Ammonia-dimethylchloramine system: Kinetic approach in an aqueous medium and comparison with the mechanism involving liquid ammonia

Article abstract of DOI:10.1002/kin.20312

After an exhaustive study of the system ammonia-dimethylchloramine in liquid ammonia, it was interesting to compare the reactivity of this system in liquid ammonia with the same system in an aqueous medium. Dimethylchloramine prepared in a pure state undergoes dehydrohalogenation in an alkaline medium: the principal products formed are N-methylmethanimine, 1,3,5- trimethylhexahydrotriazine, formaldehyde, and methylamine. The kinetics of this reaction was studied by UV, GC, and HPLC as a function of temperature, initial concentrations of sodium hydroxide, and chlorinated derivative. The reaction is of the second order and obeys an E2 mechanism (k₁ =4.2 × 10^-5 M^-1 s^-1, ΔH^# = 82 kJ mol^-1, ΔS^# = -59 J mol^-1 K^-1) The oxidation of unsymmetrical dimethylhydrazine by dimethylchloramine involves two consecutive processes. The first step follows a first-order law with respect to haloamine and hydrazine, leading to the formation of an aminonitrene intermediate (k₂ = 150 × 10^-5 M^-1 s ^-1). The second step corresponds to the conversion of aminonitrene into formaldehyde dimethylhydrazone at pH 13). This reaction follows a first-order law (k₃ = 23.5 × 10^-5 s^-1) The dimethylchloramine-ammonia interaction corresponds to a SN2 bimoiecular mechanism (k₄ = 0.9 × 10^-5 M^-1 s ^-1, pH 13, and T =25°C). The kinetic model formulated on the basis of the above reactions shows that the formation of the hydrazine in an aqueous medium comes under strong competition from the dehydrohalogenation of dimethylchloramine and the oxidation of the hydrazine formed by the original chlorinated derivative. A global model that explains the mechanisms both in an anhydrous and in an aqueous medium was elaborated.

Full text of DOI:10.1002/kin.20312

Ammonia–
Dimethylchloramine
System: Kinetic Approach
in an Aqueous Medium
and Comparison with
the Mechanism Involving
Liquid Ammonia
J. STEPHAN, V. PASQUET, M. ELKHATIB, V. GOUTELLE, H. DELALU
Laboratoire Hydrazines et Proce´de´s, UMR n◦ 5179 CNRS-ISOCHEM (groupe SNPE), Universite´ Claude Bernard Lyon 1,
Baˆtiment Berthollet, 22 Avenue Gaston Berger, F-69622 Villeurbanne Cedex, France
Received 29 May 2006; revised 30 October 2006, 24 April 2007; accepted 10 December 2007
DOI 10.1002/kin.20312
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: After an exhaustive study of the system ammonia–dimethylchloramine in liq-
uid ammonia, it was interesting to compare the reactivity of this system in liquid ammonia
with the same system in an aqueous medium. Dimethylchloramine prepared in a pure state
undergoes dehydrohalogenation in an alkaline medium: the principal products formed are
N-methylmethanimine, 1,3,5-trimethylhexahydrotriazine, formaldehyde, and methylamine.
The kinetics of this reaction was studied by UV, GC, and HPLC as a function of tempera-
ture, initial concentrations of sodium hydroxide, and chlorinated derivative. The reaction is of
the second order and obeys an E2 mechanism (k1 = 4.2 × 10−5 M−1 s−1, ꢀH◦# = 82 kJ mol−1
,
ꢀS◦# = −59 J mol−1 K−1). The oxidation of unsymmetrical dimethylhydrazine by dimethylchlo-
ramine involves two consecutive processes. The first step follows a first-order law with re-
spect to haloamine and hydrazine, leading to the formation of an aminonitrene intermediate
(k2 = 150 × 10−5 M−1 s−1). The second step corresponds to the conversion of aminonitrene into
formaldehyde dimethylhydrazone at pH 13). This reaction follows a first-order law (k3 = 23.5 ×
10−5 −1). The dimethylchloramine–ammonia interaction corresponds to a SN2 bimolecular
s
mechanism (k4 = 0.9 × 10−5 M−1
s
−1, pH 13, and T = 25◦C). The kinetic model formulated
on the basis of the above reactions shows that the formation of the hydrazine in an aqueous
Correspondence to: V. Pasquet; e-mail: veronique.pasquet@
univ-lyon1.fr.
c
ꢀ
2008 Wiley Periodicals, Inc.

AMMONIA–DIMETHYLCHLORAMINE SYSTEM
341
medium comes under strong competition from the dehydrohalogenation of dimethylchlo-
ramine and the oxidation of the hydrazine formed by the original chlorinated derivative. A
global model that explains the mechanisms both in an anhydrous and in an aqueous medium
was elaborated. ꢀ 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 340–351, 2008
C
INTRODUCTION
getting a substituted chloramine to react with ammonia
under suitable conditions (Scheme 3).
Hydrazine and its derivatives are mainly used in the
aerospace industry (Ariane IV and V, satellite apogee
engines, space shuttle), in the pharmaceutical industry
RRꢁNCl being obtained by the action of hypochlo-
rite ions on pure amine or in aqueous solution:
R
(Viagraꢀ, precursors of hypoglycemic, diuretic, and
cardiovascular drugs), in the cosmetic industry, and in
agro-chemistry [1–5]. Schemes 1 and 2 present the two
methods of industrial preparation of these compounds
[5–13]:
The first method was forbidden because of the tox-
icity of the nitrosamines [14]; the second method has
two main disadvantages, namely the low concentra-
tion of the reagents, especially chloramines, and the
great number of secondary reactions [15–17]. Several
solutions are proposed:
Reaction (3) has the advantage of producing a con-
centrated product. Whatever the nature of the amine,
two phases are obtained and the chloramine formed
is concentrated above 95% in the organic phase. The
substituted chloramines can be either distilled like
dimethylchloramine (DMC) or used directly for syn-
thesis if they are unstable like several of these substi-
tuted chloramines.
The chemistry of chloramines has been widely
studied in the literature [24,25]. In particular, it
was announced that substituted chloramines in an
acidic medium lead to chlorination reactions [26–30],
whereas in an alkaline medium the main reactions are
the formation of imines, amines, and aldehydes [31–
39]. They can also react with hydrazines to form in-
termediary diazenes that give hydrazones, tetrazenes,
amines, or other products [40–44] according to the
case.
– Working under optimum conditions of synthesis,
ensuring excess amine to kinetically favor step
(1) rather than the secondary reactions [15–21].
– Increasing the initial concentration of NH2Cl by
using a more concentrated solution of hypochlo-
rite ions [15,22,23], since the concentration of
commercial solutions of hypochlorite does not
exceed 2.2 M.
The formulation of chloramine and its reactivity on
ammonia and on the substituted amines (Scheme 2)
in different mediums (gaseous, aqueous, organic, or in
liquid ammonia) has been widely studied [15–17,45–
50]. On the contrary, the use of substituted chloramines
(the indirect Raschig process) for the preparation of
hydrazines has only been studied in liquid ammonia in
the presence of amine and/or sodium amide [51–54].
Under these conditions, the rates obtained are variable
(24%–90%) and the information is inconclusive and
contradictory.
Under the best conditions, the concentration of hy-
drazine does not exceed 5% in the primary product,
leading to delicate extraction operations.
To simplify synthesis and solve this problem, we fo-
cused on the process of substituted chloramines or the
so-called indirect Raschig process, which consists of
To elucidate the mechanism, different hydrazines
were synthesized by the indirect Raschig process in
our laboratory. This consists of introducing substi-
tuted chloramine (R1R2NCl) in liquid ammonia un-
der pressure to obtain the corresponding hydrazine
(R1R2NNH2). The first experiments carried out with
pure DMC, without the corresponding amine, led to
Scheme 1
Scheme 2
Scheme 3
International Journal of Chemical Kinetics DOI 10.1002/kin

342
STEPHAN ET AL.
very low yields. Further experiments carried out in the
presence of excess amine permit to obtain better yields,
showing that the amine plays a major role not limited
to that of a solvent.
These experimental results, in disagreement with
those reported in the literature [55], led us to propose
a new mechanistic scheme involving a chlorine trans-
fer reaction. Thus, the formation of monochloramine
(NH2Cl) and dimethylamine ((CH3)2NH) occurs first.
Monochloramine reacts immediately with the substi-
tuted amine, in agreement with the direct Raschig pro-
cess, to produce hydrazine.
These results obtained in our laboratory [56] are
very interesting froma mechanisticpoint of view. How-
ever, they show that the mechanism involved in liquid
ammonia under pressure, giving NH2Cl in the first step,
is quite similar to the direct Raschig process with all
its constraints.
As these studies do not provide any information
as to whether the synthesis can be carried out in an
aqueous medium, it was interesting to study the indirect
Raschig process in this medium because of its two main
advantages:
tains approximately 97% of the chlorinated deriva-
tive. This phase is distilled under atmospheric pressure
(T = 43◦C, purity up to 99.8% measured by gas chro-
matography and DSC). As the pure DMC is unstable,
it was stored for 2–3 h in an inert atmosphere.
Nuclear magnetic resonance (NMR) analyses were
carried out using a high-resolution Bruker AM 300
spectrometer. The analyses were carried out at 300
1
MHz for H and at 75 MHz for 13C in CDCl3 using
Si(CH3)4 as an internal standard. The infrared (IR)
and ultraviolet (UV) spectra were recorded using,
respectively, a Beckman 842 apparatus (Csi cells) and
a Cary 1E spectrometer with double beam (quartz
cells with 1-cm light pathway). The high-performance
liquid chromatography (HPLC) analyses were carried
out using an HP 1100 chromatograph equipped with
a UV detector. The column is ODS 250 × 4.6 mm
(dp = 5 µm), the mobile phase is a H2O/MeOH
mixture (75/25% v/v), and the flow rate is equal to
1 mL/min. The GC analyses were carried out using
an HP 5890 chromatograph with different columns:
usually CP-Sil 19CB (25 m, 0.25 mm, df = 0.2 µm) or
CP-Wax (25 m, 320 µm, df = 2 µm) for the amines and
volatile compounds. The gas chromatography/mass
spectrum (GC/MS) analyses were carried out using
an ionization source of 70 eV with an HP 5890 chro-
matograph coupled with an HP 5970 spectrometer.
The experimental conditions are as follows: injector
at 250◦C, oven at 40◦C for 5 min—5◦C/min—up to
100◦C—10◦C/min—up to 180◦C for 15 min, carrier
gas He (10 psi), injection 1 µL, column CP-SIL C19
(30 m, 0.25 mm, df = 0.25 µm). The thermodynamic
data were obtained using a Mettler 820 DSC apparatus.
The tests were carried out in aluminum crucibles using
20 mg raw material at a heating rate of 5◦C/min.
The use of a concentrated reagent (RRꢁNCl up to
95%) in an aqueous medium simplifies the purification
of the product and makes the process more economical.
Working with a high initial [NH3]/[RRꢁNCl] ratio
permits easy elimination by simple stripping of the
excess ammonia, which is recycled.
To examine the feasibility of this synthesis, it is
necessary to carry out a kinetic and mechanistic study
comprising
a. the stability of the substituted chloramines, either
pure or in aqueous solution,
b. the reaction between the initial substituted chlo-
ramine and the corresponding hydrazine, and
c. the reaction between the substituted chloramine
and ammonia.
The experimental device comprises two enclosures
with double wall placed one on top of the other and
connected by a ground cone. The lower enclosure of a
capacity of 200 mL comprises a series of passages for
measuring pH, temperature and nitrogen circulation,
and taking samples of the solution for analysis. The
higher enclosure (100 mL) is sealed at its base by a
valve comprising a ground plug whose male part is
topped by a rod. When raising the latter, the content of
the bulb is emptied very quickly. This allows precise
identification of the onset of the reaction. A magnetic
stirrer is used to homogenize the mixture. A weak flow
of nitrogen is maintained during the experiment, and
the temperature is controlled to within 0.1◦C.
EXPERIMENTAL PART
All the reagents were analytical grade products pro-
vided by Merck (Fontenay Sous Bois, France), Aldrich
(Lyon, France), and Prolabo RP (Briare, France). Wa-
ter was purified by an ion exchange resin, then distilled
twice in a silica apparatus and stored in nitrogen.
DMC is not available commercially. It is prepared
by chlorination of dimethylamine (25 mL, Aldrich
reagent at 40%, d = 0.898) by the hypochlorite ion
(90 mL, commercial solution at 48◦ chlorometric) un-
der stoichiometric conditions (excess amine: 2%–3%)
[16]. Two phases are obtained. The higher phase con-
The caustic solution is prepared by dissolv-
ing a known mass of sodium hydroxide in twice-
distilled water. The solutions of ammonia and 1,
1-dimethylhydrazine (UDMH) are obtained by dilu-
tion of commercial reagents of NH3 (about 30%) or
International Journal of Chemical Kinetics DOI 10.1002/kin

AMMONIA–DIMETHYLCHLORAMINE SYSTEM
343
dissolution of a known mass of UDMH with the purity
up to 98%. Their exact concentration is determined
by potentiometric titration using a standard solution of
hydrochloric acid (Prolabo RP); pH measurements are
carried out at 25◦C with a glass electrode (Tacussel
TB/HS) and a calomel electrode after standardization
with the buffer solutions with the following composi-
tions: Na2B4O7·10H2O (pH 9.2), Ca(OH)2 (pH 12.0),
0.025 M KH2PO4·Na2HPO4 (pH 6.9), 0.025 M potas-
sium phthalate (pH 4.0). The apparatus used is a Tacus-
sel ISIS 20000 pH meter. The alkaline solution con-
taining the nitrogenized reagent is then introduced into
the engine. While the thermal balance is established, an
aqueous solution of DMC is prepared by introducing
a given volume (approximately 0.5 mL) of the freshly
distilled pure product into a 250-mL graduated flask
whose temperature is maintained at 25◦C, so that its
concentration does not exceed 0.05 M. Its content is
controlled by spectrophotometry UV (λmax = 263 nm,
εmax = 354 M−1 cm−1) and by iodometry [57,58].
It is important to limit the time taken for dissolution
and setting the thermal balance to avoid any decompo-
sition of the substituted chloramine. The reagents are
then mixed according to the protocol described pre-
viously with a homogenization time of approximately
20 s.
treated with a concentrated solution of sodium hydrox-
ide until pH 12. Figure 1 presents the characteriza-
tion of the degradation products by GC/MS analysis
(column CP-Sil 19CB). Five peaks were observed
at tR = 0.97, 1.1, 1.31, 11.55, and 29.28 min. Each
peak corresponds to masses m/z 31, 43, 79, 128, and
89 g/mol, respectively.
The interpretation of the ion fragments compared
to the reference spectra shows that it corresponds
to methylamine, N-methylmethanimine, DMC, which
did not react, 1,3,5-trimethylhexahydrotriazine and tri-
oxymethylene, respectively. The corresponding reac-
tions are as follows:
RESULTS AND DISCUSSION
Reaction (5) is an equilibrium. In a more diluted
medium, the decrease of the peak relating to the imine
is observed in favor of formaldehyde and methylamine.
Indeed, a GC/MS analysis carried out on the reaction
products (T = 25◦C, [DMC] = 4 × 10−3 M, [OH−] =
0.1 M) shows the appearance of a major peak of methy-
lamine and two less significant peaks corresponding to
the imine in its monomer and trimer forms.
Since DMC is not available commercially, very little
information on its physicochemical and toxicological
properties is available. However, dichloromethylamine
and dibromomethylamine present considerable risks of
explosion [59]. To conclude a kinetic study, prelimi-
nary tests were carried out concerning the preparation,
stability, and characterization of pure DMC.
Complete characterization of DMC ((CH3)2NCl)
Stability and Characterization of DMC
1
was carried out by UV, IR, MS, H, and 13C NMR.
DMC is obtained quantitatively by chlorination in light
excess by the hypochlorite ion (reaction (3)). Thus,
by mixing equivalent volumes of NaOCl (2 M) and
(CH3)2NH (2.05 M) at 10◦C, the medium separates into
two phases of which the higher, yellow-colored phase
contains more than 95% DMC. Compared to the OCl−
ions, the yield calculated is close to 99%. This phase is
immediately purified by distillation under atmospheric
pressure (boiling point = 43◦C). The purity of DMC
determined by CPG and DSC is close to 99.8%, the
rest being essentially water.
Differential scanning calorimetry (DSC) and micro-
analysis were carried out. The mass spectrum obtained
by direct injection shows the characteristic lines of the
chlorine isotopes with a molecular peak m/z = 79. An
elementary analysis carried out on the freshly distilled
pure product indicates a good agreement between the
experimental and calculated percentages (calculated:
C 30.18%, H 7.54%, N 17.61%, Cl 44.65%. Experi-
mental: C 30.11%, H 7.51%, N 17.65%, Cl 44.71%).
The UV absorption band of DMC has a maximum
at 263 nm (ε = 354 M−1 cm−1). Its spectral properties
are coherent with those of secondary chloramines [57].
The 13C NMR spectrum shows a peak at 54.70 ppm cor-
responding to the two atoms of carbon. The 1H NMR
spectrum is characterized by a multiplate located at
Even when preserved in a nitrogen atmosphere,
DMC is still an unstable product. It degrades through
time with the formation of a pale yellow acid salt.
To characterize the products formed, the medium was
International Journal of Chemical Kinetics DOI 10.1002/kin

344
STEPHAN ET AL.
Figure 1 GC/MS spectrum of degradation products of DMC (column CP-Sil 19CB, 25 m, 0.25 mm, d = 0.2 µm).
f
1 × 10−3 and 4 × 10−3 M. The kinetic study was car-
ried out by comparing the absorbance of the mixture to
the maximum of DMC absorption. At this wavelength,
all the other products remained transparent. The evo-
lution of UV spectra as a function of time shows a
decrease of the spectrum corresponding to the chlori-
nated product at λ = 263 nm without the appearance
of any spectral interference (Fig. 2).
In parallel, GC analyses carried out on a CP-Wax–
type column, using ethanol as an internal standard (tR
= 9.75) show the reduction of the peak corresponding
to DMC (tR = 18.15) and the appearance of three new
products with tR = 3.8, 7.35, and 22.9 min. They are,
respectively, methylamine, N-methylmethanimine,
and 1,3,5-trimethylhexahydrotriazine (Fig. 3).
2.59 ppm corresponding to all the hydrogen atoms.
The DSC analysis of 20 mg of raw material gives
two peaks at −110.3◦C and 42.3◦C corresponding to
the fusion and then vaporization of DMC. The cor-
responding enthalpies are ꢀHfusion = 2 kJ mol−1 and
ꢀHvaporization = 30 kJ mol−1
.
Kinetics of the Dehydrohalogenation
of DMC
DMC is not sufficiently water soluble; thus for a kinetic
study, its concentration should not exceed 0.05 M. Also
due to its thermodegradabilily, GC analyses require
operating at relatively low temperatures. The reaction
kinetics was followed by UV, GC, and HPLC.
The experiments were carried out at 25◦C for a con-
centration of sodium hydroxide varying from 0.25 to
2 M, the concentration of DMC being fixed between
The rate of disappearance of DMC is written as
−d[DMC]/dt = k1[DMC]α[OH−]β
(I)
International Journal of Chemical Kinetics DOI 10.1002/kin

AMMONIA–DIMETHYLCHLORAMINE SYSTEM
345
Figure 2 Determination of the kinetic parameters of the dehydrohalogenation of DMC in alkaline medium (T = 25◦C). [Color
figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
ranging from 1 × 10−3 to 4 × 10−3 M. The curves ln
The kinetic parameters were determined by using
Ostwald’s method. To evaluate α, we carried out three
[DMC]0/[DMC] = f (t) in all cases are straight lines
(α = 1) of the same slope ꢁ = k1 [OH−]0β. The value
series of experiments corresponding to a constant con-
centration of OH− ions and a concentration of DMC
of β was determined at the same temperature for a
Figure 3 Dehydrohalogenation of DMC: evolution of the chromatograms according to time (T = 25◦C, [DMC]0 = 8 × 10−3 M,
[OH−]0 = 1 M): (A) 15 min, (B) 105 min, (C) end of the reaction (column CP-Wax, 25 m, 530 µm, d = 2 µm).
f
International Journal of Chemical Kinetics DOI 10.1002/kin

346
STEPHAN ET AL.
ꢀS1◦# = −59 J mol−1 K−1
Table I DMC/NaOH Reaction. Determination of the
Partial Orders (T = 25◦C)
[DMC] × 103
[OH−]
(M)
The reactivity of chloramine in an alkaline medium has
been studied previously [17,35,60–67]. The proposed
mechanism transits through a hydroxylamine that then
reacts on a second molecule of chloramine to form an
unstable hydroxylhydrazine. The latter can react with
oxygen in air to produce nitrite and peroxonitrite ions
and the nitrogen release:
(M)
ϕ (s−1
)
k (M−1 s−1
)
1
4.10
3.02
1.98
1.03
4.00
3.97
4.05
4.04
4.12
1.01
1.03
0.99
1.00
2.01
1.50
0.75
0.50
0.25
43.2 × 10−6
41.5 × 10−6
44.0 × 10−6
39.5 × 10−6
86.7 × 10−6
59.2 × 10−6
30.5 × 10−6
21.6 × 10−6
10.0 × 10−6
43.2 × 10−6
41.5 × 10−6
44.0 × 10−6
39.5 × 10−6
43.3 × 10−6
39.5 × 10−6
40.7 × 10−6
43.2 × 10−6
40.0 × 10−6
NH2Cl + OH− → NH2OH + Cl− (slow)
NH2Cl + NH2OH + OH−
→
H2N−NHOH + H2O + Cl− (fast)
H2N−NHOH → products (fast)
constant concentration of DMC of 4 × 10−3 M and a
concentration of NaOH between 0.25 and 2 M. ln ꢁ =
f (ln [OH−]) is a line passing through the origin and
from slope β = 1 (r2 = 0.997). The main results are
given in Table I. The global order of the reaction is 2,
and the rate constant k1 is equal to 4.2 × 10−5 M−1 s−1
at 25◦C.
In the case of DMC and by analogy with the mechanism
proposed in the case of chloramine, the first step can
correspond to the formation of dimethylhydroxylamine
(mechanism SN2), leading to the following reaction
pathway:
The influence of the temperature was studied be-
tween 25◦C and 45◦C for respective concentrations of
NaOH and DMC: 1 M and 4 × 10−3 M. The variation
of k according to the temperature obeys Arrhenius’
law. The curve ln k = f (1/T ) is a line of slope –E/R
and the ordinate at starting log A (r2 = 0.998). E and
A are, respectively, the energy of activation and the
Arrhenius factor (E is in kJ mol−1):
(CH3)2NCl + OH− → (CH3)2NOH + Cl−
−H2O
———
(CH3)2NOH
−→ CH2 N CH3
CH3NH2+H2C O
−→
←−
CH2 N CH3 + H2O
To confirm this mechanism, a known acid salt mass
of dimethylhydroxylamine (Aldrich product of purity
above 99%) was dissolved in an alkaline medium.
GC/MS analyses did not show any formation of imine
or hydrolysis products. In addition, when DMC reacts
with dimethylhydroxylamine under equimolar condi-
tions (10 × 10−3 M) at pH 12.9 and at a temperature of
25◦C, the release of gas and the formation of new non-
detectable products are observed due to the reaction of
dehydrohalogenation of DMC in an alkaline medium.
These results exclude the preliminary formation of
dimethylhydroxylamine. As the OH− ion has basic and
nucleophilic properties, competition between an SN2
mechanism and an E2 mechanism is possible. The E2
mechanism becomes predominant because of the steric
hindrance around the electrophilic site. The mechanism
is, therefore, a bimolecular elimination of E2 type and
can be written as
k = 2.4 × 1010 exp(−84.7/RT ) M−1 s−1
The enthalpy and the entropy of activation are con-
nected to E and A by the following expressions:
ꢀH◦# = E − RT
ꢀS◦# = R ln(Ah)/(ekBT )
where kB is Boltzmann’s constant and h is Planck’s
constant. The numerical values are as follows:
ꢀH1◦# = 82 kJ mol−1
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AMMONIA–DIMETHYLCHLORAMINE SYSTEM
347
of UDMH constant at 50 × 10−3 M (great ex-
cess) and varying the concentration of DMC from
2.5 × 10−3 to 5 × 10−3 M. In all cases, the ln
([DMC]0/[DMC]) = f (t) curves are straight lines of
the same slope passing through the origin (α ∼ 1)
with ψ = k2[UDMH]β + k1[OH−]0 (k1 = 4.2 × 10−5
Oxidation of UDMH by DMC
The oxidation of UDMH by DMC is one of the proba-
ble interactions that take place during the synthesis of
UDMH by the indirect Raschig process. This reaction
was the subject of a former study that focused only on
the characterization of the products [68]. In particular,
formaldehyde dimethylhydrazone (FDMH) is formed.
Understanding these phenomena requires evaluation
of its rate of formation. The kinetics study was carried
out at pH 13 and T = 25◦C for concentrations rang-
ing from 2 × 10−3 to 10 × 10−3 M for DMC and from
25 × 10−3 to 500 × 10−3 M for UDMH. The kinetics
was observed by HPLC and UV after dilution of the
reactional medium, and the analyses were carried out
at 236 and 260 nm. Figure 4 shows an example of the
evolution of the concentrations of DMC and FDMH
as a function of time for a mixture with an initial con-
M−1 −1). The value of β was determined under the
s
same conditions by varying the excess of UDMH. The
curve expressing ln (ψ – k1[HO−]0) as a function of
ln [UDMH]0 is a line of the ordinate at the start of ln
k2 and slope β = 0.996 (r2 = 0.998). The results are
given in Table II. Consequently, the oxidation reac-
tion (UDMH/DMC) is of first order compared to each
reagent, and the constant k2 is equal to 150 × 10−5 M−1
s
−1 at T = 25◦C.
The second stage, that is, the transformation of
dimethyldiazene into FDMH, is common to the oxida-
tion of UDMH by various oxidants (NH2Cl, DMC, O2,
etc.) [68,69,71]. It was the subject of an exhaustive lab-
oratory study: the diazene–hydrazone rearrangement
corresponds to the first-order kinetics compared to di-
azene and displays basic catalysis. At pH 12.9 and
T = 25◦C, k3 is equal to 23.5 × 10−5 s−1 [70].
centration of 5 × 10−3 M for DMC and 50 × 10−3
M
for UDMH. The concentration of DMC decreases ac-
cording to a decreasing monotonous curve, whereas
that of FDMH evolves as a function of a curve with an
inflection point. In the presence of excess UDMH, the
inflection point moves and the curves [FDMH] = f (t)
are superimposed (Fig. 5). These results are interpreted
by considering the existence of a reactional intermedi-
ary I. By analogy with the phenomena observed in the
case of the oxidation of UDMH by chloramine [69,70],
we consider that the transitory compound is dimethyl-
diazene (I). Under these conditions, the system can be
described by the two following consecutive reactions:
Kinetics of UDMH Formation by
the Action of DMC on Ammonia in
an Aqueous Medium
Assuming that the reaction is of the second order (con-
stant k4), the rate of disappearance of DMC is ex-
The rate of DMC decomposition is expressed by
relation (II) in which α and β are the partial orders and
k2 and k1 are the rate constants of the oxidation and
dehydrohalogenation reactions:
pressed by the following relationship:
−d[DMC]/dt = k4[DMC]α[NH3]β+k1[DMC][OH−]
+k2[DMC][UDMH]
(III)
−d[DMC]/dt = k2[DMC]α[UDMH]β
+ k1[DMC][OH−] (II)
During the first moments of the reaction, the third term
is negligible and Eq. (III) is reduced to the first two:
The kinetic parameters were determined by the
isolation method by maintaining the concentration
−d[DMC]/dt = k4[DMC]α[NH3]β + k1[DMC][OH−]
International Journal of Chemical Kinetics DOI 10.1002/kin

348
STEPHAN ET AL.
Figure 4 Oxidation of UDMH by DMC. Evolution of [DMC], [dimethyldiazene], and [FDMH] concentrations with time.
[DMC]0 = 5.01 × 10−3 M; [UDMH]0 = 50 × 10−3 M; [OH−]0 = 0.1 M (T = 25◦C).
while simultaneously integrating,
and by indicating the expression in brackets by ψ,
the constant k4 is obtained from the slope of curve
ln([DMC]0/[DMC]) = f (t):
ꢀ
ꢀ
[DMC]
d[DMC]
1
= −(k4[NH3]0 + k1[OH−]0)
dt
k4 = ( − k1[OH−]0)/[NH3]0
[DMC]
[DMC]0
0
Figure 5 Oxidation of UDMH by DMC. Evolution of the curves with increasing concentration of UDMH (T = 25◦C, pH 13,
[DMC] = 5 × 10−3 M).
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AMMONIA–DIMETHYLCHLORAMINE SYSTEM
349
Table II Kinetics of UDMH Oxidation by DMC.
Determination of the Rate Constant k2 (T = 25◦C,
[NaOH] = 0.1 M)
Thus, for equimolar concentrations of NaOH and NH3,
the UDMH rate would reach 16% compared to the
chlorinated derivative.
In reality, two additional reactions are present: the
oxidation of UDMH by DMC and the reaction of
UDMH condensation by formaldehyde. As seen previ-
ously, the latter product results from the dehydrohalo-
genation of DMC.
To give a quantitative account of the phenomena,
a partial kinetic model was formulated. Determina-
tion of the UDMH rate and concentrations at T = ∞
requires the resolution of a system of complex dif-
ferential equations. Their written expression can be
simplified since the transformation of the transitory
compound dimethyldiazene into hydrazone is indepen-
dent of the reagents controlling the rates. Under these
conditions, at the end of the reaction ([DMC] = 0),
the final concentrations of UDMH, FDMH, and
N-methylmethanimine are solutions of the following
system:
[DMC]
× 103 (M)
[UDMH]
× 103 (M)
k
2
ψ(s−1
)
(M−1 s−1
)
2.52
3.02
5.10
5.05
5.03
5.01
5.11
5.03
50
50
25
50
75
100
150
200
7.82 × 10−5
8.02 × 10−5
4.14 × 10−5
8.17 × 10−5
11.67 × 10−5
15.42 × 10−5
23.25 × 10−5
29.62 × 10−5
148 × 10−5
152 × 10−5
149 × 10−5
155 × 10−5
151 × 10−5
150 × 10−5
152 × 10−5
146 × 10−5
Table III Kinetics of UDMH Formation. Determination
of the Rate Constant k4 (T = 25◦C)
[NH3]
(M)
[DMC]
× 103 (M)
[NaOH]
(M)
(s−1
)
k (M−1 s−1
)
4
3.57
3.87
5.81
3.11
5.81
5.51
3.87
5.00
5.01
5.10
10.00
4.50
5.20
5.03
1.0
0.5
0.25
0.5
0.5
1.0
1.0
72 × 10−6
56 × 10−6
50 × 10−6
43 × 10−6
72 × 10−6
95 × 10−6
7.2 × 10−6
9.0 × 10−6
7.5 × 10−6
7.0 × 10−6
9.0 × 10−6
9.2 × 10−6
d[DMC]
= −k4[DMC][NH3]
dt
− k1[DMC][OH−]0
− k2[DMC][UDMH]
82 × 10−6 10.1 × 10−6
d[NH3]
= −k4[DMC][NH3]
dt
The curves obtained are straight lines at the beginning
followed by a deviation related to the influence of the
secondary reactions. The relatively constant value of
k4 according to the initial concentration of the reagents
(Table III) confirms the reactional orders.
d[UDMH]
= k4[DMC][NH3]
dt
− k2[DMC][UDMH]
d[(CH3)2N+ N−]
= k2[DMC][UDMH]
dt
Under the experimental conditions studied, the av-
erage rate constant k4 is about 0.9 × 10−5 M−1 s−1 at
T = 25◦C. Consequently, at pH 13, the rate constants
d[CH3N CH2]
= k1[DMC][OH−]0
dt
k4 (0.9 × 10−5 M−1
s
−1) and k1 (4.2 × 10−5 M−1 s−1
)
appear to be of the same order of magnitude, which
confirms, in the absence of a large excess of ammonia,
the preponderance of DMC hydrolysis to the detriment
of UDMH formation.
Knowing that [FDMH]∞ = 0.5 [(CH3)2N+ N−] ,
∞
this system was solved numerically. The theoretical
results obtained as a function of the initial contents of
DMC, NH3, and OH− are presented in Table IV.
The following comments can be made:
In a first approximation, this competition can be
expressed by the following differential equations:
d[UDMH]
= k4[DMC][NH3]0
dt
– Obtaining significant rates of UDMH requires
very high ratios [NH3]/[DMC], which consider-
ably limit the productivity of the process. The
main reaction product is imine in every case.
– These results constitute only a first approximation
because the imine breaks up into formaldehyde,
which then condenses on the UDMH formed. The
more concentrated the medium, the faster the rate
of this reaction.
d[DMC]
= −k4[DMC][NH3]0 − k1[DMC][OH−]0
dt
Hence, the relation
1
[UDMH]
=
[DMC]0
∞
k1[OH−]0
1 +
k4[NH3]
International Journal of Chemical Kinetics DOI 10.1002/kin

350
STEPHAN ET AL.
Table IV Variation of the Rate of UDMH, FDMH, and
Imine Compared to DMC as a Function of the Ratio
[NH3]/[DMC]. [NaOH] = 1 M, T = 25◦C
process in an aqueous medium cannot constitute an al-
ternative for the direct Raschig process to synthesize
dissymmetrical substituted hydrazines.
In conclusion, this work carried out in our lab-
oratory, concerning the reaction of substituted chlo-
ramines in liquid ammonia and in an aqueous solution
of ammonia, allowed us to reconsider the contradictory
results postulated in the literature: in an alkaline aque-
ous medium, the reaction mechanism remains a nucle-
ophilic attack of ammonia on the nitrogen atom of the
substituted chloramine. On the other hand, in liquid
ammonia, the nucleophilic attack does not take place
on the nitrogen atom but on the chlorine atom as a total
deactivation of the partial positive charge of nitrogen
occurs. This causes the polarity of the NCl to change,
thereby favoring a chlorine transfer mechanism lead-
ing to the formation of monochloramine and amine in
stoichiometric proportion. In these conditions, when
adding an excess of amine, hydrazine is formed with
a sufficient rate but with more constraints than in the
direct Raschig process.
Molar Rate (%)
[NH3]
(M)
[DMC]
(M)
[NH3]/
[DMC] UDMH FDMH Imine
1
1
1
1
1
1
1
0.5
0.1
2
10
20
1.11
5.51
8.88
12.92
14.98
16.22
17.05
6.82
5.11
3.70
1.99
1.13
0.60
0.25
71.58
74.03
76.31
79.09
80.51
81.36
81.94
50 × 10−3
20 × 10−3
10 × 10−3
5.0 × 10−3
2.0 × 10−3
50
100
200
500
– These results explain also the reason why it is not
possible to obtain UDMH in aqueous medium,
even under extreme conditions and confirm the
experimental results obtained by Yagil and Anbar
[54]. On the contrary, in liquid ammonia, under
the most unfavorable conditions, a rate of UDMH
equal to 8% is obtained [56].
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