The Interaction of Sulfite Ion with 2,4,6-Trinitrobenzaldehyde: Kinetic, Equilibrium, and Proton Magnetic Resonance Studies

NEVEN MARENDIC AND ALBERT RICHARD NORRIS Departr~zent of Cher,listr-y, Q~reerl's Urziver'sitj~, Kingstotl, 011tario

Received June 4 , 1973

Sulfite ion reacts with 2,4,6-trinitrobenzaldehyde in aqueous solution of ionic strength 0.14 M to yield a 1 :1 o-con~plex at low sulfite concentrations and a 2 : 1 o-complex at high sulfite concentrations. The visible absorption characteristics of these two o-complexes as well as equilibrium constants and relevant thermodynamic parameters for their formation have been determined. P.n~.r. studies demonstrate that two isomeric forms of the 1 : 1 o-complex occur in aqueous dimethylsulfoxide solution and two isomeric forms of the 2 : l o-complex occur in water. Iri both the 1 :1 and the 2 : l complexes isomers are found which contain sulfite ion attached to the C-1 carbon and one of the isomeric 2 : 1 o-complexes is postu- lated to exist in cis and trans forms. Observations are presented concerning the kinetics of the very rapid formation of the 1 :1 o-complex in water.

L'ion si~lfite reagit avec la trinitro-2,4,6 benzaldehyde en solution aqileilse de force ioniqi~e 0.14 M pour donner iln complexe I : 1 a des concentrations faibles de si~lfite et iln coniplexe 2 : 1 i des concentra- tions Clevees de sulfite. On a determine les caracteristiqi~es d'absorption de ces deux complexes ainsi qile les constantes d'kquilibres et les parametres thermodynaniiques-se rapportant a leur formation. Des etudes en r.m.p. ont montrt que deux fornies isomeriques du complexe 1 : 1 se forment en solution aqueuse de sulfoxyde de dimethyle et que deux fornies isomeriques du complexe 2 : l se forment dans I'eai~. On a trouvk dans les deux coniplexes I : 1 et 2 : 1 des isomtres qui contiennent l'ion sulfite relie ail carbone C-I. De meme on admet que I'un des con~plexes 2 : 1 isomeriques existe sous des formes cis et trans. On prksente aussi des observations se rattachant a la cinetique de formation tres rapide du complexe 1 : 1 dans I'eau. [Traduit par le journal]

Can. J. Chem., 51,3927 (1973)

Introduction In an earlier paper describing our observations

concerning the reaction of sulfite ion with 1,3,5- trinitrobenzene (1) we reported the visible absorption characteristics of the 1 : 1 o-complexes of sulfite ion with 2,4,6-trinitrotoluene and 2,4,6-trinitrobenzaldehyde. Since that time in- formation has become available concerning the visible absorption characteristics and p.m.r. spectra of a number of 1 : 1 and 2: 1 sulfite ion - 1-X-2,4,6-trinitrobenzene o-complexes (X = H, HO, CH,O, NH,, NHCH,, NH(C6H,), N(CH,),, and N(CH3)(C6H,)) (2-4), but no further data have been published concerning the sulfite ion - 2,4,6-trinitrobenzaldehyde reaction. In connection with kinetic studies of 1 : 1 com- plex formation in the reactions of sulfite ion with a number of 1-X-2,4,6-trinitrobenzenes (X = H, NH,, NHCH,, and N(CH,),), and as part of a study involving the o-complex formation reac- tions of 2,4,6-trinitrobenzaldehyde (5) we have investigated in more detail the sulfite ion- 2,4,6-trinitrobenzaldehyde reaction. The visible absorption characteristics of the 1 : 1 and 2: 1 o-complexes, equilibrium constants, and relevant

thermodynamic parameters for the formation of the 1 : I and 2: 1 o-complexes, the p.m.r. spectra of the 1 : 1 and 2: 1 o-complexes, and observa- tions regarding the kinetics of formation of the 1 : 1 o-complex in aqueous solution are now reported.

Experimental ( I ) Reagents

Sodium sulfite and potassium nitrate were both A. R. grade reagents (Fisher Scientific) and were used as received. 2,4,6-Trinitrobenzaldehyde (Aldrich Chemical Co.) was purified by carbon black treatment in benzene- ethanol solution, recrystallized several times from ethanol, and dried irz vaclro over phosphorus(V) pent- oxide, m.p. 119.0-1 19.5 "C. All other reagents used were A.R. grade or better.

Doubly distilled and deionized water was boiled for approximately 30 min to remove dissolved gases then allowed to cool to room temperature and stored under a nitrogen atmosphere. Absolute ethanol was purified by distillation from magnesium turnings in a n atmosphere of nitrogen.

A 2,4,6-trinitrobenzaldehyde stock solution (-2 x M) was prepared by dissolving a weighed amount

of dried compound in a known volume of ethanol. Solu- tions of 2,4,6-trinitrobenzaldehyde used in the kinetic and equilibrium studies were freshly prepared prior to. use by appropriate dilution (usually 1-100 ml) of a n aliquot of

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3928 CAN. J. CHEM. VOL. 51, 1973

the stock solution with 0.14 M aqueous potassium nitrate solution.

Sodium sulfite and potassium nitrate stock solutions were always prepared immediately before use. The com- position of the stock sodium sulfite solution was deter- mined each time by iodimetric titration (6). Sodium sul- fite solutions used in the kinetic and equilibrium studies were prepared by dilution of aliquots of the stock solution with 0.14 M aqueous potassium nitrate solution. The pH's of the stock solutions and the solutions used in the equilibrium and kinetic studies were checked with a pH meter and, if required, adjusted to a particular pH value by the addition of small amounts of 2 x M sodium hydroxide or hydrochloric acid. The amount of added acid or base was generally so small that neither the con- centration nor ionic strength of the solutions was affected by this procedure.

( 2 ) Procedlrres (a ) Equilibri~i~~~ Studies In most instances a I .O-cm cell containing I .50 ml of

2,4,6-trinitrobenzaldehyde test solution (approximately 2 x M ) at an ionic strength of 0.14 M (KNOB), a second cell containing 2-3 rnl of sodium sulphite solution of desired concentration and ionic strength 0.14 M (KNOB), and a syringe were placed in the thermostatted cell holder of the spectrophotometer and allowed to stand for 20 min in order to reach thermal equilibrium. A 1.50-ml sample of the sodium sulphite solution was then quickly transferred by syringe to the cell containing the 2,4,6-trinitrobenzaldehyde solution; the solution was thoroughly mixed, allowed to stand for approximately 2 min, and its spectrum recorded. When solutions of low sulfite ion concentration were required 10.0 ml of aqueous potassium nitrate solution was placed in a 5.0-cm cell and the cell and contents allowed to come to thermal equi- librium with the thermostatted cell compartment. Small volumes of the sodium sulfite and 2,4,6-trinitrobenzalde- hyde stock solutions were then added by means of microliter syringes, the solution was mixed thoroughly and its spectrum recorded. Changes in the concentrations of sodium sulfite and 2,4,6-trinitrobenzaldehyde as a result of dilution were taken into account where necessary.

( b ) Stopped-flow Kinetic Studies Solutions containing 2,4,6-trinitrobenzaldehyde (at

approximately 2 x M ) and sodium sulfite were prepared immediately prior to the kinetic run by dilution of known volumes of the corresponding stock solutions with 0.14 M aqueous potassium nitrate solution. Before each run the drive syringes were rinsed three times with the appropriate solution, filled to capacity, and allowed 20 min to reach thermal equilibrium with the bath. In the course of the kinetic measurements the concentration of 2,4,6-trinitrobenzaldehyde was generally kept constant and therefore, only the sodium sulfite solutions were changed. Each time this was done the drive syringe was flushed thoroughly with the new sodium sulfite solution before being filled for the kinetic run.

For the most part the kinetic runs were monitored by following the change in absorbance at 460 nm. In some runs absorbance changes at 500 and 580 nm were also recorded. Traces on the storage oscilloscope of absorb- ance us. time were recorded photographically when at

least two consecutive runs gave identical traces. The kinetic curves were then transferred from the photo- graphs to cm-graph paper and the values of absorbance as a function of time read from the graph paper used to construct plots of In (A, - A,) us. time; (A, and A, refer to absorbances at times infinity and t , respectively). The slopes of the In (A, - A,) us. time plots yielded pseudo first-order rate constants k,,,. As a check on this pro- cedure values of k,,, were also calculated using absorb- ance values read directly from the photographs. Values of k,,, obtained by using the two methods were, within experimental error, generally the same. Although more time-consuming the first method had the advantage of yielding accurate absorbance values at a larger number of times. As a result, the k.,, values calculated using this method generally had lower error limits associated with them.

( c ) Proton Magnetic Reso~zance Stltrlies Solutions in aqueous dimethylsulfoxide (30% v/v

water) were prepared by dissolving 2,4,6-trinitrobenzalde- hyde in dimethylsulfoxide and mixing this solution with the required amount of aqueous sodium sulfite solution. The concentration of 2,4,6-trinitrobenzaldehyde was kept constant (0.10-0.15 M) while the concentration of sodium sulfite was varied over the range 0.10-2.0 M.

Solutions in water were prepared either by dissolving solid 2,4,6-trinitrobenzaldehyde directly in an aqueous sodium sulfite solution or by dissolving 2,4,6-trinitro- benzaldehyde in carbon tetrachloride and shaking this solution with a concentrated aqueous solution of sodium sulfite. The recorded spectra were independent of the method of preparation of the sclutions. The concentra- tion of 2,4,6-trinitrobenzaldehyde was kept constant (0.10-0.12 M)and the sodii~m sulfite concentration varied over the range 0.10-1.2 M .

All chemical shifts are quoted relative to internal tetramethylsilane (TMS).

(3) I~lstr~mle~rts Optical measurements in the equilibrium studies were

carried out on a Cary 14 recording spectrophotometer using matched silica cells of 1 .O- and 5.0-cm path lengths. Temperature in the cells was maintained constant to + 0.1 "C by circulating water from a constant tempera- ture bath through a specially designed cell holder in the spectrophotometer.

Proton magnetic resonance spectra were recorded on a Bruker HX-60 spectron~eter employing the HDO peak in aqueous solution and the TMS peak in aqueous dimethyl- sulfoxide solution as the "internal lock". The probe tem- perature was generally about 20 "C.

Kinetic measurements were carried out on a Durrum- Gibson stopped-flow spectrophotometer. A closed circuit temperature control system utilizing an Ultra Kryomat TK30D constant temperature bath kept the temperature in the valve-block, drive syringes, and observation cell constant to within k0 .1 "C. Temperature-jump or con- ventional stopped-flow cells of 2.0-cm path length were used. Minimum "dead times" of the two cells were 10 and 2 ms, respectively. For most of the kinetic runs the normal mode of operations of the stopped-flow spectro- photometer was modified so as to yield an oscilloscope trace of absorbance as a function of time rather than percent transmission as a function of time.

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MARENDIC AND NORRIS: P.M.R. OF u-COMPLEXES 3929

TABLE I . Proton magnetic resonance data for 2,4,6-trinitrobenzaldehyde and the sulfite ion- 2,4,6-trinitrobenzaldehyde complexes formed in aqueous dimethylsulfoxide and

water. Relative intensities of the resonances are in parentheses

Chemical shifts ( P . P . ~ . )

Compound Solven ta Ha H. HD -

2,4,6-Trinitrobenzaldehyde A -0.52 (1) 0.83 (2) 2,4,6-Trinitrobenzaldehyde B -0.62 (1) 0.73 (2)

1 A -0.18 (1) 1 .72 (2) 2 A -0.38 (1) 1 .82 (1)

JFI.-II~ 1 .5 HZ 3 B -0.18 (1) - 4 B

cis -0.05 (1) 1.45 (1) lrarls

-- - 0.17 (1) 1.27 (1)

-. -

OSolvent designations: A, aqueous dimethylsulfoxide 30% (v/v water); B, water.

Measurements of p H were carried out with an EIL Model 23A pH meter which is capable of measuring the p H with an accuracy of f 0.02 p H units. Calibration of the p H meter with standard buffer solutions of pH 7.40 f 0.02 and pH 10.00 ? 0.02 at 25 "C was performed prior to each measurement.

Results and Discussion In aqueous dimethylsulfoxide (30% v/v water)

2,4,6-trinitrobenzaldehyde shows resonance ab- sorbance~ at .c -0.52 and 0.83 (both singlets, ratio 1 :2). The resonance absorbances charac- teristic of the complexes generated in this medium were found at .c -0.38 (singlet), -0.18 (singlet), 1.72 (singlet), 1.82 (doublet), and 4.08 p.p.m. (doublet). N o changes were observed in the number of resonance absorbances or in the relative intensities of the absorbances over a period of at least a day at room temperature.

The observed spectra are consistent with the solutions containing, in addition to free 2,4,6- trinitrobenzaldehyde, only the two isomeric 1 : 1 o-complexes 1 and 2. Assignments of the reso-

nance absorbances of the hydrogens in these two complexes, as given in Table 1, are based by analogy with the chemical shifts observed in other related 1 : 1 o-complexes. The assignment of the ring H, and Hp protons in 2 is consistent with the observed chemlcal shifts in the 1 : 1 sul-

fite ion complexes of 1,3,5-trinitrobenzene, 2,4,6-trinitroanisole, picramide, and N-methyl- and N,N-dimethylpicramide (2). The smaller upfield shift of the H, resonance in complex 1 as compared to the upfield shift of the Ha reso- nance in complex 2 has previously been observed in the corresponding complexes of 2,4,6-trinitro- anisole with both cyanide ion and methoxide ion (7-12). The observation that the aldehyde proton in 1 is upfield from the aldehyde proton in 2 is also consistent with the chemical shifts observed for the hydrogens of the methoxyl groups in the corresponding complexes (the C-1 and -3 complexes) of cyanide ion and methoxide ion with 2,4,6-trinitroanisole (7-12).

Over the range of sodium sulfite concentra- tions employed in the experiments the two 1 : 1 o-complexes 1 and 2 were present a t approxi- mately equal concentrations and no evidence could be found for the formation of any 2: 1 o-complexes. The increased stability of I : 1 as compared to 2: 1 sulfite ion - 1-X-2,4,6-trinitro- benzene o-complexes in aqueous solutions of high dimethylsulfoxide content has been attrib- uted to better solvation by dimethylsulfoxide than by water of the charge-delocalized 1: 1 o-complex (4, 13).

Complex 1 is the first 1 : 1 o-complex of sulfite ion with a I-X-2,4,6-trinitrobenzene (X other than H) in which addition has been shown to occur a t the C-1 position. In all previous adducts investigated only addition at C-3 (or -5) was observed.

In aqueous solutions a t low sulfite ion concen- trations p.m.r. spectra recorded immediately on mixing show, in addition to resonances attrib-

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3930 CAN. J. CHEM. VOL. 51, 1973

uted to the parent 2,4,6-trinitrobenzaldehyde, eight resonance absorbances located at T - 0.18, -0.05, 0.17, 1.27, 1.45, 3.95, 4.05, and 4.13 p.p.m. All the resonance absorbances appear to be singlets. In marked contrast to what is ob- served in the case of aqueous dimethylsulfoxide solutions the pattern of resonance absorbances observed in water undergoes fairly rapid changes with time. These changes are always more rapid at higher temperatures and/or higher sulfite ion concentrations. Intermediate spectra show ab- sorbance~ at T 0.35 (often, but not invariably) and at T 1.58 while the final spectra showed in all cases only a single resonance absorbance at T

1.58. The number and the positions of the initial

resonance absorbances observed could be con- sistent with solutions containing either a mixture of the complexes 2, 3, and 4 or a solution con- sisting of complex 3 and the cis and trans isomers of 4. In view of the large excess of sulfite ion to

2,4,6-trinitrobenzaldehyde normally present in solution, the large equilibrium constant asso- ciated with 2: 1 o-complex formation in aqueous solution (vide itEji.a) and the apparent insensi- tivity of the relative intensities of the various resonances attributable to the complexes to changes in sulfite ion concentration over the range 0.10-1.20 M, the experimental data seen more in accord with the latter possibility, i.e. under all conditions the solutions are thought to contain only 2: 1 o-complexes.

Assignments of the resonance absorbances of the hydrogens in complex 3 and the cis and trans isomers of 4 are given in Table 1.

For complex 3 the chemical shifts of the HI, protons are comparable to those found for the sulfite-containing 2 : 1 complexes of 1,3,5-trini- trobenzene, 2,4,6-trinitroanisole, picramide, and N-substituted picramides (2). No covalency change occurs at the C-1 position in complex 3 so the small upfield shift (AT = 0.44) of the aldehyde hydrogen in the complex relative to the aldehyde hydrogen in the parent 2,4,6-trinitro-

benzaldehyde must reflect primarily the increase in negative charge density in the C-l position of complex 3.

All the protons attributed to the cis and trans isomers of 4 show upfield shifts relative t o the resonance absorbances of the parent compound. The shifts are largest (AT = 3.32 and 3.22) for the HI, protons and smallest for the H, protons (AT = 0.54 and 0.72). The positions of the resonance absorbances of the H p protons of 4 in water are very close to those observed for the HI, protons of 2 in aqueous dimethylsulfoxide. The aldehyde hydrogens of the two isomers show larger upfield shifts (AT = 0.57 and 0.79) than that observed for the aldehyde hydrogen in complex 3 consistent with the fact that a covalency change occurs at the C-l position of both the cis and trans forms of 4 but not a t the C-1 position of 3. Though coupling between H, and Hp protons was observed for complex 2, coupling between the H, and H,, protons in the cis and trans forms of 4 could not be observed.' As a result definite assignments of the H,, H,, and Hp protons to the cis and trans forms of 4 is not possible, nor is it possible a t this stage t o say whether the cis or the trans isomer is the pre- ferred species at equilibrium. In any event the preference of the cis form over the trans form (or zjice versa) under the reaction conditions necessary to obtain the p.m.r. spectra is slight.

Even at the lowest concentrations of sulfite ion that could be used in order to obtain the p.m.r. spectra of the complexes formed in aqueous solution (-0.09 M) there was no evi- dence for the existence of 1 : 1 o-complexes such as 1 or 2. Crarnpton (2) has previously reported on the increased stability, relative to the 1 : 1 complexes, of 2 : 1 complexes of sulfite ion with 1,3,5-trinitrobenzene, 2,4,6-trinitroanisole, and picramide in aqueous solution. The presence of 2 : 1 but not 1 : 1 complexes suggests that the 2 : 1 complexes, which have far more localized charges than the 1 : 1 complexes, are considerably more strongly solvated by water than are the 1 : 1 complexes.

Complex 4 is the first 2: 1 o-complex of sulfite ion with a I-X-2,4,6-trinitrobenzene for which addition has been shown to occur at both the

'Coupling between H. and Hp protons can be resolved in the 1 : I sulfite ion - 1,3,5-trinitrobenzene complex but not in the 2: 1 sulfite ion - 1,3,5-trinitrobenzene com- plex (2).

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MARENDIC AND NORRIS: I

C-1 and the -3 (or -5) position. Previously reported 2: 1 a-complexes between sulfite ion and a variety of I-X-2,4,6-trinitrobenzenes all pos- sessed structures similar to that observed for complex 3 i.e. addition had occurred only at the C-3 and -5 positions (4, 13).

The failure to detect addition of sulfite ion at an X-substituted carbon in other 1-X-2,4,6-trinitro- benzene 1 : 1 and 2 : 1 a-complexes has been ascribed to two causes. One is steric strain effect associated with the addition of a sulfite ion at a carbon already bearing a large substituent X; the other is a specific solvation effect which arises because the sulfite ion in the complex still carries a formal negative charge and conse- quently must still be extensively solvated. Neither steric strain effects nor the solvation require- ments of the 1 : 1 a-complexes would be expected to be much different in complexes in which X = CHO as compared to those in which X = 0 - , OCH,, NHCH,, and N(CH,),. Other factors must also be important then in determining the point of addition of the sulfite ion. The electron withdrawing effect of the aldehyde group may be one such factor. Thus cyanide ion, which would be expected to have less stringent solva- tion requirements and give rise to much smaller steric strain effects than sulfite, reacts with 2,4,6-trinitrobenzaldehyde to yield only the C-1 complex even at low temperatures (5). It might be argued that, in the special case for which X = COH and the anion is sulfite ion, the elec- tron withdrawing effect of the aldehyde group together with better solvation and coplanarity with the ring of the para nitro group and the possible release of steric compression (4, 13) in the C-1 complex (1) relative to the parent com- pound might partially offset the more favorable steric and solvation requirements of the C-3 complex (2). As a result both isomeric com- plexes are observed.

If this argument is correct the relative pro- portions of the two isomeric 1 : 1 a-complexes 1 and 2 present in the system should be strongly solvent dependent and the existence of 1: 1 C-1 complexes should occur only in those cases in which X is strongly electron withdrawing and the anion is small. These ideas are currently being tested.

The nature of the species formed as a result of the decomposition of complexes 3 and 4 is not known. The resonance absorbances at T 1.58 could possibly be assigned to ring hydrogens in a

compound such as 5, but no effort was made to isolate such a species from the final reaction mixtures.

Aqueous solutions containing both 2,4,6- trinitrobenzaldehyde (-2 x lop5 M) and so- dium sulfite (10-4-10-2 M) in the p H range 7.50-10.0 are red in color though neither com- ponent alone absorbs in the region 370-700 nm under these conditions.

Studies of the visible absorption spectra of solutions containing a fixed concentration of 2,4,6-trinitrobenzaldehyde (typically -2 x M) and a variable concentration of sodium sul- fite indicate the existence of two equilibria involving 2,4,6-trinitrobenzaldehyde and sulfite ion. The first is dominant at sulfite concentra- tions below 1.3 x M while the second becomes apparent at sulfite concentrations above 1.3 x M and is complete in solu- tions containing from 0.60-0.80 M sodium sul- fite. Further changes in the visible absorption spectra are not observed as the concentration of sodium sulfite is increased above 0.80 M. Typical absorption curves are shown in Fig. 1. Sharp isosbestic points are observed at 480 and 527 nm at all temperatures over the range 0-50 "C.

The absorption characteristics of the species formed at sulfite concentrations < 1.3 x M (A,,, 458 nm, shoulder mid point a t 540 nm) are similar to those reported for the 1 : 1 a-com- plexes of sulfite ion with 1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene, 2,4,6-trinitroanisole, and picramide (1, 2, 14). The absorption charac- teristics of the species formed at high sulfite ion concentrations (A,,, 500 nm) are similar to those reported for a number of 2 : 1 sulfite ion com- plexes of 1-X-2,4,6-trinitrobenzenes, X = H, CH,O, NH,, NHCH,, and N(CH,), (2). The positions of the absorption maxima are indepen- dent of temperature over the range 0-50 "C.

The above observations suggest that the two equilibria in solution are associated with the reversible formation of 1 : 1 and 2 : 1 sulfite

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3932 CAN. J . CHEM. VOL. 51, 1973

o . O d l ' ' ' ~ ' ' ' ' L 400 5 0 0 6 0 0

W a v e l e n g t h (nrn)

W a v e l e n g t h ( n m )

FIG. 1. Absorption spectra of 2,4,6-trinitrobenzalde- hyde (1.0 x M ) in water containing the following molarities of sodium sulfite: (1) 0.0001; (2) 0.003; (3) 0.025; (4) 0.05; (5) 0.10; (6) 0.60. T, 24.0°C. Ionic strength for all solutions but (6) 0.14 M (KN03).

ion - 2,4,6-trinitrobenzaldehyde o-complexes. Schematically the processes can be formulated as

[I] 2,4,6-Trinitrobenzaldehyde + C,

and

[21 (21 + so3'- $ c2 where K, and K2 are the equilibrium constants associated with the formation of the 1 : 1 and 2: 1 complexes, respectively.

The 1 : 1 stoichiometry of the 2,4,6-trinitro- benzaldehyde - sulfite ion complex formed a t fixed 2,4,6-trinitrobenzaldehyde concentrations (- 1 x M) and concentrations of sulfite ion in the range 1.0 x to 1.3 x M was established through the use of the graphical test method reported by Coleman, Varga, and Mastin (15) and by the use of the method of con- tinuous variations (16).

Absorbance data for solutions of fixed initial concentrations of 2,4,6-trinitrobenzaldehyde

([A,]) and sodium sulfite ion concentrations 1.0 x to 1.30 x M, when plotted in the form [A,]/absorbance us. l/[Bo] (where [B,] is the initial sodium sulfite concentration), gave excellent straight lines from whose slopes and intercept values of the equilibrium con- stant for 1 : 1 o-complex formation (K,) and molar extinction coefficients ( E , ) of the complex a t the wavelength at which absorbances were recorded were calculated (Benesi-Hildebrand (B-H) method (17)). Values of K, and E , deter- mined at a number of different temperatures and wavelengths are tabulated in Table 2. The excellent linearity of the B-H plots, the non- dependence of K, on wavelength, and the con- stancy of E , as a function of temperature offer further evidence that only a single 1 : 1 o-com- plex is being studied under the quoted reaction conditions. The results of the p.m.r. studies suggest that the 1 : 1 complex could be either 1 or 2. Unfortunately, the visible absorption data cannot be used t o differentiate between these two possibilities (4, 13, 14).

At any wavelength the niolar extinction coefficient of the 2: 1 o-complex formed between 2,4,6-trinitrobenzaldehyde and sulfite ion could be found directly since conversion of 2,4,6- trinitrobenzaldehyde to the 2 : 1 complex was conlplete at high sulfite concentrations. Em- ploying molar extinction coefficients (&,) found in this manner and known K, and E , values the concentration of the 2: 1 complex and the equi- librium constant for formation of the 2: 1 com- plex could be calculated for solutions of different initial concentrations of sodium sulfite and 2,4,6-trinitrobenzaldehyde. Values of E, and K, obtained in this way are tabulated in Table 3. The K, values are independent of the wavelength a t which absorbance data are recorded while the values of E ~ , like the values o f E , , are, within experimental error, temperature independent.

Mean values of the equilibrium constants K, and K2 and mean molar extinction coefficients E,

and E~ at the wavelengths of maximum absorp- tion (458 and 500 nm, respectively) at different temperatures are given in Table 4.

Enthalpies of formation (AH0) and entropies of formation (AS0) associated with both 1 : 1 and 2 : 1 o-complex formation were obtained from plots of log K, and log K, us. 1/T. Relevant values are -2.41 + 0.16 kcal mol-' and 1 1.36 + 1.05 cal deg- ' mol-' and - 8.53 + 0.10 kcal mol-I and -21.80 f 2.36 cal deg-' mol- ' for

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MARENDIC AND NORRIS: P.M.R. O F u-COMPLEXES 3933

TABLE 2. Equilibriuni constants for 1 : I complex formation and molar extinction coefficients of the 1 : I complex determined at different temperatures and wavelengths. Ionic strength 0.14 M (KNO,)

-- - --- -

K, x (M- ' ) at wavelength (nm)' x (M- ' cm-') at wavelength (nm)'

4.0 2.52k0.12 2.50k0.12 2.46k0.14 2.18k0.03 1.13k0.02 0.86k0.02 10.0 2.31k0.10 2.25k0.12 2.22k0.13 2.17k0.02 1.13k0.02 0.84k0.01 16.7 2.12+0.10 1.98k0.12 1.99k0.12 2.19+0.03 1.14k0.02 0.86k0.01 24.0 1.84k0.09 1.82k0.11 1.85k0.15 2.20k0.02 1.16k0.01 0.87+0.02

QError limits on K and E are based on the error ltmits assoc~ated with the "least squares" slopes and intercepts or the Benesi-Hilde- brand plots.

TABLE 3. Equilibriuni constants for 2 : l complex formation and molar extinction coefficients of the 2:1 complex determined at different temperatures

and wavelengths. Ionic strength 0.14 M - - - .. .. . . -. - - pp -- -. - . -- . -

K, (M-I ) at wavelength E, x (M- ' cni-I) at (nm) wavelength (nm)

T ("C) 458 500 458 500

TABLE 4. Mean eqi~ilibrium constants (K,, K,) and molar extinction coefficients (E,, E?) at different temperatures for the TNBA - sulphite ion system in water. Ionic

strength 0.14 M (KNO,)

the A H 0 and AS0 of formation of the 1 : 1 and 2: 1 complexes respectively. The errors i11 A H 0 and AS0 are based on a "least squares" treatment of the data in the log K us. 1/T plots.

The values of AH0 and AS0 for formation of the 1 : 1 sulfite ion - 2,4,6-trinitrobenzaldehyde o-complex differ from the A H 0 and AS0 values reported for formation of the 1 : 1 sulfite ion - 1,3,5-trinitrobenzene o-complex. The lack of published thermodynamic data for the formation of other 1 : 1 and 2: 1 sulfite ion - 1-X-2,4,6- trinitrobenzene complexes precludes any detailed analysis of the thermodynamic parameters at

this time.' The large values of AH0 and AS0 associated with the formation of the 2: 1 o-com- plex may reflect the better solvation by water of the charge-localized 2 : 1 complex than the charge-delocalized 1 : 1 complex.

Stopped-flow techniques were employed to study the kinetics of the sulfite ion - 2,4,6-trini- trobenzaldehyde. These studies, which were

2Kinetic and thermodynamic studies of the formation of the 1 :1 sulfite ion complexes of picramide, N-methyl- picramide, and N,N-dimethylpicramide have been com- pleted . The results of these studies have been submitted for publication (A.R. Norris and P. J. Sheridan).

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3934 CAN. J . CHEM. VOL. 51, 1973

carried out over a range of sodium sulfite (2.6 x lo-' to 1.12 x M) and 2,4,6-trinitroben- zaldehyde (9.7 x to 2.0 x M) con- centrations, at different temperatures (10.0 to 35.0 "C) and in solutions of varying p H (7.50- 10.00), indicated there were always two reactions taking place in solution. The first was a very fast reaction (t,,, < 5 ms) which, under all the experimental conditions we were able to employ, was complete in too short a time to allow for any detailed study of its rate dependence on various reaction parameters. The absorption spectrum of the species formed at the completion of this first reaction could be recorded however, and was identical in all respects to that of a 1 : 1 sul- fite ion - 2,4,6-trinitrobenzaldehyde complex. The second reaction which was characterized by values of t+ in the range 50-100 ms could be more extensively studied. At all reaction tem- peratures the rate of this reaction was found to be independent of the initial concentration of 2,4,6-trinitrobenzaldehyde, independent of p H over the range 7.5-10.03 and linearly dependent on the initial concentration of sulfite ion at low concentrations of sodium sulfite, but indepen- dent of sulfite concentration at high sodium sulfite concentrations. The absorption spectrum of the species present at the termination of this reaction was also characteristic of 1 : 1 sulfite ion - 2,4,6-trinitrobenzaldehyde complex.

The kinetic observations seem compatible with the following scheme

k 1

[3 1 TNBA + S03Z- + A k- 1

kz [41 TNBA + SO3'- 6 B

k-2

where the first equilibrium is established much more rapidly than the second, but in which

,Separate studies were carried out to ensure that under the experimental conditions used in the kinetic and equilibrium experiments there were no complications due to the interaction of hydroxide ion and 2,4,6-trinitro- benzaldehyde. These studies indicated that over the tem- perature range 0-50 "C the formation of the hydroxide ion - 2,4,6-trinitrobenzaldehyde complex was only im- portant at p H values greater than 10. Relevant ther- modynamic, spectroscopic, and kinetic data associated with the 1 : 1 hydroxide ion - 2,4,6-trinitrobenzaldehyde o-complex are K = 5.0 x lo3 M - I at 25.0 "C (based on equilibrium studies), &430 = 2.0 + 0.1 x lo4 M - l cm-', E~~~ = 1.3 + 0.1 x lo4 M - l cm-I, k l = 5.2 x 10' M - I s- ' and k - l = 0.1 s-I . All values are quoted for solutions of ionic strength 0.14 M (KNO,).

k,/k-, << k,/k-,. A and B are formulated as isomeric 1 : 1 o-complexes differing only in the position of addition of the sulfite ion. The formation of 1 : 1 o-complexes of methoxide ion with 2,4,6-trinitroanisole has been shown to follow the above reaction scheme with the C-3 adduct being the kinetically favored species and the C-l adduct being the thermodynamically favored species (18). While p.m.r. studies of the sulfite ion - 2,4,6-trinitrobenzaldehyde reaction in aqueous solution have to be carried out under conditions such that only the isomeric 2: 1 o-complexes are formed, the isomeric 1: 1 o-complexes 1 and 2 were observed as stable entities in aqueous dimethylsulfoxide solution and may be presumed to exist as well in aqueous solution under appropriate conditions.

The lack of accurate rate constants for the second reaction over a wide range of low sulfite ion concentrations renders a detailed analysis of kinetic data in terms of the proposed scheme unwarranted at this time. The data that are available suggest k, has a value (at 25') in the range 9 x lo4 M-' s-'. This rate constant is very close to that reported by Bernasconi and Bergstrom (19) for the formation of the 1 : 1 sulfite ion - 1,3,5-trinitrobenzene complex and is in the range of the specific rate constants ob- served for 1 : 1 o-complex formation between sulfite ion and picramide, N-methylpicramide, and N,N-dimethylpicramide.4 For each of these four reactions sulfite ion attacks a t a hydrogen-bearing ring carbon. Studies using the T-jump technique are in progress in an attempt to obtain more extensive and more reliable kinetic data for both the fast and very fast reactions.

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

1. A. R. NORRIS. Can. J . Chem. 45, 175 (1967). 2. M. R. CRAMPTON. J. Chem. Soc. (B), 1341 (1967). 3. M. R. CRAMPTON and M. EL GHARINI. J. Chem. Soc.

(B), 330 (1969). 4. M. J. STRAUSS. Chem. Rev. 70, 667 (1970). 5. E. BUNCEL, A. R. NORRIS, and W. PROUDLOCK. Can.

J. Chem. 46, 2759 (1968). 6. A. I. VOGEL. A textbook of quantitative inorganic

analysis. Longmans, London. 1968. p. 343. 7. A. R. NORRIS. Can. J. Chem. 47, 2895 (1969). 8. M. R. CRAMPTON and V. GOLD. J. Chem. Soc. 3293

( 1964).

4A. R. Norris and P. Sheridan. T o be published.

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MARENDIC AND NORRIS: P.M.R. O F u-COMPLEXES 3935

9. J . H. FENDLER, E. J. FENDLER, and C. E. GRIFFIN. J. Org. Chem. 34, 689 (1969).

10. M. R. CRAMPTON and V. GOLD. J. Chem. Soc. (B), 893 (1966).

1 I. K. C. SERVIS. J. Am. Chem. Soc. 89, 1508 (1967). 12. R. FOSTER and C. A. FYFE. Tetrahedron, 21, 3363

(1965). 13. M. R. CRAMPTON. Adv. Phvs. Ore. Chem. 7. 211 -

(1969). 14. E. BUNCEL, A. R. N O R R I S , ~ ~ ~ K. E. RUSSELL. Quart.

Rev. London, 22, 123 (1968).

15. J. S. COLEMAN, L. P. VARGA, and H. MASTIN. Inorg. Chem. 9, 1015 (1970).

16. P. JOB. C.R. 180, 928 (1925). 17. H. BENESI and J. HILDEBRAND. J. Am. Chem. Soc.

71, 2703 (1949). 18. C. F. BERNASCONI. J . Am. Chem. Soc. 93, 6975

(1971). 19. C. F. BERNASCONI and R. G. BERGSTROM. J. Am.

Chem. Soc. 95, 3603 (1973).

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