SOLVOLYSIS OF NITROSTYRENES
Feb. 5 , 1961
591
drochloride solution necessary to give a 0.500 M solution a similarly diluted sample of the original buffer solution. Runs were made a t 24.97 and 38.90 f 0.02' (34.83'for the was transferred to a 250-ml. volumetric flask; the solution also was made 0.500 M with respect to pyridine; sernicarbazone). The reading of the solution on a Beckfinal addition of methanol was made at the reaction tem- man PH meter was 3.49 f 0.02 throughout the reaction. perature. The ketone solution (4.00 X 10-2M) and the The rate constants were reproducible within 1-2%; actithiosemicarbazide hydrochloride solution (2.00 X 10-2M) vation energies are believed to be accurate to f 0 . 4 kcal./ were made up in the buffered methanol. Equal volumes of mole. We wish to thank Dr. C. T. Lester of Emory University the two solutions were mixed for the reaction. Samples of for the suggestion of this problem. Some of the spectro1 ml. were removed a t intervals and diluted 100 times with ethanol-water (1: l ) ,and the optical density was read against scopic data were obtained by Miss Judith Lang.
[CONTRIBUTION FROM C O B B CHEMICAL
LABORATORY, UNIVERSITY
OF VIRGINIA, CIIARLOTTESVILLE, V A . ]
The Mechanism of Solvolysis of Nitrostyrenes BY THOMAS I. CROWELL AND ANDREW W. FRANCIS, JR. RECEIVED JULY 25, 1960 The kinetics of hydrolysis of 3,4-rnethylenedioxy-p-nitrostyrene, to piperonal and nitromethane, are characteristic of two consecutive, pseudo-first-order reactions over the pH range -0.8 to 6. The first is reversible and shows general base catalysis, with the rate constant a non-linear function of the acetate ion concentration. The rate of the second is pH dependent. A mechanism is proposed consistent with these observations.
The base-catalyzed cleavage of 3-methoxy-4hydroxy-P-nitrostyrene in strongly alkaline solution was shown by Stewart' to proceed with attack of a hydroxyl ion via the colorless intermediate nitroalcohol. The fact that this cleavage will take place slowly in acid solutionzaproves, however, that a slightly different mechanism must be possible. In the course of some preliminary work on nitrostyrene formation in acetate buffers (our previous work dealt only with amine catalysis2) we had occasion to study the hydrolysis in acid solution and can report new kinetic features. Experimental The nitrostyrene used in this work was 3,4-methylenedioxy-P-nitrostyrene (piperonylidenenitromethane), which was prepared by condensation of piperonal and nitromethane. a Buffer solutions and hydrochloric acid solutions were prepared from reagent-grade chemicals and distilled water. The ionic strength, here equal to the sodium ion concentration, was kept constant a t 0.1 M by adding sodium chloride, except for a series of runs at 0.3 M . The kinetic runs were started by adding 1 ml. of a freshly prepared 0.002 M methanol solution of nitrostyrene to the aqueous buffer components and adding water to make 100 ml. The flasks were placed in the thermostat and samples analyzed spectrophotometrically without further dilution, a t 372 mp. The optical density a t this wave length decreased as the nitrostyrene hydrolyzed, and a t first no peak a t 312 mp appeared to indicate the presence of piperonal unless the p H was as high as 6 . Piperonal gradually was produced a t lower pH, however, but only after a considerable quantity of an intermediate had formed. In very acidic solution (3 and 6 M HCl) the Xmax of the nitrostyrene shifted to 385 mp.
Results The hydrolysis of nitrostyrene showed complex kinetics, though all the reactions involved were pseudo first order because of the high dilution of the substrate in comparison with the buffer components. Figure 1 shows typical logarithmic plots of concentration vs. time. At very low pH, equilibrium is reached when about 60% of the nitro-
RATECONSTANTS (SEC, - I ) [HAC]. M 0.005 .003 .001 .I .05 .03 01 .3 .2 .12 .07
HC1, M
.
.02 .01 .008 .006 .004
.002 .25 .05
0.0012 0.012 0.12 3.0 6.0 0,0012 ,012 .006
[A-I, M 0.05 .03 .01 .1 .05 .03 .01 .3 .2 .12 .07 .02 .01
TABLE I FOR KITROSTYRENE HYDROLYSIS
$Ha
5.6 5.6 5.6 4.6 4.6 4.6 4.6 4.5h 4.5b 4.5b 4.6b 4.5b 4.5b .008 4.5b .006 4.5b .004 4.5b .002 4.5* .025 3.6 .005 3.6 2.9 1.9 0.9 -0.5 -0.8 2.9J
l.gdse 2.2+
10% lock-i 3.86 3.22 6.37 4.25 1.92 6.34 8.94 5.08 7.78 4 . 0 3 6.42 2.16 3.47 6.94 6.94 6.83 6.04 3.84 2.03 1.69 1.39 0.91 .58 3.16 4.83 1.24 2.13 0.181 0 . 4 2 .186 .42 .181 .091 ,086 1.92 0.15 .I2
lO6ki
ki/ k-i
4.36 3.72
0.83 .67
c
0.44 .42 .47 .43
0.075 .lo3 ,114 .114
.71 .65 .63 .62
0.65 .58 .44 .44 .53
c k2
> ki
"Using 2.8 X 10-6 for K H Ain~ 0.1 M NaCl (M. Kilpatrick and R. D. Eanes, J . Am. Chem. SOC.,75, 586 (1953)). *Ionic strength 0.3, K H A = ~ 3.2 X 10". kz to close to kl for accurate determination. Temperature 45'. Solvent 99% DzO.
styrene has reacted, and a plot of log (x,-x) is linear, indicating opposing first-order reactions. At a slightly higher pH, a slow reaction becomes apparent after the first equilibrium is established. This reaction is so rapid at pH 7 that the first step is rate controlling and simple first-order kinetics are obtained. These observations may be represented by the scheme
(1) R . Stewart, J . Am. Chem. Soc., 74, 4531 (1852); see also a study of nitrochalcones by E. A. Walker and J. R. Young, J . Chcm. Suc., 2045 (1957). (2) (a) T. I. Crowell and F. A. Ramirez. J . Am. Chem. Soc., 73, 2268 (1951); (b) T. I. Crowell and D. W. Peck, ibid., 75, 1075 (1953). (3) N. A. Lange and W. E. Hambourger, ibid., 53, 3865 (1931).
which is confirmed by the spectral changes described in the Experimental section.
TIIOMAS r. CROWEIL
592
10% sec. (curve A). 2 4
AND
ANDREWW.
FRANCIS,
Vol. 83
JR.
G 7
G
5 d
M
-4 G
5 3
2
1 O'--_I-__
I
I
I
I
2.0 30 l@t, sec. (curves B-E) Fig. 1.-Logarithmic plots of nitrostyrene concentration: curve A, pH 0.9; B, 3.6; C, 4 6; D, 5.6; E, 7.7.
1.0
0
0.15 0.25 [Ac-1, M. Fig. 2.-Variation of kl with acetate ion concentration: 0 , PH 3.6; 0 , pH 4.5; 0, PH 4.6; 0 , pH 5.6. 0
0.05
The value of k1 usually was calculated by assum- intermediate, presumably the nitroalcohol, in ing a value of 0.6 for h , / k - l , plotting as for a re- acetate buffers in methanol. Only a small fracversible first-order reaction and measuring the tion of the stoichiometric concentration of nitroinitial slope. By the approximation of McDaniel styrene was formed; whether this was due to unand S m ~ o t k2 , ~ then was evaluated and finally favorable equilibrium with the nitroalcohol or to k-1, this last value being an improvement on the a competing methanolysis, possibly forming a tentative assumption for kl/k-1. These constants methyl ether, was not determined. are given in Table I for all runs. Discussion The first step shows general base catalysis : We explain the above results in terms of the mechk1 depends only on the acetate ion concentration between the p€I values 3.6 and 5.6. In the absence anism shown in the accompanying diagram. The of any base except the solvent, for example in 0.1 kc M or 0.01 M hydrochloric acid solutions, k1 attains ArCH=CHS02 ( I ) + H 2 0 ArCHCHNOz a minimum value of 0.153 X 10" sec.-l. The I k- 4 OH2 + relationship between k1 and acetate ion concentration, given by the points in Fig. 2, is not a linear one. The constancy of kl/k-l a t different pH, B within the limits of experimental error, is an indication that this first step is not an ionization nor an addition of hydroxyl ion. ArCHCH2N02 (XI) ArCIICHN02 (IIa) The second step, on the other hand, proceeds at I I a rate which depends not on the acetate ion but on OH 011 the @H. The values of kz given in Table I seem proportional to hydroxyl ion concentration at HB + higher pH, but also increase in very acidic solutions. ks At 45' in 10-3M hydrochloric acid, k1 is 1.92 CHtNOz ArCHCHzNO2--t ArCHO + cII2ITO2 X sec.-l, yielding an apparent activation I I11 0- IIb energy of 22.6 kcal./mole. In 99% deuterium oxide, the corresponding value a t 45' is 0.13 X first stage of eq. 1 is separated into three steps: 10" sec.-l. The ratio &/KD is 14 =t2. an initial hydration which proceeds oiily to a slight Methano1ysis.-Many experiments were carried fxtent (kd
