15N NMR and FTIR studies of 2,4-dinitroanilines and their salts

Article abstract of DOI:10.1016/S0022-2860(00)00379-3

Twenty-two 2,4-dinitroanilines were synthesised and their pK(a) values were determined. The 2,4-dinitroanilines and their protonated forms were studied by ¹⁵N NMR spectroscopy. The relations between the ¹⁵N NMR chemical shifts and the pK(a) values of the 2,4-dinitroanilines and their salts were found to be linear. The deprotonation reaction of N-methyl-2,4- dinitroanilines and N-methyl-2,4,6-trinitroaniline by MTBD was successful only for the latter and yielded protonated MTBD molecule and the anion in which the electrons are strongly delocalised. The kinetic parameters of the 2,4-dinitroanilines in reactions with hydroxide ions in mixed solvent DMSO:water (95:5, v/v) were determinated and discussed. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0022-2860(00)00379-3

Journal of Molecular Structure 524 (2000) 217–225
www.elsevier.nl/locate/molstruc
15N NMR and FTIR studies of 2,4-dinitroanilines and their salts
B. Gierczyka, B. L⁄ e˛skaa, B. Nowak-Wydraa, G. Schroedera, G. Wojciechowskia,
F. Bartlb, B. Brzezinskia,
*
aFaculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
b
¨
´
Institute of Medical Physics and Biophysics, Universitatsklinikum Charite, Humboldt University, D-10098 Berlin, Germany
Received 22 October 1999; accepted 1 December 1999
Abstract
Twenty-two 2,4-dinitroanilines were synthesised and their pKa values were determined. The 2,4-dinitroanilines and their
protonated forms were studied by 15N NMR spectroscopy. The relations between the 15N NMR chemical shifts and the pKa
values of the 2,4-dinitroanilines and their salts were found to be linear. The deprotonation reaction of N-methyl-2,4-dinitro-
anilines and N-methyl-2,4,6-trinitroaniline by MTBD was successful only for the latter and yielded protonated MTBD molecule
and the anion in which the electrons are strongly delocalised. The kinetic parameters of the 2,4-dinitroanilines in reactions with
hydroxide ions in mixed solvent DMSO:water (95:5, v/v) were determinated and discussed. ᭧ 2000 Elsevier Science B.V. All
rights reserved.
Keywords: 2,4-Dinitroanilines; Protonated 2,4-dinitroanilines; pKa values; 15N NMR; FTIR; s-Complexes; Kinetic parameters
1. Introduction
deprotonated by strong bases, respectively.
RNH2 ϩ Hϩ O RNHϩ3
The interaction of aromatic nitro compounds with
bases can give rise to processes involving: charge
transfer, electron transfer, proton transfer and s-
complex formation, as well as to nucleophilic substi-
tution when a displaceable group is present. Acti-
vating groups are definitely decisive in the reaction
process of formation of a conjugated anion or s-
complexes. The occurrence of one, two or three elec-
tron acceptor substituents, at the ortho- or para-posi-
tions, determines the directions of the nucleophilic
attack of the base in the aromatic ring.
RNH2 ϩ B O BHϩ ϩ RNHϪ
Nitroanilines have been very interesting compounds
because in polar aprotic solvents they react with acids
and yield salts. They react also with bases and form
several products depending on the structure of the
substrate. In the case of 4-nitroaniline and N-alkyl-
4-nitroaniline interacting with nucleophilic reagent,
conjugated base (1), nucleophilic substitution product
(2) or Meinsenheimer complex (3) have been identi-
fied [1,2 and references given there].
In solvents amines may be protonated by acids and
In the reaction between N-alkyl-4-nitroaniline and a
strong base, two types of products: that of nucleo-
philic substitution (4) or Meinsenheimer complex
(5) have only been observed to form.
* Corresponding author. Tel.: ϩ 48-61-869-9181; fax: ϩ 48-61-
865-8008.
E-mail address: bbrzez@main.amu.edu.pl (B. Brzezinski).
0022-2860/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(00)00379-3

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B. Gierczyk et al. / Journal of Molecular Structure 524 (2000) 217–225
NH2
NH
Nu
Nu
H2N
H
Nu
NO2
NO2
NO2
NO2
1
2
3
R
N
R
N
NR2
NO2
H
NR
Nu
H
Nu
R2N
Nu
NO2
H
NO2
NO2
H
Nu
Nu
NO2
NO2
NO2
NO2
NO2
6
7
8
4
5
R
N
R
R
R
N
N
R
B
NO2
NO2
NO2
O2N
O2N
O2N
H
B
NO2
11
NO2
NO2
10
9
In the case of 2,4-dinitroaniline and N-substituted-
2,4-dinitroaniline reacting with a strong base, the
formation of conjugated base (6) or two possible
Meinsenheimer complexes, i.e. 1,1-s-complex (7) or
1,3-s-complex (8), has been observed. For the N,N-
dialkyl-2,4-dinitroaniline-base system only concen-
tration changes between the two s-complexes (7) or
(8) have been found.
some of the anilines has been studied by the FT-IR
and 15N NMR spectroscopic methods as well as by the
kinetic method.
2. Experimental
2.1. Synthesis of N-methyl-2,4-dinitroaniline
The reaction of 2,4,6-trinitroaniline or N-alkyl-
2,4,6-trinitroaniline with a strong base yielded a
conjugated base (9), 1,3-s-complex (10) or 1,1-s-
complex (11).
NMe2
NHMe
NHMe
NO2
O2N
or
NO2
HNO3
The reaction between N,N-dialkyl-2,4,6-trinitroani-
line and the nucleophilic reagent gave s-complexes:
1,3-s-complex (10) or 1,1-s-complex (11) have only
been identified.
This paper reports syntheses of many new deriva-
tives of 2,4-dinitroanilines and results of determina-
tion of their pKa values. The deprotonation reaction of
NO2
NO2
N,N-dimethylaniline (10 g) was nitrated with nitric
acid (80 cm3, d ˆ 1:3904 g cmϪ3) at Ϫ30ЊC for 5 h.
12 g of the mixture of the following products: N-
methyl-2,4-dinitroaniline (70%), N-methyl-2,4,6-
trinitroaniline (20%), was obtained after pouring on

B. Gierczyk et al. / Journal of Molecular Structure 524 (2000) 217–225
219
ice. The 2,4-dinitro derivative was purified by fraction
crystallisation from ethanol.
O
N
n
NO2
Cl
O
O
O
n
O2N
+
O
O
2.2. Synthesis of N,N-dimethyl-2,4-dinitroaniline
NO2
N
H
NO2
NO2
NO2
O2N
NMe2
O2N
Cl + Me2NH
n=1 1N12C4; n=2 1N15C5; n=3 1N18C6
Scheme 1.
2,4-Dinitrochlorobenzene (5 g) and 5 g of potas-
sium hydroxide were added to 70 cm3 of 40% water
solution of dimethylaniline (in the Teflon vessel,
placed in calorimetric bomb). The bomb was heated
for 12 h at 130ЊC. After cooling the mixture was
dissolved in water to obtain ca. 250 cm3 mixture and
then the solvent was evaporated under reduced pres-
sure. To the residue dissolved in 100 cm3 of water,
20 g of K2CO3 was added and the obtained mixture
was evaporated. The product was extracted three
times with 50 cm3 of acetone, the organic phase was
dried with anhydrous Na2SO4 and the solvent was
evaporated. The dark red oil was washed with
25 cm3 of 10% solution of potassium hydroxide,
next with 25% hydrochloric acid and finally with
water. The crude product was dissolved in 200 cm3
of diethyl ether, dried with anhydrous Na2SO4,
filtrated and the solvent was evaporated. The residue
was chromatographed on a silica gel column with
ether–acetone 4:1 (v/v). The obtained product was
recrystallised from methanol. About 0.8 g of the
final product was obtained.
procedure was made, depending on the agglomeration
state of crude crystal products.
(a) In the case of precipitate formation, the product
was filtered and washed with 100 cm3 hot water.
After drying on air the product was crystallised
from methanol or methanol–acetone 1:1 (v/v)
mixture.
(b) When the raw product was an oily substance,
the water was decanted and the residue was set in
ethyl acetate and dried with anhydrous Na2SO4 and
filtered off. The solvent was evaporated and the
residue was chromatographed on the silica-gel
column with diethyl ether—acetone 2:5 (v/v)
mixture. A crystalline, homogenous phase was
crystallised from acetone. In the case of the reaction
with decylamine the product was eluted from the
column with ether–hexane 2:1 mixture and crystal-
lised from ether.
2.3. Synthesis of 2,4-dinitroanilines (the monoamine
derivatives)
NO2
NR2
NO2
+
R2NH
Cl
O2N
O2N
R2NH = Et2NH, PrNH 2, Bz2NH, BzNH 2, DcNH2, 2-MeO-PhNH 2, C6H11NH2, 2-NphNH 2, PhNH
pyrrolidine, piperidine, morpholine
To the mixture of 0.015 mol of amine and 5 g of
2.4. Synthesis of the products by using azacoronands
(Scheme 1)
anhydrous sodium carbonate in 50 cm3 anhydrous
acetone, 0.01 mol of 2,4-dinitrochlorobenzene was
added and the total was refluxed for 5 h. The reaction
mixture was poured on ice and the following
The mixture of 0.006 mol of macrocyclic amine
and the 0.01 mol of tetrabutylamine acetate was

220
B. Gierczyk et al. / Journal of Molecular Structure 524 (2000) 217–225
The analytical data and 1H NMR spectra of N-amino-
methyl-crown-2,4-dinitroanilines are following:
O
O
n
O
O
N-methyl-12C4-2,4-dinitroaniline (Scheme 2, n ˆ
1; NHCH212C4); oil; 1H NMR [ꢀCD3†2C ˆ O;
TMS], 3.7–3.9 ppm, m, 15H; 7.3 ppm, d, 1H;
8.6 ppm, dd, 1H; 9.1 ppm, d, 1H; 9.0 ppm, s, N–H.
N-methyl-15C5-2,4-dinitroaniline (Scheme 2, n ˆ
2; NHCH215C5); oil; 1H NMR (ꢀCD3†2C ˆ O;
TMS), 3.7–3.9 ppm, m, 19H; 7.3 ppm, d, 1H;
8.6 ppm, dd, 1H; 9.1 ppm, d, 1H; 9.0 ppm, s, N–H.
N-methyl-18C6-2,4-dinitroaniline (Scheme 2, n ˆ
n = 1 NCH212C4
n = 2 NCH215C5
n = 3 NCH218C6
N
NO2
NO2
1
3; NHCH218C6); oil; H NMR (acetone-d6, TMS),
Scheme 2.
3.7–3.9 ppm, m, 21H; 7.3 ppm, d, 1H; 8.6 ppm, dd,
1H; 9.05 ppm, d, 1H; 8.98 ppm, s, N–H.
added to the solution of 0.008 mol of 2,4-dinitro-
chlorobenzene in 50 cm3 of anhydrous diethyl ether.
The mixture was refluxed for 24 h, cooled and the
amine salt was washed three times with water. The
diethyl ether phase was dried with anhydrous Na2SO4,
filtered off and then the solvent was evaporated. The
crude product was chromatographed on SiO2 column
with diethyl ether–hexane 10:1 (v/v). The product
(about 95% yield), chromatographically and spectro-
scopically pure, was not crystallised.
2,4-Dinitrophenylazo-15C5 was also obtained by
nitration of phenylazo-15C5 (5 g) using nitric acid
(100 cm3, d ˆ 1:3904 g cmϪ3) at Ϫ50ЊC for 30 min.
After pouring on ice and neutralisation using 30%
KOH water solution, the obtained product was crys-
tallised from ethanol (yield 90%).
2.6. pKa measurements
The pKa measurement was made following the
procedure given in Ref. [3].
2.7. Kinetic measurements
The kinetic runs were carried out with a stopped-
flow spectrophotometer (Applied Photophysics) with
the cellblock thermostated to ϩ /Ϫ0.1ЊC. The kinetic
runs are completed under pseudo-first-order condi-
tions with the base concentration in large excess.
The observed rate constants (kobs and k0obs) were calcu-
lated from the traces of absorbance vs. time. For the
fast step of s-complex formation, the rate constants
for forward (k) and backward (kϪ) reactions are
defined by the equilibrium (1), while for the slow
step (kslow) rate constants are given by Eq. (2).
2.5. Synthesis of N-methyl-crown-2,4-dinitroanilines
(see Scheme 2)
kobs ˆ k‰OHϪŠ ϩ kϪ
ꢀ1†
The structure of N-methyl-crown-2,4-dinitroanilines
is shown in Scheme 2. N-methyl-crown-2,4-dinitroani-
lines were prepared from (10 mmol) 2-aminomethyl-
crown ethers (2-aminomethyl-12-crown-4, 2-amino-
methyl-15-crown-5, 2-aminomethyl-18-crown-6; com-
mercial products of Aldrich): dissolved in 1 cm3 of
dry acetone (oxygen free) including (12 mmol) 2,4-
dinitrophenyl chloride as starting materials. The reac-
tion mixture was heated under reflux for 2 h. The
solvent was evaporated under reduced pressure. The
crude product (N-methyl-crown-2,4-dinitroanilines)
was chromatographed on IRA-410 column with
water:methanol 1:1 (v/v).
k0obs ˆ kslow‰OHϪŠ=1 ϩ K‰OHϪŠ
where K is the equilibrium constant of the fast step.
ꢀ2†
2.8. 15N NMR measurements
15N NMR spectra of the aniline were recorded in
CD3CN and those of the salts in H2SO4, respectively,
the operating frequency 30.682 MHz; at ˆ
3:0 s; d1 ˆ 2:0 s; T ˆ 293:0 K and CH3NO2 as the
internal standard. The proton decoupling procedure
was used.

B. Gierczyk et al. / Journal of Molecular Structure 524 (2000) 217–225
221
R
NO2
NO2
N
H
R= NH2, NHMe, NHEt, NHnPr, NHnBu, NHnDec,
, NHPh, NHBz,
N
O , 1N12C4, 1N15C5,
N
N
NHCH212C4,NHCH215C5, NHCH218C6, NEt2,
1N18C6
,
,
Scheme 3.
2.9. FTIR measurements
atoms of the secondary anilines are strongly
deshielded in comparison with those of tertiary
anilines. This effect is probably due to the existence
of relatively strong intramolecular hydrogen bond
between the N–H proton and the nitro group in
ortho-position. The positions of the 15N NMR signals
7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene
(MTBD) was purchased from Fluka. The mixtures of
anilines with MTBD were prepared by addition of
pure MTBD to the acetonitrile solutions of the
anilines at the concentration ratio 1:1. The concentra-
tions of all the samples were 0.1 mol dmϪ3
.
˚
The solvent was stored over 3 A molecular sieves.
All preparations and transfers of solutions were
carried out in a carefully dried glove box under
nitrogen atmosphere.
A cell with Si windows and a wedge-shaped layer
was used to avoid interferences (mean layer thickness
0.176 mm). The mid-IR spectra were taken with a
FTIR spectrophotometer IFS 113v from Bruker with
a DTGS detector (125 scans, resolution 1 cmϪ1).
3. Results and discussion
3.1. 15N NMR measurements
The twenty-two studied compounds are
summarised in Scheme 3 (studied R-2,4-dinitroben-
zenes). The pKa values of new and known compounds
are given together with 15N NMR data in Table 1. The
15N NMR chemical shifts of the bases were observed
in the region of the substituted anilines discussed
previously [4–8]. For the studied family of new 2,4-
dinitroanilines for the first time the linear dependen-
cies between the 15N chemical shifts and the pKa
values were found (Fig. 1). Fig. 1 shows that the 15N
Fig. 1. Correlation between 15N NMR chemical shifts and pKa
values of substituted anilines: (V) secondary amines; (B) proto-
nated secondary amines; (X) tertiary amines; (O) protonated tertiary
amines.

222
B. Gierczyk et al. / Journal of Molecular Structure 524 (2000) 217–225
Table 1
Table 2
15N NMR chemical shift and pKa values of R-2,4-dinitrobenzenes
Rate constants of the formation of 3s-complex in the reaction
between R-2,4-dinitrobenzenes and NaOH in DMSO:water
(95:5%, v/v) at 25ЊC
and their protonated forms
R
pKa
Free forms Protonated forms
R
k dm3 molϪ1 sϪ1
kϪ sϪ1
NHPh
8.22
Ϫ276.2
Ϫ283.8
Ϫ287.4
Ϫ302.8
Ϫ279.6
Ϫ280.3
Ϫ283.1
Ϫ272.4
Ϫ310.9
Ϫ326.4
Ϫ335.5
Ϫ345.4
Ϫ310.2
Ϫ316.1
Ϫ315.4
Ϫ305.7
NMe2
NEt2
4:28 ^ 0:17
6:54 ^ 0:08
0:074 ^ 0:002
0:0067 ^ 0:001
NHn–Bu
NHBz
8.68
9.12
9.71
8.46
8.60
8.59
8.11
NH2
0:048 ^ 0:007
0:0007 ^ 0:0001
N
O
NHMe
NHn–Pr
NHEt
N(CH2Ph)2
0:041 ^ 0:006
4:87 ^ 0:09
0:0005 ^ 0:0002
0:083 ^ 0:002
NHn–Dec
N
NH
8.71
8.73
Ϫ284.7
Ϫ282.4
Ϫ327.6
Ϫ326.8
NHn–Hex
5:78 ^ 0:14
0:098 ^ 0:003
N
O
O
NH
8.75
Ϫ280.7
Ϫ324.6
O
O
of protonated anilines are much higher (Dd ca. 30–
40 ppm) then those of other amines (ca. 10 ppm [4–
8]). This result demonstrates that the shielding effects
of Natoms in the studied compounds are relativelyhigh.
O
O
O
O
O
8.88
Ϫ287.3
Ϫ322.1
NH
O
O
O
3.2. FTIR measurements
NH
8.91
9.80
Ϫ288.4
Ϫ287
Ϫ323.4
O
O
The deprotonation reaction between N-methyl-2,4-
dinitroaniline as well as N-methyl-2,4,6-trinitroaniline
with MTBD were studied by FTIR spectroscopy. The
spectra of both anilines as well as of their 1:1 mixtures
with MTBD are given in Fig. 2a and b, respectively.
In the spectrum of the 1:1 mixture of N-methyl-2,4-
dinitroaniline with MTBD the n(N–H) vibration of
O
NEt2
Ϫ314.3
Ϫ319.1
10.23 Ϫ295.4
10.61 Ϫ304.7
N
Ϫ323.7
Ϫ288.7
Ϫ285.4
O
N
Table 3
8.14
8.10
Ϫ240.6
Ϫ238.7
N
Rate constants of the formation of the phenolate ion in the reaction
between R-2,4-dinitrobenzene and NaOH in DMSO:water (95:5%,
v/v) at 25ЊC
N
R
k dm3 molϪ1 sϪ1
O
NMe2
NEt2
N(CH2Ph) 2
0.0028
0.0026
0.0021
7.94
Ϫ233.8
Ϫ284.3
O
N
O
O
O
O
O
0.0034
0.0046
N
N
N
7.65
9.76
Ϫ226.8
Ϫ281.5
Ϫ277.4
Ϫ312.8
Me2N

B. Gierczyk et al. / Journal of Molecular Structure 524 (2000) 217–225
223
Fig. 2. FTIR spectra of (- - -) anilines and (—) their 1:1 mixtures with MTBD: (a) N-methyl-2,4-dinitroaniline; (b) N-methyl-2,4,6-trinitro-
aniline.
the aniline at 3382 cmϪ1 as well as the so-called Bohl-
mann band [9,10] of the MTBD molecule at
2845 cmϪ1 are unchanged indicating no interactions
between these molecules in the acetonitrile solution.
A completely different result is observed in the
spectrum of 1:1 mixture of N-methyl-2,4,6-trinitro-
aniline and MTBD. In this case the band at
3375 cmϪ1 is assigned to the n(N–H) ϩ vibration
of free protonated MTBD molecule and the broad
absorption in the region 3300–2500 cmϪ1 to the
'
'
NR2
NR2
O
NO2
H
NO2
NO2
fast
_
slow
+
OH
+ M
OH
NO2
NO2
NO2
NHR
NO2
NHR
NR
NO2
NO2
H
NO2
fast
_
slow
+
+ M
+ OH
+
OH
NO2
NO2
Scheme 4.

224
B. Gierczyk et al. / Journal of Molecular Structure 524 (2000) 217–225
Table 4
Table 5
Rate constants formation of conjugated bases for reaction between
R-2,4-dinitrobenzene and NaOH in DMSO:water (95:5%, v/v) at
25ЊC
Rate constants of the formation of 3s-complex for reaction of R-
2,4-dinitrobenzene with NaOH in DMSO:water (95:5%, v/v) at
25ЊC
R
k dm3 molϪ1 sϪ1
kϪ sϪ1
R
k dm3 molϪ1 sϪ1
NHMe
NHEt
NHBu
NH(CH2Ph)
NHPh
Very fast
Very fast
Very fast
23:1 ^ 0:3
350 ^ 6
NHMe
NHEt
NH(CH2Ph)
NHPh
8:18 ^ 0:20
11:41 ^ 0:20
0:056 ^ 0:005
Very small
0:8 ^ 0:2
5 ^ 2
HN
ꢀR ˆ NR0 † studied with alkali metal hydroxides in
2
0:047 ^ 0:003
0:008 ^ 0:003
a mixed solvent DMSO:water (95:5, v/v) give the s-
adducts at position 3 in the fast reversible step and in
the slow step a phenolate ion as the final product. The
neutralisation of the reaction mixture allows the isola-
tion of 2,4-dinitrophenol. On the other hand, for R ˆ
NHR0 in the fast reaction the conjugated base and in
the slow reaction the 3s compound are formed
(Scheme 4).
OCH3
NHCH212C4
NHCH215C5
NHCH218C6
Very fast
Very fast
Very fast
hydrogen bonded MTBD molecule with N-methyl-
2,4,6-trinitroaniline. This result is further
confirmed by the vanishing of the Bohlmann
band of MTBD at 2845 as well as by the doublet
band appearance at 1602 and 1626 cmϪ1, which is
very characteristic of protonated MTBD molecule
[11,12]. The FTIR spectrum of N-methyl-2,4,6-
trinitroaniline (dashed line) shows very intense
bands nas(NO2) at 1553 cmϪ1 and ns(NO2) at
1346 cmϪ1. In the spectrum of 1:1 mixture the inten-
sity of these bands decreases significantly. This result
indicates that after deprotonation of the N–H group of
the aniline molecule the formed anion is delocalised
and the nitro groups become more negative NOϪ2 ;
which is indicated by the broad bands at 1325 and
1250 cmϪ1, previously observed in the spectra of
other nitro compounds [13–16].
The number of substituents at the N atom deter-
mines the reaction direction and first of all charac-
terises the transformation rate (Tables 2–5). The
products yielded in the fast processes (i.e. in the
case of NR2 group the 3-s-adduct and in the case of
NHR the conjugated base) are formed as kinetic
products, however the final products are controlled
thermodynamically.
Acknowledgement
B. Gierczyk thanks Foundation for Polish Science
for fellowship.
References
In conclusion, the results of the FTIR measure-
ments demonstrate that the deprotonation reactions
of N-methyl-2,4-dinitroaniline can be only investi-
gated using bases stronger than the MTBD base, i.e.
alkali metal hydroxides.
´
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