear polarization is the evidence that 2,6-
dibromo-4-nitroaniline is formed through recombination
of free radicals.27,28 This confirms our hypothesis, that
the expulsion of a C-proton precedes transformation of
the ONO group into the stable nitro group, conjugated
with the amino substituent across the ring. The ONO to
NO2 rearrangement may occur in the solvent cage, since
it does not require distant migrations. Obviously, the
whole transformation cannot be entirely intramolecular,
otherwise the CIDNP effect would not have been
observed.29 However, the escape from the cage is
The catalytic effect of an acid used to be interpreted as
the result of protonation of the intact substrate molecule, in
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 115–122

122
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responsible for some side reaction accompanying the
rearrangement, whereas migration of the nitro group is
strictly intramolecular.
11. Arnett EM. Prog. Phys. Org. Chem. 1963; 1: 223.
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15. Daszkiewicz Z, Nowakowska EM, Prez˙do WW, Kyziot JB. Pol. J.
Chem. 1995; 69: 1437.
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