Qureshi,Qureshi,Rathore, and Moharnmad
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Kinetics and Mechanism of p- DimethylaminobenzaldehydediphenylamineHydrochloride Reaction in the Solid State M. Qureshl,* S. 2. Qureshl, H. S. Rathore, and Ali Mohammad Chemlstry Section, 2.H.College of Engineering and Technology,Allgarh Muslim University,Aligarh, lndia (Received April 29, 1974) Publication costs assisted by Aiigarh Muslim University
Kinetic data for the reaction between diphenylamine hydrochloride and p-dimethylaminobenzaldehydein the solid state are recorded. The experimental data fit the equation = klt, where $, is the thickness of the colored boundary, k1 is a constant, and t is the time. Many tests have been performed to ascertain the diffusion mechanism and the nature of the product obtained. A reaction mechanism is proposed and final product obtained is p-dimethylaminobenzylidenediphenyliminiumchloride.
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Solid-state reactions have some novel features which distinguish them from reactions in solution. These reactions are free from the complicating influence *ofthe solvent and their rates are comparatively slower. They are, therefore, more amenable to kinetic and mechanistic studies and very suitable for the study of weak interactions and for the discovery of new ~pecies.l-~ The development of a new technique by Rastogi, et al., has renewed interest in these reactions. They showed that is easy to follow the progress of a solid-state reaction in a glass capillary if a colored product is formed. p - Dimethylaminoben~aldehyde~?~ (p-DAB) is a useful chromogenic reagent which gives color reactions with primary aryl amines, hydrazides of carboxylic acids, pyrrole base, ergot alkaloids, steroid acids, nicotinic acid, antipyrine, novalgine, urea, etc. Qureshi, et al., 7,8 showed that p DAB can be used for the specific detection of diphenylamine. Unfortunately all these reactions have been studied only in solution. It was, therefore, decided to study the kinetics and mechanism of the solid-state reaction of p - DAB with diphenylamine hydrochloride (DPAH). The present paper summarizes the results of such a study.
Experimental Section Material. p-DAB was obtained from E. Merck. DPAH was prepared from reagent grade diphenylamine (BDH). All other reagents were of analytical grade. Apparatus. Graduated capillaries (3 mm i.d.), the Philips X-ray diffractometer, and the Reid’s apparatus were used for reaction kinetics, X-ray, and vapor pressure studies, respectively. Procedure. The reaction kinetics in the solid state was studied by the technique of Rastogi and coworkers1 in a thermostat using 100-200 mesh reactant particles. The time required for the movement of the colored boundary to a definite distance was noted. For nonporous studies a filter paper was placed at the bottom of a glass petri dish. A DPAH pellet was placed on this paper followed by a p DAB pellet and then the system was covered with a glass lid. Ion-exchange studies were performed by placing Dowex 50W-X8 resin beads in the hydrogen form inside the above petri dish containing p-DAB and DPAH pellets. The resin beads turned dark green and their color did not change on washing with ethanol. For vapor-phase studies approximately 1 m kg of DPAH was placed at the bottom of a glass beaker. The beaker was The Journal of Physical Chemistry, Vol. 79, No. 2, 1975
covered with the filter paper impregnated with p-DAB. Similarly another beaker containing p-DAB was covered with DPAH paper. On keeping the system in an evacuated desiccator for a couple of days, the p-DAB paper turned dark green while the DPAH paper turned light green. p DAB and DPAH papers were prepared by immersing filter paper circles in 1%alcoholic solutions of the reagent concerned and then drying them at 303 K. X-Ray diffractograms of DPAH and of the hard dark green product (pDAB:DPAH 1:l) were taken using nickel-filtered Cu Ka radiation. The vapor pressure of pure ethanol and ethanolic solutions of the reactants and products were measured at 311 K and 745 mm pressure (99394.920 N mP2)as usual. To determine the reaction temperature as defined by C ~ h n Rastogi’s3 ,~ procedure was used. The reaction was studied from 273 to 323 K and the temperature rise was found to be gradual.
Results and Discussion Before discussing the mechanism of this reaction in detail a few important facts may be summarized. This reaction does not occur in the absence of a strong acid, i.e., HCl or NaHS04. Feig16 has stated, “surprisingly diphenylamine gave positive reaction with p- DAB. The limit of identification was 0.6 n kg.” It seems an important remark is missing. After spotting the p- DAB with ethereal solution of diphenylamine, exposition to acid vapors is necessary to yield the yellow color. This omission was pointed by Qureshi and coworkers7 and the necessity of this step was also discussed by them. When the same reaction is carried out in the solid state by mixing equimolar quantities of p- DAB and DPAH at room temperature (303 K) a yellow product is formed instantaneously. After several hours the yellow product turns to a dark green gummy product which on keeping at room temperature for several days changes to a hard dark green solid. At higher temperatures (>323 K) the dark green gummy product forms directly. Diffusion Mechanism. For the study of formal reaction kinetics reactions may be classified as f01lows:~(i) reactions in which phase boundary processes are more rapid than diffusion; (ii) reactions in which phase boundary processes are slower than diffusion. Diffusion in solid state may involve any one of the following mechanisms: (a) bulk diffusion, (b) lattice diffusion, (c) grain boundary diffusion, (d) vapor-phase diffusion, (e) surface diffusion.
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Solid State Reaction Kinetics
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Flgure 1. Kinetic data for the reaction between DPAH and p-DAB at various temperatures: (0) 303 f 2 K; (0) 313 f 2 K; (m) 323 f 2 K.
Our results show that (1) the reaction under study is a diffusion-controlled reaction, (2) the diffusing species is Ph2NH2+Cl-, (3) the diffusion is vapor-phase diffusion. Evidence will now be presented to substantiate the above conclusions. It is clear from Figure 1 that the rate of reaction decreases with time and therefore also with an increase in l , the thickness of the product. The initially rapid increase in the colored boundary is due to the fact that the chemical reaction is much faster than the diffusion process. As the colored boundary increases diffusion needs more time and the reaction rate decreases. According to the Arrhenius equation the reaction rate increases with temperature. This holds .good in the present case for the reaction a t 303 K and the initial reactions at 313 and 323 K. However, after a few minutes the reaction rate at 323 K is slower than the reaction a t 313 K. When a sufficient quantity of the green product has been formed, this product melts a t 323 K and shrinking occurs leading to an empty space at the junction of DPAH and the product inside the capillary. The presence of the empty space decreases the rate of diffusion and hence the reaction rate is slower a t 323 K than at 313 K. The thermal studies point. to the absence of reaction temperature and rule out the possibility of bulk diffusion, lattice diffusion, grain boundary diffusion, and surface migration. Several empirical or semiempirical rate laws such as (i) l3= kit, (ii) l2= k2tr (iii) E2 = 2k$, (iv) E2 f l b = k4t, (v) l = k5t, and (vi) = k.6 log t have been proposed to describe the course of the solid-state diffusion-controlled reactions. In these equations is the thickness of the product layer; t is the time, and k 1, k 2, k 3, k 4, k 5, k 6, and b are constants. Our kinetic data fit eq 1. The plot is shown in Figure 2. The diffusing species was indicated by the following experiments. 1. Vapor-Pressure Experiments. The vapor pressures of ethanol, 0.33% ethanolic solutions of DPAH, p - DAB, the yellow product, and the dark green product were found to be 1968.40, 1757.50, 386.65, 386.65, and 878.75 N m-2, respectively. These results show that DPAH is more volatile than p - DAB while the yellow and dark green products are less volatile than DPAH. Therefore, if the reaction is controlled by vapor-phase diffusion the diffusing species is DPAH rather than the p - DAB or the products. 2. Nonporous Experiments. The experiments on filter papers impregnated with DPAH and with p-DAB also point to vapor-phase diffusion and the same diffusing species. It was observed in the course of the pellet experiments that the exposed filter paper turns dark green and the dark green product was deposited on the walls of the
J Flgure 2. Kinetic data for the lateral diffusion reaction and the test of eq 1 for the reaction between DPAH and p-DAB: (0) 303 f 2K, yellow boundary was recorded; (0)313 f 2 K, yellow boundary was recorded; (m) 323 f 2 K dark green boundary was recorded.
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petri dish. The surfaces of the p-DAB and DPAH pellets are coated with the dark green product. The thickness of this coating increases with time. The unexposed filter paper, Le., the filter paper covered with the pellet, remains unchanged. When the pellets were cut into several thin pieces no change in color was observed within the bulk of the pellets. When the DPAH and the p - DAB pellets were placed in another petri dish at a little distance from one another similar results were obtained. To summarize the green deposit on the DPAH and p - DAB pellets junction >> on the p - DAB pellet surface > DPAH pellet surface > on the walls of the petri dish > on the filter paper > on the resin beads. All these results are consistent with the postulated vapor-phase diffusion and indicate Ph2NH2+Cl- as the diffusing species. Mechanism of Chemical Interaction. By X-ray diffraction a good diffractogram containing 36 lines was obtained for DPAH. The strong lines correspond to d values of 3.43, 9.40, 4.37, 4.91, 4.53, and 6.50 A. However, no line is observed for the hard dark green product prepared by mixing the DPAH and p - DAB in 1:l mole ratio indicating that the DPAH is absent in the product and it has been consumed to form a new compound. Furthermore the spectra of the yellow and the dark green products in the visible region The Journal of Physical Chemistry, Vol. 79, No. 2, 1975
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Qureshi, Qureshi, Rathore,
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