KINETICSOF ALKYLHYDROPEROXIDE-TETRANITROMETHANE REACTION
4155
Kinetics and Mechanism for the Reaction between Alkyl Hydroperoxides and Tetranitromethanel
by William F. Sager2 and John C. Hoffsommer U . S. Naval Ordnance Laboratory, White Oak, Silver Spring, Maryland
80810
(Received April 18, 1969)
The kinetics and activation parameters for the reaction between ROOH (where R = hydrogen, ethyl, isopropyl, cumene, and t-butyl) and tetranitromethane (TNM) in water in the pH range, 5.8-8.0, at 12.50, 25.14, and 37.10’ have been investigated. Product analysis shows that ROOH reacts with TNM C(I’?Oz),
+ ROO- + HzO +C(N0z)a- + NOz- + H+ + ROH + 02
Rates of reaction were followed spectrophotometricallyfrom the trinitromethide, NF, concentration with time. Good second-order rate constants, kp, were obtained from the expression -d(TNM)/dt
= +d(NF)/dt
kl(TNM) = kz(ROO-)(TNM) = lczK,(ROOH)(TNM)(H+)-’,
where k, is the pseudo-first-order rate constant with excess hydroperoxide, and K , is the hydroperoxide acid dissociation constant. ROO- reacted with TNM loa to lo4times faster than OH-, and, in general, the rates were found to follow the Bronsted free energy relationship with pK,. Labeling experimentswith Hzl8Oindicate that nucleophilic attack by ROO- on TNM occurs at oxygen of the NO2 group rather than at carbon or nitrogen.
Introduction Tetranitromethane (TNM) has been used primarily as a color complexation reagent for the detection of carbon-carbon unsaturation in a wide variety of organic compound^.^ I n addition, T N M is known to react with hydrogen peroxide and potassium hydroxide in aqueous solution to produce n i t r ~ f o r m . ~Since unsaturated organic compounds often contain varying amounts of alkyl hydroperoxides, we thought it worthwhile to investigate the possibility of a hydroperoxideTNM interaction. Reaction between TNRI and the alkyl hydroperoxides would be theoretically interesting from a mechanistic standpoint, since reaction sites at carbon, nitrogen, as well as oxygen are all available for attack in TNM. We now wish to report our findings concerning the reaction between alkyl hydroperoxides and TNM.
Experimental Section and Results Hydroperoxides. Ninety per cent t-butyl hydroperoxide in aqueous solution was obtained commercially (Lucidol Division of Wallace and Tierman, Inc., Buffalo, IT.Y.) and was purified by distillation under reduced pressures. Ethyl hydroperoxide was prepared in 37% yield from ethyl sulfate and aqueous potassium hydroperoxide based on methods of Baeyer and Villiger,5 and Rieche and Hitz.6 The hydroperoxide was obtained as a, 53% aqueous solution by weight after distillation at reduced pressures. Isopropyl hydroperoxide was prepared in 40% yield from diisopropyl sulfate’ and aqueous potassium hy-
droperoxide based on the methods of Medvedev and Alexejewa.8 The hydroperoxide was obtained as an 8% aqueous solution after distillation a t reduced pressures. Cumene hydroperoxide was obtained commercially from the Hercules Powder Co., Wilmington, Del., and found to contain 73% active oxygen by iodometric analysis. The pure hydroperoxide was obtained by fractional distillation and a middle fraction boiling a t 65’ (0.18 mm) was c ~ l l e c t e d . ~ Dilute aqueous hydrogen peroxide solutions were prepared from both commercially available 30% (Baker Analyzed Reagent) and 90% (Becco Chemical Division, Buffalo, N. Y.) aqueous hydrogen peroxide. (1) Work taken in part from the Ph.D. dissertation of J. C. H., George Washington University, Washington, D. C., 1964. (2) Department of Chemistry, University of Illinois at Chicago. (3) A. Werner, Ber., 42, 4324 (1909); I. Ostromisslensky, ibid., 439 197 (1910);E.M.Harper and A. K. MacBeth, J. Chem. Soe., 107, 1824 (1915); D. L1. Hammick and R. P. Young, ibid., 149, 1463 (1936); E. Heilbronner, Hela. Chem. Acta, 1121 (1953); K. Miescher, ibid., 29,743 (1946); L. F. Fieser and M. Fieser, “Reagents for Organic Synthesis,” John Wiley & Sons, Inc., New York, N. Y., 1967,pp 1147-48. (4) Method of K.Schimmelschmidt, reported by K. Klager and M. B. Frankel, Aerojet Report No. 494,Aorojet Engineering Corporation, Amsa, California, p 3 (1951). See also J. W. Copenhaver and M. H.
Bigelow, “Acetylene and Carbon Monoxide Chemistry,” Reinhold Publishing Corp., New Yorlr, N. Y., 1949,p 24. ( 5 ) A. Baeyer and V. Villiger, Ber., 34,738 (1901). (6) A.Rieche and R. Hitz, ibid., 62,2469(1929). (7) W. R.Ormandy and E. C. Craven, J. SOC.Chem. Ind., 49, 363T
(1930). (8) S. Medvedev and E. N. Alexejewa, Ber., 65,132 (1932). (9) D. Barnard and K. R. Hargrave, Anal. Chim. Acta., 15, 476 (1951). Volume 79, Number 1 8 December 1969
4156
WILLIAMF. SAGERAND JOHN C. HOFFSOMMER
Table I : Experimental pK, Values for Ethyl Hydroperoxide in Water at 25.14' Buffered pH*
c
-11.09-
Wavelength, mp
260 265 270 275 280
EHA'
8.74 6.75 5.18 3.94 3.02
-11.78-
-11.56-
fA
f
PKD.
f
98.5 78.2 60.8 47.1 36.1
40.7 31.3 23.9 18.6 14.1
11.35 11.34 11.34 11.33 11.34
62.7 48.3 37.2 28.5 21.9
11.34
PKB.
11.36 11.37 11-38 11.37 11.36
11.37
P&
f
73.8 57.4 44.2 34.2 26.2
11.36 11.32 11.a2 11.31 11.31
11.33
Combined av 11,35 j=0.02 a €HA, EA, and e are extinction coefficients for the undissociated, completely dissociated, and buffered ethyl hydroperoxide. phate buffers, ionic strength = 0.100.
Tetranitromethane. Tetranitromethane (TNM) was prepared in these laboratories by nitration of acetic anhydride, and purified by distillation at reduced pressures. The freezing point of the TNM used was 14.1'. Dilute aqueous solutions of TNM were analyzed quantitatively according to the method of Glover and Landsmanlo which is based on the quantitative reaction between TNM and hydrazine to produce nitroform (A, 350 mp; E, 14,418). Hydroperoxide Analysis. Dilute aqueous stock solutions, 0.03 to 0.05 M in hydroperoxide, were analyzed iodometrically immediately before use. Solutions of hydrogen peroxide were analyzed according to the procedure of Kolthoff and Sandell," while the alkyl hydroperoxide solutions were determined in hot isopropyl alcohol solutions containing glacial acetic acid and potassium iodide.12 Dissociation Constants. Dissociation constants of the alkyl hydroperoxides in water were measured by a modification of the general procedure of Everett and Minkoff .13 Extinction coefficients of the dissociated, EA, and undissociated hydroperoxides, EHA, were determined between 260 and 310 mp in matched 5-cm quartz cells which were thermostated to within k0.05" in the cell compartment of a Beckman, Model DU, spectrophotometer. Extinction coefficients, E, of the hydroperoxides in phosphate buffers in the pH range, 10.9 to 12.10, for which the pH was known as a function of temperature14 were measured at the same temperature and wavelengths as EA and EHA. The ionic strength of each buffered solution was maintained at 0.100 by the amount of buffer added, and the pK, was log (EA calculated from the equation, pK, = pH E ) / ( € - EHA). Buffered solutions were checked with a Beckman, model G, pH meter. For each buffered hydroperoxide at a specific temperature and pH, the pK, values obtained at 4 or 5 different wavelengths were found to be essentially constant and agreed reasonably well with pK, values obtained in the buffer range, pH 10.9 to 12.0, at the same temperature. Alkyl peroxyanion extinction coefficients, EA, were obtained
+
T h e Journal of Physical Chemistry
' Phos-
by measuring the optical densities of 2 to 5 M aqueous sodium hydroxide solutions which were 2 to 3 X M in hydroperoxide. The data for ethyl hydroperoxide are illustrative of the general method employed for the determination of pK, values for all the hydroperoxides studied and are sho vn in l'able I. Although aqueous solutions of the alkyl hydroperoxides were found to be stable in the pH range 10 to 12, and at temperatures, 12.5 to 37.1') solutions of hydrogen peroxide were rapidly decomposed under these ~0nditions.l~Therefore, a modification of the procedure used for the alkyl hydroperoxides was used to obtain the pK:s of hydrogen peroxide. Optical density measurements of aqueous phosphate-buff ered solutions of hydrogen peroxide in the pH range 11 to 12 were made at 270 mp as a function of time. Extrapolated values of log optical density, at time, t = 0, were taken as log E, a t the specific buffered pH and temperature, and used to obtain the pK, from the buffer equation, as described. Plots of log optical density us. time were found to be linear to about 10 minutes depending on the pH. The pK, values and calculated thermodynamic parameters for all the hydroperoxides are shown in Table 11. Kinetics. Aliquots of aqueous stock hydroperoxide solutions together with sufficient phosphate buffer to make the ionic strength of the finally diluted solution equal to 0.1 were thermostated in 100 ml volumetric flasks at least 0.5 hr prior to each kinetic run. At the same time, separate aqueous solutions of 2.0 to 5.0 X (10) D. J. Glover and S. G. Landsman, Anal. Chem., 36, 1690 (1964). (11) I. M. Kolthoff and E. B. Sandell, "Textbook of Quantitative Inorganic Analysis," The Macmillan Co., New York, N . Y., 1948, p 630. (12) V. R. Kokatnur and M. Jelling, J. Amer. Chem. SOC.,63, 1432 (1941). (13) A. J. Everett and G. J. Minkoff, Trans. Faraday Soc., 49, 410 (1953). (14) H.T.S.Britton, "Hydrogen Ions," Vol. I, D. Van Nostrand Co., Inc., New York, N. Y., 1956,p 359. (15) F.R. Duke and T. W. Haas, J. Phys. Chem., 65,304 (1961).
4157
KINETICSOF ALKYLHYDROPEROXIDE-TETRANITROMETHANE REACTION
Table 111: Observed First-Order Rate Constants, kl,for the Rate of Disappearance of TNRl with Excess Ethyl Hydroperoxide in Aqueous Solution"
Table I1: Thermodynamic Parameters for the Dissociation of Alkyl Hydroperoxides in Water T,'C
AH,
dz 0.06
PKS, i 0.02&
kcal/mol
12.50 25.14 37.10
12.74 12.54 12.35
6 . 4 f 0.02
12.50 25.14 37.10
11.85 11.54 11.31
8.9 i0.3
Isopropyl-
12.50 25.14 37.10
12.07 11.74 11.61
7 . 5 f 1.8
-28 i 6
Curn en e-
12.50 25.14 37.10
12.60 12.33 12.07
8.8 f 0 . 3
-27 dz 1
85.5
Ethyl-
12.50 25.14 37.10
11.56 11.35 11.26
5.0 f 1.0
-35 i 3
94.6 100
R-OOH
t-Butyl-
H-
a
A#,
%Completion
cal/deg
2.6 -36 =t 1 15.9 -23 =t 1 42.8
71.4
M TNR4 were similarly thermostated. Sufficient distilled water mas added to the volumetric flasks containing the buffered hydroperoxides so that on addition of the TNR4 aliquot, the total volume of solution would be exactly 100 ml.16 The contents of the flask were thoroughly mixed, poured into dry 1-cm quartz optical cells, stoppered, and immediately placed into the cell compartment of the spectrophotometer. The compartment was specially designed so that the optical cells could be immersed in circulating distilled water maintained at the desired temperature by means of an external constant temperature bath. A calibrated and certified National Bureau of Standards thermometer which could be read to within =kO.O2"C was fitted tightly into the cell compartment. Temperatures during the kinetic runs were controlled to within =kO.O5"C. Reference cells contained the same amount of buffer and hydroperoxide as the kinetic run. Cell blanks were negligible. TNM was found to react quantitatively" with excess hydroperoxide in buffered aqueous solutions in the pH range 5.8 to 8.0 to produce yellow solutions containing trinitromethide (nitroform) and nitrite ions. From a large scale reaction between T N M and t-butyl hydroperoxide, oxygen gas and t-butyl alcohol were isolated (later section). And, although the isolation of these products was not quantitative, these products indicate the following general reaction
+ ROO- + HzO N02-
1.49 4.11 11.45 14.27 21.19 28.23 39.05 69.25 81.57 87.60 104.86 125.47 137.54 147.86 182.35 207.24 overnight,,
/GI
0.015 0,034 0.089 0.106 0.150 0.187 0.239 0,351 0.384 0.399 0.431 0.462 0.478 0.489 0.519 0.529 0.559
----f.
C(N0z)a-
+
+ H+ + ROH + Oz
(1)
whereas the rate of reaction between T N M and hydrazine was found to be practically instantaneous, the rate between TNNf and excess hydroperoxide was found to
x
IO-^, sec-1 d
0.243 0.234 0.243 0.239 0.212 0,239 0.237 0.237 0.237 0.237 0,234 0.232 0.234 0.234 0.240 0.234
... Av
Ionic strength = 0,100.
C(N0z)d
O D c~
t, m i d
0.237 =t 0,003
a TNM, 3.86 X 10-6M; ethyl hydroperoxide, 2.43 X lo-* M ; pH, 7.00; 25.14'C. * Obtained with an automatic timer, Precision Scientific Co., Chicago, Ill. ' Optical density of nitroform kl = l / t In (OD,)/(OD, - OD,). anion, C(N02)3-, a t 360 mfi.
be much slower and could be measured as a function of nitroform concentration with time from its molar absorption at 350 mp. Under pseudo-first-order conditions with excess hydroperoxide, the rates of disappearance of T N N were found to obey first-order kinetics in the buffered pH range 5.8 to 8.0. The first-order rate constants, lc1, obtained for the rate of disappearance of TNM for the reaction between 3.86 X M TNM and 2.43 X M ethyl hydroperoxide in water at pH 7.00 and 25.14" were typical and are shown in Table 111. The rate of disappearance of TNM, -d(TNM)/dt, was followed from the observed rate of nitroform ion formation, +d(NF)/dt. Concentrations of TNM at any time were known from (OD, - ODt)/eNF, where OD, and ODt were the optical density values after a t least 10 half-lives and at time, t , respectively, and ENF, the extinction coefficient of nitroform anion in water. At a constant pH, the first-order rate constants, lcl, varied with initial hydroperoxide concentration and indicated first order dependency with respect to hydroperoxide, Table IV. I n addition, the observed firstorder rate constants, ICl, were found to vary with the pH of the buffered solution. These results indicated that (16) Care must be exercised in working with aqueous solutions of
T N M , since even a t room temperatures, T N M is quite volatile and
easily lost. (17) Nitroform produced agreed within 1% to the nitroform obtained by the hydrazine method for the quantitative determination of
TNM.10
Volume 78,Number 12 December 1969
WILLIAMF. SAGER AND JOHN C. HOFFSOMMER
4158 Table IV: Order with Respect t o Hydroperoxide for Reaction with TNM R-OOH
X
10-6 M
H9.64 18.6 21.6 Ethyl25.5 12.1 t-Butyl10.8 15.7
TNM X M
PH
ki, seo-1
Ordera
3.06 4.42 3.95
7.2 7.2 7.2
0,482 0,904 1.12
1.oo
2.18 4.47
7.4 7.4
0.625 0.307
0.96
4.58 4.07
7.8 7.8
0.137
1.02
0 * 202
10-6
~1 Value of r from, (k1-1)/(k1-z) = (ROOH1/ROOHz)”, where kl-l and klbz are first-order rate constants for the respective hydroperoxide concentrations, ROOHl and ROOHZ.
the alkyl peroxyanion, ROO- was the reactive nucleophile in attack on TNM. Second-order rate constants, kz, for the rate of disappearance of TNM, -d(TNM)/dt = k~(R00-)(TNM), were calculated from the derived expression, kz = kl(H+)/K,(ROOW), in terms of ICl, the observed first-order rate constant, and K,, the acid dissociation const,ant of the alkyl hydroperoxide determined at the same ionic strength and temperature. Observed kl and calculated ICz rate constants for the reaction between hydrogen peroxide and TNM in water at 25.14OC as a function of pH and concentration are shown in Table V. Rate data and thermodynamic activation
Table V : First- and Second-Order Rate Constants, kl and kz for the Reaction Between TNM and Excess Hydrogen Peroxide in Water a t 25.14’ TNM X 10-6 Ma
HzOa X 10-2 M
pH
ki X 10-ac
3.38 3.95 4.42 4.29 2.18 3.06 4.25 4.15 4.08 4.09
0.482 2.16 1.86 1.86 1.86 0.964 3.09 0,964 0.482 4.66
7.40 7.20 7.20 7.20 7.20 7.20 7.00 7 .OO 7.00 5.86
0.403 1.12 9.904 0.904 0.938 0.482 0.850 0.273 0.146 0.123
Av
k i X 108
1.16 1.13 1.06 1.06 1.10 1.10 0.960 1.16 1.06 1.26 1.10 0.05
Concentration of TNM determined after a t least ten halflives from nitroform ion concentration. b Phosphate buffers, total ionic strength = 0,100. c In sec-1. d Inl. mol-’ see-1.
parameters for the reactions between five hydroperoxides and TNM in water are shown in Table VI. Each second-order rate constant, kz, is the average of at The Journal of Physical Chemiatry
least three kinetic runs in at least two different buffered solutions in the pH range, 5.8 to 8.0. Product Analysis. Solutions were analyzed for nitrite ion by the method of Snell and Snell.l* Under acidic conditions the nitrite ion was converted to nitrous acid. The nitrous acid produced was used to form the diazotization product of sulfanilic acid which was then coupled with l-aminonaphthalene. Nitrite ion concentration was determined spectrophotometrically by measuring the absorption of the purple coupled product at 520 mp and comparing to a standard curve. Beer’s law was found to hold in the range of nitrite ion concentraM. tion, 1.0 to 6.0 X It was essential that all peroxides be destroyed prior to the nitrite analysis to prevent oxidation of nitrite ion to nitrate under the acidic conditions of the determination. Hydrogen peroxide and t-butyl hydroperoxide were completely decomposed in aqueous solutions after adjusting the pH of the solution between 11 and 12 with dilute aqueous sodium hydroxide and warming on the steam bath for 20 min. Ethyl, isopropyl, and cumene hydroperoxides were only partially decomposed under these conditions and, consequently, the nitrite analyses were somewhat low. For example, (NOZ-)/(C(XOZ)~ratios obtained from the kinetic runs for hydrogen peroxide and t-butyl hydroperoxide were found to be 1.00 and 0.96, respectively, while ratios of 0.81, 0.48, and 0.24 were found for the kinetic runs with ethyl, isopropyl and cumene hydroperoxides. However, similar ratios were obtained on analyses of synthetic solutions containing equal concentrations of NO2- and C(N02)3- ions together with each of these hydroperoxides separately, and indicate that the ratio, (NOz-)/ (c(r\To&-was close to unity in each kinetic run. The pseudo-first-order rate constant, kl, for the formation of NOz- ion from the reaction between 5.02 X 10-5 M TNM and 4.20 X lod3M HZOZat pH 7.40 was found to be 0.28 X sec-’. This result paralleled the value obtained for the pseudo-first-order rate constant, Icl (0.25 X sec-’), for the formation of nitroform ion for this run. Nitrite analyses were determined on aliquot samples taken from the reaction solution at various time intervals. Prior to the analyses, unreacted TNM was extracted with chloroform, and unreacted hydrogen peroxide was destroyed by adjusting the pH to 11.6 and heating on the steam bath for 20 min. Oxygen gas and t-butyl alcohol were isolated and identified from a large scale reaction at room temperature between TNM and t-butyl hydroperoxide, buffered at pH, 11.56. The oxygen gas was collected and measured by displacement of water and identified by gas chromatography. Tertiary butyl alcohol was isolated by distillation, followed by extraction of the aqueous dis(18) F. D. Snell and c. T. Snell, “Colorimetric Methods of Analysis,” Vol. 2, 3rd ed, D. C., Van Nostrand Co., Inc., New York,N. Y., 1949, pp 802-804.
KINETICS OF ALKYL HYDROPEROXIDE-TETRANITROMETHANE REACTION
4159
Table VI : Second-Order Rate Constants and Activation Parameters for Reaction between Alkyl Hydroperoxides and TNM in Water T,"C
ka X 108'
AF*~
t-Butyl-
12.50 25.14 37.10
0.623 f 0.02 2.25 f 0.07 7.66 0.02
12.9 f 0.3
17.7 rt 0 . 2
15.1 f 0.5
H-
12.50 25.14 37.10
13.3 f 0.3
12.6 f 0 . 2
-.2.8 i 0.8
3.23 f 0.02
Isopropyl-
12.50 25.14 37.10
0.249 f 0.003 0.811 i 0.04 2.72 0.05
13.5 i 0.4
16.9 f 0 . 2
10.4 zt 0.5
Cumene-
12.50 25.14 37.10
0.139 f 0.006 0.435 f 0.007 1.36 f 0.010
13.9 f 0 . 2
16.1 f 0.2
6 . 5 i 1.0
Ethyl-
12.50 25.14 37.10
0.0673 f 0.001 0.217 f 0.007 1.04 f 0.03
14.3 f 0 . 2
18.8 i 0 . 2
16.5 f 0.9
R-OOH
* 0.530 f 0.02 1.10 * 0.05
*
AH+C
cal deg-1. Errors in footnotes c and d were estimated by the method of E. L. Purlee, a 1. mol-' sec-1. b kcal mol-'. kcal mol-'. R. W. Taft, and C. A. De Faeio, J.Amer. Chem. Soc., 77,837 (1955). See ale0 L. L. Schaleger and F. A. Long, in "Advances in Physical Organic Chemistry," Vol. I, V. Gold, Ed., Academic Press, New York, N. Y., 1963, pp 7-9.
Table VII:
_-TNM 0.128
Large Scale Reaction between &Butyl Hydroperoxide and TNM in Water Reaction mixture, mol" (CHs)aCOOH
pHb
C(NOds-
0.198
11.56
0,101
Reaction products, mol N020%
r
0.0873
I _ _
(CHn)sCOHc
0.097
0.034
a Reaction mixture was heterogeneous. Buffer, 0.60 mol of potassium dihydrogen phosphate and 0.90 mol of sodium hydroxide in Isolated yield. No other organic fragment other than t-butyl alcohol was found. 500 ml of water.
tillate with ether and drying over anhydrous magnesium sulfate. The pure alcohol was obtained by fractional distillation and had an identical boiling point and infrared spectrum with authentic t-butyl alcohol. Nitrite ion concentration was determined on an aliquot sample as previously described. The results of a large scale reaction between t-butyl hydroperoxide and TNM are shown, Table VII. '80 Labeling Experiments. Since TNM possesses three possible sites for nucleophilic attack by the hydroperanion nucleophile, we considered it worthwhile to investigate the labeled products resulting from the reaction between t-butyl hydroperoxide and T N M under basic conditions in HPO. After optimizing conditions in ordinary unlabeled H2160, the following experiments mol of t-butyl were carried out. A total of 4.12 X hydroperoxide (5.3 g of 7001, hydroperoxide in ordinary water) was dissolved at 20" in a solution containing 8.33 X lo-' mol Volk, 5.09 excess atom yo "normalized," HPO, 6.44 X lo-' mol of ordinary water and 0.100 mol (4.05 g 97.7%, reagent grade) of sodium hydroxide.lg To this stirred solution was added in one portion, 9.19 X mol (1.80 g) of TNM. The mixture was stirred vigorously for 0.5 hr until the gas
evolution (oxygen) stopped and a portion collected. Tertiary butyl alcohol was obtained by distillation from the reaction mixture and purified as previously described. The results of the lS0mass spectral analyses20 are shown (where the numbers indicate % 1 8 0 excess) Reaction mixture
(CH&COO-
+ C(N02)d + H2'80 * 0
0
2.69
Products
C(NO&-
+ + NOz- + (CH&COH + H+ 0 2
0.003
Obviously, the
(2)
0.024 labeling in the 0 2 and t-butyl alcohol
(19) The Hz"0 was analyzed by conversion to C2HalBOH by the basic hydrolysis of ethyl sulfate (M. Anbar, I. Dostrovsky, D. Samuel, and A. D. Yoffe, J . Chem. SOC.,3609 (1954)). Mass spectral analyses of the CzHa180H were kindly performed by F. E. Saalfeld of the Naval Research Laboratory, Washington, D. C. Result is average of ten determinations. (20) Mass analyses were performed by A. S. Ostashever, Analytical Corporation, New York, N. Y. Volume 73, Number 18 December 1969
4160
WILLIAMF. S A ~ EAND R JOHN C. HOFFSOMMER
products was practically insignificant. We then decided to look for ls0 labeling in the nitrite ion by the method of Anbar and Taube21 according to eq 3, by mass spectral analysis of N20. Also, Anbar and Taube found no exchange between NOz- and H2180 in the buffer pH range, 2.5 to 3.5
+
""02
pH 2.5-3.5 3"
Nz"0
+ N2 + H2180
Reaction mixture
+ C(N02)4 + H2180 + 0
0
2.59
Products
C(NOJ3-
+ N"02- + + (CH~)BCOH+ H+ 0 2
(4)
2.68 Also, had exchange between "02 and H2180 taken place inadvertently, before or during the reaction with hydrazoic acid, the l80labeling in NzO would be expected to be a t most, 0.16% excess ls0. Synthetic mixtures of t-butyl hydroperoxide, sodium hydroxide, nitroform, and t-butyl alcohol in water were found not to interfere with the conversion of nitrous acid to NzO with hydrazoic acid.
Discussion TNM reacts quantitatively with excess hydroperoxide to forrn trinitromethide and nitrite ions, with the liberation of 0 2 gas and formation of the corresponding alcohol (eq 1). The rate of disappearance of TNhl obeys second-order kinetics overall, first order in TNM and alkyl peroxyanion, (Tables 111 and IV) according to -d(TNRl)/dt = k2(TNM)(ROO-). Evidence for ROO- attack on TNM as the rate determining step is demonstrated by the pH dependancy on the first-order rate constant, (Table V). Logarithms of the secondorder rates, log k2, follow the Bronsted free energy The Journal of Physical Chemistry
hydrogen
3.51 8
+
i-pmpyl
CUrnene
ethyl hydroperoxide3
(3)
A total of 3.0 X mol of TNM was allowed to react with 1.40 X mol of t-butyl hydroperoxide in 0.277 mol of 5.09 excess atom % H2180,0.232 mol of ordinary distilled water and 3.70 X lod2mol of sodium hydroxide. After completion of the O2 gas evolution, one-third of the resulting mixture was buffered22 between pH 2.5-3.5 with 9.8 X mol of potassium acid phthalate, and 1.4 X loF2 mol of HC1 in 0.85 mol of ordinary water. To this stirred solution was added 1.54 X mol of sodium azide in one portion, whereupon a rapid evolution of gas (N20) ensued. Mass spectral analysesz3 showed the NzO to possess 1.34 atom yoexcess l80. The l80labeling in the original HNls02 would, therefore, be expected to be 2*G8 atom % excess l80since the HN"O2 is converted to both N2l80 and Hz"0 (eq 3). The resu1ts for the TNM-'butyl hydroperoxide reaction are summarized (where the numbers indicate % l80excess) (CH3)3COO-
0 I-butyl
A
3,O
A
';c /
2.0
relationship with p ~ , ,in general ( ~ 1) andi also ~ indicate attack on T N I I by ROO-. Although the fit of the data to a straight line is somewhat imprecise, this is not entirely unexpected since the R group of ROO- is quite close to the reactive centerof the peroxyanion oxygen, and since there is considerable variation in the entropies of activation (Table VI). In this plot, hydrogen peroxide has been statistically corrected for its two hydrogens by dividing the second-order rate constant by two. The kinetics, however, do not distinguish the point of nucleophilic attack by the peroxyanion, ROO-, which in TNM may occur at any one or combination of three diff erent atoms, namely, carbon, nitrogen, or oxygen to form I, 11,or I11 at
ROO-
+ C(N02),
-E 'low step
+
[1E00-C(N02)3] N O 2 -
(1) 0 at
I1 I1
[ROO-N]
+ C(N0z)a-
(11)0 0
II
+
[ROO-ON] C(NO2)8' Subsequent rapid decomposition of (111) the intermediates at
I, 11, or I11 by H 2 0 or OH- could yield the other products (21) M. Anbar and H. Taube, J. Amer. Chem. Soc., 76,6243 (1954). (22) The pH of the buffered solution was checked before and after the
run with a Beckman, Model G, pH meter. (23) Mass analyses by F. E. Saalfeld, Naval Research Laboratory, Wash., D. C. Atom %1*0= (14dI44)/(146/144 I44/I43 X 100,where Ii = ion current observed for m / e = i. Results are average of ten determinations.
+
~
4161
KINETICSOF ALKYLHYD~OPEROXIDE-TETRANITROMETHANE REACTION Table VIII:
Relative Rates and Activation Parameters for Nucleophilic Attack by OOH- and OH- on TNM and Beneonitrile Nucleophile
kz x 10-15
TNM~
OHOOH -
Beneonitriled
OH OOH -
3.51 11,000 0.00112 73
Substrate
*,
kz(OOH-)/kz(OH-)
3,130 65,000
AH+^
AS*“
16.7 i 0.3 12.6 f 0 . 2 20.5 0.7 22.0 f 1 . 0
-4.5 -2.8 -13.0 $5.0
f 1.0 i 0.8 f 2.0 f 4.0
‘
a k ~ 1.-mol-1 , sec-1; AH kcal mol-’; AS*, cal deg-1. Solvent, water a t 25.14’. ‘Results of D. J. Glover, this laboratory. A Solvent, 50% acetone: water, by volume, a t 50’. Data detailed report of the kinetics of the OH--TNM reaction is in preparation. of K. B. Wiberg, J.Amer. Chem. Soc., 77,2519 (1956).
I, 11, or I11
Hz0, OH-
02
+
“02
+ ROH
or HC(N02)3
Attack of TNhf by the nucleophile, OH- has been postulated to take place at either carbon or nitrogen depending on the hydroxide ion concentration.24 I n strong base, nucleophilic attack by OH- on nitrogen was suggested to account for the formation of 1 mol each of nitrate and trinitromethide ions, while in dilute base, OH- attack on carbon was rationalized to account for the formation of 4 mol of nitrite and 1 mol of carbonate ions. The details of the catalytic reaction of nitrite ion with TNM and the kinetic investigation of the hydroxide-TNM reaction will be reported separately by D. J. Glover.25 Since t-butyl hydroperoxyanion reacts with TNM in basic H2180to produce almost complete l8O labeling in the nitrite ion to the exclusion of l80labeling in the t-butyl alcohol and O2 gas, eq 2 and 4, initial nucleophilic attack by the t-butyl peroxyanion on the oxygen atom of T N M seems most plausible followed by a subsequent rapid attack by 180H- on the nitrogen of intermediate, 111,to produce the products as indicated 0 ~
l
8
0
-
Ne[
Io -0n
n
-0-R
--+
I11 RO-
f
02
+
HN1802
Rationales involving nucleophilic attack by Hz180 or 180H- on various sites of intermediates I or I1 produce either R180H or lSO2 and labeled or unlabeled nitrate ion and are at variance with experimental evidence. Alternatively, perhaps the most attractive mechanism involves attack by ROO- on the oxygen of the nitro group in TNM in a slow equilibrium step ROO-
+
O-N-C(NO,),
.1
SIOW
-L-
A rapid, concerted decomposition of intermediate IV would then form 02 gas and trinitromethide ion, C(N02)3-, as the driving force of the reaction
RO-
+
O2
+
NO’
+
C(NO,),-
NO+ then rapidly reacts with 180H- to form exclusive labeling in the nitrite, as observed
NO+ + 180H-
% r
“1Q2
Besides the convincing H2180experiments for peroxyanion attack on oxygen, some justification for considering that OOH- and OH- ions attack TKM at different sites is derived from a comparison of the activation parameters involving the substrates T N M and benzonitrile, Table VIII. Whereas the rates for nucleophilic attack by OOH- on both TNM and benzonitrile are greater by a factor of 3 to 65 X lo3 than rates for OH- attack,26the activation energy for the OOH-TNhl reaction is about 4 kcal mol-’ less than that for the OH--TNM reaction. On the other hand, the activation energies for both OOH- and OH- reacting with benzonitrile are about the same where attack occurs at carbon. The more positive entropies of activation for nucleophilic attack by OOH- relative to OH- has been rationalized2’ as a solvation effect, whereby OOH- is less solvated than OH-, thus requiring less reorientation of solvent molecules around OOH- in the transition state. With the exception of the hydroperoxyanion, all the alkyl peroxyanions possess positive entropies and have greater activation energies than the hydroperoxyanion in reaction with TNM, Table IX. AH* is found to vary fairly linearly with AS*, with a slope of 330°K, (24) E. Schmidt, Ber., 52B,400 (1919). (25) D. J. Glover, J. Phys. Chem.; 72, 1402 (1968). (26) Attributable to the so called “alpha effect;” J. 0. Edwards and R. G. Pearson, J. Amer. Chem. SOC.,84, 16 (1962);T. C.Bruice, A. Donzel, R. W. Huffman, and A. R. Butler, ibid., 89, 2106 (1967); M.J. Gregory and T. C. Bruice, ibid., 89,4400(1967). (27) C. A. Bunton in “Peroxide Reaction Mechanisms,” J. 0. Edwards, Ed., Interscience Publishers, Inc., New York, N. Y., 1962,p 25.
Volume 73, Number 1.2 December 1069
WILLIAM F. SAGERAND JOHN C. HOFFSOMMER
4162
Table IX: Relative Activation Parameters for the Reaction of Alkyl Peroxy Anions with TNM in Water
e,
AAH *,
AAS os1 deg-1
koa1 mol - 1
R-OO-
HCsH&(CHs 12i-CaHr t-CdHeC2Hs
0
0 3.5 4.3 5.1
9.3 13.2 17.9 19.3
0.2
as the isokinetic temperature28 for the reaction, and implies a single mechanism for peroxyanion attack on TNM . Although one can readily rationalize on steric grounds the greater activation energies of the alkyl peroxyanions, as a class, relative to the hydroperoxyanion in reaction with TNM, it is somewhat more difficult to explain the ordering among the alkyl peroxyanions themselves. For instance, cumene hydroperoxyanion is the most sterically crowded of the alkyl peroxyanions, and yet has the lowest activation energy for attack on TNM, while just the converse is true for the ethyl peroxyanion. A rationale for these observations is offered in terms of a proposed “activated complex” resulting from nucleophilic peroxyanion oxygen attack on the oxygen of TNM
L
II
R
J
in the ground state would account, in part, for the positive entropies observed. Maximum p electron orbital overlap of the peroxyanion oxygen with the nitro oxygen orbital is required in forming the incipient 0-0 bond at the expense of weakening the old N-C and 0-0 peroxide bonds. The driving force of the reaction could now arise by the formation of C(N02)3- ion and 0 2 gas, and subsequent reaction of NO+ with 180H- as previously outlined. Examination of molecular models for each of the peroxyanions indicate that the C-C and C-0 bond rotations, C-R (R’, R”) and (R) (R’) (R”)-C-0are free and ordinarily do not interfere with the peroxyanion oxygen except in those cases where the R groups are quite bulky as in cumene peroxyanion. Here, rotations about the C-C bonds and especially the C-0 bond are somewhat restricted in the ground state and, therefore, less energy and hence, lower AH would be required to ‘lfreeze out’’ the proper orientation for maximum oxygen p orbital electron overlap. Conversely, more energy, and greater AH* would be required for the less sterically crowded ethyl peroxyanion where C-C and C-0bond rotations in the ground state are essentially free. Similar arguments have been offered to explain the AH and AS* values obtained for the decompositions of a series of &butylp e r e ~ t e r s . ~ ~
*
*
Although all the peroxyanions would be expected to be solvated by water, the alkyl peroxyanions, ROO-, most likely would be solvated about the same extent, but somewhat less than the hydroperoxyanion, HOO-, where hydrogen bonding to itself afi well as to water is possible.
Activated complex
The rather diffuse negative charge distribution in the proposed activated transition state relative to the more oriented and concentrated charge of the peroxyanion
The Journal of Physkal Chemistry
(28) J. E.Leffler, J. Org. Chem., 20,1202 (1965). (29) J. E.Leffler and E. Grunwald, “Rates and Equilibria of Organic Reactions,” John Wiley &Sons, Inc., New York, N. Y., 1983,pp 358 See also P. D. Bartlett and R. R. Hiatt, J. Amer. Chew. Soc., 80, 1398 (1958).
