Kinetics of Proton Transfer between Ortho Substituted Benzoic Acids

ARTICLE pubs.acs.org/JPCA

Kinetics of Proton Transfer between Ortho Substituted Benzoic Acids and the Carbinol Base of Crystal Violet in Toluene. Ortho Effect on the Reactivity of Benzoic Acids in Apolar Aprotic Solvents Susanta K. Sen Gupta* and Sangeeta Mishra Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India

bS Supporting Information ABSTRACT: Apolar aprotic solvents are particularly advantageous for investigating the intrinsic ortho effect free from complications of specific solvent effects. A kinetic study for toluene-phase proton transfers between ortho F, Cl, Br, I, OMe, OEt, OPh, OAc, Me, NO2, COMe, COPh, OH, NH2, and H benzoic acids and crystal violet carbinol base has shown the forward rate constant (log kþ1) is the most appropriate reactivity parameter in toluene. log kþ1 (toluene) as compared to other reported reactivity parameters in benzene, toluene, or chlorobenzene has been found more sensitive to the ortho substituent effect. The regression results of the correlation of log kþ1 (toluene) of the acids (except OH and NH2 substituted ones) according to seven ortho effect models are all very significant, and the best result is given by Fujita
Nishioka’s model. The overall analysis reveals that a substituent’s ortho effect pattern is a 58:24:18 ratio of its ordinary electrical, proximity electrical, and steric effects and that the proximity electrical effect is the major component to account for the peculiarity of the substituent’s ortho effect. The results further favor the transmission of this effect mainly through the molecular cavity. The effect may, however, be outweighed by the steric component for bulky enough substituents, e.g., Me. The enhanced strength exhibited by salicylic acid in toluene has been quantitatively described using Pytela
Li
ska’s σHBi parameter. The abnormally high log kþ1 observed for anthranilic acid in toluene has been ascribed to a very extensive homoconjugation in its acid
acid anion complex induced by the acid’s three hydrogen bond donors.

’ INTRODUCTION Ortho substituted benzoic acids and other benzene derivatives behave peculiarly compared to their meta or para isomers.1,2 The ortho substituent effect or the ortho effect on the acidity of benzoic acid involves possible proximity interactions besides the normal polar ones and is too complex to be correlated by simple forms of the Hammett equation.3
16 For probing this peculiarity, it seems particularly advantageous to use an apolar aprotic solvent (dielectric constant, εr < 15; dipole moment, μ < 8.3  10
30 C m; and Dimroth
Reichardt’s normalized solvent 17 as the medium for reacpolarity parameter, EN T ca. 0.0
0.3) tions of these acids. In such solvents, specific solvation due to the solvent’s own acidity and/or basicity is reduced to a minimum. However, acidity measurements in an apolar aprotic solvent require a reference base. The colorless carbinol base of crystal violet dye, has been found to be a very useful reference base for determining the relative strengths of a variety of oxygen acids in benzene, toluene, and other apolar aprotic solvents.18
27 A very remarkable feature of acid
dye carbinol base reactions in an apolar aprotic solvent is their abnormally slow kinetics. Time scales of proton transfers between a variety of oxygen acids r 2011 American Chemical Society

ranging from simple aliphatic and aromatic carboxylic acids, fluoro and nitro alcohols, and phenols to complex hydrogen spiroborates corresponding to dicarboxy, hydroxyl carboxy, and dihydroxy acids, and colorless carbinol bases derived from crystal violet and several other triarylmethane dyes in different apolar aprotic solvents are as long as several tens to several hundreds of seconds.18
27 In fact, kinetics of all such reactions are conveniently monitored by ordinary spectrophotometry of the colored ion-pair product. A mechanism for such abnormally slow proton transfers has been elucidated in terms of a fast equilibrium between an acid and a dye carbinol base to form an intermediate hydrogen bonded complex, followed by a rate limiting proton transfer along the hydrogen bond to form a colored ion-pair product under the combined influence of the monomeric acid catalyst, the nonreactive cyclic dimeric acid inhibitor, and the hyperacidic open chain dimeric and trimeric acid catalysts.27 Another example of such a slow minute scale proton transfer is Received: November 13, 2010 Revised: March 25, 2011 Published: April 19, 2011 4616

dx.doi.org/10.1021/jp110851s | J. Phys. Chem. A 2011, 115, 4616–4623

The Journal of Physical Chemistry A the rate determining step in the reactions of aliphatic and aromatic carboxylic acids with diazodiphenylmethane in different classes of solvents.11,28
33 Further examples of slow proton transfers are the reactions between bromophenol blue and substituted pyridines in apolar aprotic solvents, time scales of which, however, are much shorter, in the range of milliseconds.34 An interesting result of an earlier study of toluene-phase acid
crystal violet carbinol base reactions for a small set of four ortho substituted benzoic acids is that compared to log K (toluene) or pKa (water), log kþ1 (forward rate constant in toluene) has a significantly higher sensitivity to the ortho effect.25 It may be noted that two other ortho effect studies based on the equilibrium constant data for benzoic acids in benzene38 and toluene40 have been reported. The advantages of the virtual absence of specific solute
solvent interactions, easy monitoring of kinetics, and the enhanced sensitivity of the forward rate constant toward the ortho effect make studying proton-transfer kinetics in an apolar aprotic solvent particularly appropriate for the analysis of the ortho effect on the reactivity/acidity of benzoic acids. Several correlation models based on the extended Hammett methodology, e.g., the Charton models,6,35 the Fujita
Nishioka models7,36 and the Pytela
Li
ska model,8 have been proposed toward understanding the nature and composition of the ortho effect. Despite a large number of equilibrium6
8,25,36
40 and kine6
8,11,12,25,27,33,41
43 studies complemented by spectroscopic14,44
48 tic and theoretical13,14,16,49
52 studies of the ortho effect in benzoic acids and their derivatives, there are only three reports, to the best of the authors’ knowledge, of kinetic studies for ortho substituted benzoic acids in an apolar aprotic solvent.25,27,33 While the first two reports25,27 concern a limited number of ortho substituted benzoic acids (five and two, respectively) but with due considerations to complex effects of acid concentrations on the rate constants of their reactions in apolar aprotic solvents, the third one33 though involves as many as 27 ortho substituted benzoic acids and neglects the pronounced dependence of the rate constants of their reactions on acid concentrations. The two limitations from using a small number of ortho substituted benzoic acids and from neglecting concentration effects of the acids in an apolar aprotic solvent need remedying for a proper analysis of the specific solvent effect free ortho effect. The present work is directed at this task. We report here the results of detailed kinetics of toluene-phase reactions of a set of 15 ortho substituted benzoic acids (including the unsubstituted one) with crystal violet carbinol base for a range of acid concentrations. Kinetic data for six acids of the set, viz., ortho Me, OMe, OH, Cl, NO2, and H benzoic acids reported earlier,25 were reexamined, and the data for chloro, hydroxy, and methoxy substituted acids needed revision. We report further a comparison of the results of quantitative analysis of the ortho effect on the forward rate constant, log kþ1 (the most appropriate rate coefficient found) of the reactions by seven extended Hammett-type models.6
8,35,36 The substituents chosen are both simple and symmetric ones (Me, F, Cl, Br, and I), and those allowing more than one conformation (NO2, COMe, and COPh), hydrogen bonding in the undissociated acid only (OMe, OEt, OPh, and OCOMe), hydrogen bonding in the dissociated acid anion (OH and NH2), or intramolecular cyclization (COMe and COPh).

ARTICLE

Figure 1. Structure of crystal violet carbinol base.

the reference apolar aprotic solvent because of its relatively low toxicity. Crystal violet carbinol base, [4,40 ,400 -tris(dimethylamino)triphenyl]carbinol (Figure 1), which is colorless, was employed as the reference base. Crystal violet carbinol base was prepared by precipitating a 100 mL aqueous solution containing ∼100 mg of crystal violet dye [4,40 ,400 -tris(dimethylamino)triphenyl chloride] with ∼2 M NaOH followed by extraction of the carbinol base precipitate into ∼200 mL of toluene. The toluene extract was separated and subjected to freeze-drying to obtain crystal violet carbinol base as a powder (mp 180
C). Elemental analysis results (C, 76.85%; H, 7.88%; N, 10.94% as against C, 77.08%, H, 8.02%; N, 10.79% required for C25H31N3O), IR peak data in Nujol (3350 cm
1, bonded OH), 1H NMR signals in carbon tetrachloride (2.90 ppm, s, 18 H, 3 N(CH3)2; 6.55 ppm, d, 6 H, J = 10 Hz, aromatic H; 7.00 ppm, d, 6 H, J = 10 Hz, aromatic H; 7.28 ppm, s, 1H, C
OH), and freezing point depression in benzene confirmed the structure of the carbinol base (Figure 1). All stock solutions of crystal violet carbinol base and ortho substituted benzoic acids were prepared by dissolving their weighed amounts in dry toluene. Kinetics of the reaction between an acid, HA (CHA ∼ 10
4
10
2 M, higher molarity order being employed for relatively weaker acids and lower order for stronger acids), and the colorless carbinol base of crystal violet, dye
OH (2.5  10
5 M), in dry toluene, producing a colored ion associate, dyeþA
, were monitored by absorbance measurements of the ion associate at 610 nm in a Teflon stoppered quartz cuvette using a 160 A Shimadzu UV
vis spectrophotometer at 28.0 ( 0.1
C. The sampling time for absobance recordings during the reaction was every 5
10 s. A time of 40
60 min was required to attain the reaction equilibrium. The molar absorptivity (ɛ) of crystal violet cation (dyeþ) in toluene was found to be 3.3  104 dm3 mol
1 cm
1 at 28.0
C.

’ RESULTS AND DISCUSSION ’ EXPERIMENTAL SECTION The chemicals used were of either analytical reagent grade or highly purified by standard procedures. Toluene was chosen as

Acid
Dye Carbinol Base Reaction Kinetics in Toluene. Ortho substituted benzoic acids (HA) react with crystal violet carbinol base (dye
OH) in toluene to form colored ion 4617

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Table 1. Equilibrium Parameters (log K, n) for ToluenePhase Reactions of Ortho Substituted Benzoic Acids with Crystal Violet Carbinol Base at 28.0 ( 0.1
C

where the acid exponent (n) takes into consideration the possibility of HA existing effectively as a n-mer due to formation of open chain and cyclic acid dimers in toluene, the apolar aprotic solvent.38 It has been argued that the water molecule formed (eq1) is hydrogen bonded to one of the species of the system and that they exist as one entity.18
27,39,40,53
58 Equation 1 can be reexpressed as

significance on R better than 0.1% and t-level of significance on the regression coefficient, k, better than 0.1%). The rate constants of relatively more reactive acids, o-nitrobenzoic and salicylic acids for which the lower limit of CHA was extended close to CD, were obtained by an appropriate second-order procedure. The toluene-phase rate constant (k) data, though independent of initial CD, exhibit a complex dependence on CHA (vide Supporting Information) and conform very well to the following: k = kRCHA þ kβCHA2 þ kγCHA3, indicating the occurrence of general acid catalysis in the reaction (eq 2), an acid playing the dual role of the acid substrate and the catalyst. kR, kβ, and kγ are the coefficients of catalysis by the monomer acid, dimer (open chain homoconjugated) acid, and trimer (open chain homoconjugated) acid, respectively. In particular, kβ represents the net effect of catalysis by the hyperacidic open chain dimer over inhibition by the nonreactive cyclic dimer acid. This is expected in view of increasing participation of hyperacidic homoconjugated complex acids in an apolar (also a dipolar) aprotic solvent with increasing CHA.27,29,30,38,59 Participation of such homoconjugated open chain dimers has been noted for many other acids in apolar (also dipolar) aprotic media while studying not only reactions with dye carbinol bases18
27 but also ethyl diazoacetate
carboxylic acid interactions,60,61 the inversion of l-menthone,62 and diazodiphenylmethane
carboxylic acid reactions.29,30 Assuming both the forward and the reverse steps of the equilibrium (eq 2) are influenced by the acid, eq2 can be rewritten as

nHA þ D h DHþ A


D þ f HA h DHþ A
þ rHA

substituent

n

a

substituent a

log K

n

H

1.78

OEt

2.55

1.12

Me

1.96 b

1.19 b

OPh

1.94 a

1.04 a

NO2

5.38 b

1.33 b

OAc

3.43

1.08

F

2.36

a

0.94 a

COMe

4.32

1.80

Cl Br

2.85 a 3.04 a

0.98 a 0.98 a

COPh OH

2.83 4.81

0.92 1.10

I

3.53 a

1.10 a

NH2

5.34

2.22

a

1.18 a

OMe a

log K

2.87

0.98

Reference 40. b Reference 25.

associates (dyeþA
) nHA þ dye
OH h dyeþ A
þ H2 O

ð1Þ

ð2Þ

where D and DHþA
represent the colorless carbinol base of crystal violet and the colored ion associate, respectively. It has been reported that absorbance data for the reaction equilibrium (eq2) for 10 members of the present set of ortho substituted benzoic acids in toluene fitted the expression: K ¼

½DHþ A
 ½D½HAn

ð3Þ

where K is the association constant.25,40 Values of log K and n for the 15 acids chosen here (Table 1) were determined as the intercept and regression coefficient, respectively, of the linear regression of log[Aeq/(b
Aeq)] on log [HA] (n = 7
8, R2= 0.994
0.996, F(481.41
969.36)-level significance on R better than 0.1%), where Aeq and b are the absorbances of DHþA
at 610 nm at equilibrium and when converted totally from D (by the use of excess HA) respectively and [HA] = CHA. As seen, values of n are close to or greater than unity. A similar behavior was observed for many other acids in apolar aprotic solvents.18
27,38
40,54
58 This has been ascribed to overlapping participations of open chain dimeric and multimeric homoconjugated complex acids H(A 3 3 3 (HA)m) (m g 1) in the reaction with the carbinol base (D) in apolar aprotic media.18
27,38
40,54
58 The value of n for an acid depends upon its concentration, temperature, and solvent.24,38,56
58 The time course of the absorbance data (At) for the reaction, eq 2 under the condition CHA . CD (vide Experimental Section), conforms very well to pseudo-first-order reversible kinetics for all of the acids of the set: At ¼ Aeq
ðAeq
A0 Þe
kt

ð4Þ

where A0 is the absorbance at t = 0. The overall rate constant, k, was determined from the exponential regression of At on t (n = 15
20; R2 = 0.996
0.999, F(1241.23
8246.17)-level of

ð5Þ

where f and r are the individual acid exponents for the forward and reverse steps of the equilibrium, respectively. f can be interpreted as the weighted or effective mean aggregation number of HA species, HA, H(A 3 3 3 (HA)m) (m g 1) reacting with D in the forward step. r is thus the number of HA molecules released from the effective f-mer, (HA)f after the equilibrium, eq 2, has been attained. If kþ1 and k
1 represent the rate constants for the forward and reverse steps, respectively, it follows that k ¼ kþ1 ½HAf þ k
1 ½HAr

ð6Þ

K ¼ kþ1 =k
1

ð7Þ

n ¼ f
r

ð8Þ

Equations 6
8 lead to a useful relation:   k log ¼ f log ½HA þ log k
1 K þ ½HA
n

ð9Þ

where K and n were already determined from equilibrium measurements (Table 1). log k
1 and f were determined as the intercept and the regression coefficient, respectively, of the linear regression of the left-hand side of eq 9 on log [HA] for each member of the set of ortho substituted benzoic acids (n = 7
8, R2 = 0.996
0.998, F(761.45
2134.14)-level of significance on R better than 0.1%). Values of the other two kinetic parameters, log kþ1 and r, were obtained using eqs 7 and 8, respectively. Values of log kþ1, log k
1, f, and r for the acids are compiled in Table 2 along with their pKa (H2O), log KBHA (benzene), and log k (chlorobenzene) (vide infra) As seen from Table 2, kþ1. k
1 and f . r. Values of f, as expected, are nonintegral and greater than unity. Furthermore, log kþ1 appears much more susceptible to substituent effects than log k
1. It may be noted that whereas the association constant, log K, for a given n is determined by the interplay of four parameters, viz., kþ1, 4618

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Table 2. Individual Rate Constants (log kþ1, log k
1), and Acid Exponents (f, r) for Toluene-Phase Reactions of Ortho Substituted Benzoic Acids with Crystal Violet Carbinol Base at 28.0 ( 0.1
C along with pKa (H2O) at 25
C, log KBHA (Benzene) at 25
C, and log k (Chlorobenzene) at 30.0
C for the Acid log k
1

f

r

pKa b (water)

log KBHA b (benzene)

H

1.80

0.02

1.46

0.48

4.20

5.26

0.34

Me

1.99 a

0.03 a

1.67 a

0.48 a

3.91

4.94


0.004

NO2

7.25 a

1.87 a

2.05 a

0.72 a

2.17

7.44

3.13

F

2.93

0.57

1.54

0.60

3.27

5.77

1.15

Cl Br

4.56 5.16

1.71 2.12

1.86 1.98

0.88 1.00

2.94 2.85

6.08 6.17

1.52 1.64

I

4.73

1.20

1.82

0.72

2.86

6.22

OMe

2.28


0.59

1.46

0.28

4.09

3.7

OEt

1.96


0.59

1.32

0.22

4.21

OPh

2.08

0.14

1.72

0.68

3.53

OAc

4.50

1.07

1.74

0.66

3.48

COMe

5.26

0.94

2.70

0.90

3.41 c

COPh OH

4.62 4.46

1.79
0.35

1.88 1.34

0.98 0.24

14.66

9.32

6.44

4.24

NH2 a

log k d (chlorobenzene)

log kþ1

substituent

1.68
1.16
1.52 0.33

5.72

1.22

3.54 3.00

6.44 7.45

1.91 3.13

4.95

4.91

0.15

Reference 25. b Reference 38. c Corrected value of the keto acid form.68 d Reference 33.

k
1, f, and r, the forward rate constant, log kþ1, for a given f is independent of any other parameter. Moreover, log kþ1, as compared to log K, has higher sensitivity to the effects of the substituents (Tables 1 and 2). log kþ1 thus appears to be the most appropriate parameter for a quantitative study of structural effects of ortho substituents on chemical reactivities of benzoic acids in toluene or other similar apolar aprotic solvents. Results of quantitative analysis of the ortho effect on log kþ1 (toluene) on the basis of a number of multiparametric models proposed by Charton, Fujita
Nishioka, and Pytela
Li
ska are presented in the following. Quantitative Analysis of the Ortho Effect on ToluenePhase Reactivities of Benzoic Acids Using Extended Hammett-Type Models. The following well-known models proposed for the quantification of structural effects of ortho substituents on reactivities of benzoic acids and other benzene derivatives have been employed to correlate the log kþ1 data set in Table 2 using relevant substituent parameters by multiple linear regression analysis. (1) Charton's model:6 log kortho ¼ RσI þ βσ R þ Ψv þ h

ð10Þ

where kortho stands for reactivity or physicochemical data of ortho substituted compounds. σI and σR are the localized (field) and delocalized (resonance) electrical effect parameters, respectively, of a substituent, while v is a monoparametric steric effect parameter based on the van der Walls radius of the substituent. (2) Charton's modified models:35 (a) log kortho ¼ Rσ I þ βσ R þ Φv þ h

ð11Þ

where v of eq 10 is modified to a multiparametric steric effect parameter, v*. (b) LDRS model log kortho ¼ Lσl þ Dσd þ Rσ e þ Sv þ h

ð12Þ

where σl is simply an alternative symbol of σI of eqs 10 and 11. σd is the intrinsic delocalized (resonance) electrical effect parameter which represents the delocalized electrical effect in a system with zero electronic demand. σe is the electronic demand sensitivity parameter which adjusts the delocalized effect of a group to meet the electronic demand of the system. σd and σe are related to σR of eqs 10 and 11 as35 σR = 0.380σe þ σd. The steric effect is described by the v parameter of eq 10. (c) CRS model log kortho ¼ Cσc50 þ Rσe þ Sv þ h

ð13Þ

is a simplified version of eq 12, where σl and σd are replaced by a composite parameter, σc50, defined as σc50 = 0.50σl þ 0.50σd. (3) Fujita
Nishioka's model: log kortho ¼ Fσ op þ f F þ δEs þ h

ð14Þ

where σo is considered equal to σp (i.e., σortho  σpara) and represents a substituent’s ordinary electrical effect. F, the Swain
Lupton
Hansch field constant of a substituent,63,69 describes its proximity electrical effect, while Es, the Taft
Kutter
Hansch parameter which is based on the mean van der Walls radius of the substituent,3c,64 is employed as its steric effect parameter. (4) Fujita
Nishioka's reformulated model: log kortho ¼ f F þ rR þ δEs þ h

ð15Þ

The reformulation of eq 14 follows from the relation7 σp = 0.92F þ R, where R is the substituents’s Swain
Lupton
Hansch resonance constant.63,69 (5) Pytela
Li
ska’s model: log kortho ¼ Fo σ o i þ Fs σ s i þ FHB σ HB i þ h

ð16Þ

This model unlike the other models includes a substituent’s intramolecular hydrogen bonding effect as a significant component of its ortho effect. The three parameters (σoi, σsi, and σHBi) were determined for 29 substituents by applying the method of conjugated deviations to a large number of data sets of log 4619

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Table 3. Substituent Parameters Employed for the Ortho Effect Analysis by Different Models, Equations 10
16 substituent

σop a

σoi c 0.000

σI (or σl) d

σR d

σd d

σc50 d

σe d 0

Fh

Rh

H

0

0

0

0

Me


0.17


0.150


0.01


0.16


0.14


0.15


0.030

0.01

0


0.18

0

NO2

0.78

1.400

0.67

0.10

0.18

0.85


0.077

0.65

0.13

F

0.06

0.425

0.54


0.48


0.48

0.06


0.041

0.45

Cl

0.23

0.576

0.47


0.25


0.28

0.19


0.011

Br

0.23

0.596

0.47


0.25


0.27

0.20

I

0.18

0.554

0.40


0.16


0.20

OMe OEt


0.27
0.24


0.211
0.186

0.30 0.28


0.58
0.57

vi

v*d

Es a 0

σsic

σHBi c

0

0

0

0

0.52

0.52


1.24

0.133

0

0.57(^) j

0.35(^) l


1.01(^)

0.288

0


0.39

0.27

0.27


0.46

0.198

0

0.42


0.19

0.55

0.55


0.97

0.430

0


0.018

0.45


0.22

0.65

0.65


1.16

0.489

0

0.20


0.057

0.42


0.24

0.78

0.78


1.40

0.521

0


0.55
0.55


0.25
0.27


0.064
0.070

0.29 0.26


0.56
0.50

0.32 0.32

0.36 0.48


0.55
0.55

0.257 0.212

0 0

OPh


0.32

0.240

0.40


0.48


0.51


0.11


0.083

0.37


0.40

0.32

0.57


0.55

0.356

0

OAc

0.31

0.405

0.38


0.23


0.24

0.14


0.005

0.42


0.11

0.32

0.50


0.55

0.143

0

COMe

0.50

0.30

0.20

0.25

0.05


0.095

0.33

0.17

0.50(^)

0.50(^)


0.88 j

0.31

0.12

0.50(^) k

0.50(^) l


0.88 j


0.62


0.57


0.22


0.044

0.33


0.70

0.32

0.32


0.55

0.392

0.695


0.80


0.68


0.51


0.13

0.08


0.74

0.35

0.35


0.61

0.235

0.153

COPh OH

0.43 b
0.37

0.28 e
0.088

0.35

0.15 g

0.24f NH2


0.66


0.788

0.17

Reference 7. b Reference 69. c Reference 8. d Reference 35. e Taft’s σF.69 f Reference 11, g Calculated as σop
σI(σF).69 h Recalculated values.69 i Reference 70. j Calculated as per ref 7. k References 11 and 70. l Calculated as per refs 7 and 35.

a

Benzoic acids with simple ortho substituents, H, Me, F, Cl, Br, and I, which are monatomic or tetracoordinate symmetric top and free from conformational and intramolecular hydrogen bonding effects, are the straight choices for the correlation analysis. The same is true for OMe, OEt, OPh, and OAc substituents whose intramolecular hydrogen bonding effect in the undissociated acid is, however, not as significant as to be necessary to express it explicitly.8 Ortho COMe and COPh benzoic acids cyclize intramolecularly to an equilibrium mixture of open chain keto acid and cyclic lactol tautomers with the keto acid content, nearly 25 and 100%, respectively, in an apolar aprotic medium.65
67 While reacting with crystal violet carbinol base, the equilibrium mixture would, however, act entirely as the stronger acid form, the keto acid. The three planar π-bonded substituents chosen, the above two (i.e., COMe, COPh) and NO2, show steric effects which may have strong conformational dependence. Values of the steric effect parameters calculated for the two extreme conformations, viz., coplanar ( , maximum dimension) with and orthogonal (^, minimum dimension) to, the reaction site are substantially different.7,11,64 log kþ1 data corresponding to these three substituents each (Table 2) have been found to correlate very well with the steric parameter (^) of minimum dimension for the orthogonal conformation rather than with the parameter ( ) for the coplanar one. The same inference was also made for the ortho NO2 group during proton-transfer studies of benzoic acids in water and benzene.7,36 The data set of log kþ1 can thus justifiably include the data corresponding to these three groups for correlation by all these models, eqs 10
16. The case of ortho OH substituent is an exception. It exerts intramolecular hydrogen bonding effects in the undissociated salicylic acid and more strongly in the salicylate anion. The effect in the anion is strong enough as to be necessary to express it explicitly as a hydrogen bonding effect parameter. This particular case may thus be treated by Pytela
Li
ska’s model which has a separate σHBi parameter, eq 16. Regression Analysis Results. Multiple linear regression analysis of log kþ1 data of the ortho substituted benzoic acids )

)

kortho: σoi describes the electrical effect and its values were optimized according to an isoparameter relation between σoi and σmi. σoi correlates very well with the σI and σR constants. σsi describes the steric effect and is not significantly related to any of the known quantities of this type. σHBi describes the intramolecular hydrogen bonding between the oxygen atom of the carboxylate anion and the ortho substituent, and it has nonzero values for substituents such as OH, NH2, and NHCOCH3 etc. Values of the parameters in eqs 10
16, viz., σI = σl, σR, σd, σe, σc50, σop, σoi, F, R, v, v*, Es, σsi, and σHBi, of the substituents selected and references of their sources are given in Table 3. Coefficients of the substituent parameters and the intercepts (h) in the above equations for log kþ1 data (Table 2) are evaluated by the method of regression analysis. Choice of log kþ1 Data Set for Correlation Analysis. As shown already, the appropriate parameter for quantifying the toluene-phase reactivity of an acid is its log kþ1 for a given f. As seen from Table 2, excepting anthranilic acid, all the other acids have their f around 2.0 ( 0.7. f of anthranilic acid is comparatively too large, 6.44. Too large are also its log kþ1, log k
1, and r (Table 2). In fact, acids having one hydrogen bond donor center have their f-exponent within 2.0 ( 0.7 (Table 2). It seems that the presence of as many as three hydrogen bond donor centers in anthranilic acid causes extensive enough homoconjugation in its hyperacidic open chain complex, H(A 3 3 3 (HA)m)27 at the acid concertrations employed (10
2 to 10
3 M) and leads to such a high f-exponent (6.44). It may be noted that at concentrations below 10
3 M, the reaction of anthranilic acid is too slow to monitor, whereas the reaction rate of salicylic acid is easily measured at concentrations as low as 10
5 M. Due to the use of such low concentrations (∼10
5 M), salicylic acid despite having two hydrogen bond donor centers has a normal f-exponent (1.34) (Table 2). The log kþ1 data of anthranilic acid was accordingly excluded from the log kþ1 data set while correlating them in the light of the above models: eqs 10
16. The ortho substituted benzoic acids, as already stated, have different categories of substituents. It is thus worth examining the justifiability of log kþ1 (toluene) of all these acids (Table 2) for analysis by the ortho effect models.

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Table 4. Results of Regression Analysis for log kþ1 (Toluene) by Different Ortho Effect Models model (eq)

na

best regression equation

F-level significance on R

t-level (two-tail) significance on

R2

F

(better than)

regression coefficients (better than)

(10)

5:67σ I þ 3:92σ R þ 1:97v þ 1:79

ð17Þ

13

0.939

46.51

0.1%

R, 0.1%; β, 0.1%; ψ, 5%

(11)

6:14σI þ 4:43σ R þ 159v þ 1:85

ð18Þ

13

0.929

39.30

0.1%

R, 0.1%; β, 0.1%; φ, 10%

(12)

5:49σ l þ 3:38σd þ 2:17v þ 1:75

ð19Þ

12

0.945

45.44

0.1%

L, 0.1%; D, 0.1%; S, 5%

(13)

4:30σ c50 þ 2:51v þ 2:13

ð20Þ

12

0.922

53.08

0.1%

C, 0.1%; S, 2%

(14)

3:60σ op þ 2:81F
0:93Es þ 1:63 ð21Þ

13

0.979

139.76

0.1%

F, 0.1%; f, 0.1%; δ, 0.5%

(15)

6:20F þ 3:92R
1:06Es þ 1:57

ð22Þ

13

0.957

66.64

0.1%

f, 0.1%; r, 0.1%; δ, 2%

(16)

3:50σ o þ 2:41

ð23Þ

11

0.869

59.75

0.1%

Fo, 0.1%

(16)

3:50σ o þ 3:49σ HB þ 2:41

0.872

30.66

0.1%

Fo, 0.1%; FHB, 2%

i

i

i

ð24Þ

12

þ

a

n = 11 (ortho H, Me, OMe, OEt, OPh, OAc, F, Cl, Br, I, and NO2 benzoic acids); n = 12 (n = 11 plus ortho COMe benzoic acid); n = 13 (n = 12 plus ortho COPh benzoic acid); n = 12þ (n = 11 plus salicylic acid)

(Table 2) was carried out using the correlation eqs 10
16. The best regression equations thus obtained, eqs 17
24, and statistics of the results are given in Table 4. As seen from Table 4, the regression results from all the models are very significant. A comparison of statistics of the results in eqs 17
24 in Table 4 shows eq 21 the most significant one. Thus, Fujita
Nishioka’s model, eq 14 appears to be the best one for correlating log kþ1 (toluene). Its regression eq 21 explains 97.9% of the variance (Table 4). Further statistical results from the models eq 14 and eq 21 show that the relative contribution of the ordinary electrical (%F), proximity electrical (%f), and steric (%δ) effects in the overall ortho substituent effects on the basis of their partial regression (beta) coefficients (0.70, 0.29, and
0.21) are in the ratio of 58:24:18. The ordinary electrical effect is, obviously, the largest contributor to the ortho effect. However, according to an earlier report25 based on the log kþ1 data of ortho Me, NO2, Cl, OMe, and H benzoic acids, the proximity electrical effect is the major factor contributing to the ortho effect. As stated under the Introduction, while reexamining log kþ1 data of these five acids, the values for two, viz., Cl and OMe substituted acids, needed significant revision. On incorporation of these revised data, the ordinary electrical effect is again found to be the largest contributor, doing away with the confusion about the predominant component of the ortho effect. Thus, the toluene-phase reactivity of an ortho substituted benzoic acid (log kþ1) is virtually equally sensitive to the ordinary electrical and total proximity effects (F ≈ f þ |δ|) of the substituents. The analysis shows further that for most of the substituents the major component of the total proximity effect is the proximity electrical effect rather than the steric effect (fF > δEs). The opposite is, however, found for the Me substituent (δEs > fF). Other exceptions are I, COMe, and COPh substituents for which fF ≈ δEs. The sign of the regression coefficients (F, f, and δ) in eq 21 corresponds to the reaction eq 2, being subject to both fieldassisted and steric acceleration by the ortho substituents. This is exactly what one would expect in terms of the mechanism of the reaction eq 2, the rate controlling step of which is HA catalyzed formation of a benzoate anion as DHþA
from a hydrogen bonded complex, D 3 3 3 HA.27 This is favored not only by stronger electron-withdrawing substituents but also by bulkier ones apparently due to a greater relief in the steric strain in the more easily twistable acid anion during its formation from the acid molecule.

It is obvious that log kþ1 is a useful and sufficiently sensitive parameter for analyzing the ortho effect on reactivities of benzoic acids in toluene and other apolar aprotic solvents. The present analysis relates to the ortho effect where complications due to specific solvation and conformational dependence, intramolecular cyclization, or intramolecular hydrogen bonding in the undissociated acid are minimal. The resulting pattern of ortho effect components is obviously of more intrinsic nature than the one based on the reactivity/acidity measurements of the acids in a polar protic or a dipolar aprotic solvent. log kþ1 vs Other Reactivity Parameters in Apolar Aprotic Solvents. As stated earlier, other reactivity parameters for ortho substituted benzoic acids in such media have been reported.33,38,40 These include log K (the association constant for reactions with crystal violet cabinol base in toluene)40 (Table 1), log KBHA (the association constant for reactions with 1,3diphenylguanidine in the presence of bromophthalein magenta E, the indicator acid in benzene)38 (Table 2), and log k (the rate coefficient for reactions with diazodiphenylmethane in chlorobenzene)33 (Table 2). While correlating the data for these reactivity parameters with Fujita
Nishioka’s model, eq 14, significant regression results are obtained for log K (toluene) with σop and F only, and log KBHA (benzene) or log k (chlorobenzene) with σop only. The correlations between log kþ1 (toluene) and log K (toluene), log KBHA (benzene), or log k (chlorobenzene), log kþ1 ¼ 1:50 log K
0:73 ðn ¼ 13;

R 2 ¼ 0:791;

ð25Þ

Fð49:63Þ-level significance on R

better than 0:1%Þ log kþ1 ¼ 1:47 log KBHA
4:51 ðn ¼ 10; R 2 ¼ 0:733;

ð26Þ

Fð21:91Þ-level of significance on R

better than 0:5%Þ log kþ1 ¼ 1:14 log k þ 2:68 ðn ¼ 12; R 2 ¼ 0:766;

ð27Þ

Fð32:65Þ-level of significance on R

better than 0:1%Þ 4621

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intramolecular hydrogen bonding effect in the salicylate anion plays an overwhelming role in enhancing the reactivity (log kþ1) of salicylic acid in toluene. In fact, as estimated by the partial regression coefficients for σoi and σHBi from the results of the models eq 16 and eq 24 (0.95 and 0.40), the percent contribution of hydrogen bonding effect (σHBi) on log kþ1 is as large 30%. This effect with its overwhelming role is, in fact, observed though in different degrees in media ranging from highly polar protic water to the solvent free gas phase.8,11,14,37

Figure 2. A comparison of correlations of log kþ1 (toluene) on log K (toluene), log KBHA (benzene), and log k (chlorobenzene).

show that as compared to log K (toluene), log KBHA (benzene), or log k (chlorobenzene), log kþ1 (toluene) is distinctly more sensitive to the ortho effect. The three correlations eqs 25
27 are compared in Figure 2. log kþ1 (Toluene) vs pKa (Water). It is obvious that the reactivity parameter log kþ1 is a very satisfactory and sensitive measure of the strength of acids in toluene or other apolar aprotic solvents. It is of interest to see the effect of changing the solvent from apolar toluene to highly polar water on the sensitivity of acidities to the ortho effect. On correlating the pKa (H2O) data for the same 13 acids (employed in log kþ1 correlation) with Fujita
Nishioka’s model, eq 14, pKa ðH2 OÞ ¼
0:55σop
1:94F þ 0:48Es þ 4:52 ð28Þ ðn ¼ 13,

R 2 ¼ 0:868Þ

with the F(19.76)-level of significance on R better than 0.1% and the t-level of significance on F, f, and δ better than 10, 0.5, and 5%, respectively. A comparison between the results, eqs 21 and 27, shows readily that whereas the sensitivity to a substituent’s ordinary electrical effect (F) increases as much as ∼6 times on changing the solvent from water to toluene, the sensitivity to its proximity electrical effect (f) increases ∼1.5 times only. This favors strongly the transmission of a substituent’s proximity electrical effect as a direct field effect mainly through the molecular cavity rather through the solvent. The simultaneous enhancement in the sensitivity of a substituent’s steric effects indicates a greater ease in the relief of the steric strain during acid to anion conversion in toluene as compared to H2O, seemingly the anion being more easily twistable in toluene than in strongly solvating water. The overall correlation, log kþ1 ðtolueneÞ ¼
2:46pKa ðH2 OÞ þ 12:18 ðn ¼ 13; R 2 ¼ 0:756;

’ CONCLUSIONS Ortho substituted benzoic acid
crystal violet carbinol base reactions in toluene follow general acid-catalyzed pseudo-firstorder reversible kinetics. The forward rate constant (log kþ1) of the reactions is found to be the appropriate toluene-phase reactivity/acidity parameter for the acids. The ortho effect on log kþ1 was analyzed using a number of correlation models. The regression results are all very significant, and the best regression equation is given by Fujita
Nishioka’s model. The findings are consistent with the reaction mechanism involving the acid anion formation from an acid
carbinol hydrogen bonded complex in the rate determining step.27 log kþ1 is found to be equally sensitive to a substituent’s ordinary electrical and total proximity effects consisting of its proximity electrical and steric effects. A substituent’s steric effect is generally subordinate to its proximity electrical effect barring certain exceptions, e.g., Me and others. Interestingly, log kþ1 (toluene) is ∼2.5 times as sensitive as pKa (H2O) to structural effects of ortho substituents. The results show further that, unlike the ordinary electrical effect, the proximity electrical effect is transmitted mainly through the molecular cavity rather through the solvent. The case of salicylic acid is an exception. Its enhanced acidity in toluene was quantitatively described using Pytela
Li
ska’s σHBi parameter. The exceedingly high value observed for the toluenephase acidity of anthranilic acid and extensively homoconjugated acid complex formation seem related to the acid’s three hydrogen bond donors and the use of its relatively large concentrations. ’ ASSOCIATED CONTENT

bS

Supporting Information. Tables S1
S15 listing the experimental overall rate constant, k (min
1), vs acid concentration, CHA (M), data at 28.0 ( 0.1
C for reactions of 15 ortho substituted benzoic acids with Crystal Violet carbinol base (the reference base) in toluene. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (91) 93 35 41 70 50.

ð29Þ

Fð34:14Þ-level of significance on R

better than 0:1%Þ shows log kþ1 (toluene) is ∼2.5 times as sensitive as pKa (water) to the ortho effect. Case of Salicylic Acid. When the data set was extended to log kþ1 of salicylic acid and correlated with Pytela
Li
ska’s model, eq 16, the regression results, eq 24, demonstrate clearly that

’ ACKNOWLEDGMENT The financial support extended by University Grants Commission, New Delhi, is gratefully acknowledged. We thank Mr. Satish Kumar for his assistance in computer work. ’ REFERENCES (1) Hofmann, A. W. Ber. 1872, 5, 704–719. (2) Meyer, V.; Sudborough, J. J. Ber. 1894, 27, 1580–1592. 4622

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