CORRESPONDENCE
On the Kinetics of Aromatic Nitration Comments @ Lyle F. Albright
T
he article on this topic by R. C. Miller, D. S. Noyce, and T. Vermeulen, published by INDUSTRIAL AND ENGINEERING CHEMISTRY 56, 6, 43 (1964) describes attempts to devise a chemical model that is applicable to all aromatic nitrations. T h e particular model that is proposed may be correct, but I feel that additional investigations are needed to prove or disprove the mechanism and model proposed. T h e nitronium ion mechanism postulated by the authors may have limitations and may apply only to nitrations with concentrated acid. I n addition, it seems that the experimental methods are open to question on several major points : -the question of eliminating mass transfer resistances. -the question of solubility of reactant and product in the mixed acids.
Reactor Design. Although the reactor used by the present authors is not completely described, the use of a glass reactor a t 350 r.p.m. probably does not give a degree of agitation anywhere near comparable with that obtained by Kobe and associates (7, 3) or White and associates (2, 6 ) . An attempt has been made to estimate the Reynolds number for agitation in the present system. T h e acid phase would undoubtedly be continuous in the reactor, and density and viscosity of the mixture can be approximated. For a 2-inch impeller, the Reynolds number would be, a t most, about 20,000. Such a value for a flat-blade impeller would likely not give good agitation. T h e authors report that the organic and acid phases separated rapidly (within a matter of seconds). This observation implies that the liquid phases may not have been well emulsified.
Nitronium Ion Mechanism
I t is generally recognized that many aromatic nitrations involve nitronium ions. However, the nitronium ion mechanism may apply only to those nitrations made with concentrated acids. Several investigators point out that nitronium ions may not exist in relatively weak mixed acids such as are normally used for nitrating easily nitrated aromatics such as toluene, benzene, and phenol. Buntin et al. (4)suggest instead that the nitrosonium ion may be the active nitrating ion in the case of weak acids containing nitrous acid. Brennecke and Kobe (3)indicate also that the mechanism of nitration changes when weak acids are used. Hence, plots such as Figures 7 and 8 of the present paper probably should not be drawn without showing a line or area of discontinuity. Mass Transfer Resistances
Comparison with O t h e r Data. Several experimental studies of nitration of aromatic systems have shown that mass transfer resistances are important at much higher agitator speeds than the speeds used in the work under discussion. In the nitration of toluene, the work of McKinley and White ( 6 ) can be compared with that of Kobe and associates (7, 3). McKinley and White found only a 4% variation in reaction rate in varying impeller speed from 1200 to 2000 r.p.m., and, therefore, seemed to have eliminated mass transfer resistances. Yet Kobe’s work, apparently a t higher agitation, gave results higher by about a factor of four. I n indicating a difference of 18yo in the reaction rate constant between run 26 (100-150 r.p.m.) and run 27 (350 r.p.m.), the authors d o not seem to have proved elimination of mass transfer resistances. Again, in the nitration of benzene, Biggs and White (2) used agitator speeds as high as 2810 r.p.m., yet they indicate that “it is not clear whether or not the effects of mass transfer have been completely eliminated for all values of the phase ratios investigated. ..”
.
Solubility in Mixed Acids
Solubility of the reactant and product in the mixed acid is undoubtedly a n important consideration when the reaction mechanism is to be considered. Apparently, all measurements were made a t 20’ C., but nitration runs were all made a t 4580’ C. Solubility probably increases a t these higher temperatures, and would tend to increase the apparent activation energy. Calculation of Energy of Activation
In the data presented, there is evidence that the calculated energy of activation actually varied with temperature, rate of nitration, or both. T h e value of 18.1 kcal. per gram-mole was calculated (page 48) using runs 1 2 and 13 and runs 52 and 53. If run 14 were included with runs 12 and 13, the activation energy would decrease somewhat with increased temperature. Several other series of runs could be used for calculating the energy of activation. Runs 35 and 35a indicate a value of COMPARISON OF VALUES OF k
Run
1 2 4
5 6 8 10 15
1
Table I
I
Figure 5
1.17 0.00 0.96 0.75
0.75 0.6 1.1 0.95
1.43
1.25 0.95
0.86 0.66
0.70 VOL. 5 7
0.78 0.85
NO. 1 0 O C T O B E R 1 9 6 5
53
about 23 kcal. per gram-mole. Two other series of runs! 57 and 57R>and 33 and 36, both a t higher ranges of temperature, indicate energies of activation lo\ver than 18 kcal. per grammole. I n t\vo cases: the temperature ranges of these additional series of runs were low; as a result, the accuracy in calculating the energy of activation is probably decreased. Yet the temperature difference betiveen runs 35 and 35a is essentially identical to that of runs 52 and 53. The authors state (page 48) that “18.1 kcal. per gram-mole activation energy indicates that mass transfer resistance is not appreciable.” T h e activation energy for the results of McKinley and White \vas 14 kcal. per gram-mole, xvhich is quite similar. There is doubt, therefore, whether the present arguments are any more correct than those of McKinley and White. T h e relatively high calculated activation energies may actually be caused by a combination of the following three factors: resistances to chemical reaction, mass transfer resistances, and changes of the aromatic solubility in the acid phase as the temperature increases.
increase as the reaction progresses? Perhaps the chemical model chosen is not the correct one, and the k-values d o not represent the true chemical reactions. A comparison of Figure 5 with Table I also implies that energy of activation varied; the k values. corrected to 50”C . as shown in Table I , were plotted in Figure 5. Yet eight of the 14 runs made a t 50’ C. indicate poor agreement between Table I and Figure 5 of the original article, as shown in the table here. The comparison certainly raises doubt concerning the accuracy of the data, or the technique of correcting k values to 50’ C. using 18.1 kcal. per gram-mole as the activation energy. I n Figures 7 and 8. the data of McKinley and White (6) ivere used with those of the present investigation. If the two sets of data correlate well, it supports the contention that mass transfer resistances were not eliminated in the present investigation, because I accept the data of Kobe and asrociates ( 1 . 3) as a closer representation of the true chemical reactions than those of McKinley and White.
Reaction Rates
LITERATURE CITED
Consideration of the data and graphs presented casts further doubt on the accuracy of the data or the method of correcting k-values using activation energy. T h e fact that the data shown in Figures 3 and 4 is plotted as a straight line does not in itself verify the model postulated. T h e authors report that such plots were concave upward for acids stronger than 64 mole 7, in sulfuric acid. TYhy does k
(1) Barduhn, A . J.. K o b e . K. A , : IND. ENO. CHEM.48, 1305 (1956). ( 2 ) Biggs, R . D.. Lt‘hite, R . R..AZChE J . 2 , 26 (1956). (3) Brennecke, H . 1 4 . . Kobe, K . A , , IND.Esc. CHEM.48 1298 ( 1 9 5 6 ) . (4) Buntin, C . h.. Hughes. E . D.. Ingold, C. K., Jacobs: D. I. H., Jones. 51. H.. M i n k o f f ,G . J.. Reid. R . 1.. J . Chem. Soc. (Brit.) 1950, p p . 2628-57, (5) Gillespie, F. J., Millen, D. J., Quart. Reu. (London) 2, 2 7 7 ( 1 9 4 8 ) . (6) McKinley. C.. White, R . R.. T m n s . .4m.Znst. C h m E n q r . 40, 143 (1944).
Lyle F. Albright i s Professor of Chemical E n g i n e e r i q , Purdue Fniversity, Lafayette, Ind.
Reply of R. C. lIMiller, B. S.Ncyce, and T. Vermeulen
S
ome reaction-rate papers contain “bad” kinetics and others “good“ (;.e.: valid) kinetics. Professor Albright differs from us regarding the category of the present paper. The questions we faced in deciding whether to release the results were these : --Would the conclusions dralvn be different if the study were extended? -Should publication be delayed until further data could be obtained? Because of the ivay the conclusions fit. we assessed their validity as showing 1OO:l odds that the NO2+ mechanism applies to “ordinary” nitration reactions over the entire mixedacid range. TVhen we tardily encountered Den0 and Stein’s results for homogeneous nitration, in which the same mechanism Lyas supported, the odds were raised to about IO4:1 in its favor. (The presence of N 0 2 + throughout the mixed-acid range in an amount sufficient to produce nitration has now a thermodynamic proof. in terms of interpolated “03, H+, and H 2 0 activities, that is essentially irrefutable. T h e N O + mechanism is claimed by its advocates to apply only to extremely reactive aromatic species.) I n other words, there is no known aspect of gross kinetic behavior over a relative-rate range of 109 that cannot be explained to well within a factor of 10 by the NOz+ mechanism. Should this information, and our basis for it, have been withheld for another three, or ten, years? Far better to exhibit it and allow others also either to refine or to reject it. I t should be noted that the k contours do not depend upon the NOn’ mechanism. Professor Albright’s criticism of the paper centers upon his belief that mass transfer contributes in a major way to the observed rates; but our data appear to us to lie in a n effectiveness factor range of 80 to 90y0or better. Uncertainty in the activation energy reduces the absolute reliability of rates meas54
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
ured away from 50’ C., but not by any order of magnitude. T h e results of McKinley and White are not uniformly incorrect, but are most in doubt in their region of fastest reaction. The work of Kobe and coworkers was not used because of unresolved discrepancies that exist between the studies by Barduhn and those by Brennecke. Despite its possible iveaknesses, the study we have reported provides virtually the only basis yet available for predicting the effect of mixed-acid composition, throughout the whole range, upon the chemical nitration rate. I t \vi11 most often be used for short-range extrapolations of known data, and ill be least subject to error in such use. T h e over-all vie\r of relative reactivity we provide is subject to some correction even for the same compound (HFX), but will almost certainly be substantiated qualitatively for any other aromatic compound of low reactivity that is studied in the future. Thus, our objective \vas to draw conclusions about the chemical nature of nitration reactions. ,4separate study should be conducted of the mass-transfer behavior of these reactions, and it is quite possible that hexafluoroxylene or pentafluorobenzene will be a n ideal material for those studies. For such purposes a very narrow range of acid compositions \\,odd be studied with a more highly purified reactant, with its solubility estimated (it probably cannot be measured under reaction conditions) as a function of temperature, with numerous runs made a t different temperatures and a t different stirring speeds, with the degree of emulsification measured photographically. and viscosities and diffusivities determined externally.
R. C. M i l l e r is with the C‘nion Oil Go. of California. D . S. .\‘ojce and T . Vermeuien are at the CTniversitj of California. Berkeley,
Calif.
