I
HAROLD H. BIEBER' and W, FRED SCHURIG Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
ERect of Process C o n d i f i ~ n s
....
Formation of Isomers in Nitration of Chlorobenzene Proportion of p-nitrochlorobenzene formed is a function of nitration temperature and strength of final spent sulfuric acid
A in the chemistry of benzene derivatives is the relative rates MAJOR PROBLEM
of isomer formation and the influence of process conditions on their final distribution. In general, the relative amounts of ortho-, meta-, and para- substitution are related to the reaction rates a t the corresponding carbon atoms. The reaction rates are, in turn, dependent on several factors, chief of which are: (1) relative degree of activation of carbon atoms undergoing substitution; (2) statistical factors; and (3) steric factors depending upon the nature of the initially substituted group. Ingold and others ( 7 , 2, 77, 72) Francis and others (5) have studied orienting effects in benzene substitutions; Lapworth and Robinson (74) have concerned themselves with the ortho-para ratio in aromatic substitution; and R i and Eyring (76) have attempted to predict orientation in terms of the dipole moments and the charge distribution on the molecule. However, very little information is available on the effect of process variables on the distribution of isomers. Jones and Russell (73) have investigated the effect of temperature on the proportions of isomers formed in the mononitration of toluene. Holleman and others (8, 9 ) have studied the effect of different nitrating solvents. Data on other nitration variables, such as mixed acid cornposition, nitration time cycle, and ratio of Present address, Hoffmann-La Roche Inc., Nutley, N. J.
832
nitric acid to hydrocarbon, are for the most part unavailable. It was the purpose of this investigation to determine the effect of nitration process variables on the isomers formed during the nitration of chlorobenzene with a mixture of nitric and sulfuric acids. Nitrating Equipment and Raw Materials
hTitrations were carried out in 3-Iiter, three-necked stirring flask equipped with a thermometer, reflux condenser, and constant-rate mixed-acid feed device. Agitation was provided by a glass flagtype stirrer, 3.7 X 9 X 0.3 cm., fitted into the flask through a packing gland, and connected to a variable-speed motor running at 685 r.p.m. The flask was set in a water bath, which was cooled with ice or heated to provide temperature regulation for all nitrations from 15" to 90" C. Nitrations at 100' C were regulated by a heated calcium chloride bath. Nitration at -5" C. was carried out using a mixture of ice and brine in the regulating bath. The chlorobenzene used throughout this work was a technical grade, waterwhite liquid having an over-all distillation rangeof131.3-131.7° C. T o ensure a minimum quantity of water in the chlorobenzene, it was stored over a desiccant for at least 48 hours before nitration. The nitrating acids were prepared
INDUSTRIAL AND ENGINEERING CHEMISTRY
from C.P. sulfuric and nitric acids, mixed in glass or porcelain equipment to eliminate metals contamination. The temperature of the mixing operation was maintained below 30" C., to keep formation of nitrosylsulfuric acid to a minimum. The chlorobenzene (143.0 i- 1 grams) was first weighed into the nitrating flask. The acid feeder was then filled with sufficient mixed acid to give the required nitric acid-chlorobenzene ratio. The stirrer was adjusted to 685 r.p.m., the chlorobenzene brought to the nitration temperature, and the mixed acid fed into the flask at a constant rate. The acid feed system was designed so that the feed time was 120 minutes. Throughout feeding the contents of the nitrating flask were maintained within =+=lo C. of the previously determined operating temperature. Following the acid feed, the reaction mixture was stirred for 2 hours at the reaction temperature, cooled to below 70' C., and then allowed to settle. The nitrochlorobenzene oil layer was separated from the spent acid and washed at 70" C. with two 100-ml. portions of hot water, neutralized with 100 ml. of 27, sodium carbonate solution, and finally washed with 100 ml. of hot water. The spent acid, after being separated from the oil layer, was diluted in approximately an equal weight of ice and combined with the wash waters from the nitrochlorobenzene washes. The com-
CHEMICAL PROCESSES bined aqueous layers were extracted with three 200-ml. portions of chloroform to recover any dissolved nitrochlorobenzene. After the chloroform extract had been neutralized and washed it was evaporated on a steam bath to dryness. The remaining oil was then added to the main portion of washed nitrochlorobenzene. This was fractionated a t a reduced pressure of about 28 inches of mercury through a fourplate Synder column "4 inch in diameter, until the vapor temperature a t the top of the column reached 90' C. This removed residual chloroform, water, and a part of the unreacted chlorobenzene. The remaining bottoms fraction was analyzed as required.
/ j
58
f
t
c
c)
56
Methods of Ana Iyo is
p"
Point of First Crystallization. The point of first crystallization (PFC) was determined by slowly cooling a sample of nitrochlorobenzene with agitation, in a modified Thiele tube. I n the vicinity of the crystal point very small amounts of seed crystals of p-nitrochlorobenzene were added to prevent supercooling. The point of first crystallization was taken as the first temperature a t which formed crystals did not dissolve. The determinations were done in triplicate, the reproducibility being 1 0 . 1 ' C. The per cent p-nitrochlorobenzene in the mixture was then determined, using the data of Holleman (7). A section of his phase diagram has been enlarged (Figure 1). As the phase diagram was constructed for the two-component system p-nitrochlorobenzene (PNCB)-o-ni t r o c h l o r o benzene (ONCB), and the final product contained, as contaminants, chlorobenzene, dinitrochlorobenzene, and mnitrochlorobenzene, modifications were made to account for their presence. I n all cases it was assumed that the rn-nitrochlorobenzene had the same freezing point depression as o-nitrochlorobenzene. If this was not entirely true, the fact that the meta content was small minimized any errors. The extent to which chlorobenzene lowers the point of first crystallization of a p-nitrochlorobenzene mixture was determined experimentally, using purified p-nitrochlorobenzene (PFC = 82.60' C.), o-nitrochlorobenzene (PFC = 31.95' C,), and varying amounts of chlorobenzene. The value of the point of first crystallization depression due to chlorobenzene was determined to be 0.93' C. per per cent present. This value was employed in correcting all the data for the presence of chlorobenzene. For the most part, the dinitrochlorobenzene (DNCB) formed in nitration was so small that the assumption made for the m-nitrochlorobenzene held equally well. However, nitrations made with
"60
62
64
66
68 70 72 P-NITROCHLOROBENZENE %
74
76
,
Figure 1.
I
Enlarged section of Holleman phase diagram NHz
No2 id02
a mixed acid yielding an 80% spent acid contained significant amounts of dinitrochlorobenzene. T o obtain the true ratio of the mononitrated isomers formed, it was then necessary to know the ratio of o-nitrochlorobenzene and p-nitrochlorobenzene which was nitrated to give the dinitrochlorobenzene. This was determined experimentally by nitrating a mixture of pure o-nitrochlorobenzene and p-nitrochlorobenzene having a point of first crystallization at 52.40' C. Ten per cent of the theoretical quantity of nitric acid required was used. The reaction was run at 90' C., with a mixed acid which yielded a final spent acid strength of 80% sulfuric acid. A material balance indicated that the initial rate of formation of dinitrochlorobenzene from a mixture of 62.6% p-nitrochlorobenzene and 37.470 o-nitrochlorobenzene under the above conditions is such that the ratio of o-nitrochlorobenzene consumed is 15.7 to 1. Wherever corrections for dinitrochlorobenzene in the nitrochlorobenzene mixture were deemed necessary, this ratio was used. Dinitrochlorobenzene. When an alcohol solution of mononitrochlorobenzene and aniline is refluxed, any dinitrochlorobenzene present reacts with aniline to form a condensation product and an amount of hydrochloric acid equivalent to the dinitrochlorobenzene. Mononitrochlorobenzene does not react in this manner a t so low a temperature and in so short a time.
A sample of the nitrochlorobenzene to be analyzed was dissolved in 50 ml. of neutral alcohol (specially denatured 3A) and refluxed for 2 hours. The mixture was drowned in water and extracted with 100-ml. portions of benzene until the color was removed. The residual benzene was boiled off the aqueous layer and, after being cooled, titrated with 0.1Nsodium hydroxide to a phenolphthalein end point. The results obtained are reproducible to =k2% of the amount determined. Unnitrated Chlorobenzene. A known amount of the nitrated product was dissolved in p-cymene and the mixture distilled through a fractionating column. The p-cymene (boiling point 178' C.) aids in complete removal of the lower boiling monochlorobenzene (boiling point 132' C.) from the higher boiling nitrochlorobenzene (boiling point 240' C.). A measured volume of the distillate [which contains all the chlorobenzene (MCB) from the sample and p-cymene] was collected at 25' C. and its density determined. Its chlorobenzene content was calculated by use of a straight-line equation which relates the measured density of the distillate with the volume per cent of chlorobenzene in the distillate. The weight per cent of chlorobenzene in the original sample is easily calculated from the weight of chlorobenzene found and the weight of the original sample. VOL. 49, NO. 5
M A Y 1957
833
ln Q
x
01
N
rn0 W
.I
2 d
m
2 a w
0 0
m
2 z 2 a ln
0
h
25
w
'CJ
Y
3 s f
'5
? m
"9 lnm
h h
-2
W
? m
h
"8
w
? m
2 m N
h - 0
::
p'? m m
h h
0: 0
m
k?
lnm
h h
6
0
ln a
2 3
2 a
5
N m
2 3
0: 0 ln ln
2 h N
2 w m
2 v) 0
2 .-I
'0
2 -3 01
2
6 01
834
INDUSTRIAL AND ENGINEERING CHEMISTRY
0 . e
m-Nitrochlorobenzene. The meta isomer was determined by an infrared technique developed by the Spectrophotometric Laboratories, American Cyanamid Co., Bound Brook, N. J. Briefly this is accomplished by measuring the absorbance of the sample, with an 0.002-inch potassium bromide cell at 810 cm.-1, and subtracting from this value the absorbance of the solvent used to make the solution (benzene) and the interfering components 0-, p-, and dinitrochlorobenzene, and chlorobenzene. The net absorbance is then converted to rn-nitrochlorobenzene by using the exuerimentallv determined linear relationship that 5% chlorobenzene is equal to an absorbance of 0.0620 for a 20% volumetric solution a t 810 cm.-l with a 0.002-inch cell. Determination of m-nitrochlorobenzene by this method is within =t0.2% meta in the range analyzed.
I 69
#
-
68-
I
I
I
I
I
I
I
n
"
I v
-
30%-
6.2
g 267p
65-
#
-
-
66-
-
h
0 70*C.
-
U
-
-
N
PP
i
35-
0
-
n
34-
-
t
s
g33-
5::1 7 k
,
,;
6
3 0 ~
9
I
,
,
12
15
18
I 21
24
4
27
NITRIC ACID CONCENTRATION IN MIXED ACID, %
Figure 2.
Effect of nitric acid concentration of mixed acid
Process Variables
The variables studied to determine their influence on the isomer distribution in the nitration of chlorobenzene with mixtures of nitric and sulfuric acids were: nitric acid content of the mixed acid, ratio of nitric acid to chlorobenzene, sulfuric acid strength, nitration temperature, and nitration time cycle. T o obtain a more precise value of the isomers formed during nitration, the nitrochlorobenzenes soluble in the acid layer were recovered by the chloroform extraction procedure described. Final removal of chloroform from the combined nitrochlorobenzene fractions resulted in some loss of chlorobenzene and made it impossible to obtain an accurate chlorobenzene material balance. However, a nitric acid material balance was obtained (Table I). This balance is, at best, approximate, because of inherent nitric acid losses during the nitration, solubility in the oil layer, and losses during separation of the acid and oil layers. Every effort was made to minimize these losses, but this became exceedingly difficult as the nitration tempera__
66
Figure 3.
E
71-
i W W
e
63.4
63.2
62.7
Trace 0.1
Trace
36.6
37.3
36.8
Effect of sulfuric acid strength
t
- 7 0 % S.A.
x - BO%S.A.
69-
0
i!
II. Effect of Varying Nitric Acid-
Chlorobenzene Ratios Nitration temp., O C. 75 75 75 Mixed acid feed, hours 1.5 1.5 1.5 Reaction time, hours 1 1 1 Excess HNOa. % 1 3 5 Final spent acid strength 75.0 75.0 75.0 99 Product yield, % 98 100 0.6 0.0 0.0 MCB, % PFC dry, O C. 52.5 52.9 .52.4 MCB correction 0.6 0.0 0.0 PFC corrected 53.1 52.9 52.4
z + SPENT ACID CONCENTRATION, %
Y
Table
\
0-75YoS.A. 0- 85%S.A.
8
3 #mI W 1 65 7-V
El
':
63-
z
Q
-
-
SPENT ACID CONCENTRATION,P/o)
Figure 5. Relation of slope of para isomer-temperature curve and spent acid strength
ture was increased. In spite of these limitations, material balances accounting for better than 9570 of the nitric acid charged were obtained for all but one run. Nitric Acid Concentration of Mixed Acid. For nitration of chlorobenzene, Hough (70) recommends a mixed acid containing 18.0y0 nitric acid, 71.0% sulfuric acid, and l l . O ~ O water. To determine whether this factor is a critical variable, a series of runs was made in which the nitric acid concentration of the mixed acid was varied from 3 to 25%, while the final sulfuric acid strength of the spent acid was kept relatively constant (Table I, runs 6, 20, 22, 8, 21, and 23, and Figure 2). Within the limits of this investigation, the concentration of nitric acid in the mixed acid does not influence the final distribution of the isomers of nitrochlorobenzene. Molar Ratio of Nitric Acid to Chlorobenzene. Three nitrations a t 75' C. were carried out, in which the molar
excess of nitric acid was varied from 1 to 5y0,and the mixed acid composition was unchanged. The data, summarized in Table 11, indicate that this variable has little, if any, effect on the final isomer distribution within the range investigated. For the most part, all nitrations employed a nitric acid-chlorobenzene molar ratio of 1.06 to 1, and it was assumed that this slight excess of nitric acid had no influence on the final isomer ratio. Sulfuric Acid Strength. The commercial mixed acid used for nitrating chlorobenzene contains approximately 18% nitric acid and 71% sulfuric acid (70). Thus, the ratio of sulfuric acid to water (on a nitric acid-free basis) varies from 0.865 a t the beginning of the nitration to 0.815 at the completion of the reaction. T o reduce this variation, mixed acids containing on the order of 3.001, nitric acid were used. As nitric acid concentration had no effect on the final isomer ratio, it was felt that this approach would give truer isomer ratios
Table 111. Temperature Effects in Nitration of Chlorobenzene Concn. of HzSOc Nitration Literature Isomer in Final Spent Temp., Isomer Distribution, % Distribution Acid, yo O c. Para Ortho Meta Para Ortho 50 66.76 31.84 1.4 70 70 65.51 33.09 1.4 90 64.61 33.89 1.5 66 (f5) 34 110 62.91 35.49 1.6 15 68.90 30.40 75 0.7 30 68.29 31.31 0.4 65.14 (4) 34.86 50 66.02 33.08 0.9 63.53 ( 4 ) 36.47 70 64.49 34.61 0.9 90 1.1 62.76 36.14 1 .o 110 61.97 37.03 80 -5 70.50 29.50 0.0 15 69.26 30.14 0.6 66.06 (4) 33.94 30 67.66 31.84 0.5 65.20 (4) 34.20 50 65.44 34.26 0.3 64.05 (4.) 35.95 70 64.18 34.82 1.0 1.2 90 62.68 36.12 50 85 64.55 34.85 0.6 70 63.03 36.27 0.7
for a given acid strength. Four mixed acids, with sulfuric acid strengths varying from 70 to 85%, were studied at several temperatures (in Table I and Figure 3). The data indicate that at a given nitration temperature the variation of each isomer formed can be expressed as a linear function of the ratio of the sulfuric acid to water content of the nitrating acid. Temperature of Nitration. Each mixed acid type investigated was studied over a range of temperatures (Tables I and 11, and Figure 4). The variation of any isomer formed is a straight-line function of the nitration temperature, if the sulfuric acid-water ratio is held constant. The available published data are included in Table 111. It is believed that the differences are due to the fact that the data of this work are corrected for the presence of dinitrochlorobenzene and m-nitrochlorobenzene. These were assumed to be negligible, in published data. Holleman (7) obtained 73.1% pnitrochlorobenzene at -30' C. If the data of this paper are extrapolated to this temperature, a value of 72.7y0 pnitrochlorobenzene is obtained. In another experiment, Holleman obtained 69.9y0 @-nitrochlorobenzene for a 0' C. nitration; in this work a comparable experiment yielded 70.3% of the para isomer. The straight-line relationships shown in Figure 4 may be expressed mathematically as : 70% HzSOa in final spent acid: % PNCB = -0.056t 70.4
+
where t
=
757" HzS04 in final spent acid: % PNCB = -0.073t 70.4 807' HzSOa in final spent acid: % PNCB = -0.082t 70.2
+ +
85y0HzSOa in final spent acid: '% PNCB = -0.096t 70.0
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
(2) (3)
(4)
The value for pnitrochlorobenzene obtained from these equations, as a function of the temperature, is expressed on a meta-free basis. If the slopes of the temperature lines are plotted against the spent acid strength, a straight line also results (Figure 5): which follows the equation : Slope
=
-0.0027 (yosulfuric acid in spent acid) 0.13 ( 5 )
+
If this relationship is used and a constant of 70.20 assumed, Equations 1, 2, 3, and 4 may be rewritten as a single equation, expressing the p-nitrochlorobenzene formation in terms of the two critical nitration variables:
% ' PNCB
= [ -0.0027
(70sulfuric acid
in spent acid) t33A
(1)
nitration temperature, ' C.
+ 0.131 t + 70.2
(6)
CHEMICAL PROCESSES I
By using Equation 12 and the values of A,/Ao from Figure 6, the corresponding differences of the entropies of activation between the para and ortho positions are obtained, Hammett (6) lists 0.5 for thiS entropy difference.
I
I
.40
-
Summary
m
u z
+
7 0 % SA.
z n
0
75%SA
W
x
EO%S A
m .30 0
0 J
.2(
.o
A study of the factors influencing the distribution of isomers formed during the nitration of chlorobenzene with a mixture of nitric and sulfuric acids showed that, for the most part, the proportion of p-nitrochlorobenzene formed is a function of the nitration temperature and the strength of the final spent sulfuric acid. The nitric acid concentration, nitration time, and nitric acid-chlorobenzene ratios u p to 1.06 to 1 have little, if any, influence on the distribution of nitrochlorobenzene.
I
!S
0030
OM(
.0035 I/TEMPERATURE
Figure 6.
Arrhenius equation relationship
Within the limits of the experimental work, this equation predicts the pnitrochlorobenzene content within 1%. The equation, however. should not be extrapolated outside these limits. Nitration Time Cycle. Two similar nitrations, one for 2.5 hours and the other for 7.5 hours, indicated that the total nitration cycle had no significant effect on the distribution of isomers at the end of the nitration (Table IV).
Effect of Reaction Timle on Isomer Formation
75 Nitration temp., O C. 75 7.5 Reaction time, hours 2.5 3 Excess nitric acid, % 3 75 Final spent sulfuric acid, % 75 100 Product yield, % 99 0.0 Unreacted chlorobenzene, % 0.0 Point of first crystallization, C. 52.9 52.9 0.1 Dinitrochlorobenzene,% ’ 0.1 p-Nitrochlorobenzene, yo 63.2 63.2 O
Discussion
of Data
If it is assumed that the variation of isomer distribution with temperature is primarily a function of the activation energy, the data obtained may be checked for consistency by using the Arrhenius equation. This application of the concept of kinetic activation to predict the orientation of substituted groups on the benzene ring has been developed by Bradfield and Jones (3). If the Arrhenius equation is written for the three possible points of introduction of the nitrogroup, the following expressions result : 2Az-EelRT
(7)
= 2Ame-Es/RT
(8)
KPars = A ~ c - E P / R T were If = rate formation
(9)
Korthoi=
-
EO E p , Sulfuric Acid Cal./ Strength, % Mole Spent 70 75 80
E
T
Table IV.
Acknowledgment
Constants Obtained from Figure 6
693.3 693.3 693.3
=
ASP’ 1.32 1.35 1.45
-
AS.+
Ap/Ao
0.55 0.60 0.75
literature Cited
energy of activation
= absolute temperature
Dividing Equation 9 by Equation 7 and taking logarithms, 2.303 log
% kv
=
2.303 log
A
-E
2Ao
-/-
- Ep 7
E,,
The above equation is plotted in Figure 6. The assumption was made that the ratio k,/k, is proportional to the final para-ortho ratio in the mononitrochlorobenzene mixture. A family of parallel lines results from a plot of log (PNCB/DNCB) us. 1/T, indicating good agreement of the data with Equation 10. By virtue of the fact that the family of lines are parallel, the activation energy term (Eo E,) is independent of spent acid concentration. Also included in the data is the difference in entropy of activation (ASp* ASo*) for the para-ortho nitro substitution on chlorobenzene. These values are calculated from the relationships:
-
-
The authors thank the American Cyanamid Co. for use of its facilities for carrying out the experimental work. They are also indebted to A. G. Hill, F. H. Megson, and J. H. Thelen, American Cyanamid Co., for suggestions and criticisms during this investigation.
(1) Benford, G. A,, Ingold, C. K., J. Chem. SOC.1938, 929-55. (2) Bird, M. L., Ingold, C. K., Zbid., 1938,
918. (3) Bradfield, E. W., Jones, B., Ibid., 1928. 1006.
(4) Dey,B: B., others, J . Sci. Ind. Research (India) 3, 95-6 (1944). (5) Francis, A. W., Andrews, D. H., Johnston, John, J . Am. Chern. SOC. 48, 1624 (1 926); (6) Hammett, L. P., ‘(Physical Organic Chemistrv.” McGraw-Hill. New York, 1940. (7) Holleman, A. F., Rcc. trav. chim. 19, 191 (1900). (8) Holleman, A. F., Hartogs, J. C., Lindin, T. van der, Bcr. 4 4 , 713 (1911).. (9) Holleman, A. F., Rinkes, I. J., Rec. trav. chim. 30. 49 (1911). (10) Hough, A., Ckem. and Met, Eng. 23, 668 (1920). (11) In old, C. K., Ann. Repts., 129-149 r1927) K., others, J. Chern. Soc., (12) Ingold, 1938, 905-29. (13 ) Jones, W. W., Russell, M., Ibid., 1947, 921-3. (14) Lapworth, A., Robinson, R., Mcm. Proc. Manchester Lit. Phil. SOC.7 2 , 43 (10271. (15) McCormack, H., IWD. END.CHEM.19, 1333 (1927). (16) Ri, T., Eyring, ,H., J . Cham. Phys. 8, 433 (1940).
6.
\ - - -
I -
RECEIVED for review November 30, 1956 ACCEPTED March 1, 1957
and dividing to obtain A,/A,
Division of Industrial and Engineering Chemistry,Chemical Processes Symposium, 130th Meeting, ACS, Atlantic City, N. J., September 1956. VOL. 49, NO. 5
MAY 1967
A37
