Ind. Eng. Chem. Process Des. Dev. 1981, 20, 404-407
404
Kinetics of Nitration of Aromatic Compounds with 'Mixed Acid": Reinterpretation of Published Data The nitration reaction is generally assumed to be Wit order in nitric add and also first order in aromatlc compounds. This assumption is based on nnratiin studies with sulfuric add as the reaction medium and where very low concentrations of reactants were employed and the acidity of the system remained practically constant. However,
in the case of nitrations performed under industrially relevant conditions, the acld strength changes significantly and the value of the rate constant changes severalfold. The assumption of firstorder behavior with respect to aromatic compounds is also open to doubt. An attempt has been made, therefore, to reexamine the published kinetic data with the help of the HR acidity function. Introduction The nitration of aromatic compounds is commonly affected by a mixture of nitric and sulfuric acids, called "mixed acid" and the subject has attracted the attention of several investigators in the recent past (Albright and Hanson, 1976). The intrinsic rate expression used by many of the investigators is given by = k2CHN03CAr (1) The first-order behavior with respect to nitric acid and aromatic substrate was observed by various investigators in the homogeneous acid phase nitration by employingvery low concentrations of reactants. The concentration of the aromatic compound was of the order of lob mol L-' while mol that of nitric acid was approximately equal to 3 X L-'. In the nitration reaction, nitronium ion is known to be the real electrophilic species and its concentration is almost proportional to that of nitric acid as the acid strength of the reaction medium practically remains constant. Hence, eq 1 is valid under such conditions. In the case of heterogeneous mixed acid nitrations, however, the acid strength changes significantly and eq 1 may not be applicable. The use of eq 1for the interpretation of kinetic data results in widely varying values of kp. It is for this reason that empirical correlations have been reported in the literature to relate the rate of nitration with the acid phase composition. For example, McKinley and White (1944) have used eq 2 to express the rate of nitration of toluene.
R = k(XT)(N)'
(2)
The value of P, the order of reaction with respect to nitric acid, varies from 0.6 to as high as 18. In the nitration of benzene, Biggs and White (1956) have proposed a linear relationship between the apparent rate constant obtained by eq 1 and a factor representing the composition of the acid phase on the log log scale. Sehiefferle et al. (1976) have found eq 3 as a suitable expression to correlate the rate of nitration of benzene.
R
a
H,0.36k1'16(hCAr)0'6(CHNOs)4'45
(3)
Hanson and co-workers have studied the macrokinetics of heterogeneous nitration of toluene (1971) and chlorobenzene (1974) with mixed acid of composition 15 mol % HN03, 30 mol % H2S04, and 55 mol % H20. However, no consideration has been given to the change in the acidity of the acid phase and its effect on the change in the rate constant. From the foregoing discussion it is seen that the composition of the acid phase is an important factor and a small change in the proportion of sulfuric acid or nitric acid in the acid mixture significantly affects the rate of nitration. It is believed that nitronium ion is the real electrophilic species involved in nitration. Thus it would be highly desirable to know the relationship between the 0 198-430518111120-0404$01.25/0
composition of the acid phase and the concentration of nitronium ion. The direct determination of nitronium ion concentration is not possible for mixed acid, which is commonly used for mononitration, as it is below the limits of spectroscopic determination. To overcome this problem, the similarity of behavior of tsiphenylcarbinol or its derivatives in sulfuric acid with that of nitric acid could be used (Westheimer and Kharasch, 1946; Lowen et al., 1950; Den0 and Stein, 1956). The ionization of triphenylcarbinol, or its substituted derivative, represented by ROH, in sulfuric acid to produce the corresponding triphenylcarbonium ion is analogous to the ionization of nitric acid to produce nitronium ion
+ H+ e R+ + H2O HN03 + H+ * NO2+ + H2O ROH
(4) (5)
The measurement of the ionization ratios cR+/cRoHby colorimeter for indicators undergoing ionization accordmg to eq 4 has become the basis of an acidity function. The acidity function is denoted by H R and is defined as
HR= - P ~ R O H - log C R + / ~ R O H (6) Relationship between k2and HR.The validity of expression (1)for the homogeneous acid phase nitration suggests the interaction of the nitronium ion with the aromatic substrate to be the slow, rate-limiting step. The rate expression, therefore, should have been rate = kCNO2+CArHYNO2+YArH (7) From the use of eq 1and the concept of acidity function the following relationship can be readily derived
Thus if other terms are assumed to be constant, a linear relationship should exist between log k2 and - H R with a slope of unity. This has been ascertained for nitration of various substrates such as nitrobenzene, p-chloronitrobenzene, phenyltrimethylammonium ion, etc., for homogeneous nitration in the medium of sulfuric acid. A similar treatment is warranted for the nitration of aromatic compounds with mixed acid under industrially relevant conditions. However, no information is available for the values of acidity function for mixtures of nitric acid and sulfuric acid. It was, therefore, thought desirable to obtain such data. Experimental Section Triphenylcarbinol was used for the determination of values of H R acidity function of mixtures of nitric acid and sulfuric acid. The reagent was prepared by reaction of phenylmagnesium bromide with methyl benzoate as reported by Bachmann and Hetzner (1941). 0 1981 American Chemical Society
I d . Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 405 1 >or
Table I. Experimental and Predicted Values of -HR for Synthetic Mixtures of Nitric Acid and Sulfuric Acid -HR no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
CH,SO,
3.501 3.501 3.501 3.50.1 3.501 3.501 3.501 3.501 3.874 4.201 4.551 4.878 5.275 5.625 5.998
CHNO, 4.949 5.360 5.771 6.144 6.592 7.115 7.787 4.342 4.342 4.342 4.342 4.342 4.342 4.342 4.342
9
obsd
6.462 6.687 6.912 7.112 7.362 7.662 8.022 6.112 6.485 6.810 7.150 7.475 7.860 8.210 8.575
6.452 6.688 6.924 7.139 7.396 7.697 8.084 6.102 6.487 6.824 7.184 7.521 7.929 8.290 8.675
l'f
I
Synthetic mixtures of nitric and sulfuric acids were prepared and the desired quantity of triphenylcarbinol added so as to have its concentration of the order of mol L-l. Absorbance values of only those compositions were determined which were reasonably stable (no change in absorbance values over a period of 3-4 h) and are listed in Table I. For other compositions of mixed acid, the indicator was not stable and the direct measurement could not be made. However, the variation of acidity function with the composition of acid mixture could be correlated satisfactorily by the following empirical equation (Table
I)*
-HR = 1.03CH2SOt+ 0.575CHNOa
10 (-H.I
I
11
,
l
12
13
Figure 1. Data of McKinley and White: plot of log RJXTCmh HR.
v8.
(9)
The same expression was assumed to be applicable for the compositions of mixed acid used for nitration studies, and the respective values of HR were predicted. Discussion Correlation of Apparent Rate Constant with HR: In the literature there are extensive kinetic data available for the nitration of aromatic compounds such as benzene, toluene, etc. However, in order to avoid complicationsdue to the role of mass transfer, only the data pertaining to those cases where the kinetic regime is known to prevail were considered. In the case of toluene, the intrinsic rates of nitration are extremely high and it may not be possible to eliminate mass transfer effects, particularly when nitrating mixtures of very high acid strength are used. However, for nitrating mixtures containing less than, say, 30 mol % sulfuric acid, the intrinsic rates could be low enough to permit elimination of mass transfer effecta with efficient agitation. In a recent review, Strachan (1976) has inferred that for the data of McKinley and White, the kinetic regime is most likely to prevail. The plot of (-HR) vs. log Ra/XyCHNOafor the data of McKinley and White for the nitration of toluene is shown in Figure 1. Most of the points lie close to the straight line with a slope of 1.04. Similarly, the plot of -HRagainst the values of log Ra/XB-CHNos for the data of Biggs and White for the nitration of benzene is a straight line as expected and is shown in Figure 2. Thus there seems to be good agreement between the observed and expected nature of the plot. The Intrinsic Rate Expression As in the case of homogeneous nitration, it has been assumed that the reaction of nitronium ion with aromatic species is the rate-controlling step under the industrially relevant conditions. In such a situation, the reaction is expected to be first order with respect to aromatic com-
9.rV
I
I
10
11
I 12
Figure 2. Data of Biggs and White: plot of log R J X & m h
VB. HR.
pounds. However, a close scrutiny of the data indicates that the reaction under industrially relevant conditions is not first order with respect to aromatic species. For example, Schiefferle and co-workers have reported the order with respect to aromatic compounds as 0.6 for the nitration of benzene with mixed acid under the conditions of a kinetic regime. Further, the examination of their plot of rate of reaction against composition of the organic phase indicates that, up to 40% conversion based on benzene, the reaction rate might be independent of the organic phase composition (Figure 3). This could be possible if the formation of nitronium ion is the rate-limiting step, particularly when the conversion levels based on aromatic compounds are low. Another important observation which strengthens doubt about the validity of first-order behavior with respect to aromatic compounds is the difference in relative rates of nitration of toluene, benzene, and chlorobenzene for homogeneous nitration and for nitration under industrial conditions. In the homogeneous nitration with sulfuric acid as the reaction medium where the nitration reaction is known to be first order with respect to aromatic compounds, Coombes et al. have reported the rate of nitration of toluene to be 25 times greater than that of benzene. Similarly, benzene reacts 30 times faster than chlorobenzene (Coombes et al., 1969). Thus the reactivities of
406
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
Figure 4. Data of McKinley and White: plot of log R,/CH,, (-HR + log c H f l ) *
Figure 3. Nitration of benzene: data of Schiefferle and co-workers.
toluene, benzene, and chlorobenzene show a severalfold difference when the attack of nitronium ion on the aromatic substrate is the rate-limiting step. In contrast to these observations, in commercial nitrations the rate of nitration of toluene has been reported to be 1.3 to 4 times that of benzene (McKinley and White, 1944; Hanson et al., 1971; Schielferle, 1976), while the rate of nitration of chlorobenzene is about 0.25 times that of benzene (Hanson et al., 1974). The ratio of rate of nitration of toluene to that of benzene is also reported to vary from 1.3 to 2.8 as the conversion level increases (Hanson et al., 1971). The solubility of aromatic compounds like benzene or toluene in mixed acid is of the order of mol L-I. This corresponds to values of concentration approximately 100 times more than those used in studies with homogeneous acid phase nitration. This is quite an important difference between the two processes and may render the interaction of nitronium ion with aromatic species faster than formation of the nitronium ion, making the latter a rate-limiting step particularly for low levels of conversion. In view of these considerations, it was thought desirable to reexamine the kinetic data. A simplified scheme for the nitration of aromatic compounds may be presented as HN03 + H+ NO2++ H 2 0 (10) NO2++ ArH
-
ArN02 + H+
(11)
If the rate of formation of nitronium ion is the ratelimiting step, the nitronium ion formed may rapidly disappear by reaction 11 and hence the reverse reaction as represented by eq 10 may be ignored. The rate expression would then be given by rate = k0CH+CHNO3YHNO3YH+ (12) If the nitration is assumed to be zero order with respect to aromatic compounds and first order with respect to nitric acid, we have rate = k'exptCHNO8 (13) The relationship between koexptand H R may be readily derived and we have log k'expt =
Thus if other terms are assumed to be constant, a linear relationship should exist between log koexptand (-HR + log CHzO)with a slope of unity.
,
o
l 10
/ I-HR +log CHz0l
va.
13
-
Figure 5. Data of Biggs and White: plot of log R,/CH,, + 1% CHz0).
vs. (-HR
Reexamination of Data. The plot of log R,/Cm, vs. + log CH#) for the data of McKinley and White for the nitration of toluene is shown in Figure 4 and is a straight line with a slope of 1.09. Points corresponding to runs where the conversion based on toluene exceeds 90% deviate significantly from the straight line. Under such conditions the concentration of aromatic compound is low and probably the interaction of nitronium ion and aromatic compound assumes importance. The plot of log Ra/Cm, vs. (-HR + log CHzO) for the data of Biggs and White for the nitration of benzene is shown in Figure 5 and the agreement between the actual and expected natures of the plot is fairly good. In the studies the conversion based on benzene was not more than 60%. It may appear rather contradictory that the same kinetic data could be correlated satisfactorily whether formation of nitronium ion is assumed to be the rate-controlling step or its interaction with aromatic compound. However, it may be due to the limited accuracy of the proposed correlation. I t is also likely that the difference between the rate of formation of nitronium ion and that of its interaction with the aromatic compound may not be of appreciable magnitude under the industrially relevant conditions. (-HR
Ind. Eng. Chem. Process Des. Dev. 1901, 20, 407-409
Conclusion
It may be concluded that the concept of the HRacidity function could be used in a satisfactory manner to correlate the effect of change in composition of the acid phase on the apparent rate constant in nitration reactions performed under industrially relevant conditions. It appears that formation of nitronium ion is the rate-limiting step in the nitration of benzene and toluene at lower levels of conversion. Nomenclature
C = concentration in mol L-'
H, = volume fraction of acid phase in the emulsion
hCh = interfacial concentration of benzene in mol L-' k, k2 = reaction velocity constant in L mol-' min-' K = equilibrium constant k", keXpt= reaction velocity constant in min-' N = mole percent nitric acid in the acid phase P = a constant depending upon the sulfuric acid concentration in the acid phase pK = log of equilibrium constant r, R = rate of nitration in mol L-' min-' R, = rate of nitration in mol L-' min-' based on acid phase X = mole fraction of unnitrated aromatic compound in the organic phase Y = Activity Subscripts Ar = aromatic compound B = benzene H+ = hydrogen ion
401
H20 = water HN03 = nitric acid H2Sy4= sulfuric acid NO2 = nitronium ion R+ = triphenylcarbonium ion ROH = triphenylcarbinol T = toluene L i t e r a t u r e Cited Aibright, L. F.; Henson. C. "Industrial and Laboratory Nltratlons", Symposium Series, American Chemical Soclety, 1976. Bachmann, W. E.; Hetzner, H. P. In "Organic Syntheses", Collect. Vol. 111, Wiiey: New Y a k , 1941; p 839. Biggs, R. D.; White, R. R. A I C M J . 1958, 2(1), 26. Chedln, J. C. I?. Aced. Sci., Ser. B 1935, 200, 1397. Coombes, R. G.; Moodie, R. B.; SchoHeld, K. J . Chem. Soc. 61989, 800. Deno, N. C.; Stein, R. J . Am. chem. Soc. 1958, 78, 578. Henson, C.; Marsland, J. G.; Wilson, 0. Chem. Eng. Sci. 4971, 26, 1513. Henson, C.; Marsland, J. 0.; Naz, M. A. Chem. Eng. Sci. 1974, 29, 297. Lowen, A. M.; Muray. M. A.; Williams, 0. J . Chem. Soc. 1950, 3318. McKinley, C.; white, R. R. Trans. AIChEl944, 40, 143. Schefferle, D. F.; Henson, C.; Aibrlght, L. F. In "Industrial and Laboratory Nltratlons", Albrlght, L. F.; Henson, C., Ed.; Symposium Serbs, American Chemical Soclety, 1976; Chapter 11. Strachan, A. N. In "Industrial and Laboratory Nitrations", Aibrlght. L. F.; Henson, C., Ed.; Symposlum Series, Amerlcan Chemical Society, 1976; Chapter 13. Westheimer, F. H.; Kharasch, M. S. J . Am. Chem. Soc. 1948, 88, 1871.
Department of Chemical Technology Jayant M. Kanhere Uniuersity of Bombay Sampatraj B. Chandalia* Matunga Bombay 400019, India Receiued for reuieur April 10, 1980 Accepted December 16, 1980
UNIFAC Group Contribution Method for Silicone Compounds The UNIFAC group contribution method is applied to solutions of silicone compounds. Reliable vapor-liquid equilibrium data are used to estimate interaction parameters between six common groups and two groups containing silicon. A general purpose optimization package is used to estimate the interaction parameters.
Introduction
In yi = In y;
In the recent years a new method has been developed for predicting thermodynamic properties of liquid mixtures. This method, called UNIFAC, was proposed in 1975 by Fredenslund et al. (1975). The significant contribution of UNIFAC is its capability of predicting activity coefficients for mixtures for which no experimental data are available. Liquid mixtures of silicone compounds, especially polymers, have found numerous applications (Morton, 1973). Surprisingly, few phase equilibrium data have been published. Hence, the application of UNIFAC in this area is appropriate. Theoretical Background
The group contribution concept put forward by Langmuir represents the basis of the UNIFAC method. It considers the liquid mixture to consist of functional groups such as CH3, COOH or OH rather than molecules of various components. Hence the thermodynamic properties of the mixture are determined by the properties of the groups. The assumptions involved in the derivation of the UNIFAC model are clearly stated in a recent monograph (Fredenslund et al., 1977). The activity coefficiets are divided into a combinatorial part and a residual part 0196-4305/81/1120-Q407$01.25/0
+ In yiR
(1)
The combinatorial part is expressed by the equation
di
z
4i
In y i = In - + - qi In - + li - - Cx;l; xi 2 di Xi i fli
z
z = 10
li = -(ri - qi) - (ri - 1 ) ; 2
(2) (2a)
The parameters ri and qi are functions of the group van der Waals volumes and surface areas, vk and Ak, respectively. ri
CVk('Rk;
qi = CV("Qk k
k
Rk = Vk/15.17;
Qk = Ak/(2.5 x 10')
(3)
(3a)
V k ( i ) is the number of groups of type k in molecule i. The residual part is
In yp = &(i) k
[In rk - In
@ 1981 American Chemical Society
rkCi)]
(4)
