May 20, 1955
2447
2,4-DINITROCHLOROBENZENE AND %-BUTYLAMINE [CONTRIBUTION FROM THE
RESEARCH LABORATORIES OF THE SPRAGUE ELECTRIC CO.]
Nucleophilic Displacement Reactions in Aromatic Systems. 11. Catalysis of the Reaction of 2,4-Dinitrochlorobenzene and n-Butylamine by Triethylamine in Chloroform BY SIDNEY D. Ross AND RAYMOND C. PETERSEN RECEIVED DECEMBER 23, 1957 I t is shown that, in the presence of triethylamine, the over-all rate equation for the reaction of 8,~-dinitrocliolorobr:nzene and n-butylamine is given by d productldt = kI(RCl)(n-BuXHz) 4- k3(RCl)(n-BuSH2)2 kS(RCl)(n-BuSH?)(Et&) All three of these rate constants have been evaluated, and the mechanism has been discussed
+
I n a previous report from this Laboratory' it was shown that the correct rate expression for the reaction of 2,4-dinitrochlorobenzene and n-butylamine is d Productldt = kI(RCl)(n-BuNHz) -t k3(RCl)(n-BuNH2)'
(I)
It was suggested that the function of the second amine molecule in the third-order term of equation 1 was to facilitate product formation by removal of a proton from an intermediate of the type I
Results and Discussion If a triethylamine molecule can perform the same function as the second n-butylamine molecule in the third-order term of equation 1, the over-all rate, 11, will be given by
+
1.' = kl(RCl)(n-BuNHp) f ~ ~ ( R C ~ ) ( V L - R U P \ T H Z ) ~ ~ ~ ( R C ~ ) ( T Z - B U N H Z )(2) (E~~N)
and the expression for the specific rate is kp = ki
+ ka(n-B~NHp)+ k4(EtSX)
(3)
The k2 in equation 3 is the second-order rate conH stant for an individual run. If we make a series of measurements a t constant initial 2,4-dinitrochlorobenzene and n-butylamine concentrations and vary the initial triethylamine concentrations and plot the kz's lis. the initial triethylamine concentrations, we should obtain a straight line with the slope equal I/ to kd and the intercept equal to K1 k3 (n-BuNH2). N+ Similarly, for a plot of kz's 2s. initial n-butylamine / \ concentrations for a series of measurements ai: con-0 0- I stant chloride and triethylamine concentr :Lt'ions Such a mechanism had been proposed previously2 and varying n-butylamine concentrations, the reto account for the fact that triethylamine, which sultant straight line should have a slope equal to k3 does not react to an appreciable extent with 2,4- and an intercept equal to kl k4(Et3N). 'Thus, dinitrochlorobenzene, nevertheless increases the from the two series of measurements it should be rate of reaction of this chloride with methylamine possible to evaluate all three of the constants, k l , in alcohol. The reaction of 2,4-dinitrochloroben- ks and kd. zene with n-butylamine in chloroform represents a There is, however, a complication in determining favorable system for a quantitative study of the the kz's. The products in this reaction are 2,4catalysis by triethylamine, since, in this reaction, dinitro-N-n-butylaniline and hydrogen chloride, the third-order term makes a relatively large con- which combines rapidly with a second amine moletribution to the total product formation and since cule to form the amine hydrochloride. In the abtriethylamine does not react with the chloride in sence of triethylamine, the hydrogen chloride rethis solvent to a measurable extent. acts with a second n-butylamine molecule, and the correct second-order rate equation is Experimental
v
+
+
Materials.-Baker and Adamson Reagent Grade chloroform, Lot KO. NO'iOX0.59, was used without purification. This solvent contains 0.7570 ethanol as a stabilizer. Eastman Kodak Co. a7hite label n-butylamine and triethylamine were distilled twice from calcium hydride, and middle fractions of b.p. 77 and 89', respectively, were used. 2,4Dinitrochlorobenzene, Eastman Kodak Co. white lab$, was crystallized two times from absolute ethanol; m.p. 51 Rate Measurements .--Separate, determinate solutions of 2,4-dinitrochlorobenzene and the two amines in chloroform were made up a t the temperature of the measurements, 24.8 + 0.1". The solutions mere mixed at zero time, and aliquots mere withdrawn a t appropriate time intervals. The aliquots were quenched by partitioning them between benzene (50 ml.) and 4 N nitric acid (2.5 ml.) and extracting the benzene solution two additional times with water. They were then analyzed for chloride ion by the Volhard method.
.
(1) F o r the first p a p e r in this series see S. D. Ross a n d M . Finkelstein, THISJ O U R N A L , 79, 6547 (1957). (2) 0. L. Brady a n d F. R. Cropper, J . Chcm. Soc., 607 (1960).
where A0 is the initial 2,4-dinitrochlorobenzeneconcentration, Bo is the initial n-butylamine co'ncentration and D is the product, chloride ion or 2,4dinitro-N-n-butylaniline. In the presence of triethylamine, equation 4 is correct only in the limiting case that all the hydrogen chloride is bound by nbutylamine. The alternate limiting case is the one where all the hydrogen chloride formed reacts with triethylamine, and the corresponding rate expression is 2.303 Ao(Bo - D ) kpt = ___ (5) BO - A0 log 7 ) I n actual fact neither of these limiting equations is appropriate, since n-butylamine and triethylamine are bases of comparable strength. In water n-
2448
SIDNEY
D.
ROSS A N D
RAYMOND c. PETERSEN
VOl. 80
butylamine has a pKa of 10.48 and triethylamine has a pKa of 10.65.3 The concentration of nbutylamine a t any time, t, is given by
mole-l sec.-l. I n Fig. 2, kz is plotted against initial n-butylamine concentration for the two sets of data a t constant initial triethylamine concentration. For the upper line, the initial triethylB = Bo - f ( D ) (6) where f ( D ) is a complicated function involving not amine concentration is approximately constant a t only D but also the equilibrium constants for the 0.57 AT; for the lower line the triethylamine concentration is 0.142 M. I n drawing the upper line reactions we have intentionally favored the two points at the %-BuSHn 4-HCl n-BuNHa', C1(7) higher initial n-butylamine concentrations, since our data for the run a t 0.2000 M n-butylamine Et& -/- HC1 Et3NHf, C1(8) proved difficult to extrapolate. For the data a t This difficulty can be obviated by plotting nieas- 0.57 M triethylamine, the slope, k3, is equal t o ured values of A D l A t vs. t for each run and extrap- 4.86 X lop4 l.:! mole-2 set.-', and the intercept, olating to zero time to obtain values of (dD/dt), = k1 k4(EtgN)0,is equal to 3.98 X loT41. mole-' The second-order rate constant is then given by sec.-l. For the data a t 0.142 M triethylamine, the slope, k,, is equal t o 5.22 X L2 mole-2 (9) set.-', and the intercept, kl kq(Et3N)o, is equal In Table I we have compiled the rate constants to 3.05 X l o p 4 1. mole-'. sec.-l. Combining and for six runs a t constant n-butylamine concentration averaging these three sets of data, the following and varying triethylamine concentration and for values are obtained for the three rate constants : I. mole-' set.-'; ka = 5.0 X two sets of three runs a t constant triethylamine con- k1 = 2.5 X mole-2 set.-'; kq = 2.9 X 1.2 m o k 2 centration and varying n-butylamine concentrations. The most reliable values for the rate con- sec. -l. These values of kl and kg are to be compared with stants are those calculated from equation 9 and our previously reported' values for these two congiven in the last column of Table I. The application of either equation 4 or equation 5 to our data stants. The earlier measurements, made without results in a satisfactory linear plot in every case, triethylamine present in the system, gave values 1. mole-' sec.-l for kl and 5.3 X and the rate constants obtained with these two of 2.0 X mole-:! set.-' for ka. The agreement is equations have been included in Table I for purposes of comparison. Some pertinent, previously reasonable. Nevertheless, i t is of interest to deterreported' runs, in the absence of triethylamine, are mine whether or not the use of equation 4 to calalso given in Table I, and the rate constants have culate the second-order rate constants a t zero time introduces a significant error even when tribeen recalculated using equation 9. ethylamine is absent. The rate constants have, TABLE I therefore, been recalculated using equation 9. The resultant values are given in Table I. X plot of k:! RATESO F REACTION O F 2,4-DINITROCHLOROBESZESE AND %BUTYLAMISE IS THE PRESESCE AND ABSENCE OF TRI- vs. the initial n-butylamine concentration is again The least squares values are 2.4 X lo-* 1. ETHYLAMINE I N CHLOROFORX CONTAINING O. i 5 % ETHANOLlinear. mole-' sec.-l for the intercept k1 and 5.7 X 1 0 - ~ AS STABILIZER AT 24.8 0.1" 2,4-D1l a 2mole-2 set.-' for the slope ka. nitrobi X 10' kz X 10' k2 X 10' It is now pertinent to ask which set of values for rhlorofrom from from Triethylbenzene, =-Butyleq. 4, eq. 5 , eq. 9, k1 and k3 is the most reliable. When no triethyl1. mole-' 1. mole-1 1. mole-' amine, mole amine, mole I. -1 sec. - 1 sec. sec.-1 amine is present, the rate equation is 1. -1 mole 1.
+
+
*
-1
-1
0,05097 .05060 .05026 .05113 ,05111 ,05056 ,04975 .05119 .05016 .05265 ,05165 .05141 ,05111 ,05061 ,05168 ,05108
0.2003 ,1978 .1993 ,1987 ,1980 ,2000 ,3977 .5018 ,2998 .3986 .lo38 .1866 .1985 .4961 .go18 .9997
0.0420 .1422 .2141 .3529 .3565 .5653 .I420 .1426 .5712 .5711
3.29 3.91 4.14 4.60 4.65 5.10 4.07 5.0s 5.20 5.53 2.47 3.25 3.07 4.37 6.22 7.45
2.94 3.57 3.69 4.19 4.14 4.60 4.4s 4.87 4.92 5.27
3.58 4.08 4.16 4.59 4.58 5.14 5.14 5.66 6.43 5.92 2.75 3.63 3.53 5.34 7.12 i.90
dD/dt
ki(Ao
- D)(Bo - 2 0 ) + kj(Ao -
D)(Bo - 2 0 ) '
(10)
This equation can be integrated to give
klt
+ k 4 B o - 2-40]
(11)
The second term on the left in equation 11 will normally be small and insensitive to the values of kl and ka. Hence it can be ignored in the first approximation. I n using 11, D was first obtained as a function of t for all six of the runs without triethylamine. The first term on the left was then plotted against (Bo - 2Ao) for points from each run a t a Figure 1 presents a plot of kz vs. the initial tri- fixed time. The time was so chosen that the points ethylamine concentrations for the measurements represented less than lOy0 reaction a t one extreme a t constant n-butylamine concentration. The least and more than 90% reaction a t the other extreme. squares value for the slope, k4, is 2.86 X 1.2 Values of k1 and k3 were then obtained from the mole-:! set.-' and for the intercept, 81 k3(n- intercept and slope, respectively, of the straight BuNHz)", the resultant value is 3.56 X 1.- line plot obtained. Using these initial values of kl and ks the second term on the left of equation 11 (3) H. K. Hall, Jr.. THISJOURNAL, 79, 5 4 4 1 (1957).
+
May 20, 1958 I
2,4-DINITROCHLOROBENZENE AND
X
2449
I
--1 4 t 2
%-BUTYLAMINE
O,/
p/
2 31
Fig. 1.-A
~
~
L2--d
--
0.1 0.2 0.3 0.4 0.5 0.6 Triethylamine, mole/liter. plot of k2 ZJS. the initial triethylamine concentration.
was calculated, and the procedure was repeated until the values of kl and ka became constant. This treatment resulted finally in values of 2.2 X 1. mole-’ sec.-l for k1 and 5.4 X 1.2 mole-2 set.-' for k3. These calculations involve neither an assumption about the rate a t zero time nor an extrapolation to zero time. They include experiments with an almost tenfold variation in the initial n-butylamine concentration and reaction percentages of less than 10% and more than 90%. These, therefore, are the most reliable single values for kl and k3. It is, moreover, encouraging that we can include all of our values, even those determined in the presence of triethylamine, and still set limits of 2.23 =t 0.25 X 1. mole-l set.-' for kl and 5.35 =t 0.35 X 1.2 mole-2 set.-' for ka. Our results, in the presence of triethylamine, are thus consistent with the rate equation dD/dt = ki(A)(B)
+ ka(A)(BI2 + h ( A ) ( B ) ( E )
(12)
where E is triethylamine. This equation can be derived either by assuming an equilibrium between A, B and an intermediate, I, or by assuming a steady state concentration of I. For the latter assumption, the steady state concentration of I is eiven bv u where the rate constants correspond to the reactions k‘i A + B + I
k’-
I+D
1
I
0.3
0.4
0.5
0.6
n-BuNH2, mole/liter. Fig. 2.-Plots of kz us. the initial n-butylamine concentra. tion. For the upper line the initial triethylamine concentrations are 0.57 M; for the lower line the initial triethylamine concentrations are 0.142 M.
The rate equation resulting from a combination of equations 13 through 18 is
An equation of the form 12 can then be obtained by applying the assumption k’-1
>> k‘i
+ k’s(B) + k’4(E)
(20)
which is equivalent to assuming an equilibrium among A, B and I, or by applying to the denominator of the term in parentheses of equation 19 either of the assumptions k‘i
>> kl-1
&’-I
+ k’s(B) + k’4(E)
+ k’z >> k’a(B) + k’4(E)
(21) (22)
It should be noted that if k’-1 > > k’*, assumption 22 is identical to 20. Otherwise i t should be noted that the application of 21 or 22 to the numerator of equation 19 as well as to the denominator leads to the simple second-order rate expression dD/dt = K(A)(B)
(23)
(17) (18)
NORTHADAMS,MASS.
(15) (16)
k‘: I + B + D k’4 I + E + D
0.2
in contradiction to the experimental facts. This is not sufficient reason to discard these two possibilities, since the numerator of the term in parentheses of equation 19 is more sensitive to variatioris in B and E than is the denominator. Thus our experiments do not permit any choice among these assumptions.
(14)
1
I-A+B k ’r
I
0.1
