Kinetic and Equilibrium Studies of the Reactions of Cyanide Ion with 1,3,5-Trinitrobenzene, 2,4,6-Trinitroanisole, and 2,4,6-Trinitrotoluene

in Isopropanol

LEONG HUAT GAN AND ALBERT RICHARD NORRIS Department of Chernistry, Queen's University, Kingston, Ontario

Received July 10, 1973

Equilibrium constants for the formation of 1 : 1 cyanide ion o-complexes with 1,3,5-trinitrobenzene, 2,4,6-trinitroanisole, and 2,4,6-trinitrotoluene have been determined spectrophotometrically over a range of temperatures. Standard enthalpy ( A H 0 ) and entropy (AS0) changes associated with each reaction have been evaluated. The kinetics of formation of the o-complexes have been investigated by means of a stopped-flow technique and the activation parameters characterizing the formation of each complex have been determined. Evidence is presented which indicates the cyanide ion - 2,4,6-trinitroanisole o-complex formed in isopropanol contains the cyanide ion bonded exclusively at the C-3 position.

Les constantes dlCquilibres pour la formation d'un complexe-o 1 : 1 ion cyanure - trinitro-1,3,5 ben- zene, trinitro-2,4,6 anisole et trinitro-2,4,6 toluene ont t t t dtterminies dans un intervalle d e temptratures a I'aide de la spectrophotornCtrie. Les changements d'enthalpie standard (AH0) et d'entropie (AS0) asso- ciCs a chaque reaction ont Ctt Cvalues. La cinktique de formation des complexes-o a 6t6 t tudite a I'aide de la technique a dtbit interrompu et les parametres d'activation se rattachant a la formation de chaque complexe ont ete dtterminks. On apporte des preuves pour montrer que le complexe-cr ion cyanure - trinitro-2,4,6 anisole form6 dans I'isopropanol contient I'ion cyanure lie exclusivement a la position C-3.

[Traduit par le journal] Can. J. Chem., 52,8 (1974)

Introduction CH30 X 0CH3 \ / I

Cyanide ion is known to react with 1,3,5- o~N--$I-Noz 0 2 ~ 9 ..I- LO H trinitrobenzene and a number of related I-X- x

2,4,6-trinitrobenzenes (X = CH,, OCH,, and NO2 NO2 CHO) in acetone, chloroform, and various 3 - 4 alcohols to form o-complexes ,of 1 : 1 stoichiom- etry ( I 6 ) . For = CHO, of 'yanide but the thermodynamically more stable o-com- ion occurs exclusively at the C-1 position to plex in this system is (X = 0CH3) The yield 1 while with 2,4,6-trinitrotoluene, addition influence of the nature of the solvent and the seems to occur exclusively at the C-3 position t o properties of the group on the relative stabili- yield o-complex 2 (2). With 2,4,6-trinitroanisole ties of isomeric : o-complexes of this type is

both the C-1 and the C-3 adducts 3 (X = CN) and 4 (X = CN) are formed in deuterochloro- form (3). Studies over a range of temperatures suggest that 4 (X = CN) is the thermodynam- ically more stable species (3). In the reactions of 2,4,6-trinitroanisole with methoxide ion in methanol both the C-1 and C-3 adducts 3 (X = OCH,) and 4 (X = OCH,) are also formed

still largely unknown. As part of a series of kinetic studies of the

reactions of bases and nitroaromatic compounds (12, 13), and as an extension of previously reported equilibrium studies of the reactions of cyanide with 1,3,5-trinitrobenzene in a number of alcohols (14), we have investigated the kinetics of o-complex formation between cyanide ion and 1,3,5-trinitrobenzene, 2,4,6-trinitroanisole, and 2,4,6-trinitrotoluene in isopropanol.

Evidence is presented to show that the only o-complex formed between cyanide ion and 2,4,6-trinitroanisole in isopropanol is the C-3 adduct 4 (X = CN). Specific rate constants and activation parameters for the formation of 2, 4 (X = CN), and 5 (X = CN) have been deter-

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

CAN AND NORRIS: REAC TlONS O F CYANIDE ION 9

mined together with the standard enthalpy (AH') and entropy (AS0) changes associated with their formation.

Experimental Reagents

Eastman 1,3,5-trinitrobenzene was recrystallized twice from an ethanol-water mixture and dried to constant weight il: vacuo over phosphorus(V) pentoxide, m.p. 122 "C, (lit. m.p. 122.5 "C (15)).

Eastman 2,4,6-trinitrotoluene was recrystallized twice from absolute ethanol and dried in vacuo over phos- phorus(V) pentoxide, m.p. 89-82 "C, (lit. m.p. 82 "C (1 5)).

Eastman 2,4,6-trinitroanisole was recrystallized twice from carbon tetrachloride and dried in vacuo over phos- phorus(V) pentoxide, m.p. 68-69 "C, (lit. m.p. 68 "C (15)).

Eastman tetraphenylarsonium chloride was dried to constant weight in vacrto over phosphorus(V) pentoxide.

Tetraphenylarsonium cyanide was prepared as pre- viously described (14) and dried to constant weight in vacuo over phosphorus(V) pentoxide. Tetraphenylar- sonium nitrite was prepared in a similar fashion but using sodium nitrite in place of sodium cyanide in both the initial preparation and the recrystallization pro- cedures. After two recrystallizations the compound was dried to constant weight in vacuo over phosphorus(V) pentoxide.

All other reagents used in the study were A.R. grade or better.

Isopropanol was dried by refluxing over Linde 4A Molecular Sieve overnight and then distilled under a dry nitrogen atmosphere. Only the middle portion of the distillate was retained.

Stock solutions of the nitroaromatic compounds and the tetraphenylarsonium salts were prepared by dis- solving weighed quantities of each reagent in a known volume of isopropanol. Solutions of lower concentra- tions used in the kinetic and equilibrium studies were prepared by dilution.

Instruments The instruments used to record absorption spectra and

to carry out kinetic studies using the stopped-flow technique have been described previously (16). For vir- tually all of the kinetic runs the normal mode of opera- tion of the stopped-flow spectrophotometer was modified, as described previously (16), so as to yield an oscilloscope trace of absorbance as a function of time rather than % transmittance as a function of time (17).

Procedures (a ) Determination of Molar Extinction Coefficients (E) An empty 0.10-cm cell and a 1.0-cm cell filled with

2-3 ml of a concentrated solution of one of the reactants were placed in the sample cell compartment for thermo- statting (about 20 min was required). A small volume

(usually less than 0.20 ml) of solution containing the second reactant was then introduced by means of a Micrometer syringe1 into the contents of the 1.0-cm cell. The resulting solution was quickly mixed, a portion of it transferred to the 0.10-cm cell and the absorbances of the solution recorded at a number of wavelengths. A matched 0.10-cm cell filled with isopropanol was used as a refer- ence.

In all the systems the basic reagent was present in excess in order to minimize any neutralization effects due to absorbed carbon dioxide. At the highest concentra- tions of cyanide ion employed a slow decrease in absorb- ance was sometimes found to occur after the maximum absorbance had been reached. In these cases the absorb- ance was corrected by extrapolating the absorbance us. tlme curve back to the time of mixing. The correction in most cases was small; less than 4% ',f the total absorb- ance.

Because of the high equilibrium constants associated with these reactions greater than 99% of t he nitroaro- matic compound was converted to complex under the reaction conditions employed. The molar extinction coefficient of the complex (E) was therefore evaluated from the slope of the plot of absorbance us. initial con- centration of nitroaromatic compound. For all the sys- tems studied molar extinction coefficients a t the absorp- tion maxima were found t o be temperature independent over the temperature range studied (5-50 "C).

(b ) Determination of Eq~tilibririrr~ Constants ( K ) The experimental procedure was identical t o that used

in determining the molar extinction coefficients except that all solutions contained nearly equimolar amounts of tetraphenylarsonium cyanide and nitroaromatic com- oound. Eauilibrium constants were calculated using the - expression shown in eq. 1 where A is the measured

absorbance at equilibrium, E is the molar extinction coefficient of the com~lex a t the wavelength a t which the absorbance was measired, I is the path length of the cell (0.1 cm), and [CN-lo and [Nl0 represent t he total con- centrations of cyanide ion and nitroaromatic compound in solution.

( c ) Determination of Psertdo First-order Rate Consta~tts (kOD,)

Solutions of the nitroaromatic compound in isopro- panol M) and tetraphenylarsonium cyanide in isopropanol (10-4-10-2 M ) were prepared immediately prior to each run by dilution of known volumes of the corresponding stock solutions with isopropanol. The drive syringes were filled and traces of absorbance ( A ) us. time and kOb, obtained as described previously (16).

The reactions of cyanide ion with I ,3,5-trinitrobenzene, 2,4,6-trinitroanisole, and 2,4,6-trinitrotoluene were mon- itored by following changes in absorbance a t 540, 518, and 535 nm, respectively.

( d ) Determination of .Activity Coefficients ot 25.0 "C The activity coefficients of 1,3,5-trinitrobenzene and

2,4,6-trinitrotoluene in isopropanol solutions containing

'Roger Gilmont Instruments, Inc., Great Neck, N.Y.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

10 CAN. J . CHEM. VOL. 52, 1974

TABLE 1. Spectral properties of some 1 : 1 cyanide ion o-complexes in isopropanol at 25.0 "C

Wavelengths Molar extinction coefficients of maximum at the absorption maximaa

absorption (nm) x (M-I cm-I 1 Nitroaromatic

compound XI A2 E I E2

"Error limits are standard errors of the mean based on at least four determinations.

tetraphenylarsonium chloride were determined by a solubility method (18, 19).

Volumetric flasks (25 ml) containing solid 1,3,5-trini- trobenzene or 2,4,6-trinitrotoluene and a known amount of tetraphenylarsonium chloride in isopropanol were attached to a shaker. The shaker was immersed in a water bath thermostatted at 25.0°C and the flasks shaken vigorously for 3 days. The saturated solution was then decanted into a test tube and centrifuged. A portion of

I

I the clear supernatant solution was withdrawn by means of a 2.0-ml Micrometer syringe' and diluted to the desired concentration for absorbance measurements on the Cary

I 14. T o ensure the solution was saturated the measurement for each solution was repeated after 24 h. Thz amount of

I 1,3,5-trinitrobenzene or 2,4,6-trinitrotoluene in solution was determined from its absorbances at 300 and 310 nm.

Attempts to determine the solubility of 2,4,6-trini- 1 troanisole in a similar fashion were unsuccessful because of the very slow precipitation of tetraphenylarsonium

1 picrate from the solution.

I Results and Discussion 1 When isopropanol solutions containing tetra-

phenylarsoiium cyanide and 1,3,5-trinitroben- zene (TNB), 2,4,6-trinitroanisole (TNA), or 2,4,6-trinitrotoluene (TNT) were mixed, deep red-colored solutions resulted. The visible absorption spectra of the solutions suggested that the colored species present were 1 : 1 cyanide ion o-complexes of the 1-X-2,4,6-trinitroben- zenes (X = H, OCH,, CH,) (1-4). The results of the equilibrium and kinetic studies were fully consistent with this formulation for the stoichiometrv of the colored s~ecies. The wave- lengths of maximum absorption and the molar extinction coefficients at the absorption maxima of the 1: 1 cyanide ion o-complexes of 1,3,5- trinitrobenzene, 2,4,6-trinitroanisole, and 2,4,6- trinitrotoluene in isopropanol at 25.0°C are given in Table 1. The visible absorption curves of the three complexes are shown in Fig. 1.

An equilibrium constant of 10.0 + 0.3 x lo3 M-' has been reported for the formation of the cyanide ion - 1,3,5-trinitrobenzene o-complex

FIG. 1. Visible absorption spectra in isopropanol a t 25.0 "C of 1 : 1 cyanide ion o-complexes of (i) 1,3,5-trini- trobenzene (-.-.), (ii) 2,4,6-trinitrotoluene (-), and (iii) 2,4,6-trinitroanisole (-..-).

at 25.0°C (14). The same authors report an enthalpy of reaction based on calorimetric mea- surements (AH,,,), of -7. l k 0.6 kcal mol- ' and a AS' of 5.5 f 3.0 cal deg-' mol-'. Our studies have yielded an average equilibrium constant for formation of 5 (X = CN) at 25.0 "C of5.1 f 0.1 x 104M-' . Employing the reported AH,,, of -7.1 + 0.6 kcal mol-' we obtain a AS0 of 2.5 + 3.0 cal deg-' mol-'. There is no obvious reason why the equilibrium constants should differ. Fading reactions and complica- tions due to alcoholysis of the cyanide ion are not problems in this system (1 6). Perhaps at the high ratio of concentration of cyanide ion t o concentration of 1,3,5-trinitrobenzene used in earlier studies some degree of 2: 1 ' o-complex formation resulted. This complication would be reflected in lower values of E , and E,. We note though that our E, value (Table 1) is not signi- ficantly greater than the value of E, previously reported (1 4).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

GAN AND NORRIS: REAC

The equilibrium constants for the formation of the cyanide ion - 2,4,6-trinitroanisole o-com- plex were found to be 1.24 f 0.03 x lo4, 1.18 0.04 x lo4, and 1.12 + 0.05 x lo4 M-' at 4.6, 15.0, and 25.0 "C, respectively.' Based on the variation of the equilibrium constants with temperature AH0 was calculated to be -0.81 + 0.55 kcal mol-'. AS0 for the reaction was 15.8 + 2.0 cal deg-' mol-'.

The equilibrium constants for formation of the cyanide ion - 2,4,6-trinitrotoluene o-complex were determined to be 3.1 1 f 0.16 x lo4, 2.01 + 0.09 x lo4, and 1.50 + 0.01 x lo4 M-' at 9.1, 25.0, and 34.9 "C. The resulting AH' and AS0 associated with the complex formation reaction were calculated to be -4.90 +_ 0.30 kcal mol-' and 3.4 + 1.1 cal deg-' mol-', respectively.

Equilibrium constants for 1 : 1 o-complex formation with cyanide ion at 25.0 "C increase in the order TNA < TNT < TNB.

The variation in K is, however, very small. A far greater variation in K for complex formation is found for the sulfite ion complexes of TNA, TNB, and TNT in aqueous solution where the order of the equilibrium constants is T N T < TNA < TNB and the maximum K is a factor of about 40 times larger than the minimum K (20, 21). For the formation of ethoxide, n-prop- oxide, and isopropoxide o-complexes of TNB and TNT in the corresponding alcohols, the equilibrium constants for complex formation with TNB are uniformly a factor of about 10' times larger than the equilibrium constants for o-complex formation with TNT (22). For the methoxide ion complexes of TNB and TNA (4, X = OCH,) in methanol at 25.0°C, the equilibrium constants favor the formation of the TNB complex by a factor of approximately 6 (12, 23).

For the three complex formation reactions studied more negative AH0 values are accom- panied by more negative AS0 values and a plot of AH0 us. AS0 is reasonably linear. The least negative values of both AH0 and AS0 are asso- ciated with the formation of the cyanide ion - TNA o-complex. It has been argued that for the reaction of sulfite ion with a number of 1-X-2,4,6-trinitrobenzenes (X = CHO, NH,, NHCH,, and N(CH,),) decreasing AH0 and

ZErrors in K are standard deviations of the mean based on at least four determinations.

:TIONS O F CYANIDE ION 1 1

AS0 reflect decreasing solvation of the resulting 1 : 1 o-complex (24). By analogy we might expect that of the three complexes under consideration here the cyanide ion - 2,4,6-trinitroanisole com- plex is least strongly solvated by isopropanol. This is a surprising result since the steric effects associated with the methoxyl group might have been expected to lead to greater rotation of the ortho nitro groups out of the plane of the ring and hence greater overall solvation of the o-complex in this case. There are two reasons perhaps why analogies cannot be so simply drawn. First, because of its size isopropanol, unlike water, may be no more effective in sol- vating a nitro group rotated out of the plane of the ring than a nitro group in the plane of the ring. Second, the interpretation of trends in AH0 and AS0 in terms of trends in solvation effects of the resulting o-complexes assumes that the starting nitroaromatic compounds have comparable properties. 2,4,6-Trinitroanisole has been estimated to be about 4 kcal mol-' more stable than 1,3,5-trinitrobenzene as a result of ground state stabilization through resonance structures such as 6a and 6b (23).

In methanol, AS0 for the formation of 5 (X = OCH,) and 4 (X = OCH,) are the same within experimental error while the A H 0 values are 1.0 + 1.6 and 2.2 + 1.5 kcal mol-', respec- tively (I 1, 12, 23).

For the three cyanide ion - 1-X-2,4,6-trinitro- benzene interactions studied, plots of In (A, - A,) us. time were linear to greater than 90% of reaction at all cyanide ion concentrations. At the high concentrations of cyanide ion to con- centration of nitroaromatic compound used in the kinetic studies a slight fading reaction was observed in the TNT - cyanide ion system., On the time scale involved in the kinetic study this

3This "fading reaction" was probably due to the slow formation of a 2:l o-complex as only one absorption maximum in the visible region, characteristic of a 2 : 1 o-complex of this type (4, 6), remained after approxi- mateIy 10 min. The nature of the species formed is under investigation.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

12 C A N . J . CHEM. VOL. 5 2 , 1974

fading reaction did not appear to interfere to any appreciable extent with the formation of the 1 : 1 o-complex. In some instances a small correction to the infinite absorbance value (A,) was made by extrapolating the linear portion of the absorb- ance us. time portion of the fading reaction plot back to the time of mixing. By analogy to the reaction of 2,4,6-trinitroanisole with methoxide ion in methanol ( l l ) , two reaction traces of absorbance us. time might have been expected in the reaction of 2,4,6-trinitroanisole with cyanide ion. The faster of the two reactions would corre- spond to formation of o-complex 4 (X = CN) while the slower reaction would be associated with the formation of o-complex 3 (X = CN). A careful check of absorbance us. time traces at different wavelengths and over a range of cyanide ion concentrations showed that only one reac- tion was occurring in the solutions containing 2,4,6-trinitroanisole and cyanide ion. The rapidity of the reaction suggests that the o-complex formed is 4 (X = CN).

For the reversible formation of 1 : 1 o-complex according to eq. 2 (where X = H, OCH,, and

CH,) carried out under reaction conditions such that [CN-1, >> [l-X-2,4,6-trinitrobenzene], ([ 1, represents the total concentration), the k,,, ob- tained from the slope of the In (A, - A,) us. time plot is related to the specific rate constants k, and k - , and the [CN-1, as shown in eq. 3.

Plots of k,,, us. [CN-1, were strictly linear for the reaction of cyanide ion and 2,4,6-trinitro- anisole up to a [CN-1, of 1.20 x M (Fig. 2). However for both the 1,3,5-trinitrobenzene - and 2,4,6-trinitrotoluene - cyanide ion reactions, where the maximum [CN-1, was much less than 1.2 x M, k,,, us. [CN-1, plots were linear at low concentrations of cyanide ion but curved slightly upward at higher concentrations of cyanide ion (Figs. 3 and 4). In both systems curvature of the k,,, us. [CN-1, plots seemed to become more pronounced the higher the tem- perature.

The slopes (k,) and intercepts (k-,) of the k,,, us. [CN-1, plots for the reaction of cyanide ion and 2,4,6-trinitroanisole and the slopes (k,) of the k,,, us. [CN-1, plots (obtained at low

FIG. 2. kOhi a s a function of the [+,AsCN] for the reactions of 2,4,6-trinitroanisole with cyanide ion in isopropanol at a number of temperatures.

FIG. 3. kobr as a function of the [+4AsCN] for the reactions of 1,3,5-trinitrobenzene with cyanide ion in isopropanol at a number of temperatures.

FIG. 4. kUb, a s a function of the [$,AsCN] for the re- actions of 2,4,6-trinitrotoluene with cyanide ion in isopropanol at a number of temperatures.

concentrations of cyanide ion) for the reactions of cyanide and 1,3,5-trinitrobenzene and 2,4,6- trinitrotoluene, are listed in Table 2. Values of the intercepts of the k,,, us. [CN-1, plots for the reactions of cyanide ion with 1,3,5-trinitroben- zene and 2,4,6-trinitrotoluene were too small t o permit an accurate evaluation of k - , values in these systems. Values of k- , at 25.0 "C, cal- culated from the known values of K and k, a t 25.0 "C, are given in Table 3.

Equilibrium constants (K,) and specific rate constants k , and k- , associated with the forma- tion of a number of 2,4,6-trinitroanisole and

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

C A N A N D NORRIS: REACTIONS O F CYANIDE ION 13

TABLE 2. Specific rate constants for the reactions of 1,3,5-trinitrobenzene, 2,4,6-trinitroanisole, and 2,4,6-trinitrotoluene with cyanide ion in isopropanol"

kl(M-' s-') k- l (s - l )

TCC) 1,3,5-Trinitrobenzene 2,4,6-Trinitroanisole 2,4,6-Trinitrotoluene 2,4,6-Trinitroanisole

'Error limits on kl and k-l are based on the errors associated with the "least-squares" slopes and intercepts of the kab, US. [CN-lo plots. In the 1.3.5-trinitrobenzene - cyanide ion and 2,4,6-trinitrotoluene - cyanide ion systems only points at low [CN-lo were used in evaluating the slopes and intercepts.

1,3,5-trinitrobenzene o-complexes at 25.0 "C are given in Table 4. For the formation of o-com- plexes 5 (X = OCH,) and 4 (X = OCH,) in methanol values of K, differ by a factor of about 6, values of k - , differ by a factor of about 7 and values of k - , are approximately the same. For the formation of o-complexes 5 (X = OCH,) and 3 (X = OCH,) in methanol the latter o-complex is seen to be more stable but k , for its formation is a factor of about 500 times smaller and k - , for its reversion to reactants about lo5 times smaller than the corresponding values associ- ated with the formation and decomposition of 5 (X = OCH,) (6, 10, 12). For the formation of the cyanide ion o-ccmplex of 1,3,5-trinitrobenzene 5 (X = CN) and the cyanide ion complex of 2,4,6-trinitroanisole in isopropanol, values of K, differ by a factor of about 5, values of k , differ by a factor of about 6, and values of k - , are approximately the same. The similarity in the pattern to that observed in the case of forma- tion of 5 (X = OCH,) and 4 (X = OCH,) is strong evidence for the formulation of the cyanide ion - 2,4,6-trinitroanisole complex as the C-3 adduct (4, X = CN).

The question arises as to why 4 (X = CN) should be more stable than 3 (X = CN). Bernas- coni (23).has argued that 3 (X = OCH,) is more stable than 4 (X = OCH,) by virtue of a stabi- lizing effect due to double bond - no bond res- onance associated with the geminal alkoxy groups attached to the sp3 carbon in the complex. No equivalent double bond - no bond resonance structures can be drawn when methoxide ion is replaced by cyanide ion hence such a stabilizing effect does not operate in 3 (X = CN). On the other hand, as has been pointed out by Bernas- coni (23), the C-3 complex formed by TNA can

benefit from resonance stabilization involving the methoxyl group (4a ++ 4b (X = CN), there- fore 4 (X = CN) could be more stable on this account.

1 *

Solvation effects must also play a major role in determining relative stabilities. The energies associated with solvation effects are evidently comparable to those associated with the reso- nance energies discussed above since in deutero- chloroform both 3 (X = CN) and 4 (X = CN) exist with the evidence being that 4 (X = CN) is the thermodynamically more stable species in this solvent (3).

Plots of In k , us. 1/T yielded the activation parameters AH* and AS* listed in Table 3.4 Enthalpies of activation are, within experimental error, the same for the reactions of cyanide ion with 1,3,5-trinitrobenzene and 2,4,6-trinitro- toluene. The lower AH* for the reaction of cya- nide ion and 2,4,6-trinitroanisole seems to sup- port Bernasconi's suggestion that the C-3 com- plex formed from 2,4,6-trinitroanisole can benefit from resonance stabilization 4a ++ 46 (X = CN). Values of AS* for the reactions of cyanide ion with 2,4,6-trinitrotoluene and 2,4,6-trinitro- anisole are more negative by about 10 cal deg-'

4There is no apparent curvature in the In k us. 1/T plot for the data from the cyanide ion - 2,4,6-trinitroanisole reaction. Curvature might have been expected if the formations of 3 (X = CN) and 4 (X = CN) were taking place at roughly comparable rates.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

TABLE 3. Kinetic and thermodynamic parameters for the formation of some I-X-2,4,6-trinitrobenzene-cyanide ion a-complexes in isopropanol at 25.0 "C

k~ k-1" K x AH^*^ AH-,". AH0 AS,*" ASD AGO, a-Complex ( - 1 - 1 ) ( -1 ) ( - 1 ) (kcal mol-') (kcal mol-1) (kcal mol-1) (cal deg-1 mol-1) (cal deg-1 mol-1) (cal deg-1 mol-1) (kcal mol-1)

- -

5 (X = CN) 2450 0.048 5.1 11.7_+0.5 18.8f 1.3 -7.1 f 0.60 -1.2+1.4 2.8f3.4 2.3f 3.0 -6.43f 0.02

4 (X = CN) 344 0.031 1.12 9.3k0.3 10.1k0.8 -0.81f0.55 -11.4k0.9 -27.2k2.9 15.82 2.0 -5.53+0.03

2 32.6 0.002 2.01 11.8k1.0 16.7+0.7 -4.9f 0.3 -9.4f 1.4 -12.8_+2.5 3 .42 1.1 -5.87f0.03

'Calculated from kl/$. bError limits for AH are based on "least-squares" slopes obtained in plots of log k, us. 1/T. CAH-~* = AH^* -'AHO. *Error limits for As1* are based on "least-squares" intercepts, obtained at 1/T = 0, in plots of log kl us. 1/T.

= ASl* - ASO. JAG0 = -RT In K. 'From ref. 2.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

GAN AND NORRIS: REACTIONS OF CYANIDE ION

TABLE 4. Kinetic and equilibrium data for the formation of some 1 : l a-complexes of 1,3,5-trinitrobenzene and 2,4,6-trinitroanisole at 25.0°C

K kl k-1 a-Complex Solvent (M-') (M-I sbl) (s-l)

-

5 (X = OCH,) Methanol 16.6 7700" 357" (7070)b (305)b

5 (X = CN) Isopropanol 51 000 2450 344

4 (X = 0CH3) Methanol 2.71" 950 350 4 (X = CN) Isopropanol 1 1 200 344 0.031

3 (X = OCH,) Methanol 17 OOOd 17.3 1 x l o 4

mol-' than the AS* for the reaction of cyanide ion with 1,3,5-trinitrobenzene. This is the opposite trend to that observed for the AS0 values. For the formation of 4 (X = OCH,) and 5 (X = OCH,) in methanol the value of AS* was found to be 5 cal deg-' mol-' smaller in the former case.

There was no curvature in the k,,, us. [CN-1, plots for the cyanide ion - 1,3,5-trinitrobenzene reaction when the reactions were carried out in solutions maintained at a constant ionic strength (p) of 0.007 M with tetraphenylarsoniurn nitrite. From the slopes of the k,,, us. [CN-1, plots, values of k, were determined to be 1.30 + 0.08 x lo3, 2.75 i 0.10 x lo3, and 5.46 + 0.10 x 10, M-' s-' at 14.6, 25.0, and 34.9 "C, respectively. At a constant [CN-1, but varying concentration of total salt in the system the k,,, us. [total salt] was found to increase linearly at low [total salt] then become independent of [total salt] at high con- centrations of total salt. The variation in k,,, is not large: at [CN-1, = 1.96 x M and over the range of added salt 0 to 2.8 x M, k,,, increased from 4.4 to 6.7 s-', i.e. a maxi- mum increase of roughly 50%. Within experi- mental error, tetraphenylarsoniurn chloride and tetraphenylarsoniurn nitrite had the same effect on the variation of k,,, with [total salt]. The experimental results indicate therefore that the upward curvature in the k,,, us. [CN-1, plots at high [CN-1, is not due to ion-pair effects in- volving the association of tetraphenylarsonium ion and cyanide ion and that effects associated with the presence of the counter anion in solu-

observed for several SN2 reactions involving anionic nucleophiles reacting with alkyl or aryl halides (18, 25, 26) and with activated aromatic compounds to form o-complexes (28, 29). The observed salt effects cannot generally be inter- preted by using qualitative extensions of the Debye-Huckel theory since the theory predicts that the salt effects should be small and negative for the anion-molecule reactions (1 8, 25).

The kinetic salt eff'ect may be best described by the Brernsted-Bjerrum equation (29). For the interaction of TNB and cyanide ion the equation gives [4], where k2S is the second-order rate con-

stant a t a given salt concentration, k20 is the limiting value of the second-order rate constant for zero value of all ion concentrations and YTNB, yCN-, and y* are the activity coefficients of TNB, cyanide ion, and the transition state, respectively. It is evident from eq. 4 that rate enhancement in the presence of salt is due to an increase in the activity coefficient ratio yTNByCN-/y*. The effect of salts on the activity coefficients of nonelectrolyte solutes has been discussed in detail in a review by Long and McDevitt (1 9).

The results of solubility studies of TNB and TNT in isopropanol at 25.0°C are given in Fig: 5 in the form of a plot of 1 + log S/So us. the concentration of tetraphenylarsoniurn chlo- ride. The salt is found to "salt in" both sub- strates, i.e. it decreases yTNB. The overall in-

tion are not important. The observed rate crease in rate of the cyanide ion - 1,3,5-trinitro- increases seem to be due to a specific effect in- benzene reaction in the presence of added tetra- volving the tetraphenylarsonium cation. phenylarsonium ion is therefore the result of an

Electrolyte effects on rate constants have been increase in the ycN-/-y* ratio.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

16 C A N . J . CHEM. VOL. 52, 1974

FIG. 5. Solubilities of 1,3.5-trinitrobenzene and 2,4,6- trinitrotoluene in isopropanol at 25.0°C as a function of the concentration of tetraphenylarsonium chloride in solution. S and So represent solubilities at a given con- centration of salt and zero concentration of salt respec- tively. (1 + log S/So = 1 + 102 y/yo where y refers to the activity coefficient of the nitroaromatic compound).

In the course of a study of the reactions of 1-X-2,4-dinitrobenzene (X = C1, F) with hy- droxide ion, Bunton and Robinson (18, 25) found that an ion of low charge density such as tetramethylammonium ion "salted in" the sub- strates but nevertheless increased the rate of the reactions. The overall rate enhancement was attributed to a large increase in the y0,,-/y* ratio. The increase in yo,-/y* was taken to imply that the major effect of tetramethylam- monium chloride was to stabilize the large anionic transition state relative to the small hydroxide ion. Hostetler and Reinheimer (26) have attributed the rate enhancement for the reaction of benzyltrimethylammonium meth- oxide and activated aromatic halides to the association of the R,N+ with the nitro group or with the n-electron system of the activated halide in the transition state (7 and 8).

In the case of the tetraphenylarsonium cation, the transition state for the formation of o-com- plex 4 (X = CN) may be stabilized by a weak interaction of the type proposed by Hostetler and Reinheimer. That an interaction of some type does exist is suggested by the observation

I

that in deuterochloroform at 22OC there is a linear relationship between the chemical shift

I I of the TNB ring protons and the mol % of tetra-

phenylarsonium nitrite in solution. More detailed studies of this interaction are planned.

The nonlinear plot of k,,, us. [CN-1, for the reaction of 2,4,6-trinitrotoluene and cyanide ion is intelligible in terms of the same type of salt effect operating as in the T N B - cyanide ion reaction. Linear plots of k,,, us. [CN-1, for the cyanide ion 2,4,6-trinitroanisole reaction indi- cate that the term yTNAycN-/y* is approximately unity. This may arise either because of a negli- gible effect of the cation on both yTNA and the ratio ycN-/y* or because the effects of the cation on yTNA and yCN-/y* are almost equal and cancel each other out. I n the absence of solubility data for 2,4,6-trinitroanisole in isopro- pan01 in the presence of tetraphenylarsonium chloride, a clear choice between these two possibilities cannot be made. However, steric considerations suggest that the former possibility is much more likely. The size of the substituents X in our series of 1-X-2,4,6-trinitrobenzenes decrease in the order C H 3 0 > CH, > H. By analogy to what occurs in 2,4,6-trinitrophenetole (X = C,H,O) where the ortho nitro groups a re extensively twisted out of the plane of the ring (30) the methoxyl group would also be expected to lead to rotation of the ortho nitro groups ou t of the plane of the ring. This could lead to a much larger distance of closest approach of the tetraphenylarsonium cation and the nitroaro- matic compound and hence a much smaller stabilization effect in both the ground state a n d the activated complex leading to 4 (X = CN).

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

1. A. R. NORRIS. Can. J. Chem. 45, 2703 (1967). 2. E. BUNCEL, A. R. NORRIS, and W . PROUDLOCK. Can.

J. Chem. 46, 2759 (1968). 3. A. R. NORRIS. Can. J . Chem. 47, 2895 (1969). 4. E. BUNCEL, A. R. NORRIS, and K. E. RUSSELL. Q.

Rev. Lond. 22, 123 (1968). 5. M. R. CRAMPTON. Adv. Phys. Org. Chem. 7, 211

(1969). 6. M. J. STRAUSS. Chem. Rev. 70, 667 (1970). 7. V. GOLD and C. H. ROCHESTER. J. Chem. Soc. (B),

1692 (1964). 8. K. L. SERVIS. J. Am. Chem. Soc. 87, 5495 (1965). 9. M. R. CRAMPTON and V. GOLD. J. Chem. Soc. (B),

893 (1966). 10. J. H. FENDLER, E. J. FENDLER, and C. E. GRIFFIN.

J. Org. Chern. 34, 689 (1969). 11. C. F. BERNASCONI. J. Am. Chem. Soc. 93, 6975

(1971).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.

GAN AND NORRIS: REACTIONS OF CYANIDE ION 17

12. L. H. GAN and A. R. NORRIS. Can. J. Chem. 49,2490 (1971).

13. E. BUNCEL, A. R. NORRIS, K. E. RUSSELL, and R. TUCKER. J. Am. Chem. Soc. 94, 1646 (1972).

14. E. BUNCEL, A. R. NORRIS, K. E. RUSSELL, and W. PROUDLOCK. Can. J. Chem. 47, 4129 (1969).

15. J. R. A. POLLOCK and R. STEVENS (Editors). Dictio- nary of organic compounds. 4th Ed. Eyre and Spot- tiswoode Ltd., London. 1965.

16. L. H . GAN and A. R. NORRIS. Can. J. Chem. 52, I (1974).

17. P. G. ASHMORE, B. P. LEVITT, and B. A. THRUSH. Trans. Faraday Soc. 52, 530 (1956).

18. C. A. BUNTON and L. ROBINSON. J. Am. Chem. Soc. 90, 5965 (1968).

19. F. A. LONG and W. F. MCDEVITT. Chem. Rev. 51,119 (1952).

20. A. R. NORRIS. Can. J . Chem. 45, 175 (1967). 21. M. R. CRAMPTON. J. Chem. Soc. (B), 1341 (1967). 22. E. BUNCEL, A. R. NORRIS, and K. E. RUSSELL.

International conference on the mechanisms of reac- tions in solution. Canterbury, England. Paper B-11. 1970.

23. C. F. BERNASCONI. J. Am. Chem. Soc. 92, 4682 (1970).

24. E . BUNCEL, A. R. NORRIS, K. E. RUSSELL, and P. J. SHERIDAN. Can. J. Chem.

25. C. A. BUNTON and L. ROBINSON. J. Org. Chem. 34, 780 (1969).

26. W. H O S T E T L E R ~ ~ ~ J . D. REINHEIMER. J. Org. Chem. 33, 35 10 (1968).

27. J. B. AINSCOUCH and E. F . CALDIN. J . Chem. Soc. 2528 (1956).

28. C. F . BERNASCONI and R. G. BERGSTROM. J. Org. Chem. 36, 1325 (1971).

29. A. FROST and R. G. PEARSON. Kinetics and mechan- ism. 2nd Ed. John Wiley and Sons, Inc., New York. 1953.

30. C. M. GRAMACCIOLI, R. DESTRO, andM. SIMONETTA. Acta Crystallogr. 24, 129 (1968).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV O

F T

EC

HN

OL

OG

Y S

YD

NE

Y o

n 11

/09/

14Fo

r pe

rson

al u

se o

nly.