КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 6, с. 689-695
УДК 541.124:547.822.7
KINETIC AND MECHANISTIC STUDY OF N-AMINOPIPERIDINE FORMATION VIA THE RASCHIG PROCESS © 2013 C. Darwich, M. Elkhatib, V. Pasquet*, Н. Delalu
Laboratoire Hydrazines et Composés Energétiques Polyazotés, Université Claude Bernard Lyon 1, France
*E-mail: veronique.pasquet@univ-lyon1.fr Received 06.02.2012
Formation of N-aminopiperidine (NAPP) in the reaction of monochloramine with piperidine was studied by varying the reagents concentrations, pH and temperature. The study was carried out in diluted solutions, recording simultaneously monochloramine concentration by UV spectrophotometry at 243 nm and hydrazine concentration at 237 nm after treatment with formaldehyde. The presence of two competitive reactions: formation of NAPP and a complex parallel reaction limiting the yield of hydrazine, was established. Reaction products were characterized by GC/MS analysis. The rate constant of NAPP formation and activation parameters were determined, k1 = 56 x 10-3 M-1 s-1 (25°C) and k1 = 9.3 x 106 exp(-46.5/RT) M-1 s-1, respectively.
DOI: 10.7868/S0453881113060038
N-aminopiperidine (NAPP) is currently used [1, 2] as a precursor in pharmaceutical, cosmetics and photographic industries and also for plant protection. It is a component of drugs used particularly for smoking cessation and obesity. Moreover, NAPP has applications in the field of crop protection where it is used to prepare herbicide derived from tetrazolinone. Other uses are reported [3]: manufacture of paper and transparent film recording, elaboration of inhibitor gels and resistant to amine-based solvent.
The methods for NAPP synthesis described in the literature are essentially based on the use of urea [4—6] or nitrosamines [7—17]. The first method, requiring several steps, is not compatible with a continuous process, the second one presents a great toxicity due to the carcinogenic effect of nitrosamines, and hence becomes a problem for industrialization. In our laboratory, another way has been developed: the reaction of hydroxylamine-O-sulfonic acid (HOSA) with piperidine [18]. The first results are promising. However, it presents some disadvantages related to the instability of HOSA that leads to many side-reactions and lowers the yield.
Another way is the Raschig process. This is a selective synthesis using neither organic solvent nor polluting reagents. Moreover, contrary to the reaction with participation of HOSA, it is particularly suitable for continuous industrial process. However, it has disadvantages related to low hydrazine content in the synthesis solutions due to the high dilution of the reagents and to the existence of numerous side-reactions, which imposes non-stoichiometric conditions at all stages of the synthesis.
The Raschig process is represented by the two following basic steps: (i) monochloramine (NH2Cl) formation
NH3 + OCl--> NH2Cl + OH-
and (ii) hydrazine formation
R1R 2NH + NH2Cl + OH-->
-> R1R 2NNH2 + H2O + Cl
Despite its disadvantages, the Raschig process remains the most economical method and lends itself to the development of a continuous industrial process [19].
This challenge needs a thorough study of each step of the basic and side-reactions. The study of the oxidation of NAPP with monochloramine [20] and the chlorine atom transfer reaction between monochloramine and piperidine [21], which is one of the principal side-reactions observed in the synthesis of NAPP via the Raschig process, are already published.
In this paper, we report a kinetic and mechanistic study of NAPP formation via the monochloramine-pi-peridine interaction.
EXPERIMENTAL
Reagents and apparatus
All the reagents and salts used were reagent grade products from "Aldrich" and "Prolabo RP." Water was passed through an ion-exchange resin, then distilled twice, deoxygenated, and stored under nitrogen. Monochloramine is unstable in water and was therefore prepared in situ at -10°C in reaction of sodium hypochlorite (2 M, 25 mL) and with NH3/NH4Cl
aqueous solution ([NH4Cl] = 2.3 M, [NH3] = 3.6 M, 20 mL) in the presence of diethyl ether (40 mL). The organic layer (0.8—1.0 M of NH2Cl) was shaken and washed several times with aliquots of distilled water. Aqueous solution of monochloramine was obtained by re-extraction from the ethereal phase.
The apparatus used for NAPP synthesis consisted of two thermostated vessels, one on the top of the other and joined by a conical fitting. The lower reactor (200 cm3) contained a magnetic stirrer and had inlets to allow pH and temperature measurements, the influx of circulating nitrogen and the removal of aliquots for analysis. Because of the sensitivity of hydrazine to oxidation upon exposure to air, the mixture was monitored by an oxygen-sensitive electrode connected to a numerical indicator. The upper cylindrical vessel (100 cm3) had a temperature sensor. It was blocked at its base by a large diameter needle valve integrated in the thermo-stated jacket. This setup allowed rapid introduction of the ampoule contents into the reactor and therefore precise definition of the start of the reaction. A slightly reduced pressure was maintained throughout the reaction mixture and the temperatures of the two vessels were defined to ±0.1 °C.
Reaction conditions
The reaction of NAPP formation was carried out in alkaline medium, at pH 12.89 ([NaOH] = 0.1 M) and T = 25°C. In order to minimize side-reactions [20], piperidine (PP) was used in excess with respect to
[PP]
chloramine (1.5 < —-—-— < 45). Concentrations of [NH2Cl]
reagents ranged between 1 x 10-3 and 9 x 10-2 M.
Procedure and analysis
Piperidine was dissolved in deoxygenated water and introduced into the lower reactor. The pH value was adjusted by addition of sodium hydroxide and/or a buffer solution. When thermal equilibrium was reached, the aqueous solution of monochloramine of identical pH was added from the upper vessel.
The concentration of monochloramine was monitored at 243 nm, the maximum of its ultraviolet absorption (snh2c:i = 458 M-1 cm-1), either by UV spectrophotometry using a Cary 1E double-beam spectrophotometer or by HPLC using a HP 1100 chromatograph equipped with a Diode Array Detector. As PP is not transparent in the UV spectral range and was present in excess in the reaction medium, the reference cell in the experiments monitored by UV spectrophotometry was filled with a PP aqueous solution of identical concentration and pH as the reaction medium. For experiments monitored by HPLC, the separation was carried out on a 150 x 3 mm ODS XDB-C8 column (dP = = 5 |m) using MeOH/H2O (70/30) as mobile phase (rate flow = 0.5 mL/min). The monochloramine con-
centration was determined with the use of previous calibration of column by standard solutions of NH2Cl iodometrically titrated.
Concentrations of NAPP formed was also followed by UV after derivation by the method developed in our laboratory [22]: NAPP itself is transparent to UV in the studied range (220-350 nm), therefore aliquots were treated with formaldehyde (40-fold excess) in order to convert NAPP into its hydrazone (FNAPP), which has an absorption maximum in UV at 237 nm (sFNAPP = 4485 M-1 cm-1).
GC/MS analyses were carried out on a chromatograph coupled to a mass spectrometer HP 5970 equipped with a CP-Sil C19 column (30 m, 250 |m i.d., df = = 1.5 |m), oven temperature rising from 30 up to 200°C with a heating rate of 5°C/min. Methodological details on the apparatus and the experimental procedure have been described elsewhere [23, 24].
RESULTS AND DISCUSSION
Kinetic study of the monochloramine—piperidine interaction
Rate laws. The rate of the NAPP formation is expressed by the following relation:
V = d[NAPPJ = ^i[NH2Cl]a[PP]p, (1)
dt
where k1, a and P are the rate constant of NAPP formation and partial reaction orders, respectively.
In order to determine partial orders and rate constant, we have measured the instantaneous evolution of chloramine and NAPP concentrations. Figure 1a shows the successive UV spectra recorded at different times. The evolution of the spectra is intricate and involves several steps: during the first step, the chloram-ine absorption at 243 nm decreases and shifts toward higher wavelengths. At the same time, an isosbestic point at 277 nm appears (Fig. 1b). During the final step, a new absorption peak appears, increases slowly, and shifts toward the lower wavelengths and then stabilizes at 237 nm (Fig. 1a).
These results show that it is not possible to follow directly monochloramine concentration up to the end of the reaction because of the interference of these several steps. Hence, in order to determine the rate laws, a method based on the use of the isosbestic point and concentration-time curves was established, limiting the measurements to the half time of the reaction.
The presence of an isosbestic point requires a defined stoichiometry between two chromogenic compounds, monochloramine and its instantaneous product. It excludes any subsequent slow reaction involving one of the two compounds. This result is surprising as the expected product, i. e. NAPP (C5H10NNH2) does not show any absorption in the UV range under study. It cannot be an intermediate compound between NH2Cl and hydrazine because of the immediate for-
mation of the latter. The isosbestic point can only be due to a parallel or competitive reaction between the same reagents giving a chromophore P:
NH2Cl +
/ NH —( 1
NNH2 + HCl,
NH2Cl +
NH
P+Z p-
Under these conditions, the absorbance of the reaction medium at any wavelength X can be written as follows (l = 1 cm):
A = s NH2CI[NH2C1] + s P[P]-
(2)
By deriving Eq. (2), we obtain: dA_ci d[NH2Cl]
- NH2C1 ■
+ O ^ d[P]
+ Op "
P dt
^ (3)
dt 2 dt Taking into account the expressions for the reaction rate, that gives:
dAx_ x /d[Pl d[CsH1oNNH 2
— -"В nh2ci + dt
+ Fx d[P]
+ О P --
P dt
(4)
As the ratio d[C5H10NNH2]/d[P] is equal to ^/£2, the Eq.
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