Kinetic Studies of the Reaction of Picryl Chloride with Sodium Methoxide in Methanol

LEONG HUAT GAN AND ALBERT RICHARD NORRIS Depart~nent of Chemistry, Queen's Ut~iversity, Kingston, Otztnrio

Received June 20, 1973

Picrylchloride reacts rapidly with methoxide ion in methanol to form a o-complexcontainingmethoxide ion attached to the C-3 position of the picryl chloride. Thermodynamic and kinetic parameterscharac- terizing this reaction are presented and compared with corresponding parameters for the analogous reaction of methoxide ion and 2,4,6-trinitroanisole in methanol. Details concerning the much slower formation of the o-complex containing methoxide ion added to the C-l position of picryl chlorideare also presented.

Le chlorure de picryle rtagit rapidement avec I'ion mtthoxyde dans le methanol pour conduire a un complexe-o contenant l'ion mtthoxyde relit a la position C-3 du chlorure de picryle. O n presente les parametres thermodynamiques et cinetiques caracttrisant cette rtaction et on les compare avec ceux de la rtaction analogue de l'ion mithoxyde avec le trinitro-2,4,6 anisole dans le mtthanol. On presente aussi les details se rattachant a la formation beaucoup plus lente du complexe-o qui contient I'ion mithoxyde relit a la position C-1 du chlorure de picryle. [Traduit pa r le journal]

Can. J. Chem., 52, 18 (1974)

Introduction The substitution of chloride ion in picryl

chloride by hydroxide or alkoxide ion is postu- lated to involve the intermediate o-complex l(1). There are questions, however, concerning the relative stability of this o-complex and whether the initial color observed in these systems is due to 1 or the presence of small amounts of the isomeric o-complex 2 (2, 3).

Gaboriaud and Schaal (4) interpreted their kinetic results from a study of the reaction of picryl chloride with hydroxide ion in aqueous solution in terms of a rapid preequilibrium formation of 1 (R = H) followed by the slow release of chloride from 1 to yield picric acid. The color observed in solution was attributed to the presence of small amounts of 1 (R = H). One problem with this interpretation is that it requires the hydroxide ion to be a better leaving group than the chloride ion. This seems unlikely (1, 3).

We have investigated the reaction of picryl chloride with methoxide ion in methanol in an attempt to obtain more detailed information on the sequence of steps in the substitution process.

The results obtained implicate 2 rather than 1 (R = CH,) as the color producing species early in the reaction and shed some light on the validity of the reaction sequence proposed by Gaboriaud and Schaal for the picryl chloride - hydroxide ion reaction.

Experimental Reagents

Eastman 2,4,6-trinitrochlorobenzene (picryl chloride) was recrystallized twice from absolute ethanol and dried in uacuo over phosphorus(V) pentoxide, m.p., 83.0°C (lit. m.p., 83.0 "C (5)).

Analytical grade sodium chloride (Fisher Scientific) was recrystallized from water and dried to constant weight it1 uacuo a t 100 "C over silica gel.

Reagent grade methanol (Fisher Scientific) was dis- tilled under nitrogen from magnesium and iodine accord- ing to the described by Vogel (6). The middle portion of the distillate was collected and stored under a dry nitrogen atmosphere.

All other reagents used were A.R. grade or better. Stock solutions of picryl chloride (-lo-* M ) were

prepared by dissolving an accurately weighed amount o f the compound in 25.0 ml of methanol.

Stock solutions of sodiun~ methoxide were prepared by adding freshly cut sodium metal, prewashed in a separate portion of dry methanol, to pure methanol maintained in an atmosphere of dry nitrogen. Solutions thus prepared were standardized by titration with 0.1 NHCl to the phenolphthalein end point. The concentration of sodium methoxide in the stock solution was usually 0.05 t o 0.10 M. Exposure of the solutions to air was kept t o a minimum to prevent the absorption of water and carbon dioxide.

Instr~rmet~ts The instruments used to record absorption spectra

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GAN AND NORRIS: REACTION O F PICRYL CHLORIDE 19

and to carry out kinetic studies, using the stopped- flow technique, have been described previously (7).

An Orion1 solid state chloride ion selective electrode was used in conjunction with an Orion Ionanalyzer double-junction reference electrode and an Orion Model 801 digital pH/mV meter to determine the concentration of chloride ion in solution. A Moseley strip chart recordsr (Model 7100B) connected to the meter was used to record e.m.f. as a function of time.

Calibration of the chloride ion selective electrode with solutions of recrystallized sodium chloride in methanol showed that, at 3.7 "C, the cell e.m.f. could be related to

of picryl chloride in methanol was put in a thermostatted bath kept at 3.7 + 0.1 "C. The reference and chloride ion selective electrodes were immersed in the solution, which was stirred by means of a mechanical stirrer, and the system allowed to reach thermal equilibrium. A small volume (1-3 ml) of sodium methoxide solution pre- thermostatted a t 3.7 "C was quickly added and the cell e.m.f. recorded as a function of time on the strip chart recorder.

Results and Discussion the concentration of chloride ion in solution by eq. 1. When sodium rnethoxide in methanol (10-4 to [ l l E(mV) = constant - 53.0 log [CI-] 10-'M) was added to a dilute solution of

The concentration range covered in the calibration pro- picryl chloride M) in methanol a red

cedure was from to 12 in sodium color resulted. At any wavelength in the region chloride. 350-525 nm the absorbance of the solution was

Procedures Equilibrium and Kinetic Studies Using the

Durrum-Gibson Stopped-folv Spectrophotometer Solutions containing picryl chloride (approximately

M ) and sodium rnethoxide to lo-' M ) were prepared immediately prior to each run by dilution of known volumes of the corresponding stock solutions with distilled methanol. Before each run the drive syringes were each rinsed three times with the appropriate solu- tion, filled to capacity, and allowed to stand for 20 min in order to reach thermal equilibrium with the bath. In the course of the kinetic and equilibrium studies the concentration of picryl chloride generally was kept con- stant and therefore only the sodium methoxide solut~ons were changed. Each time this was done the drive syringe was flushed thoroughly with the new sodium rnethoxide solution before being filled for the kinetic run.

For the equilibrium studies the spectrophotometer was used in the conventional way to obtain values of absor- bance at infinite time (A,) associated with both the fast initial reaction and the slower secondary reaction. Values of A , associated with the second reaction were checked by running the reaction under the same experi- mental conditions and recording the A , value and the spectrum of the solution on the Cary 14; A , values agreed in all cases to within experimental error.

For most of the kinetic runs the normal mode of opera- tion of the spectrophotometer was modified, as described previously (7), so as to yield an oscilloscope trace of absorbance as a function of time rather than % transmit- tance as a function of time (8). Traces on the storage oscilloscope of absorbance us. time were recorded photo- graphically when at least two consecutive runs gave identical traces. In the case of the fast reaction an epidiascope was used to magnify the photograph so that absorbances at different times could be measured more accurately. For the slower second reaction absorbances at different times were read directly from the photo- graphs. All kinetic runs were carried out with sodium methoxide concentrations much greater than the con- centration of picryl chloride.

Kinetic Studies oj' Chloride Ion Release A 150-11-11 beaker containing about 80 ml of a solution

seen to increase rapidly after mixing of the two components, reach a maximum absorbance within 1 min and then remain constant for at least 20 min. A very slow decrease in absorbance was observed after that time; this fading reaction was more rapid at higher temperatures and at higher base concentrations. The species in solu- tion in the time interval between 1 and 20 min after mixing had absorption maxima and molar extinction coefficients identical to those reported for the 1,l-dimethoxy-2,4,6-trinitrocyclohexa- dienate anion (3) (9, 10)

NO2

3

When the interaction of picryl chloride (P) with sodium methoxide (in excess) was studied using the stopped-flow spectrophotometer, two distinct reaction traces were observed on the oscilloscope. The rates of the two reactions were sufficiently different (t,,, values of approximately 5 ms and 10 s, respectively, at 25.0°C) that separate studies of each reaction could be made.

For a series of solutions offixed picryl chloride (-3 x M) and varying sodium methoxide concentration (I x lo - , to 7 x lo-' M) values of A , at 480 nm were recorded from the absorbance us. time traces associated with the first reaction. If the reaction taking place in solution at this stage is represented by eq. 2

'Orion Research Incorporated, Cambridge, Mass. and the absorbance at 480 nm is due solely to

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20 CAN. J . CHEM. VOL. 5 2 , 1974

absorption by the 1 :I o-complex then plots of the experimental data in the form [P],/A, us. .I/[CH,O-1, should be linear with intercepts equal to I/E and slopes equal to ~ / K E (1 1). K represents the equilibrium constant for complex formation (K = k,/k-,), E represents the molar extinction coefficient of the complex at 480 nm and [PI, and [CH,O-1, represent the total con- centrations of picryl chloride and methoxide ion respectively. Plots of [P],/A, us. l/[CH,O-lo for this system were linear in all cases. However, because K associated with this reaction is small, and large ratios of [CH,O-],/[P], could not be used in the experimental runs, fairly large errors are associated with both the K and E determined in this way (12).

As a result E was chosen as 1.50 x lo4 M-I cm-' over the temperature range studied2 and K calculated for each run using the expression shown in eq. 3. The calculated K was constant

over a wide range of sodium methoxide con- centrations. This is further confirmation that the colored species has a 1 :1 stoichiometry. The equilibrium constants associated with the forma- tion of this l :I species were found to be l .53 f 0.03, 2.03 f 0.03, and 2.66 f 0.05 M-I at 7.5, 15.4, and 25.0 "C respectively. The error limits on K represent the standard deviations of the mean of at least 10 determinations.

The visible absorption spectrum of the species formed in the first reaction was obtained by measuring at different wavelengths in the 350 to 550 nm region A, values associated with the first reaction. Since picryl chloride absorbs to some extent below 450 nm, the absorption curve so obtained had to be corrected for residual absorption due to unreacted picryl chloride. This was done with the aid of separately determined values of E for picryl chloride and use of the known K for this reaction to calculate the con- centration of unreacted picryl chloride at equi- librium. A typical absorption curve obtained

'This choice of E was based on the mean values of E obtained from the [P]/A, os. l/[CH,O-]o plots. The value of E a t 480 nm for 3 is 1.63 x lo4 M-' cm-I (9). Molar extinction coefficients are generally found to be temperature independent over the temperature range studied (2).

FIG. 1. (i) Absorption curve (as measured from A , values) resulting from the fast reaction between picryl chloride and sodium methoxide in methanol at 10 "C: [picryl c h l ~ r i d e ] ~ = 3.97 x 10-4M; [NaOCH310 = 1.33 x

M ; 1 = 2.0 cm. (ii) Absorption curve due to un- reacted picryl chloride in solution. (iii) Absorption curve due to the species formed by methoxide ion addition a t the C-3 position of picryl chloride (G-complex 2).

from the A, values and the curve obtained after correction for absorption by picryl chloride are shown in Fig. 1. The positions of the absorption maxima (424 and 475 nm) were independent of temperature and of the concentration of sodium methoxide used in the experiment. While the absorption spectrum is characteristic of the 1 : 1 o-complex it is not diagnostic of C-1 or C-3 methoxide ion addition (2,3). Crampton et al. (13) have reported however that the C-3 adduct of methoxide ion and picryl chloride in methanol- dimethylsulfoxide has an absorption maximum at 424nm and a shoulder at 480nm. The similarity of the two absorption spectra strongly suggests that our fast reaction in methanol involves addition of methoxide at the C-3 posi- tion in picryl chloride to form 2 (R = CH,).

For the fast reaction traces of absorbance us. time gave rise to linear In (A, - A,) us. time plots to greater than 90% completion of reac- tion. For the reversible formation of 1 :1 o- complex according to eq. 2, the slope of the In (A, - A,) us. time plot (k,,,) is related to the specific rate constants k, and k - , and the con- centration of methoxide ion by the relation

Plots of k,,, us. [CH,O-1, were linear over the methoxide ion concentration range 5 x to 7 x 10-'M. Since k- , values for this reaction were large and the term k,[CH,O-1, small in comparison to k.. , in all cases the slopes of the k,,, us. [CH,O-1, plots were small. Conse-

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GAN AND NORRIS: REACTION OF PICRYL CHLORIDE 21

quently k-, was obtained fairly accurately from k2l

such a plot but k , (obtained from the slope) had [61 1 - 2,4,6-Trinitroanisole + C1- (fast)

a larger error limit associated with it. More reliable values of k , could be obtained from the with the step illvolving the formation of 1 being expression K = k,/k-, . Values of k, and k - , rate determining, then the rate of appearance of at 25.0°C and thermodynamic and kinetic chloride ion in solution can be expressed by the parameters associated with the first reaction are equation presented in Table 1 together with the corre- sponding data for the formation of 4, the C-3 C71 log {CPIO - Cc~-Itl =

complex of methoxide ion and 2,4,6-trinitro- anisole (14). Detailed comparisons strongly

support the conclusion, arrived at on the basis of spectral studies, that the 1 :1 a-complex first formed in this system is 2. The smaller AH' asso- ciated with the formation of 4 may be due in part to resonance stabilization involving the methoxy group (5a-56) as suggested by Bernasconi (14). A greater steric effect

associated with the methoxy group could also lead to increased solvation of 4, as compared to 2, and thus a smaller AH0 for the formation of 4. The more positive AS0 for formation of 2 is consistent with this possibility.

The second reaction trace observed on the stopped-flow spectrophotometer did not give rise to linear In ( A , - A,) us. time plots. In addition there was associated with this part of the reac- tion the appearance of chloride ion in solution.

The liberation of chloride ion was followed at 3.7 "C as described in the Experimental section. If one assumes the following sequence of reactions shown in eqs. 2, 5, and 6

For a series of runs with different [PI, and [CH30-1, the experimental traces of cell e.m.f. as a function of time were used in conjunction with eq. 1 to calculate values of log {[PI, - [CI-I,) as a function of time. Based on the slopes of the log {[PI, - [Cl-I,) us. time plots and a value of K of 1.41 M - ' at 3.7 "C, the average value of k, at 3.7 "C, for a series of runs in which [CH30-1, covered the range 3.6 x to 5.4 x M, was 3.7 0.3 M-'s- ' ; k , was independent of [PI, over the range 3.39 x M to 6.70 x M. The rapidity of chloride ion formation limited detailed studies to this low temperature. Studies at higher temperature indi- cated that, under typicai reaction conditions, >90% of the chloride ion was released within about 1 min of mixing of the reagents.

The final spectrum of the solution indicates that all the picryl chloride initially present has been converted to a-complex 3. If one assumes, as above, that reactions 2, 5, and 6 are occurring together with reaction 8 to form 3

k3 [8] 2,4,6-Trinitroanisole + CH30- + 3

k - 3

then

where B = [CH30-1. If negligible amounts of 1 and 4 are present in solution then 2 and 3 are the only species absorbing at 480 nm. The ab- sorbance due to 3 at any time t can be expressed as

KPoBo

ClOl At - 1 + KB, E2z

(A3)t = 1 - KBoE2

(1 + KB0)E3

Since reaction 8 goes to completion (K = 7700

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CAN. J . CHEM. '

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G A N A N D NORRIS: REACT1 ON OF PICRYL CHLORIDE 23

M-' at 25.0 "C (9)) [3],/[P], = (A,),/(A,), and eq. 9 can be written in the form

For each kinetic run 7 to 10 experimental absorbance readings (A,) covering from 25 to 75% completion of the reaction were taken to calculate (A,),. A nonlinear "least-squares" program was used together with the known (A,),, (A,),, K, and Bo values to fit eq. 11. Approximate values of k, and k, were supplied initially and the program executed to yield values of k, and k, which gave the least sum of squares of the residuals among these points. Typical calculated values of k, and k, over a range of sodium methoxide concentrations at 25.0 "C are given in Table 2. Mean values of k, and k, determined at different temperatures are given in Table 3.

From the data in Table 3 the value of k, at

TABLE 2. Specific rate constants k, and k3 for the interaction of picryl chloride with sodium methoxide

at 25.0 "C ---- --

[NaOCH3] x 102(M) k l (M-Is- ' ) k2 (M-Is- ')

TABLE 3. Average specific rate constants k2 and k3 for the interaction of

picryl chloride with sodium methoxide in methanol

--

T("C) k2 (M-Is- ') k, (M-I s-')

3.7 "C is calculated to be 3.90 M- ' s-'. This is in excellent agreement with the k, value of 3.7 + 0.3 M - ' s-' determined by following the rate of chloride ion release at 3.7 "C. The close- ness of the k, values determined by the two different methods offers support for the formu- lated mechanisric scheme (reactions 2, 5, 6, and 8) and the assumptions made in the derivations of eqs. 10 and 11. The enthalpy and entropy of activation associated with reaction 5 are 14.0 f 1.3 kcal mol-' and -0.7 f 4.4 cal deg-' mol-', respectively. Miller (15) has predicted a AH* of 12.9 kcal mol-' for this reaction.

A further indication that the formulated mechanistic scheme is correct comes from the observed value of k, which, based on the above method of analysis, is found to be 14.0 f 1.2 M- ' s- ' at 25.0°C. From a study of the direct reaction of niethoxide ion and 2,4,6-trinitro- anisole to form 3, Fendler et al. (9) reported a k, of 17.3 M- ' s-' at 25.0 "C, a AH* of 12.9 f 1.0 kcal mol-', and a AS* of -9.4 f 2.0 cal deg- ' mol- '. Our values of AH* and AS* for formation of 3 (reaction 8) were 10.7 & 1.5 kcal mol-' and - 12.8 + 3.6 cal deg-' mol-', re- spectively. The agreement is well within experi- mental error.

All the experimental results are consistent with the methoxide ion substitution of chloride ion in picryl chloride proceeding via reactions 2, 5, 6, and 8. The specific rate constants k, and k - , for formation and decomposition of the C-3 adducts 2 (R = CH,) and 4 are very similar, while the specific rate constants' for formation of the C-1 adducts 1 (R = CH,) and 3 differ by at least a factor of 2 at 25.0 "C. The similarity in the specific rate constants k, and k - , for the reactions of hydroxide ion with picryl chloride and 2,4,6-trinitroanisole in aqueous solution (4) strongly suggests that these constants are asso- ciated with the formation of 2 (R = H) and 6 respectively rather than 1 (R = H) and 7.

Financial support by the National Research Council of Canada is gratefully acknowledged.

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24 CAN. J . CHEM. VOL. 5 2 , 1974

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nary of organic compounds. 4th ed. Eyre and Spot- 12. D. A. DERANLEAU. J. Am. Chem. Soc. 91, 4050 tiswoode Ltd., London. 1965. (1969).

6. A. I. VOGEL. A textbook of practical organic chemis- 13. M. R. CRAMPTON, M. A. EL GHARIANI, and H. A. try. 3rd ed. Longmans, Green, London. 1968. p. 169. KHAN. Chem. Commun. 834 (1971).

7. L. H. GAN and A. E. NORRIS. Can. J . Chem. 52, 1 14. C. F. BERNASCONI. J . Am. Chem. Soc. 93, 6975 (1974). (1971).

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