I n d . Eng. C h e m . Res. 1989, 28, 38-43
38
Formation of Grignard Reagents from the Reaction of Cyclopentyl Bromide and a Rotating Disk of Magnesium in Diethyl Ether: A Comparison of the Chemical Reaction Rate and the Mass Transport Ph. Hasler and W. Richarz* Department of Chemical Engineering, Swiss Federal Institute of Technology E T H , CH-8092 Zurich, Switzerland
Studies of the reaction of cyclopentyl bromide with a rotating disk of magnesium in diethyl ether showed that a t a temperature of 25.8 "C the overall rate constant (k0J is influenced but not limited by mass transport (Reynolds number > 20000). For a surface area of 7.07 cm2,the chemical reaction rate (12,) has been determined as 9.0 X s-', while the mass-transport coefficient (k,) varied in the range 1.7 X 10-4-7.1 X s-'. With a electrochemical test reaction, additional measurements have been made to determine the hydrodynamic flow behavior on rough disks. The mass-transport coefficient (k,) and the chemical reaction rate constant (k,) of the Grignard reaction have been measured as absolute values. 1. Introduction Investigations to determine the mechanism of formation of the Grignard reagents have attracted scientists (e.g., Kharasch and Reinmuth (1954),Tamura and Kochi (19711, Nutzel (1973), Bodewitz et al. (1975), Lawrence and Whitesides (1980),Ashby (1980), Lindsell(l982)) for many years. But only a few papers (Rogers et al., 198Oa-c; Root et al., 1981) deal with the overall kinetics. Thus, there is insufficient information for the design of the equipment. We describe an improved technique to measure absolute rather than relative kinetic data such as chemical reaction rate constants and mass-transport coefficients for the reaction of cyclopentylbromide with a rotating disk of magnesium in diethyl ether at a temperature of 25.8 "C.
Scheme In
2. Experiments A simplified reaction scheme for the formation of the Grignard reagent and its byproducts from alkyl halides is illustrated in Scheme I. The rate-limiting step is the abstraction of an electron from magnesium (Rogers et al., 1980a-c; Lindsell, 1982; Sergeev et al., 1983) (step 1). Excluding one, all the other reactions are much faster due to the radical nature of these reactions. A Wurtz-type reaction can possibly occur between the Grignard reagent and the organic halide (step 5 ) . Starting with a 0.15 M solution of Grignard reagent and cyclopentylbromide, however, no additional yield of dicyclopentyl was obtained at 25 "C.Therefore, it can be assumed that a Wurtz-type side reaction can only be of minor significance under the experimental conditions used here. The Schlenk (1931) equilibrium 2RMgX e MgR2 + MgX2 (1)
concentration of the alkyl halide, and k,* the heterogenous chemical reaction rate constant. Assuming that the overall reaction content (k,) depends on both the chemical reaction and mass transport, the reaction rate equation can be written according to the two-film theory as dcRx/dt = -krCRXs = -km(CRX - CRXJ
K = ([MgX2][MgR2])/[RMgXI2i= 0.002 can be neglected for the reaction of cyclopentylbromide with magnesium in diethyl ether (Smith and Becker, 1966). The formation of the Grignard reagent in the model equation leads to, for the chemical reaction rate, dcRx/dt = -kr*AMgCRXs = -krCRXs where
cRX
(2)
is the bulk concentration, cRXsthe surface
i
R(tH)
R-R
R(-H)
"Mg = magnesium, RX = alkyl halide, RR = dialkyl, RMgX = Grignard reagent, R(-H) = alkene, R(+H) = alkane.
dCRx/dt = -kovc~x= - l / ( l / k m
+ l/k,)cRx
where k, is the mass-transport coefficient and k, the chemical reaction rate constant of the formation of the Grignard reagent. Our experiments confirm that the overall reaction rate is first order in alkyl halide and pseudo zero order in magnesium (see also Rogers et al. (1980a-c) and H ill et al. (1980)). It is further assumed that, after activation and initial alkyl halide consumption due to reaction, the magnesium surface is of uniform activity and the area remains constant during reaction. Since the rotating disk system offers the advantage of well-defined mass-transport characteristics (Levich, 1962; Riddiford, 1966), at least for smooth disks, it has been chosen as a suitable method to measure both the mass transport and the chemical reaction rate. The mass-transport characteristics are usually written as dimensionless equations. This leads for a smooth disk in the laminar flow region and for large values of tht Schmidt number (Levich, 1962)
Shl = 0.62Re0.5S~0.33 *To whom all correspondence should be addressed. 0888-5885/89/2628-0038$01.50/0
(3)
(4)
The dimensionless groups are as follows: Sherwood num0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 39 410 ' 4
k ov
Is 'I
500
Figure 1. Apparatus for the Grignard reaction of cyclopentylbromide with a rotating disk of magnesium in diethyl ether. (1) Rotating speed controlled motor: 100-6000 rpm, h0.3%. (2) Precision stirring shaft, maximum eccentrity < 0.1%. (3) Mounting with two high-precision ball bearings. (4) Double-walled glass flask, maximum volume: 500 mL. (5) Magnesium disk, diameter: 3.0 cm. (6) Stick with merging paper to scratch magnesium. (7) Nitrogen input. (8)Liquids input, nitrogen output. (9) Mercury thermometer. (10) Thermostatic unit.
ber (Sh) = k,r/D, where k , = k,'V/A; Reynolds number (Re) = 2 r w P f v ; Schmidt number (Sc) = u/D where r is the disk radius (cm),D is the diffusion coefficient (cm2/s),o is the rotations per second (s-l), v is the kinematic viscosity (cm2/s), k , is the rate constant (cmfs), k,' is the masstransport coefficient of a smooth disk (s-l), V is the volume of solution (cm3),and A is the magnesium surface area (cm2). Liu and Stewart (1972) have calculated eq 4 for moderate and small values of the Schmidt number to obtain Shl = aRe0,5Sc1/3
(5) where a = l.Of(1.6117 0.4803S~-'/~ + 0.2339S~-~/~...). Due to the stochiometric dissolution and the pitting of magnesium (Hill et al., 1980, Root et al., 1981), the disk surface becomes roughened, thereby changing the hydrodynamic flow behavior across the surface (Rogers and Taylor, 1963; Daguenet and Robert, 1968; Cornet et al., 1969). One can formulate an equation for the Sherwood number of rough disks analogous to smooth disks as
+
+
S h = a'ReaSc@ (6) The exponent p remains constant; thus p = 0.33. The preexponential factor ( a ' ) is a complex function of the Schmidt number, the effective surface area, and the degree of roughness of the disk. The critical Reynolds number for grinded, rough disks where the flow is changing from laminar to turbulent (Cornet et al., 1969) is about Re, = 80000. All experiments were carried out at Reynolds numbers less than this value. By introducing a hydrodynamic flow correction factor ( f ) which is defined as f = S h / S h l = a'Rea/aReQ,5
(7)
one can compare the observed mass-transport coefficient (k,) with the mass-transport coefficient of a smooth disk (km*) in the laminar flow region at a specific Schmidt number. Apparatus for the Grignard Reaction. The experiments were carried out by using the apparatus shown in Figure 1. The magnesium disks had a diameter of 3.0 cm and were embedded in a PVDF stirrer with epoxy resin. The shape of the stirrer has been designed to avoid dis-
low
w [rpm]
Figure 2. Overall reaction constant (k,) from the Grignard reaction a t 25.8 "C for various rotating speeds ( w ) .
turbances of the flow characteristics across the surface from the upper volume of solution (Riddiford, 1966). A merging plate and small amounts of iodine and cyclopentylbromide were used to initiate reaction at 600 rpm. The experiments were carried out at 25.8 "C. Once the initial reaction was complete, the rotating speed was altered to the desired speed and additional cyclopentylbromide was added. Speed values have been chosen randomly. Gas chromatographic analysis of the liquid with calibrated response factors has been used to obtain the concentration data employed for the kinetic calculations. Apparatus for the Hydrodynamic Measurements. The experiments were carried out by using an apparatus similar to the one shown in Figure 1. The shape of the apparatus is the same as for the Grignard reactions. For the measurements of the hydrodynamic flow behavior of the rough disks, the electrochemical reduction of alkaline K3Fe111(CN)6 to K,Fen(CN)6was chosen as a suitable test reaction. This reaction is fully mass transport limited (Selman and Tobias, 1978). Electrochemical reaction rates can easily be measured by the current flowing through the cell. Therefore, the measurements and control of the parameters were performed with a personal computer. Since magnesium is not stable in aqueous solution, platinized nickel duplicates were made of the original disks. This process involved several steps: the manufacture of a silicone rubber negative from the original magnesium disk analogous to galvanoforming procedures (DiBari, 1984); coating of the nonconducting negative with a thin film of high-conductive silver lacquer; electrodeposition of nickel on the negative (Brugger, 1984); electrochemical platination to avoid surface passivation. 3. Results Grignard Experiments. Table I and Figure 2 show the calculated kinetic data from the experiments according to eq 3. The Reynolds number varies between 20000 and 78 000, and the temperature is kept constant at 25.8 OC. Above a Reynolds number of about 85 000, vortexing became severe. Figure 2 shows that there is a scattering of the data when the overall reaction constant (kov) is plotted versus the rotating speed (a).Some scatter in the data points is due to experimental errors. However, as we will show, most of the deviations are mainly caused by the different hydrodynamic flow behavior across the magnesium as a result of the surface roughening. Therefore, it is not advisable to calculate a chemical reaction rate from plots of the inverse overall reaction constant (1/ k,) versus the inverse
40 Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 Table I. Overall Reaction Constant (kJ from the Reaction of Cyclopentylbromide with a Rotating Disk of Magnesium in Diethyl Ether at 25.8 "C (Equation 3) w,
expt 9 7 6 2 1 8 10 4 6 3 11 12 13
*Pm 259 409 489 501 604 655 750 841 900 962 1050 1053 1057
Re
k,,, s-' 1.31 x 10-4 1.79 x 10-4 2.57 x 10-4 2.20 x 10-4 2.51 x 10-4 3.36 x 10-4 3.32 x 10-4 3.59 x lo4 3.20 x 10-4 2.71 x 10-4 3.14 x 10-4 3.98 x 10-4 3.76 x
19 350 30 550 36 530 37 430 45 120 48 930 56 030 62 830 67 230 71 870 78 440 78 660 78 960
3
square root of the rotating speed ( ~ / w O . ~ ) . Hydrodynamic Flow Experiments. As a test of the apparatus, the Shenvood numbers have been measured as a function of the Reynolds and the Schmidt numbers for a smooth, platinized nickel electrode in the laminar flow region. The viscosity of the solution (0.5 M NaOH, 0.01 M K3Fe*11(CN)6,0.02 M K,Fe"(CN),) and the diffusion coefficients of the hexacyanoferro ions are u
+ 3.9022 X
= 1.7728 X
D = 3.8180
X
- 4.182 X 10-4T
lo4
10*T2 (8)
+ 1.0191 X lO-'T + 1.4982 X 10-9T2 (9)
where the temperature (T) is in "C and the kinematic viscosity and the diffusion coefficient are in cm2/s. These data have been measured with a rotating disk system too (Bourne et al., 1985) and are valid in the temperahre region from 15 to 35 "C. For a smooth, platinized nickel electrode, the function to determine Sh = f(Re,Sc) was measured as
Shl = 0.58Re0.5Sc0.33
(10)
The error is less than 3% to the theoretical value (Liu and Stewart, 1972) and suggests that the equipment has been properly designed to yield reliable data. The hydrodynamic flow correction factor (f,eq 7 ) has been modified according to the measured data as f = ~Re"/0.58Re~.~
Table 11. Measurement of the Function Sh = f(Re,Sc) (Equation 5) and the Mean Hydrodynamic Flow Factor f (at a Temperature of 25 "C and at the Reynolds Number of the Corresponding Grignard Experiment, Equation 6) for Platinized Nickel Electrodes in Alkaline Solution (0.5 M 0.02 M K4Fe11(CN)6) NaOH, 0.01 M K3Fe111(CN)6, expt a' a Re f 0 0.579 0.499 9 19 350 1.150 0.640 0.504 30 550 1.404 0.332 0.586 5 36 530 1.753 0.263 0.628 2 37 430 1.232 0.244 0.601 45 120 1.405 1 0.218 0.622 48 930 2.169 8 0.325 0.625 2.347 10 56 030 0.243 0.657 4 62 830 2.151 0.145 0.693 67 230 2.097 6 0.338 0.614 3 71 870 1.378 0.212 0.618 11 78 440 1.812 0.296 0.611 2.340 12 78 660 0.410 0.606 1311 78 960 1.864 0.247 0.630 1.812 78 960 0.351 0.596 1312 78 960 1.820 0.258 1313 0.624
(11)
For every electrode (platinized nickel duplicate of the magnesium disk), a series of experiments at different Reynolds (25 speed values) and Schmidt (4 temperatures) numbers was carried out. The temperature has also been varied to determine the proper condition of the solution (no oxygen effect, no poisoning of solution) and completely degreased surface of the platinized nickel electrode. The hydrodynamic flow correction factors (f ) were calculated a t the corresponding Reynolds number of the Grignard experiment and are listed in Table I1 as mean values. Experiment 0 corresponds to the measurements of the smooth electrode. All T values (mean value/standard deviation) are larger than 200. The mean preexponential factor (a', 6) varies between 0.14 and 0.64, t.he exponent of the Reynolds number ( a )varies between 0.5 and 0.69. Experiment 13 has been used as a test of the reproducibility of the duplicate manufacturing. Starting with the magnesium disk three silicone rubber negatives were made, and of each, one platinized nickel duplicate was prepared. The comparison of the mean hydrodynamic flow
factors (f) of experiments 13/1, 13/2, and 13/3 indicates that the reproducibility of the duplicate manufacturing is satisfactory. The ratios of the observed mass-transport coefficients (hm)of the rough disks to the corresponding mass-transport coefficient (&*) of a smooth disk is defined as the hydrodynamic flow correction factor cf, eq 7). Analyzing the data in Table 11, one notices that the mass-transport coefficients (k,) are up to 2 times higher than the masstransport coefficients (&*) of the corresponding smooth disk. Assuming the hydrodynamic flow behavior is partly laminar and partly turbulent, one may formulate a equation for the Sherwood number as
+
Sh = ~ u R e ~ . ~ s(1 c -~ ~. )~0~. 0 1 9 8 R e ~ ~ (12) ~Sc~~~~ laminar flow turbulent flow
where K is the fraction of laminar flow. However, in the region investigated, eq 12 did not fit the experimental data better than eq 6. Chemical Rate Constant and Mass-Transport Coefficients for the Grignard Reaction of Cyclopentylbromide and Magnesium in Diethyl Ether. The data in Table I1 indicate that the exponent of the Reynolds number ( a )and the factor a'(eq 6) are not constant over the range of rotating speed (w) used. Therefore, it is not possible to determine the chemical rate constant ( k J graphically. We have calculated k, as the difference of the overall reaction rate constant ( k 0 J and the true masstransport coefficient (&, eq 3). The latter can be obtained by multiplying the mass-transport coefficient of a smooth disk (&*) with the hydrodynamic flow correction factor (f,eq 11). K,' has been derived from measurements of the diffusion coefficient ( D ) and the kinematic viscosity (v) of cyclopentylbromide in diethyl ether (Hasler, 1986). The diffusion coefficient has been determined by the diaphragm method (Stokes, 1950; Janz and Mayor, 1966; Dunlop et al., 1972) and leads to D = 4.71 x IOw5cm2/s at 25.8 "C. The kinematic (standard deviation: 4.6 X viscosity was measured with a capillary viscometer to give a value of v = 3.15 X cm/s (standard deviation: 1.5 X 10-j) at 25.8 "C. This yields a Schmidt number of S c = 66.9. The mass-transport coefficientsof the smooth disk (km*) have been calculated by using eq 5 (Table 111). To obtain the effective mass-transport coefficient in the Grignard reactions (k,), the mass-transport coefficient of the smooth disk (&*) (Table 111) has been multiplied by
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 41 Table 111. Determination of the Mass-Transport Coefficient of a Smooth Disk (km')with Measured Physical Properties and Function Sb = f(Re,Sc)for Cyclopentylbromide in Diethyl Ether at 25.8 "C Re km', s-l expt 19 350 1.495 X lo4 9 1.879 X 30 550 7 2.055 X lo4 36 530 5 37 430 2.080 X lo4 2 2.284 X lo4 45 120 1 2.378 X lo4 48 930 8 2.545 X lo4 56 030 10 62 830 2.695 X 10"' 4 67 230 2.787 X lo4 6 2.882 X 10"' 71 870 3 3.011 X lo4 78 440 11 3.015 X 78 660 12 3.021 X 78 970 13 Table IV. Calculation of the Chemical Reaction Rate Constant (k,)for the Reaction of Cyclopentylbromide with a Rotating Disk of Magnesium in Diethyl Ether at 25.8 O C expt k,, s-l k,, s d k, 9 1.72 X lo4 1.31 X lo4 5.50 x 10-4 7 1.79 X 10"' 2.64 X lo4 5.57 x 10-4 2.57 X lo4 8.97 X lo4 5 3.60 X lo4 2.56 X 10-j 15.55 X 2.20 x 10-4 2 11.52 X 3.21 X lo4 2.51 x 10-4 1 8 9.64 x 10-4 3.36 x 10-4 5.16 X 10"' 7.47 x 10-4 10 5.97 x 10-4 3.32 x 10-4 4 5.80 x 10-4 9.43 x 10-4 3.59 x 10-4 6 5.84 X lo4 7.07 X lo4 3.20 X lo4 8.53 X 10"' 3 2.71 X lo4 3.97 x 10"' 3.14 X lo4 11 5.46 X lo4 7.40 X lo4 3.98 x 10-4 7.06 x 10-4 9.13 x 10-4 12 11.63 X 3.75 x 10-4 5.53 x 10-4 13 ~~
~
~
the corresponding hydrodynamic flow factor cf) (Table 11). Using the overall rate constant (kov) (Table I), one calculates the desired chemical reaction rate constant (k,)by using eq 3. The data are listed in Table IV. The mean value of the chemical rate constant (k,)is 9.0 X s-l (standard deviation: 2.7 X lo4 s-l). This leads for a surface area of 7.07 cm2 to a heterogeneous chemical reaction rate constant (k,*)of 1.3 X s-l cm-2 at 25.8 OC. 4. Discussion The kinetic parameters of the Grignard reaction of cyclopentylbromide in diethyl ether have been measured as absolute values by use of a rotating disk of magnesium in the laminar flow region (Reynolds number < 80000). The chemical reaction rate constant (k,)is found to be in the same order of magnitude as the mass-transport coefficients (k,). The assumption made in previous publications (Rogers et al., 1980a-c) to neglect the influence of the chemical reaction rate constant on the overall reaction rate constant (12,) is not consistent with the results presented here. When the experiments are carried out a t low Reynolds numbers and close to the boiling point of the ethereal solution, there is still a significant influence of the chemical reaction on the overall reaction rate. Therefore, it seems not possible to measure purely mass-transport-limited reaction rates in the presence of convective forces, a t least for cyclopentylbromide as a substrate in diethyl ether solution. However, it should be possible to measure mainly chemically controlled reaction rates at low temperatures and much higher Reynolds number (>300000). For rough disks, a Reynolds number larger than 4 0 000 should change the hydrodynamics from laminar to completely turbulent flow, where the exponent of the Reynolds num-
ber (CY) of 0.8 will be obtained. Measurements in the turbulent flow region would make the time-consuming experiments to determine the hydrodynamic flow correction factor cf) unnecessary. When the experiments are to be carried out in the laminar flow region, the consideration of the flow correction factors significantly increases the accuracy of the chemical reaction rate constant. Neglecting the influence of the roughness of the disks results in a chemical reaction rate constant of -1.7 X s-l with a standard deviation of 3.2 x The initial stages of the Grignard reaction still remain largely uncertain. Nevertheless, the entire magnesium surface was active during the experiment as shown by straight lines resulting from plots of In cRX versus time. The final shapes of the disks resembled those of comparable experiments (Rogers and Taylor, 1963), but it was not possible to reproduce the shape of a disk nor to find a correlation between the rotating speed and the hydrodynamic flow correction factor ( f ) . The present measurements show that for cyclopentylbromide, which reacts near to mass-transport-limited conditions in diethyl ether solution, the chemical reaction has to be taken into consideration when measuring kinetic data. 5. Conclusions Examining the data in Table IV, one finds a chemical rate constant (k,)of the Grignard reaction investigated that is in the same order of magnitude as the mass-transport coefficients (k,). The magnesium disks had a surface area of 7.07 cm2. The mass-transport coefficients of the rough disks (k,) vary in the range 1.7 x lo4 (Re = 19450) to 7.1 X s-l (Re = 78 700),while the chemical reaction rate constant (k,) is 9.0 X s-l at 25.8 OC. In order to get more accurate values of the chemical reaction rate constant, it would be necessary to alter the rotating speed significantly, thereby shifting the quotient k , / k , to smaller values. 6. Experimental Section
General Method and Procedures. Magnesium disks with a diameter of 3 cm were made of polycrystalline magnesium of ordinary quality (99.9% Mg, 0.08% Fe, 0.01% Mn, determined by AAS). Before their use in the Grignard experiments, the disks were polished with a 600-mesh merging paper at 5000 rpm until the surface was bright and smooth. Cyclopentylbromide (Fluka, purum) was purified by vacuum distillation over activated carbon, yielding a purity of 99.8%. For the Grignard experiments, only freshly distilled cyclopentylbromide was used. The solvent used for the kinetic studies, diethyl ether, was refluxed for several hours over sodium-potassium-eutectica and then distilled in dried bottles. As an internal standard, benzene (Fluka, puriss. p.a.) was used. All liquids were stored over activated molecular sieves (4A) and under nitrogen atmosphere. Solutions for the electrochemicaltest reaction were made of quartz distilled water, potassium ferricyanide (0.02 M, Fluka puriss. p.a.1, potassium ferrocyanide (0.01 M, Fluka puriss. p.a.1, and sodium hydroxide (0.5 M, Merck Titrisol). The solution was gassed with helium and stored under helium atmosphere. For the reaction, only freshly prepared solution was taken. Nickel duplicates of the original magnesium disks were made by producing a silicone rubber (silicosehlRTV110 + KA-1) negative, followed by coating it with a thin film (0.5 l m ) of a high-conductive silver lacquer (L200, Demetron). Nickel was electrodeposited at 50 OC with a high-speed plating solution (50 g/L boric acid, 660 g/L nickel(I1) sulfamate, 9.3 g/L nickel(I1)
42 Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989
chloride, 1g/L sodium lauryl sulfate; all puriss p.a. grade). Finally, the nickel (thickness: 3 mm) was electroplated with a platinum film (0.6 pm, platinum solution K, Degussa). The anode was made of nickel; the anodic surface was 20 times the cathodic surface. The electrochemical cell was powered by a routine potentiostat (AMEL 549), and the reduction potential was -600 mV (two-electrode system). Measurements and control of parameters were performed with a personal computer (Olivetti M24SP, data acquisition hardware Burr-Brown PCI20000). Procedure for the Kinetics of Grignard Reaction. A 500-mL, double-walled flask was used. All glass particles were heat dried before used. After the high-precision stirring shaft (powered by MEL4000 motor, Alowag Switzerland) was mounted with the embedded (Araldit standard epoxy resin) magnesium disk, the apparatus was inertisized for several hours with dried nitrogen (99.9999% purity, C0.1 ppm of water). Diethyl ether (470 mL) was introduced under slight pressure and 7 mL of internal standard benzene added. The solution was thermostated, and the water amount was determined which typically was 3-5 ppm (Karl Fischer coulometer 562, Metrohm). The initial amounts of sublimed iodine (less than 0.1 mmol) and cyclopentylbromide (6 mmol) were added to initiate reaction at 600 rpm. The magnesium disk was scratched from time to time with a 600-mesh merging plate. After the initial reaction had completed, 40 mmol of cyclopentylbromide was added to start the experiment. A total of six samples for analysis were withdrawn with slight vacuum to capped vials. Time intervals from one sample to the other were estimated to give equal alkyl halide concentration differences. The samples were cooled to 0 “C, and then the Grignard reagent was hydrolyzed carefully by adding the required amount of water dropwise. Analysis of the hydrolyzed Grignard solution was performed by gas chromatography (Hewlett-Packard HP5880A with FID detector and automatic sampler HP7671A). Each sample (1.5 mL) was injected 3 times. Column material was 25% methylsilcone SE30 on Chromosorb (30/60 mesh). Detector signals were calibrated over the entire concentration range for the internal standard, cyclopentylbromide, cyclopentane, and bicyclopentane (accuracy of concentration: 0.2% 1. Procedure for the Measurements of the Hydrodynamic Flow Behavior of Rough Disks. Before mounting the platinized nickel electrode to the apparatus, it was degreased cathodically in alkaline solution. Nondegreased electrodes gave erratic results. The determination of the function Sh = f ( R e )was performed a t four temperatures (15.0, 22.1, 28.8, and 35.0 OC). The rotating speed varied from 200 to 2300 rpm. The function Sh = f(Sc) has been determined at 25 temperatures between 15 and 35 “C (with temperature corrected Reynolds number). Each Sherwood number has been measured 10 times. The Reynolds and the Sherwood numbers have been defined on the active surface area.
Acknowledgment We thank Hoffmann La-Roche Co., Basle, Switzerland, for the financial support of this project. Registry No. Mg, 7439-95-4; cyclopentyl bromide, 137-43-9.
Literature Cited Ashby, E. C. “A Detailed Description of the Mechanism of Reaction of Grignard Reagents with Ketones”. Pure Appl. Chem. 1980,52, 545-549. Bodewitz, H. W. H. J.; Blomberg, C.; Bickelhaupt, F. “The Formation of Grignard Compounds-111. The Influence of the Solvent”. Tetrahedron 1975, 31, 1053-1063.
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Tamura, M.; Kochi, J. “Couplingof Grignard Reagents with Organic Halides.” Synthesis 1971, 3, 303-305. Received f o r review January 5, 1988 Revised manuscript received August 16, 1988 Accepted August 29, 1988
Modeling of the Thermal n-Octane Oxidation in the Liquid Phase Felix Garcia-Ochoa,* Arturo Romero, and Jose Querol Departamento de Ingenieria Qutmica, Facultad de CC Qutmicas, Universidad Complutense, 28040 Madrid, Spain
Thermal n-octane oxidation in the liquid phase is studied. Molecular schemes of reactions with two possibilities of kinetic equations used for lumping compounds are tested. Parameters are calculated by a multiresponse linear method, and discrimination among kinetic models is carried out applying both statistical and physical criteria. The production rate and distribution of the products of interest, such as ketones, alcohols, and acids, are described, after the relationships of the parameters with temperature and oxygen partial pressure are established by nonlinear regression. Hydrocarbon oxidation by oxygen from the air is a process widely used to obtain intermediate petrochemical products, such as hydroperoxides, alcohols, ketones, and acids (Toland, 1960). The liquid-phase oxidation provides some advantages, such as a greater intermediate product yield, better conversion control, and smaller equipment operating under less severe operational conditions (Prengle and Barona, 1970). This is the process by which methyl ethyl ketone; cyclohexanone; phenol; acetic, adipic, and terephthalic acids; vinyl acetate; propylene oxide; and cumene hydroperoxide, among others, are commercially produced (Saunby and Kiff, 1976). Many studies of the hydrocarbon oxidation mechanism have been made. Nevertheless, a small number of kinetic studies have comparatively been carried out. This is due to the complexity of the system: it is a gas-liquid reaction, the network is very complex, there are probably hundreds of reactions taking place, and the intrinsical mechanism seem to be through a chain free radical with branched degeneration (Dawkins, 1966). Most of the works only offer conversion and distribution product data for different operational conditons (Rumbea et al., 1975). Only some of them (Camacho et al., 1980a,b) determine the kinetic equation of one of the molecular steps (formation or decomposition of hydroperoxide). Very few works (Chen et al., 1985; Cavalieri d’0ro et al., 1980) have analyzed a molecular network of reactions, and these have supposed first-order reactions. For the reactor design, an overall kinetic equation is not usually enough, because the product distribution is often the most important aspect of the design. The aim of this work is to show how to obtain a set of kinetic equations, for a simplified molecular scheme, that is, for a set of reactions yielding the products of interest. This set of simple equations, including the operational conditions, can be utilized for reactor design and for product distribution prediction. Similar problems are present for other interesting technical network reactions, e.g., hydrocarbon cracking/pyrolysis, for which Froment has given different methods of study (Sundaram and Froment, 1977, 1978).
Experimental Section Runs were carried out in an installation where the oxygen partial pressure and the temperature were fixed and 0888-5885/89/2628-0043$01.50/0
Table I. Chemicals Detected by Analysis a n d Species Created by Lumping detected products lumDed species hydroperoxides hydroperoxide 2-octanone ketones 3-octanone 4-octanone 2-octanol alcohols 3-octanol 4-octanol acetic acids propionic butyric valerianic caproic
controlled. The reactor was a glass tube, 20-mm inside diameter, filled with 20 cm3 of liquid n-octane through which a gas stream (oxygen + nitrogen) was bubbled. The total pressure was maintained constant at 1.934 atm. Samplings were made every 20,60,120,180, and 240 min and immediately analyzed. Hydroperoxide overall content was determined by iodometry (Mair and Graupner, 1964), and gas-liquid chromatography (FFAP + H,PO, column, and FI detector) was used for the rest of the products. Eleven peaks were detected, corresponding to the compounds in Table I. Experimental Data. A set of runs was previously carried out varying the gas stream flow (2,6, and 10 L/h) at 0.20 atm of oxygen pressure and 135,140, and 145 “C. Under these conditions, the hydrocarbon conversion and product distribution were the same. Thus, a flow of 8 L/h was adopted for the rest of the runs. By measuring the outlet oxygen content, it was detected that the partial pressure remained constant inside the reactor. Then, 9 runs were carried out varying T (135,140, and 145 “C) and Po (0.1,0.2, and 0.4 atm). Another run, at 135 “C and 0.3 atm, was made. Analysis of the Results. A qualitative analysis of the results showed the following main conclusions: (a) Conversion of hydrocarbon increases with temperature, as shown in Figure 1. (b) The oxygen partial pressure hardly has an influence at low temperature, but this effect was greater at high temperature (Figure 1). (c) The product distribution, when data at the same overall conversion are compared, was hardly affected by 0 1989 American Chemical Society
